Texas Instruments | AN-1451 LM4935 Automatic Gain Control (AGC) (Rev. A) | Application notes | Texas Instruments AN-1451 LM4935 Automatic Gain Control (AGC) (Rev. A) Application notes

Texas Instruments AN-1451 LM4935 Automatic Gain Control (AGC) (Rev. A) Application notes
Application Report
SNAA028A – April 2006 – Revised May 2013
AN-1451 LM4935 Automatic Gain Control (AGC) Guide
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ABSTRACT
This report provides information on the operation and settings of the automatic gain control circuit.
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Contents
Automatic Gain Control (AGC) Overview ................................................................................
AGC Operation ..............................................................................................................
AGC Settings Explained ...................................................................................................
3.1
AGC_TARGET (bits 3:1(0x08h)) .................................................................................
3.2
AGC_FRAME_TIME (bits 5:3(0x07h)) ..........................................................................
3.3
AGC_HOLDTIME (bits 4:0(0x0Ah)) .............................................................................
3.4
AGC_ATTACK (bits 7:5(0x0Ah)) .................................................................................
3.5
AGC_DECAY (bits 6:4(0x09h)) ..................................................................................
3.6
AGC_MAX_GAIN (bits 3:0(0x09h)) ..............................................................................
3.7
AGC_TIGHT (bit 7(0x09h)) .......................................................................................
3.8
ADC_SAMPLERATE (bits 5:4(0x06h)) ..........................................................................
3.9
NOISE_GATE_THRES (bits 7:5(0x08h)) .......................................................................
3.10 NOISE_GATE_ON (bit 4(0x08h)) ................................................................................
Recommended AGC Settings .............................................................................................
Revision History .............................................................................................................
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List of Figures
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AGC Loop .................................................................................................................... 2
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AGC Operation Example ................................................................................................... 3
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A Noisy Audio Signal with NOISE_GATE_ON Disabled............................................................... 5
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The Same Audio Signal (Figure 3) with NOISE_GATE_ON Enabled
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AN-1451 LM4935 Automatic Gain Control (AGC) Guide
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1
Automatic Gain Control (AGC) Overview
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Automatic Gain Control (AGC) Overview
A microphone is typically used in an environment where the level of the audio source is unknown. The
LM4935 features an Automatic Gain Control (AGC) circuit that addresses this situation by compensation
for significant variations in level from a microphone’s input. It maintains a fixed level during recording or
playback. The following scenarios showcase the real world benefits of the AGC circuit:
Scenario 1:
A user talks too loudly or too close to the microphone. This causes a clipped voice signal at the output of
the microphone preamplifier.
Without AGC
The user at the receiving end hears a clipped signal. This clipped signal sounds distorted. Further, it may
damage the receiving-end transducer or may cause hearing loss.
With AGC
The AGC automatically lowers the microphone preamplifier gain. This prevents voice signal clipping. The
resulting undistorted, unclipped voice signal is then presented to the user on the receiving end.
Scenario 2:
A user talks too softly or too far away from the microphone.
Without AGC
The resultant low-level voice signal may not be heard clearly or at all by the user at the receiving end.
With AGC
The AGC will automatically increase the gain of the microphone preamplifier to a level that is heard clearly
by the user at the receiving end.
As shown in the scenarios above, the AGC operates by automatically adjusting the gain of the microphone
preamplifier to maintain a pre-defined target level at the amplifier's output. The AGC loop is shown in
Figure 1.
MIC
MIC
PREAMP
ADC
AGC
Figure 1. AGC Loop
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AGC Operation
The AGC circuit adjusts the level of the input applied to the ADC. This optimizes the dynamic range of the
voice data when the level of the source is unknown. A target level for the output is set so that any
transients on the microphone input will not clip during normal operation. The AGC circuit compares the
output of the ADC to this target level and increases or decreases the gain of the microphone preamplifier
to compensate. The ADC's full dynamic range is realized when the AGC optimizes microphone
preamplifier output signal magnitude.
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AN-1451 LM4935 Automatic Gain Control (AGC) Guide
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AGC Settings Explained
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To ensure that the AGC circuit does not reduce the quality of the speech by constantly modulating the
microphone preamplifier gain, the ADC output is passed through an envelope detector. The envelope
detector frames the output of the ADC into time segments roughly equal to the phonemes found in speech
(AGC_FRAMETIME). To calculate this, the AGC circuit must also know the ADC's sample rate
(ADC_SAMPLERATE). If, after a programmable number of these segments (AGC_HOLDTIME), the level
is consistently below target, the microphone amplifier's gain is increased at a programmable rate
(AGC_DECAY). If the signal ever exceeds the target level (AGC_TARGET), the gain of the microphone is
reduced immediately at a programmable rate (AGC_ATTACK). The AGC's operation is demonstrated in
Figure 2.
(1) Decay hold time, (2) Slow Decay, (3) Quick Attack
(2)
(3)
(1)
target level
peak detection
and ADC output
attack
decay
microphone gain
12 dB
12 dB
14 dB
signal below target
10 dB
signal above target
Figure 2. AGC Operation Example
The ADC input signal shown in Figure 2 is initially at a low level. After the hold time has finished, the gain
rises ((1)→(2)). Later, the microphone output signal's amplitude increases and reaches the threshold for a
gain reduction. This reduction is programmed to decrease the gain at a faster rate ((2)→(3)), to allow the
elimination of typical popping noises.
Only ADC outputs that are considered signal (rather than noise) are used to adjust the microphone
preamplifier gain. The signal-to-noise ratio (SNR) of the expected input signal is set by
NOISE_GATE_THRESHOLD. In some situations, it is preferable to remove audio considered to be solely
background noise from the audio output; for example the background noise during a conference calls.
This can be done by setting NOISE_GATE_ON. This does not affect the performance of the AGC
algorithm.
The AGC algorithm should not be used where high amplitude background noise is present. If the type of
input data, application, and microphone is known, the AGC will typically not be required for good
performance. The AGC is intended for use with inputs with a large dynamic range or unknown nominal
level. When setting NOISE_GATE_THRESHOLD, be aware that in some mobile phone scenarios the
ADC SNR will be dictated by the microphone performance rather than the ADC or the signal. Microphone
preamplifier gain changes are performed on zero crossings ensuring no zipper noise. Enable the ADC's
high pass filter to eliminate DC offsets, wind noise, and pop sounds from the output of the ADC.
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AGC Settings Explained
3.1
AGC_TARGET (bits 3:1(0x08h))
This is the predefined audio signal magnitude target level at the input of the ADC that is maintained by the
AGC algorithm. This target level can be programmed from –6dBFS to –20dBFS in 2dBFS steps. There is
a delay between when the AGC attenuates the ADC input and when it reads the ADC output. During this
delay, the ADC input signal may clip if there is not enough headroom available. The headroom should be
approximately the difference between the average signal level and the level of the expected transients. In
order to maximize the dynamic range of the ADC, the reference level of the expected transients should be
set at the maximum input level of the ADC (Full scale = 1Vrms). Therefore, it is recommended to set
AGC_TARGET at the difference between the average signal level and 0dBFS. For typical speech
patterns, an AGC_TARGET setting of -12dBFS or -14dBFS is a good starting point.
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AGC Settings Explained
3.2
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AGC_FRAME_TIME (bits 5:3(0x07h))
This sets the frame time to be used by the AGC algorithm. In a given frame, the AGC’s peak detector
determines the peak value of the incoming microphone audio signal level and compares this value to the
target value of the AGC (AGC_TARGET) in order to adjust the microphone preamplifier’s gain. The length
of a frame determines the sample rate of the AGC. The frame time can be set from 96ms to 1000ms, an
adjustment range for a wide variety of speech patterns.
3.3
AGC_HOLDTIME (bits 4:0(0x0Ah))
This bit sets the amount of delay (by controlling the number of frames passed to the ADC) before the AGC
algorithm begins to adjust the gain of the microphone preamplifier. The hold time can be set from 0 to 31
frames. The length of each frame is set by AGC_FRAME_TIME. For example, an AGC_HOLDTIME set
for 2 frames and an AGC_FRAMETIME set for 96ms gives a total delay time of 192ms.
Delay Time = (AGC_HOLDTIME) x (AGC_FRAME_TIME)
Delay Time = (2) x (96ms) = 192ms
For this particular example, the AGC algorithm will not adjust the microphone preamplifier gain until a
delay of 192ms after a signal is present on the ADC.
3.4
AGC_ATTACK (bits 7:5(0x0Ah))
Setting this bit sets the rate at which the AGC will reduce the microphone preamplifier’s gain when the
input signal level is larger than the target level set by AGC_TARGET. Whenever the input level is greater
than the target level, the AGC immediately decreases the microphone preamplifier gain on the next zero
crossing. The attack time is the amount of time between each step to decrease the gain. It has a range
from 32ms to 4096ms.
3.5
AGC_DECAY (bits 6:4(0x09h))
This bit sets the rate at which the AGC will increase the microphone preamplifier gain when the input
signal level is smaller than the target level set by AGC_TARGET. Whenever the input signal level is lesser
than the target level, the AGC waits for the hold time and then increases the microphone preamplifier gain
on the next zero crossing. To ensure maximum SNR, the AGC does not increase the microphone
preamplifier gain when the input signal level is below the noise floor as set by the NOISE_GATE_THRES.
The decay time is the amount of time between each step to increase the gain. It has a range from 32ms to
4096ms.
3.6
AGC_MAX_GAIN (bits 3:0(0x09h))
This bit sets the maximum gain that the AGC can apply to the microphone preamplifier. The maximum
gain can be set from 6dB to 36dB in 2dB steps.
3.7
AGC_TIGHT (bit 7(0x09h))
Setting this bit allows the AGC gain algorithm to control the microphone preamplifier gain so that the
output signal level closely matches the AGC target level.
The AGC is used to optimize the microphone preamplifier output signal level, whether intended for the
LM4935's analog output stages or as an input signal for the ADC. When the analog path is used, this bit
should be cleared. This allows greater dynamic range for the audio signal at the output stages. When the
microphone input is routed to the ADC for recording or playback, the AGC_TIGHT bit should be set to
ensure that the audio signal tightly adheres to the target level.
3.8
ADC_SAMPLERATE (bits 5:4(0x06h))
This variable is used by the AGC algorithm. It informs the AGC circuit about the closest expected ADC
sample rate. This bit does not set the actual ADC sample rate. ADC_SAMPLERATE can be set to 8kHz,
12kHz, 16kHz, or 24kHz. It is recommended that the selected ADC_SAMPLERATE setting closely
matches the actual ADC sample rate so that the AGC works properly.
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AN-1451 LM4935 Automatic Gain Control (AGC) Guide
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SNAA028A – April 2006 – Revised May 2013
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Recommended AGC Settings
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3.9
NOISE_GATE_THRES (bits 7:5(0x08h))
This setting tells the AGC magnitude of the expected background noise level relative to the peak signal
level. The presence of signals below this level will be removed from the ADC output and will not produce
an AGC gain change. This prevents background noise from being unnecessarily amplified during periods
of silence in a conversation. The noise gate threshold level can be set from –72dB to –30dB. This level
must be defined even if the noise gate is not in use, because this level is required by the AGC algorithm.
3.10 NOISE_GATE_ON (bit 4(0x08h))
Setting this bit will mute cause the amplifier to mute signals that are below the noise gate threshold level
defined by NOISE_GATE_THRES. When the noise gate is enabled and no signal is detected, the noise
gate waits for a delay time equal to (AGC_FRAMETIME) x (AGC_HOLDTIME) to pass and then mutes
any signal below the noise gate threshold level. When a signal larger than the noise gate threshold level is
detected, the noise gate unmutes the signal on the next zero crossing. Figure 3 and Figure 4 show the
effect of the noise gate on the same audio signal.
Figure 3. A Noisy Audio Signal with NOISE_GATE_ON Disabled
Figure 4. The Same Audio Signal (Figure 3) with NOISE_GATE_ON Enabled
When using the noise gate, there is a tradeoff between eliminating background noise during moments of
silence and maintaining the original relevant audio information. Figure 4 shows the elimination of
background noise found from the noisy waveform shown in Figure 3. However, some of the audio
transients were also reduced. This is a result of the noise gate's action: once it detects a signal larger than
the noise gate threshold, the gate unmutes the signal on the next zero crossing. Setting
NOISE_GATE_THRES to its minimum of –72dB reduces the possibility of relevant audio information being
muted, but may increase the chance of unwanted background noise leaking through.
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Recommended AGC Settings
Here are the recommended starting points for tuning the AGC for voice conversations.
AGC_TARGET = -12dB
AGC_FRAMETIME = 96ms
AGC_HOLDTIME = 1 frame
AGC_ATTACK = 64ms
AGC_DECAY = 64ms
AGC_MAX_GAIN = 36dB
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Revision History
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AGC_TIGHT = Enabled
ADC_SAMPLERATE = closest sample rate to the actual sample rate of the ADC
NOISE_GATE_THRES = -72dBFS
NOISE_GATE_ON = Enabled
The effect of the AGC is highly subjective and system dependent, therefore further tuning may be
required.
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Revision History
Rev
Date
Description
0.1
04/24/06
Initial release.
AN-1451 LM4935 Automatic Gain Control (AGC) Guide
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