TSC2100 Datasheet, PDF (29/71 Pages) Texas Instruments – PROGRAMMABLE TOUCH SCREEN CONTROLLER WITH INTEGRATED STEREO AUDIO CODEC AND HEADPHONE/SPEAKER AMPLIFIER

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TSC2100
SLAS378− NOVEMBER 2003
Automatic Gain Control (AGC)
Automatic gain control (AGC) can be used to maintain nominally constant output signal amplitude when recording speech
signals. This circuity 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.
Target gain represents the nominal output level at which the AGC attempts to hold the ADC output signal level. The
TSC2100 allows programming of eight different target gains, which can be programmed from −5.5 dB to −24 dB relative
to a full-scale signal. Since the TSC2100 reacts to the signal absolute average and not to peak levels, it is recommended
that the larger gain be set with enough margin to avoid clipping at the occurrence of loud sounds.
Attack time determines how quickly the AGC circuitry reduces the PGA gain when the input signal is too loud. It can be
varied from 8 ms to 20 ms.
Decay time determines how quickly the PGA gain is increased when the input signal is too low. It can be varied in the range
from 100 ms to 500 ms.
Noise threshold determines level below which if the input speech average value falls, AGC considers it as a silence and
hence brings down the gain to 0 dB in steps of 0.5 dB every FS and sets noise threshold flag. The gain stays at 0 dB unless
the input speech signal average rises above noise threshold setting. This ensures that noise does not get gained up in the
absence of speech. Noise threshold level in the AGC algorithm is programmable from −60 dB to −90 dB relative to full scale.
This operation includes debounce and hysteresis to avoid the AGC gain from cycling between high gain and 0 dB when
signals are near the noise threshold level. When noise threshold flag is set, status of gain applied by AGC and saturation
flag should be ignored.
Maximum PGA applicable allows user to restrict maximum gain applied by AGC. This can be used for limiting PGA gain
in situations where environmental noise is greater than programmed noise threshold. It can be programmed from 0 dB to
59.5 dB in steps of 0.5 dB.
See Table 3 for various AGC programming options.
Input
Signal
Output
Signal
AGC
Gain
Target Gain
Decay Time
Figure 27. AGC Characteristics
Attack Time
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