LM4935RLEVAL datasheet(30/91 Pages) TI1

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(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

LM4935, LM4935RLEVAL

SNAS296D – OCTOBER 2005 – REVISED MARCH 2006

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4.2.15 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.

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

the

ADC

into

time

segments

roughly

equal

the

phonemes

found

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 4-11. 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)

→ (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

noise

ratio

the

expected

input

signal

set

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.

The AGC algorithm should not be used where very large background noise is present. If the type of input

data, application and microphone is known then the AGC will typically not be required for good

performance, it 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. Gain changes to the

microphone are performed on zero crossings. To eliminate DC offsets, wind noise, and pop sounds from

the output of the ADC, the ADC's HPF should always be enabled.

4.2.16 MIC_1 CONFIGURATION REGISTER

This register is used to control the microphone configuration.

Application Information

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