Texas Instruments LM49370RLEVAL Evaluation Board LM49370RLEVAL/NOPB LM49370RLEVAL/NOPB 데이터 시트
제품 코드
LM49370RLEVAL/NOPB
(1)
(1) Decay hold time, (2) Slow Decay, (3) Quick Attack
(2)
(3)
12 dB
10 dB
14 dB
12 dB
signal below target
attack
decay
microphone gain
target level
signal above target
peak detection
and ADC output
SNAS356D – FEBRUARY 2007 – REVISED MARCH 2012
Table 16. AGC_3 (0x0Ah) (continued)
Bits
Field
Description
7:5
AGC_ATTACK
This programs the speed at which the AGC will reduce gains if it detects the input level is too large.
AGC_ATTACK
Step Time (ms)
000
2
32
001
2
64
010
2
128
011
2
256
100
2
512
101
2
1024
110
2
2048
111
2
4096
AGC OVERVIEW
The Automatic Gain Control (AGC) system can be used to optimize the dynamic range of the ADC for voice data
when the level of the source is unknown. A target level for the output is set so that any transients on the input
won’t clip during normal operation. The AGC circuit then compares the output of the ADC to this level and
increases or decreases the gain of the microphone preamplifier to compensate. If the audio from the microphone
is to be output digitally through the ADC then the full dynamic range of the ADC can be used automatically. If the
output is through the analog mixer then the ADC is used to monitor the microphone level. In this case, the analog
dynamic range is less important than the absolute level, so AGC_TIGHT should be set to tie transients closely to
the target level.
when the level of the source is unknown. A target level for the output is set so that any transients on the input
won’t clip during normal operation. The AGC circuit then compares the output of the ADC to this level and
increases or decreases the gain of the microphone preamplifier to compensate. If the audio from the microphone
is to be output digitally through the ADC then the full dynamic range of the ADC can be used automatically. If the
output is through the analog mixer then the ADC is used to monitor the microphone level. In this case, the analog
dynamic range is less important than the absolute level, so AGC_TIGHT should be set to tie transients closely to
the target level.
To ensure that the system doesn’t reduce the quality of the speech by constantly modulating the microphone
preamplifier gain, the ADC output is passed through an envelope detector. This frames the output of the ADC
into time segments roughly equal to the phonemes found in speech (AGC_FRAME_TIME). To calculate this, the
circuit must also know the sample rate of the data from the ADC (ADC_SAMPLERATE). If after a programmable
number of these segments (AGC_HOLDTIME), the level is consistently below target, the gain will be increased
at a programmable rate (AGC_DECAY). If the signal ever exceeds the target level (AGC_TARGET) then the gain
of the microphone is reduced immediately at a programmable rate (AGC_ATTACK). This is demonstrated below:
preamplifier gain, the ADC output is passed through an envelope detector. This frames the output of the ADC
into time segments roughly equal to the phonemes found in speech (AGC_FRAME_TIME). To calculate this, the
circuit must also know the sample rate of the data from the ADC (ADC_SAMPLERATE). If after a programmable
number of these segments (AGC_HOLDTIME), the level is consistently below target, the gain will be increased
at a programmable rate (AGC_DECAY). If the signal ever exceeds the target level (AGC_TARGET) then the gain
of the microphone is reduced immediately at a programmable rate (AGC_ATTACK). This is demonstrated below:
Figure 15. AGC Operation Example
The signal in the above example starts with a small analog input which, after the hold time has timed out, triggers
a rise in the gain [(1)
a rise in the gain [(1)
→
(2)]. After some time the real analog input increases and it reaches the threshold for a
gain reduction which decreases 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 of the expected input signal is set by NOISE_GATE_THRESHOLD. In some
situations it is preferable to remove audio considered to be consisting solely of background noise from the audio
output; for example conference calls. This can be done by setting NOISE_GATE_ON. This does not affect the
performance of the AGC algorithm.
gain. The signal to noise ratio of the expected input signal is set by NOISE_GATE_THRESHOLD. In some
situations it is preferable to remove audio considered to be consisting solely of background noise from the audio
output; for example conference calls. This can be done by setting NOISE_GATE_ON. This does not affect the
performance of the AGC algorithm.
Copyright © 2007–2012, Texas Instruments Incorporated
27
Product Folder Links: