Digital Audio Corporation MicroDAC IV User`s manual

MicroDAC IV
PORTABLE PROGRAMMABLE DIGITAL FILTER
CONFIGURATION SOFTWARE
USER’S MANUAL
DIGITAL AUDIO CORPORATION
A DRI COMPANY
MicroDAC IV
PORTABLE PROGRAMMABLE DIGITAL FILTER
CONFIGURATION SOFTWARE
User’s Manual
March 31, 2003
Document Number: 990398
Version 1.0
DIGITAL AUDIO CORPORATION
A DRI COMPANY
5121 Holly Ridge Drive
Raleigh, NC 27612
Phone:
919 782 6767
Fax:
919 782 6766
sales@dacaudio.com
www.dacaudio.com
Copyright © 1999 by Digital Audio Corporation.
All rights reserved.
TABLE OF CONTENTS
1. OVERVIEW ..................................................................................................................................................... 2
1.1 Introduction ............................................................................................................................................... 2
2. INSTALLATION............................................................................................................................................... 4
2.1 Software Requirements ................................................................................................................................ 4
2.2 Installing the Software .................................................................................................................................. 4
3. MICRODAC IV CONFIGURATION CONTROL PANEL ........................................................................................ 5
3.1 MicroDAC IV Control Panel ..................................................................................................................... 5
3.2 Customizing Filter Sets ............................................................................................................................ 5
3.3 Input Limiter .............................................................................................................................................. 6
3.4 Input or Output Highpass Filter ..................................................................................................................... 7
3.5 Output Lowpass Filter .............................................................................................................................. 7
3.6 Output Automatic Gain Control ..................................................................................................................... 7
3.7 Panel Locking Controls ............................................................................................................................ 7
3.7.1 Utilities Menu Option........................................................................................................... 8
3.7.2 Panel Lock Settings Window............................................................................................... 8
3.8 Program MicroDAC IV Button.................................................................................................................. 9
4. PROGRAMMING THE MICRODAC IV ........................................................................................................11
4.1 Selecting the correct COM Port....................................................................................................................11
4.2 Transferring the Filter Configurations ...........................................................................................................11
5. STORING FILTER CONFIGURATIONS .............................................................................................................13
5.1 Save File Dialog Box ...............................................................................................................................13
6. RECALLING FILTER CONFIGURATIONS .........................................................................................................15
6.1 Open File Dialog Box ..............................................................................................................................15
7. FILTER CONTROL WINDOWS ....................................................................................................................17
7.1 One channel adaptive filter .....................................................................................................................17
7.2 Lowpass filter ...........................................................................................................................................18
7.3 Highpass filter ..........................................................................................................................................19
7.4 Bandpass filter .........................................................................................................................................20
7.5 Bandstop Filter.........................................................................................................................................22
7.6 Comb Filter ..............................................................................................................................................23
7.7 Notch Filter...............................................................................................................................................24
7.8 Slot Filter ..................................................................................................................................................25
7.9 20-Band Graphic Equalizer.....................................................................................................................27
7.10 Spectral Graphic Equalizer ...................................................................................................................27
7.11 Imported Coeffecient File......................................................................................................................31
7.11.1 Coefficient File Format ...................................................................................................... 31
7.12 Pass Thru Filter .....................................................................................................................................33
7.13 Two Channel Adaptive Filter ......................................................................................................................33
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8. ADJUSTING MAXIMUM TAPS FOR EACH STAGE............................................................................................35
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LIST OF FIGURES
Figure 3-1 MicroDAC IV Control Panel .................................................................................................................... 5
Figure 3-2 Input Limiter Controls............................................................................................................................. 6
Figure 3-3 Highpass Filter....................................................................................................................................... 7
Figure 3-4 Lowpass Filter........................................................................................................................................ 7
Figure 3-5 Automatic Gain Control .......................................................................................................................... 7
Figure 3-6 Utilities Menu ......................................................................................................................................... 8
Figure 3-7 Panel Lock Settings Windows................................................................................................................. 9
Figure 4-1: A Typical Computer’s Serial Ports.........................................................................................................11
Figure 4-2: MicroDAC IV Transfer Window .............................................................................................................11
Figure 4-3: MicroDAC IV Transfer Window .............................................................................................................12
Figure 5-1: Save As Setup File Window..................................................................................................................13
Figure 6-1: Open Setup File Window......................................................................................................................15
Figure 7-1 1CH Adaptive Filter Control Window......................................................................................................17
Figure 7-2: Lowpass Filter Control Window.............................................................................................................18
Figure 7-3: Lowpass Filter Graphical Description ....................................................................................................19
Figure 7-4: Highpass Filter Control Window............................................................................................................19
Figure 7-5: Highpass Filter Graphical Description ...................................................................................................20
Figure 7-6: Bandpass Filter Control Window...........................................................................................................21
Figure 7-7: Bandpass Filter Graphical Description ..................................................................................................21
Figure 7-8: Bandstop Filter Control Window............................................................................................................22
Figure 7-9: Bandstop Filter Graphical Description ...................................................................................................23
Figure 7-10: Comb Filter Control Window ...............................................................................................................23
Figure 7-11: Comb Filter Graphical Description.......................................................................................................24
Figure 7-12: Notch Filter Control Window ...............................................................................................................25
Figure 7-13: Notch Filter Graphical Description.......................................................................................................25
Figure 7-14: Slot Filter Control Window ..................................................................................................................26
Figure 7-15: Slot Filter Graphical Description..........................................................................................................26
Figure 7-16: 20-Band Graphic Equalizer Control Window........................................................................................27
Figure 7-17: Spectral Graphic Equalizer Window ....................................................................................................28
Figure 7-18: New Spectral Graphic Equalizer Display .............................................................................................28
Figure 7-19: Spectral Graphic Draw in Progress .....................................................................................................29
Figure 7-20: Completed Spectral Graphic Draw......................................................................................................29
Figure 7-21: Spectral Graphic Edit Window ............................................................................................................29
Figure 7-22: Spectral Graphic Define Edit Region...................................................................................................30
Figure 7-23: Spectral Edit In Progress....................................................................................................................30
Figure 7-24: Completed Spectral Graphic Edit........................................................................................................30
Figure 7-25: Normalized Spectral Graphic Equalizer ...............................................................................................31
Figure 7-26: Open Imported Coefficient File Dialog Box..........................................................................................31
Figure 7-27: Pass Thru Filter Selection...................................................................................................................33
Figure 7-28 Two Channel Adaptive Filter Control Window.......................................................................................34
Figure 8-1 Adjust Maximum Taps Button................................................................................................................35
Figure 8-2 Adjust Maximum Taps Control Screen ...................................................................................................35
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1
1. OVERVIEW
1.1 Introduction
The MicroDAC IV Configuration Software allows for complete setup and control of the three preset
adaptive filters and four configurable filters, each consisting of up to two stages. The software provides an
easy to use, intuitive interface for configuring the filter settings and transferring them to the MicroDAC IV.
The Configuration Software provides:
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Choice of two stages within each filter for a total of 3600 taps
Graphical representation of the actual audio path through the digital filters
Configurable filter tap distribution between adaptive filter stages
13 different filter types to choose from
Selectable Input Limiter, Lowpass Filter, Highpass Filter, and Output Automatic Gain Control
(AGC) for each filter setup
Storable configuration settings for later recall and use
Automatic COM Port detection when transferring filter to the MicroDAC IV
Front panel locking utility to protect accidental front panel switch changes
Online help for quick and easy access
Each filter can be configured in a Mono Single (one stage), Mono Series (two stages in series), or StereoLinked (two stages in parallel) mode. The stages within each filter can be independently selected from
one of the following filter types:
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One Channel Adaptive Filter
Two Channel Adaptive Filter (first stage in Mono modes only)
Lowpass Filter
Highpass Filter
Bandpass Filter
Bandstop Filter
Comb Filter
Notch Filter
Slot Filter
20 Band Graphic Equalizer
Spectral Graphic Equalizer
Pass Thru Filter (digital bypass)
Floating-point format coefficient text files can be imported as well.
A description of each filter type, its application and a definition of the control parameters can be found in
the following sections.
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3
2. INSTALLATION
2.1 Software Requirements
This Windows software-based product consists of an installation CD, User’s manual, and communications
cable. Supported operating systems include Windows 95/98/ME/2000/NT/XP. The PC should have the
following minimum capabilities:
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200MHz Pentium III processor
32MB RAM
10 MB of free hard disk space storage
CD-ROM drive for installing the software
800x600 resolution SVGA monitor
2.2 Installing the Software
To install the MicroDAC IV software, simply insert the MicroDAC IV Installation CD into your CD-ROM
drive1.
Often times after the installation is complete you will be instructed to reboot your system. Once the
system has rebooted you can run the MicroDAC IV software by clicking on the Start button, then the
MicroDAC IV menu, then the MicroDAC IV icon.
1
If the installation software does not automatically run after several seconds, you will need to manually start the
installation as follows:
With the MicroDAC IV Installation CD in your CD-ROM drive click on the Start menu and then the Run option. In the
window that appears, type:
X:\setup.exe
And click OK or press the Enter key. “X” would be the drive letter of your CD-ROM. The installation program will then start;
follow the directions given in the installation program.
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3. MICRODAC IV CONFIGURATION CONTROL PANEL
3.1 MicroDAC IV Control Panel
Figure 3-1 MicroDAC IV Control Panel
At the top of the MicroDAC IV Control Panel is a Menu Bar. From the File menu, you can choose to
create a New2 filter configuration, Open a filter configuration file, Save a file As a different or similar file
name, Program the MicroDAC IV, adjust the Panel Lock Settings to be programmed, use the Utilities
to directly control the panel lock settings, select a COM Port, or Exit the software. From the Help menu
item you can view the Contents of the software help file, Search for Help On a topic in this help file, or
read information About the MicroDAC IV software.
Below the Menu Bar is the Toolbar. Each small icon allows a quick shortcut to some of the Menu Bar
items. Each icon represents the menu function of New, Open, and Save As, respectively. Beside the
tool icons is the Program MicroDAC IV button, and on the rightmost portion of the toolbar is the Panel
Lock Setting indicator.
3.2 Customizing Filter Sets
The MicroDAC IV Control Panel shows a graphical representation of the audio path through the different
processes in the MicroDAC IV. As each different filter type is selected, and features such as the Limiter,
Highpass Filter, Lowpass Filter, and Output AGC are selected or removed, the Control Panel will update
its block diagram display.
The MicroDAC IV Control Panel allows each of the seven filter setups to be configured. Each setup can
be selected by clicking on the "tabs" that correspond to the labels on the front panel MODE switch of the
MicroDAC IV.
There are up to two Stages for each filter. The Stage Mode radio buttons control how the stages are
configured. The options are described as follows:
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The New menu option allows you to reset the first three filter modes to their factory defaults. Modes A, B, C, and D
will be configured as Mono Series, Stage 1 filter set to Pass Thru, and no AGC. This is a quick and easy way to
configure an entirely new filter configuration.
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Mono Single configures the MicroDAC IV to apply a single large stage (Stage 1) of filtering to the
Left Input only. When using the 2CH Adaptive filter, the Right Input is used as the cancellation
reference. The output of Stage 1 is routed to both the Left and Right Outputs.
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Mono Series configures the MicroDAC IV to split its filtering resources among two smaller stages
(Stage 1 and 2), applying them in series to the Left Input. When using the 2CH Adaptive filter, the
Right Input is used as the cancellation reference (the 2CH Adaptive filter is only available on
Stage 1.) The output of Stage 1 is routed to the input of Stage 2, and the output of Stage 2 is
routed to both the Left and Right Outputs.
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Stereo Linked configures the MicroDAC IV to apply the two stages (Stage 1 and 2) to a stereo
signal. The Left Input is routed through Stage 1 and the Right Input is routed through Stage 2.
The outputs of Stage 1 and 2 are routed to the Left and Right Outputs, respectively. The 2CH
Adaptive filter is not available with the Stereo Linked option. The Stage 2 filter is identical to
Stage 1; any changes made to the settings of Stage 1 will also be applied to Stage 2.
The stages can implement any of the following filter types:
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One Channel Adaptive Filter
Lowpass Filter
Highpass Filter
Bandpass Filter
Bandstop Filter
Comb Filter
Notch Filter
Slot Filter
20 Band Graphic Equalizer
Spectral Graphic Equalizer
Imported Coefficient File
Two Channel Adaptive Filter3
Pass Thru Filter
Once the filter type is selected, click on the Control button to bring up the control window for that filter.
This window allows you to customize the parameters for desired filter performance. After selecting the
values for each parameter, click on the OK button.
Refer to Appendix A for more information on the filter types and full explanations of their parameters.
3.3 Input Limiter
The Input Limiter controls are shown in Figure 3-2. The Input Limiter helps prevent the input signals
from distorting on overload and is “linked” so that an overload on either input channel will cause the levels
to be reduced equally on both input channels. Selecting the “Limiter In” radio button will enable the Input
Limiter for the filter setup, while “Limiter Out” will disable this feature.
Figure 3-2 Input Limiter Controls
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Not available in Stage 2
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3.4 Input or Output Highpass Filter
The Highpass Filter controls are shown in Figure 3-3. The 200 Hz Highpass Filter is used to remove
any low frequency noises that may need to be removed before processing (“Input HPF”) or after
processing (“Output HPF”). Selecting “No HPF” will disable this feature. This filter is separate from the
Highpass Filter described in Appendix A, and has no adjustable parameters.
Figure 3-3 Highpass Filter
3.5 Output Lowpass Filter
The output Lowpass Filter controls are shown in Figure 3-4. This adjustable filter is used to remove high
frequency hiss. The selectable cutoff frequencies are:
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2500 Hz
3200 Hz
3500 Hz
3900 Hz
4500 Hz
5000 Hz
This filter is applied after Stage 1 and 2, and is separate from the Lowpass Filter described in Appendix A.
Figure 3-4 Lowpass Filter
3.6 Output Automatic Gain Control
The output AGC (Automatic Gain Control) radio buttons are shown below. Each filter setup can have the
AGC turned off via the “No AGC” option, or have “10dB”, “20dB”, or “30dB” of AGC applied to the output
signal. When the AGC is active, it will boost a low level output signal by up to the maximum amount
specified.
Figure 3-5 Automatic Gain Control
3.7 Panel Locking Controls
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The MicroDAC IV Configuration Software allows the front panel controls to be locked to prevent
accidental tampering by nontechnical operators. This feature is useful when the MicroDAC IV is deployed
in a tactical setting. There are three locking options, described as follows:
3.7.1
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No Lock: This option unlocks all the front panel controls, allowing the user to adjust the input
levels and select any filtering mode via the MODE switch.
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Mode Lock: This option forces the unit to operate only with the settings for a specified
MODE, regardless of the actual switch positon. The Input Level controls, however, are still
adjustable.
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Lock All: This option locks both the MODE switch and the Input Level controls; thus, all front
panel controls are completely disabled, and changing them will have no effect.
Utilities Menu Option
There are two methods of adjusting the panel lock settings. One way is using the Panel Lock Settings
Window (See Section 3.7.2). The other way is via the Utilities option in the File menu (Figure 3-6).
Figure 3-6 Utilities Menu
The three options within the Utilities menu allow the MicroDAC IV panel controls to be locked or
unlocked without having to reprogram the entire unit. When selecting one of the “Lock” options, the unit
is locked to the current front panel settings. For example, to lock the MODE switch to “B”, first turn the
MODE switch to “B”, then within the Utilities menu choose Lock Mode Switch. This will work even if the
front panel is currently locked.
3.7.2
Panel Lock Settings Window
The second method is to use the Panel Lock Settings control window shown in Figure 3-7. This window
can be accessed either by clicking on the Panel Lock Settings label in the top right corner of the
MicroDAC IV Control Panel, or via the Panel Lock Settings option in the File menu.
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Figure 3-7 Panel Lock Settings Windows
The lock setting to be programmed into the MicroDAC along with the filter settings can be selected by
clicking on appropriate radio button in the “Panel Locked Mode” area of the window. When the Mode
Switch Lock option is selected, the “Mode Switch Setting” area of the window will be enabled, allowing
the desired MODE switch setting to be specified. When the Mode Switch and Level Controls Locked
option is selected the, “Level Controls” area of the window will be enabled. There, the desired Input Level
to be programmed into the MicroDAC IV can be specified.
The Retrieve Level Settings button will retrieve the current Input Level settings from the MicroDAC IV.
This feature is useful for empirically determining the desired level settings using the actual front panel
controls. Should you attempt to retrieve the Input Levels from a MicroDAC IV that has the panel lock set
to “Lock All”, you will receive a message informing you to first unlock the front panel via the Utilities menu
option.
3.8 Program MicroDAC IV Button
See Section 4.
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4. PROGRAMMING THE MICRODAC IV
4.1 Selecting the correct COM Port
The COM Port menu option within the File menu allows you to select the COM Port on which the
software will first attempt to communicate with MicroDAC IV when programming it. If you are unsure
which COM Port your MicroDAC IV is connected to, then leave it set at the default option of COM 1, and
the software will automatically search all COM Ports until the MicroDAC IV is found. An example of a
computer’s COM Port connections is shown in Figure 4-1.
Figure 4-1: A Typical Computer’s Serial Ports
4.2 Transferring the Filter Configurations
The Program MicroDAC IV button on the toolbar starts the process of searching for a MicroDAC IV and
transferring the filter settings to the MicroDAC IV via the serial port. This function can also be selected
through the File option in the Menu Bar. After the Program MicroDAC IV button is pressed, you will see
a window appear that asks you if you are prepared to program all seven filters. This is to remind you that
you only need to program the MicroDAC IV one time once you have all the filters set up the way you want
them.
Figure 4-2: MicroDAC IV Transfer Window
Several messages will appear in the transfer window ito inform you when the software is using the Auto
COM Port detection feature to find the MicroDAC IV, calculating the coefficients, and finally programming
the MicroDAC IV. The COM port that is being used to transfer the settings will appear in the lower right
corner of the Transferring Filters window. As the filters are transferred, the progress bar will display how
much of the total transfer has been completed. Once the transfer is complete, the window will disappear
and the Main Control Window will become active again.
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Figure 4-3: MicroDAC IV Transfer Window
Programming of the MicroDAC IV can be stopped at any time by pressing the Cancel button. A message
window may then appear to inform you that the download has been aborted and that the filters stored in
MicroDAC IV are no longer valid. Because the download was stopped before it was completed, the filter
settings stored in the MicroDAC IV may no longer be usable. It is advised that a valid configuration be
transferred to the MicroDAC IV by repeating the Program MicroDAC IV procedure mentioned above.
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5. STORING FILTER CONFIGURATIONS
5.1 Save File Dialog Box
Application:
To save time configuring the MicroDAC IV control settings, complete setups may easily be stored
to disk setup files for future recall.
Store a setup to a disk file as follows:
1.
Click on File on the MicroDAC IV menu bar. When the pulldown menu appears, click on
Save As (or click on the "floppy disk" icon on the toolbar). This will cause the following
window to appear:
Figure 5-1: Save As Setup File Window
2.
If you desire to place the setup file into a different drive (such as a floppy drive) or
directory, use the Save in selection box. If you select a drive that is not ready, an error
message will be generated.
3.
You will need to specify a filename for the setup. Click on the File Name text box, then
type the desired filename. All setup filenames must have the .MDC extension; thus, the
.MDC extension is automatically included in the text box.
4.
Click on Save to store the setup file with the selected filename to the specified drive and
directory. (The Open as read-only checkbox is not applicable, and is thus ignored.)
NOTE: The file "default.mdc" is the configuration file stored in the installation directory that holds the
shutdown information for the MicroDAC IV software. Whenever the MicroDAC IV software is closed, the
current screen settings are stored in this file. The next time the MicroDAC IV software is run, these
settings will be retrieved, enabling you to pick up where you left off.
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6. RECALLING FILTER CONFIGURATIONS
6.1 Open File Dialog Box
Application:
To save time configuring control settings in the MicroDAC IV control settings, complete setups
previously stored may be recalled from disk files with a few simple mouse clicks.
Open a setup from a disk file as follows:
1.
Click on File on the MicroDAC IV menu bar. When the pulldown menu appears, click on
Open (or click on the "open file folder" icon on the toolbar). This will cause the following
window to appear:
Figure 6-1: Open Setup File Window
2.
If you desire to open the setup file from a different drive (such as a floppy drive) or
directory, use the Look in selection box. If you select a drive that is not ready, an error
message will be generated.
3.
Once the desired setup file name has been found, either double-click it or click on Open
to recall it. The file will then be loaded.
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7. FILTER CONTROL WINDOWS
7.1 One channel adaptive filter
Application:
The 1CH Adaptive filter is used to automatically cancel predictable and convolutional noises from
the input audio. Predictable noises include tones, hum, buzz, engine/motor noise, and, to some
degree, music. Convolutional noises include echoes, reverberations, and room acoustics.
Figure 7-1 1CH Adaptive Filter Control Window
Description of controls is as follows:
Filter Size:
Used to set the number of FIR filter taps (filter order) in the adaptive filter. Filter
size is indicated both in taps and in milliseconds. Minimum Filter Size is 16 taps,
but can be set to as high as 3600 taps (if in Mono Single mode, See Section A-8.
).
Small filters are most effective with simple noises such as tones and music.
Larger filters should be used with complex noises such as severe reverberations
and raspy power hums. A nominal filter size of 256 taps is a good overall
general recommendation.
Adapt Rate:
Used to set the rate at which the filter adapts to changing signal conditions
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(Mathematically known as "Mu"). A Mu of 1 x 2 provides very slow adaptation,
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while a Mu of 32767 x 2 provides fastest adaptation.
Larger adapt rates should be used with changing noises such as music;
whereas, smaller adapt rates are acceptable for stable tones and reverberations.
Larger adapt rates sometimes affect voice quality, as the filter may attack
sustained vowel sounds.
Prediction
Span:
Sets the number of samples in the prediction
span delay line. Prediction span is indicated both in samples and in milliseconds,
and can be adjusted from 1 to 999 samples. Shorter prediction spans allow
maximum noise removal, while longer prediction spans preserve voice
naturalness and quality. A prediction span of 2 or 3 samples is normally
recommended.
Adapt Mode:
Selects Auto(matic) or Fixed adaptation rate. Auto is recommended. When
Fixed is selected, the specified Adapt Rate Mu is applied to the filter at all times.
However, when Auto is selected, the specified Adapt Rate is continuously
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rescaled depending upon the input signal level. Overall convergence rate is
faster with Auto.
Adapt Algorithm:
Selects the adaptive algorithm to use when processing the audio. The Least
Absolute Value (LAV) algorithm offers faster, but less accurate convergence.
The Least Mean Square (LMS) algorithm does not converge as fast as the LAV,
but offers a more accurate convergence and better maintains the final solution.
Processor
Output:
Used to select the output of the filter. The
normal setting is Residue, as this is the processed audio with the predicted noise
signal removed. The Predict output is the predicted noise signal. For example, if
a 1CH Adaptive Filter was being used to remove a tone, the Processor Output
choice of Predict would output only the the subtracted tone.
Crash Detect:
Activates the built-in “crash” detection feature in the MicroDAC IV. This feature
automatically detects when the adaptive filter has “crashed” and clears the filter.
Checking this box enables this feature.
7.2 Lowpass filter
Application:
The Lowpass filter is used to decrease the energy level (lower the volume) of all signal
frequencies above a specified Cutoff Frequency, thus reducing high-frequency noises, such as
tape hiss, from the input audio. The Lowpass filter is sometimes called a "hiss filter."
The Cutoff Frequency is usually set above the voice frequency range so that the voice signal will
not be disturbed. While listening to the filter output audio, the Cutoff Frequency can be
incrementally lowered from its maximum frequency until the quality of the voice just begins to be
affected, achieving maximum elimination of high-frequency noise.
The amount of volume reduction above the Cutoff Frequency can further be controlled by
adjusting the Stopband Attenuation setting (maximum volume reduction is 90dB). The slope at
which the volume is reduced from normal (at the Cutoff Frequency) to the minimum volume
(specified by Stopband Attenuation) can also be controlled by adjusting the Transition Slope
setting.
Figure 7-2: Lowpass Filter Control Window
Description of controls is as follows:
Cutoff
Frequency:
Specifies frequency in Hertz above which all
Signals are attenuated. Frequencies below this cutoff are unaffected. Minimum
Cutoff Frequency is 100 Hz, while the maximum Cutoff Frequency is 5400 Hz.
Cutoff Frequency can be adjusted in 1 Hz steps.
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Stopband
Attenuation:
Specifies amount in dB by which frequencies above
the Cutoff Frequency are ultimately attenuated. Stopband attenuation is adjustable
from 0dB to 90dB in 1 dB steps.
Transition
Slope:
Specifies slope at which frequencies above the
Cutoff Frequency are rolled off in dB per octave. Sharpest roll off occurs when
Transition Slope is set to maximum, while gentlest roll off occurs when Transition
Slope is set to minimum. Sharp roll-offs may cause the voice to sound hollow but
will allow more precise removal of high frequency noises. Note that the indicated
value changes depending upon Cutoff Frequency.
A graphical description of the lowpass filter and its controls is given below.
Figure 7-3: Lowpass Filter Graphical Description
7.3 Highpass filter
Application:
The Highpass filter is used to decrease the energy level (lower the volume) of all signal
frequencies below a specified Cutoff Frequency, thus reducing low-frequency noises, such as
tape or acoustic room rumble, from the input audio (The Highpass filter is sometimes called a
"rumble filter").
The Cutoff Frequency is usually set below the voice frequency range (somewhere below 300 Hz)
so that the voice signal will not be disturbed. While listening to the filter output audio, the Cutoff
Frequency, initially set to 100 Hz, can be incrementally increased until the quality of the voice just
begins to be affected, achieving maximum elimination of low-frequency noise.
The amount of volume reduction below the Cutoff Frequency can further be controlled by
adjusting the Stopband Attenuation setting (maximum volume reduction is 90dB). The slope at
which the volume is reduced from normal (at the Cutoff Frequency) to the minimum volume
(specified by Stopband Attenuation) can also be controlled by adjusting the Transition Slope
setting.
Figure 7-4: Highpass Filter Control Window
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Description of controls is as follows:
Cutoff
Frequency:
Specifies frequency in Hertz below which all
signals are attenuated. Frequencies above this cutoff are unaffected. Minimum
Cutoff Frequency is 100 Hz, while the maximum Cutoff Frequency is 5400 Hz.
Cutoff Frequency can be adjusted in 1 Hz steps.
Stopband
Attenuation:
Specifies amount in dB by which frequencies below
the Cutoff Frequency are ultimately attenuated. Stopband attenuation is adjustable
from 0dB to 90dB in 1 dB steps.
Transition
Slope:
Specifies slope at which frequencies below the
Cutoff Frequency are attenuated in dB per octave. Sharpest attenuation occurs
when Transition Slope is set to maximum, while gentlest attenuation occurs when
Transition Slope is set to minimum. Note that the indicated value changes
depending upon Cutoff Frequency.
A graphical description of the highpass filter and its controls is shown below.
Figure 7-5: Highpass Filter Graphical Description
7.4 Bandpass filter
Application:
The Bandpass filter is used to decrease the energy level (lower the volume) of all signal
frequencies below a specified Lower Cutoff Frequency and above a specified Upper Cutoff
Frequency, thus combining the functions of a seriesed Lowpass and Highpass filter into a single
filter. The signal region between the Lower Cutoff Frequency and the Upper Cutoff Frequency is
called the passband region. The Bandpass filter is useful for simultaneously reducing both lowfrequency rumble and high-frequency hiss.
The Lower Cutoff Frequency is usually set below the voice frequency range (somewhere below
300 Hz) so that the voice signal will not be disturbed. While listening to the filter output audio, the
Lower Cutoff Frequency, minimum of 0 Hz, can be incrementally increased until the quality of the
voice just begins to be affected, achieving maximum elimination of low-frequency noise.
The Upper Cutoff Frequency is usually set above the voice frequency range (somewhere above
3000 Hz) so that the voice signal will not be disturbed. While listening to the filter output audio,
the Upper Cutoff Frequency, minimum setting of 100Hz over the Lower Cutoff Frequency, can be
incrementally lowered until the quality of the voice just begins to be affected, achieving maximum
elimination of high-frequency noise.
The amount of volume reduction outside the passband region can further be controlled by
adjusting the Stopband Attenuation setting (maximum volume reduction is 90dB). The slope at
which the volume is reduced from normal (at each Cutoff Frequency) to the minimum volume
(specified by Stopband Attenuation) can also be controlled by adjusting the Transition Slope
setting.
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Figure 7-6: Bandpass Filter Control Window
Description of controls is as follows:
Lower
Cutoff
Frequency:
Specifies frequency in Hertz below which all
signals are attenuated. Frequencies between this
cutoff and the Upper Cutoff Frequency are Unaffected. Minimum Lower Cutoff
Frequency is 0 Hz, while the maximum Lower Cutoff Frequency is 100 Hz below the
Upper Cutoff Frequency. Lower Cutoff Frequency can be adjusted in 1 Hz steps.
NOTE: The Lower Cutoff Frequency can never be set higher than 100 Hz below the
Upper Cutoff Frequency.
Upper
Cutoff
Frequency:
Specifies frequency in Hertz above which all
signals are attenuated. Frequencies between this
cutoff and the Lower Cutoff Frequency are unaffected. Minimum Upper Cutoff
Frequency is 100 Hz above the Lower Cutoff Frequency, while the maximum Upper
Cutoff Frequency is 5400 Hz. Upper Cutoff Frequency can be adjusted in 1 Hz
steps.
NOTE: The Upper Cutoff Frequency can never be set lower than 100 Hz above the
Lower Cutoff Frequency.
Transition
Slope:
Specifies slope at which frequencies below the
Lower Cutoff Frequency and above the Upper Cutoff Frequency are attenuated in
dB per octave. Sharpest attenuation occurs when Transition Slope is set to
maximum, while gentlest attenuation occurs when Transition Slope is set to
minimum. Note that the indicated value changes depending upon Cutoff Frequency.
Also, note that the Lower and Upper Transition Slopes always have different values;
this is because the frequency width of an octave is proportional to Cutoff Frequency.
Stopband
Attenuation:
Specifies amount in dB by which frequencies below
the Lower Cutoff Frequency and above the Upper Cutoff Frequency are ultimately
attenuated. Stopband Attenuation is adjustable from 0dB to 90dB in 1 dB steps.
A graphical description of the Bandpass filter and its controls follows in the figure below.
Figure 7-7: Bandpass Filter Graphical Description
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7.5 Bandstop Filter
Application:
The Bandstop filter is used to decrease the energy level (lower the volume) of all signal
frequencies above a specified Lower Cutoff Frequency and below a specified Upper Cutoff
Frequency. The signal region between the Lower Cutoff Frequency and the Upper Cutoff
Frequency is called the stopband region. The Bandstop filter is useful for removing in-band noise
from the input signal.
The Lower Cutoff Frequency is usually set below the frequency range of the noise, while the
Upper Cutoff Frequency is set above the frequency range of the noise. While listening to the filter
output audio, the Lower and Upper Cutoff Frequencies can be incrementally adjusted to achieve
maximum elimination of noise while minimizing loss of voice.
The amount of volume reduction in the stopband region can further be controlled by adjusting the
Stopband Attenuation setting (maximum volume reduction is 90dB). The slope at which the
volume is reduced from normal (at each Cutoff Frequency) to the minimum volume (specified by
Stopband Attenuation) can also be controlled by adjusting the Transition Slope setting.
Figure 7-8: Bandstop Filter Control Window
Description of controls is as follows:
Lower Cutoff
Frequency:
Specifies frequency in Hertz below which no
signals are attenuated. Frequencies between this cutoff and the Upper Cutoff
Frequency are attenuated. Minimum Lower Cutoff Frequency is 0 Hz, while the
maximum Lower Cutoff Frequency is 100 Hz below the Upper Cutoff Frequency.
Lower Cutoff Frequency can be adjusted in 1 Hz steps.
NOTE: The Lower Cutoff Frequency can never be set higher than 100 Hz below
the Upper Cutoff Frequency.
Upper Cutoff
Frequency:
Specifies frequency in Hertz above which no
signals are attenuated. Frequencies between this cutoff and the Lower Cutoff
Frequency are attenuated. Minimum Upper Cutoff Frequency is 100 Hz above
the Lower Cutoff Frequency, while the maximum Upper Cutoff Frequency is 5400
Hz. Upper Cutoff Frequency can be adjusted in 1 Hz steps.
NOTE: The Upper Cutoff Frequency can never be set lower than 100 Hz above
the Lower Cutoff Frequency.
Transition
Slope:
Specifies slope at which frequencies above the
Lower Cutoff Frequency and below the Upper Cutoff Frequency are attenuated in
dB per octave. Sharpest attenuation occurs when Transition Slope is set to
maximum, while gentlest attenuation occurs when Transition Slope is set to
minimum. Note that the indicated value changes depending upon Cutoff
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Frequency. Also, note that the Lower and Upper Transition Slopes always have
different values; this is because the frequency width of an octave is proportional
to Cutoff Frequency.
Stopband
Attenuation:
Specifies amount in dB by which frequencies
above the Lower Cutoff Frequency and below the Upper Cutoff Frequency are
attenuated. Stopband attenuation is adjustable from 0dB to 90dB in 1 dB steps.
A graphical description of the Bandstop filter and its controls follows in the figure below.
Figure 7-9: Bandstop Filter Graphical Description
7.6 Comb Filter
Application:
The Comb filter is used to remove, or "notch out", harmonically related noises (noises which
have exactly equally-spaced frequency components), such as power-line hum, constant-speed
motor/generator noises, etc., from the input audio. The filter response consists of a series of
equally-spaced notches which resemble a hair comb, hence the name "Comb filter".
Adjust the Fundamental Notch Frequency to the desired spacing between notches (also known
as "fundamental frequency"). Set the Notch Limit to the frequency beyond which you do not want
any more notches. Set the Notch Depth to the amount in dB by which noise frequency
components are to be reduced.
Normally, the Notch Harmonics option will be set to All, causing frequencies at all multiples of the
Fundamental Notch Frequency (within the Notch Limit) to be reduced. However, certain types of
noises have only the odd or even harmonic components present. In these situations, set the
Notch Harmonics option to either Odd or Even.
Figure 7-10: Comb Filter Control Window
Description of controls is as follows:
Fundamental
Notch
Frequency
Specifies fundamental frequency in Hertz of
comb filter.Notches are generated at multiples,
or harmonics, of this frequency.
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Notch Limit:
Specifies frequency in Hertz above which no notches are generated. Minimum
Notch Limit is 2 and 1/2 times the Fundamental Notch Frequency, while the
maximum Notch Limit setting is 5400 Hz. Notch Limit is adjustable in 1 Hz steps.
Notch Depth:
Depth of notches that are generated. Notch Depth is adjustable from 0 dB to 90
dB in 1 dB steps.
Notch Harmonics:
Specifies whether notches will be generated at All, Odd, or Even multiples, or
harmonics, of the Comb Frequency. If, for example, the Comb Frequency is set
to 60.000 Hz, then selecting All will generate notches at 60 Hz, 120 Hz, 180 Hz,
240 Hz, 300 Hz, etc. Selecting Odd will generate notches at 60 Hz, 180 Hz, 300
Hz, etc. Selecting Even will generate notches at 120 Hz, 240 Hz, 360 Hz etc.
Hint: In most instances, Odd will be sufficient to remove all hum harmonics.
A graphical description of the Comb filter and its controls follows in the figure below.
Figure 7-11: Comb Filter Graphical Description
7.7 Notch Filter
Application:
The Notch filter is used to remove, or "notch out", a narrow-band noise, such as a tone or a
whistle, from the input audio with minimal effect to the remaining audio. The Notch filter works
best with stable noise sources which have constant frequency; if the frequency of the noise
source varies, then the 1CH Adaptive filter is recommended.
To properly utilize the Notch filter, you will first need to identify the frequency of the noise; this is
best done using a Spectrum Analyzer.
Initially set the Notch Depth to 90 dB and the Notch Width to the narrowest possible value. Next,
set the Notch Frequency to the noise frequency. Fine adjustment of the Notch Frequency may be
necessary to place the notch precisely on top of the noise signal and achieve maximum reduction
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of the noise. This is best done by adjusting the Notch Frequency up or down 1 Hz at a time while
listening to the Notch filter output on the headphones.
Often, the noise frequency will not remain absolutely constant but will vary slightly due to
modulation, recorder wow and flutter, and acoustic "beating." Therefore, you may need to
increase the Notch Width from its minimum setting to keep the noise within the notch.
For maximum noise reduction, set the Notch Depth to 90dB. It is best to adjust the Notch Depth
up from 90 dB until the tone is observed, then increase the depth 5 dB.
Figure 7-12: Notch Filter Control Window
Description of controls is as follows:
Notch Frequency:
Specifies frequency in Hertz which is to be removed from the input audio.
Minimum Notch Frequency is 0 Hz, while maximum Notch Frequency is 5400 Hz.
Notch Frequency is adjustable in 1 Hz steps.
Notch Depth:
Depth of the notch that is generated. Notch Depth is adjustable from 0dB to
90dB in 1 dB steps.
Notch Width:
Width of the generated notch in Hertz.
Hint: A notch filter works best for stable tones. It has a sharp bottom. If a flatter bottom
(stopband) is needed to accommodate some variation in tone frequency, the bandstop filter may
be prefered. Also, a 1CH Adaptive filter is useful in tracking varying tones.
A graphical description of the Notch filter and its controls follows in the figure below..
Figure 7-13: Notch Filter Graphical Description
7.8 Slot Filter
Application:
The Slot filter is used to isolate, or "slot", a single-frequency signal, such as a tone or a whistle, in
the input audio, attenuating all other audio. This is the exact opposite of the Notch filter function.
25
To properly utilize the Slot filter, you will first need to identify the frequency of the signal to be
isolated; this is best done using a Spectrum Analyzer.
Once the frequency of the signal has been identified, initially set Stopband Attenuation to 90 dB
and the Slot Width to the narrowest possible value. Next, set the Slot Frequency to the signal
frequency. Fine adjustment of the Slot Frequency may be necessary to place the slot right on top
of the signal. This is best done by adjusting the Slot Frequency up or down 1 Hz at a time while
listening to the Slot filter output on the headphones.
Usually, the signal frequency will not remain constant but will vary slightly due to modulation,
recorder wow and flutter, and acoustic "beating". Therefore, you may need to increase the Slot
Width from its minimum setting to avoid having the signal move in and out of the slot.
Figure 7-14: Slot Filter Control Window
To optimize background noise reduction for your application, set the Stopband Attenuation to
60dB. If, however, you wish to leave a small amount of the background noise mixed in with the
isolated signal, adjust the Stopband Attenuation to the desired value.
Description of controls is as follows:
Slot Frequency:
Specifies frequency in Hertz which is to be enhanced in the input audio.
Minimum Slot Frequency is 30 Hz, while maximum Slot Frequency is 5400 Hz.
Slot Frequency is adjustable in 1 Hz steps.
Stopband
Attenuation:
Specifies amount in dB by which frequencies
other than the Slot Frequency are attenuated. Stopband attenuation is
adjustable from 0dB to 90dB in 1 dB steps.
Slot Width:
Width of the generated slot in Hertz.
A graphical description of the Slot filter and its controls follows in the figure below. Note that the slot
width is defined at its -6 dB points.
Figure 7-15: Slot Filter Graphical Description
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7.9 20-Band Graphic Equalizer
Application:
The 20-band Graphic Equalizer is an easy-to-use linear-phase FIR digital filter that is used to
reshape the spectrum of the final output signal. Reshaping is accomplished with twenty vertical
scroll bars (also called "slider" controls) which adjust the attenuation of each frequency band.
These controls are very similar to the slider controls found on analog graphic equalizers found on
many consumer stereo systems, and thus should be very familiar to even the novice user.
Figure 7-16: 20-Band Graphic Equalizer Control Window
Description of controls/indicators is as follows:
Slider controls:
The twenty vertical scroll bar "slider" controls are used to set the frequency
response of the equalizer. Each slider can set the gain of its frequency band
to any value between 0dB and -40 dB in 1dB steps.
Center Frequency:
Note that the Center Frequency of each band is labelled underneath each
slider, and that bands are more closely spaced at low frequencies.
Gain Indication:
Above each slider control, the gain for that frequency band is given. The
gain can also be visualized graphically by the position of the slider control.
Normalize Button:
This button instantly shifts all slider controls up together until the top slider is
at 0dB. After normalization, the relative positioning of the sliders remains the
same. This allows the digital equalizer to implement the desired equalization
curve with minimum signal loss.
Make All 0dB
Button:
This button instantly moves the slider
controls for all bands to 0dB, defeating the entire equalizer. This is a useful
feature when it is desired to reset all sliders from scratch.
All Down 1dB
Button:
This button shifts all sliders down by 1dB
from their current positions; no slider, however, will be allowed to go below 40dB. This button allows the user to shift the entire equalizer curve down so
that there will be room to move one or more sliders up relative to the others.
7.10 Spectral Graphic Equalizer
Application:
In some applications, it may be necessary to precisely reshape the spectrum of input audio prior
to passing it through successive DSP filter stages. For example, if the audio is from a
microphone which has an unusual frequency response curve (for example, a microphone
acoustically modified as a result of concealment), a compensation filter that reshapes the audio to
a normal spectral shape might be desirable.
27
The Spectral Graphic Equalizer is essentially a 115-band graphic equalizer; however, instead of
having 115 separate slider controls, it allows the user to precisely draw the desired filter shape on
the computer screen, using the mouse, with as much or as little detail as desired..
The Edit feature allows the user to make readjustments to the filter shape, while the Normalize
button allows the user to shift the entire filter curve up until the highest point is at 0dB.
Figure 7-17: Spectral Graphic Equalizer Window
Description of controls/indicators is as follows:
Filter Display:
Graphically displays the current shape of the filter. Also used in conjunction
with the mouse to draw a new filter shape or to edit an existing one (see
New, Edit, and Normalize button descriptions). A grid is provided to assist
the user in visually judging frequency and attenuation at any point in the
display.
Freq and Atten
Readouts:
Used to precisely readout the frequency in
Hertz and attenuation in dB at any point in the filter curve. Hold the left
Mouse button down while editing the curve or drawing a new curve. These
readouts below the grid indicate precise frequencies and attenuations.
Releasing the button draws the segment.
The New, Edit, and Normalize buttons are used to graphically manipulate the shape of the filter curve.
Their functions are complex, and thus are best illustrated in the following mini-tutorial:
1.
From the MicroDAC IV Filter Configuration Tabs, select the Spectral Graphic Equalizer for
Stage 1 of any filter. Set the AGC setting to No AGC.
2.
Click on the Control button for Stage 1 to bring up the Spectral Graphic Equalizer control
window. When used for the first time, the control window will be the that of the previous
figure.
3.
Click on the New button to draw a new filter. The screen will now appear as follows:
Figure 7-18: New Spectral Graphic Equalizer Display
Had you accidentally clicked the New button, you could click on Abort to restore the previous
filter.
4.
You should now notice that all the buttons on the control window have been replaced by a
simple Abort botton. Clicking on Abort at any time prior to completing the curve draw
restores the previous filter..
28
To draw the new filter curve, you will need to carefully click the mouse cursor on points within
the filter display area which correspond to the desired attenuations at the desired
frequencies.
While the mouse click button is held down, the Freq and Atten readouts will be updated as
the mouse is moved; you can use this feature to place points in the filter curve at exact
frequencies and attenuations. When the mouse click button is released, a line segment will
be drawn from the last defined point on the curve to the current mouse cursor position.
For this example, placing points at precise frequencies and attenuations is not required; draw
the curve approximately as shown in the figure below using mouse clicks.
Note that the very first click always sets the 0 Hz attenuation starting point.
Hint: To advance the frequency a single step move the cursor to the left ofthe last frequency
position and click to mouse. The curve will advanced the smallest step at the specified
attenuation.
Figure 7-19: Spectral Graphic Draw in Progress
5.
Complete drawing the filter curve as shown below by drawing points all the way to the right
edge of the filter display area.
Figure 7-20: Completed Spectral Graphic Draw
When you have drawn the last point (must be at or beyond the right edge of the filter display
area), the the Spectral Graphic control window will return to normal appearance.
6.
Suppose you decide that you would like to remove the "dip" which occurs in the filter curve at
approximately 3000 Hz in the figure, above. Click on Edit to bring up the following display:
Figure 7-21: Spectral Graphic Edit Window
29
In this window, you can make the entire filter curve drop by a specified amount prior to editing
the curve. This can be used to create headroom which can be used to increase the gain
(decrease the attenuation) in one portion of the curve relative to the rest of the curve. For
now, select a drop of 0dB (No Drop) and click on Proceed.
7.
You should now notice that all the buttons on the control window have been replaced with a
single Abort button, which permits returning to the pre-Edit filter.
To edit out the dip, you will first need to define the edit region by carefully specifying the left
and right edges of the portion of the filter curve that you wish to modify. Click your mouse to
the left and to the right of the dip to produce the following display:
Figure 7-22: Spectral Graphic Define Edit Region
8.
Now, draw in the new portion of the filter curve using mouse clicks as in Step 4, above,
roughly as shown below.
Figure 7-23: Spectral Edit In Progress
9.
Complete drawing the new portion of the filter curve as shown below by drawing points all the
way to the right edge of the edit region:
Figure 7-24: Completed Spectral Graphic Edit
When you have drawn the last point (must be at or beyond the right edge of the edit region),
the mouse cursor will change to an "hourglass" shape for a few seconds while the filter is
being recalculated. When the calculations are complete, the mouse cursor and the buttons in
the Spectral Graphic control window will return to normal appearance.
10.
Normalizing the filter places the highest point on the filter curve at 0 dB. Doing so minimizes
loss in the filter and preserves system dynamic range. Now normalize the filter curve to 0dB
by clicking the Normalize button. You should see the mouse cursor change to the
"hourglass" shape for a few seconds; when the normalization calculations are complete, the
filter shape should appear as follows:
30
Figure 7-25: Normalized Spectral Graphic Equalizer
This completes the Spectral Graphic Equalizer mini-tutorial.
7.11 Imported Coeffecient File
Application:
The Imported Coefficient file is available for inputing filters created via other synthesis software.
Several math software packages (such as Matlab) are available from other vendors. These
packages provide high level math functions for computing complex filters. Once the filters have
been calculated the results can be stored in floating point format in a standard text file. The
MicroDAC IV Configuration Software is able to import these files and convert them into the format
necessary for transfer into the MicroDAC IV.
Figure 7-26: Open Imported Coefficient File Dialog Box
Description of controls are follows:
Open File Dialog:
Box
The only control available for this filter is the
Open File Dialog Box that appears when the Control button is pressed. This
window allows the user to select the filter file from any location on the local
computer or network. The default file extension is ".txt". If this is not the file
extension of the coeffiecient file, then click on the Files of Type selection box to
choose "All Files (*.*)". This option will allow you to see all files in the current
directory. Once the appropriate file is selected press OK. (The "Open as readonly" is not applicable to the coefficient file and is to be ignored.)
7.11.1 Coefficient File Format
The coefficient file format is restricted to only accept floating point numbers in the range of +1.0 and -1.0.
If a coefficient exceeds this limitation the software will clip the coefficient to its closest maximum. Each
coefficient needs to be in standard decimal notation or scientific notation. The following are some
examples of floating point coefficients in decimal format:
0.3455
31
0.0008975
Scientific notation is in the form:
mantissaE[+-]exponent
The following are some examples of floating point coefficients in scientific format:
3.25E-003
5.36E-2
1.25E-004
8.1234E2
=
=
=
=
0.003250
0.0536
0.000125
812.34 (clipped to +1.0 by the MicroDAC IV software)
Only one coefficient may be located on a line. The coefficients can be comma-separated, but must still
be stored on to a line. The MicroDAC IV only uses the maximum number of coefficients available for
each stage. If a file contains more than this number of coefficients those remaining are ignored.
Conversely if a file contains less than the maximum number of available coefficients the remaining
coefficients are set to zero. If the coefficient file is not set in the correct format the MicroDAC IV software
will not load the file and a popup message box will inform the user.
When the Imported Coefficient File is first selected, its default coefficients are set up as a digital bypass.
Once a valid coefficient file is loaded, its data becomes part of the MicroDAC IV configuration, which can
be stored in a settings file for later retrieval without the need of the original coefficient file.
NOTE: Keep in mind that the sample frequency will affect the results of the filter. The floating point filter
coefficients are normally set up using a specific sampling frequency. If the coefficients are used in a filter
with a different sampling frequency the filter will be skewed. The sample frequency of the MicroDAC IV is
12000Hz.
Below is an example coefficient file containing 53 filter coefficients:
4.387280E-003
1.262311E-002
7.795490E-003
-5.957981E-003
-1.444848E-002
-7.919611E-003
7.957611E-003
1.674714E-002
8.028165E-003
-1.060535E-002
-1.978514E-002
-8.120655E-003
1.430844E-002
2.407519E-002
8.196839E-003
-1.991908E-002
-3.074622E-002
-8.256326E-003
2.956892E-002
4.286449E-002
8.298883E-003
-5.050633E-002
-7.269745E-002
-8.324551E-003
1.334762E-001
2.797442E-001
3.416667E-001
2.797442E-001
1.334762E-001
-8.324551E-003
32
-7.269745E-002
-5.050633E-002
8.298883E-003
4.286449E-002
2.956892E-002
-8.256326E-003
-3.074622E-002
-1.991908E-002
8.196839E-003
2.407519E-002
1.430844E-002
-8.120655E-003
-1.978514E-002
-1.060535E-002
8.028165E-003
1.674714E-002
7.957611E-003
-7.919611E-003
-1.444848E-002
-5.957981E-003
7.795490E-003
1.262311E-002
4.387280E-003
7.12 Pass Thru Filter
Application:
The Pass Thru Filter is used to pass audio through the MicroDAC IV without applying any filtering
algorithms within the stages (except by the bandwidth limitations of the sampling frequency). An
easy way to bandlimit the audio is to configure a Mono Single or Stereo-Linked Stage Mode. The
audio is only affected by the Limiter, Input Highpass Filter, Output Lowpass Hiss Filter, and the
Output AGC. These features can be used in conjunction with the Pass Thru filters for signal level
adjustment.
Figure 7-27: Pass Thru Filter Selection
There are no control options for a Pass Thru filter.and the Control button is deactivated when the Pass
Thru filter is selected for a stage.
7.13 Two Channel Adaptive Filter
Application:
The 2CH Adaptive filter is used to automatically cancel from the Primary (Left) input any audio which
matches the Reference (Right) input. For example, the Primary (Left) input is microphone audio with
desired voices masked by radio or TV sound. The radio/TV interference can be cancelled in realtime if the original broadcast audio, usually available from a second receiver, is simultaneously
connected to the Reference (Right) input.
33
Figure 7-28 Two Channel Adaptive Filter Control Window
Description of controls (Figure 7-28) is as follows:
Filter Size:
Used to set the number of FIR filter taps in the adaptive filter. Filter size is indicated
both in taps (filter order) and in milliseconds. Minimum Filter Size is 16 taps, but
can be set to as high as 3600 (Mono Single mode, see Section A-8. ). Normally, the
maximum filter size possible is used in the 2CH adaptive filter
Adapt Rate:
Used to set the rate at which the adaptive filter adapts to changing signal conditions
(mathematically known as Mu). A Mu of 1 x 2-14 provides very slow adaptation,
while a Mu of 256 x 2-14 provides fastest adaptation. As a rule set this rate to
maximum initially, to establish convergence, then back off to a mid value to maintain
cancellation.
Delay:
Sets the number of audio samples in the delay line. Delay is indicated both in
samples and in milliseconds. Minimum Delay is no samples (0), but can be set to
as high as 999 samples.
Adapt Mode:
Selects Auto(matic) or Fixed adaptation rate. Auto is recommended. When Fixed
is selected, the specified Adapt Rate Mu is applied to the filter at all times. When
Auto is selected, the specified Adapt Rate is continuously power normalized
depending upon the input signal level. The Auto mode generally results in faster
convergence for a given Mu.
Processor Output:
Selects Residue or Filter output. The Residue output is the normal output selection,
which is the signal left over after the Reference signal has been cancelled from the
Primary signal. The Filter output is the modified Reference signal being subtracted
from the Primary signal.
Crash Detect:
Activates the built-in “crash” detection feature in the MicroDAC IV. This feature
automatically detects when the adaptive filter has “crashed” and clears the filter.
Checking this box enables this feature.
Reference
Compensation:
Reference Compensation automatically adjusts
the Reference Level to bring it to an optimal level for cancellation. It can apply a
maximum gain of 18 dB. Checking this box enables this feature.
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8. ADJUSTING MAXIMUM TAPS FOR EACH STAGE
A feature available on the MicroDAC IV is the ability to trade filter taps between stages in Mono Series
mode when one of the filters is an adaptive filter. To adjust the number of taps available, first display the
control screen for the adaptive filter in the Mono Series configuration. If the Stage Mode is Mono Series
the Adjust Maximum Size button will be displayed within the Filter Size section of the control screen
(see Figure 8-1).
Figure 8-1 Adjust Maximum Taps Button
Figure 8-2 Adjust Maximum Taps Control Screen
Clicking this button displays the window shown in Figure 8-2. This window allows the maximum of 3600
to be distributed between both Stages. This might be necessary if an adaptive filter size larger than 1800
taps is required. The minimum adaptive filter size is 16 taps. The minimum fixed filter size is 1000 taps.
Clicking the Equal Size button distributes the taps equally between both stages (1800 each). Use the
scroll bar to adjust the number of taps for each stage.
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36
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