Matrix Oral History Tutorial
Audio Technology
Frequency response
Polar Patterns
Proximity effect
Cabling and phantom power
Recording techniques
Recording environment
Microphone placement
Signal processing and special
Audio Technology / A/D conversion and digital audio signal
In order to perform acoustic analysis on recorded speech data or to
deliver audio on-line, the audio signal has to be converted into a
digital audio file format, such as Wav or Aiff. Analog recordings have
to be digitized and digital recordings need to be transferred to a
personal computer via a digital audio file transfer interface. This is an
important, yet often underestimated, stage in the process of
preparing audio data for analysis.
The main goal of A/D conversion (digitization) is to obtain the best
possible digital representation of the original analog waveform.
Without going into too much technical detail of the digitization
process, one should choose a sample rate that will capture a broad
range of frequencies and a bit-depth that will allow a wide dynamic
range and a negligible amount of quantization noise. These goals can
be achieved by means of a premium-quality, stand-alone A/D
converter operating at the sample rate of at least 48,000 Hz and a 24bit resolution. It is absolutely crucial not to use a PCI multimedia
sound card, as they are built from inferior-quality electronic
components and, more importantly, allow electrostatic noise and
distortion to leak into the captured acoustic signal:
Recording devices
Analog recorders
Digital recorders
DAT recorders
Minidisk recorders
PC card and CD-R recorders
Hard disk recorders
A-to-D conversion
Improving audio digitization
Digitization workflow
Analysis and Delivery
Preparing files for analysis and
Digital restoration
LPC in acoustic analysis
Spectrum of typical electrostatic noise generated by computer
The A/D converter, such as Lucid AD 9624, should offer a variety of
sample rates, oversampling, high quality anti-aliasing filters, and AES/
EBU and S/PDIF digital outputs. Both AES/EBU (Audio Engineering
Society/European Broadcasting Union) and S/PDIF (Sony/Philips
Digital Interface) are fairly common on high-end digital audio devices.
In addition, S/PDIF is used on a variety of consumer-level products,
such as CD players, minidisk players, etc. It is also a common
interface used on PCI digital I/O cards, which is why it is probably a
better choice for most digital audio transfer applications. (1 of 3)6/20/2005 3:30:39 PM
Matrix Oral History Tutorial
S/PDIF (IEC-958)
110 ohm shielded
TP 75 ohm coaxial or fiber
3-pin XLR
RCA (or BNC)
Signal level
Max. Resolution
24 bits
24 bits
The analog playback device (such as TASCAM 122 mkIII) should be
connected to the A/D converter. One should make sure that the
output levels on the tape deck match the input levels on the A/D
converter. It is recommended to use balanced XLR line level interface
(+24 dBu min. gain, +7 dBu max. gain, 65k ohm impedance). If the
tape deck does not have this kind of output interface, a signal level
transformer (such as Ebtech Line shifter PHOTO>>) and a preamplifier should be used.
The A/D converter needs to be connected to a PCI (though USB and
FireWire are becoming common) digital audio I/O card (such as
Midiman Delta DiO 2496 via a S/PDIF interface). The digital I/O card
should be selected as the recording interface in the audio recording
software (such as Sonic Foundry Sound Forge 5.0 on a PC or BIAS
Peak VST on a Mac). The digital audio signal should be captured with
this software and saved either as Wav (PC) or Aiff (Mac) file at the
sample rate and bit depth that the A/D converter was set to. It is also
possible to capture digital audio signal directly into acoustic analysis
software, such as CSL or Praat, though it is not recommended due to
the fact that specialized recording and processing software offers
considerable more control over the incoming signal. It should also be
mentioned that USB Pre may be used as a high-quality, stand-alone
A/D converter.
In this case the digital audio signal is transferred to a PC via the USB
interface, which eliminates the need to install a separate PCI digital I/
O card and makes it possible to capture digital audio on a laptop. In
addition, USB Pre has a pair of tape-level inputs, to which a cassette
deck can be directly connected.
Sensitivity (typical,
for 0 dB FS) (2 of 3)6/20/2005 3:30:39 PM
Clip Level
(1% THD)
Impedance (actual)
Matrix Oral History Tutorial
min. gain
-10 dBu
-53 dBu
-12 dBu
(195 mV
2k ohm activebalanced
+24 dBu
+7 dBu
+24 dBu
(12.3 V
65k ohm active
+8 dBu
-9 dBu
+9 dBu
(2.2 V
10k ohm unbalanced
+8 dBu
-9 dBu
+9 dBu
(2.2 v
110k ohm
Summary of typical signal level types.
Audio Technology
Home | Downloads | Contact | Matrix
Last updated: 2002 (3 of 3)6/20/2005 3:30:39 PM
Download PDF
Similar pages