Digital AudioDataSerialInterface
ADC
+
+
DIN_x
DOUT_x
BCLK_x
WCLK_x
DINL
DINR
DOUTL
DOUTR
ADC
AGC
AGC
RecordPath
RecordPath
Effects
Effects
SW-D1
SW-D2
SW-D4
SW-D3
Left-Channel
AnalogInputs
Right-Channel
AnalogInputs
PGA
0dB–59.5dB,
0.5-dBSteps
PGA
0dB–59.5dB,
0.5-dBSteps
Volume
Control
Volume
Control
DAC
Powered
Down
DAC
Powered
Down
DAC
L
DAC
R
B0173-02
AUTOMATIC GAIN CONTROL (AGC)
TLV320AIC34
SLAS538A – OCTOBER 2007 – REVISED NOVEMBER 2007
Figure 26. Record-Only Mode With Digital Processing Path Enabled
An automatic gain control (AGC) circuit is included with the ADC and can be used to maintain nominally constant
output signal amplitude when recording speech signals (it can be fully disabled if not desired). This circuitry
automatically adjusts the PGA gain as the input signal becomes overly loud or very weak, such as when a
person speaking into a microphone moves closer or farther from the microphone. The AGC algorithm has several
programmable settings, including target gain, attack and decay time constants, noise threshold, and maximum
PGA gain applicable that allow the algorithm to be fine-tuned for any particular application. The algorithm uses
the absolute average of the signal (which is the average of the absolute value of the signal) as a measure of the
nominal amplitude of the output signal.
Note that completely independent AGC circuitry is included with each ADC channel with entirely independent
control over the algorithm from one channel to the next. This is attractive in cases where two microphones are
used in a system, but may have different placement in the end equipment and require different dynamic
performance for optimal system operation.
Target level represents the nominal output level at which the AGC attempts to hold the ADC output signal level.
The TLV320AIC34 allows programming of eight different target levels, which can be programmed from – 5.5 dB
to – 24 dB relative to a full-scale signal. Because the device reacts to the signal absolute average and not to peak
levels, it is recommended that the target level be set with enough margin to avoid clipping at the occurrence of
loud sounds.
Copyright © 2007, Texas Instruments Incorporated Submit Documentation Feedback 35
Product Folder Link(s): TLV320AIC34
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