The device includes an automatic gain controller (AGC) for ADC recording that must be used only for the AC-coupled input configuration. As shown in Figure 88, the AGC can be used to maintain a nominally constant output level when recording speech. Instead of manually setting the channel gain in AGC mode, the circuitry automatically adjusts the channel gain when the input signal becomes overly loud or very weak, such as when a person speaking into a microphone moves closer to or farther from the microphone. The AGC algorithm has several programmable parameters, including target level, maximum gain allowed, attack and release (or decay) time constants, and noise thresholds that allow the algorithm to be fine-tuned for any particular application.
The target level (AGC_LVL) represents the nominal output level at which the AGC attempts to hold the ADC output signal level. The PCM6xx0-Q1 allows programming of different target levels, which can be programmed from –6 dB to –36 dB relative to a full-scale signal, and the AGC_LVL default value is set to –34 dB. The target level is recommended to be set with enough margin to prevent clipping when loud sounds occur. Table 42 lists the AGC target level configuration settings.
|P0_R112_D[7:4] : AGC_LVL[3:0]||AGC TARGET LEVEL FOR OUTPUT|
|0000||The AGC target level is the –6-dB output signal level|
|0001||The AGC target level is the –8-dB output signal level|
|0010||The AGC target level is the –10-dB output signal level|
|1110 (default)||The AGC target level is the –34-dB output signal level|
|1111||The AGC target level is the –36-dB output signal level|
The maximum gain allowed (AGC_MAXGAIN) gives flexibility to the designer to restrict the maximum gain applied by the AGC. This feature limits the channel gain in situations where environmental noise is greater than the programmed noise threshold. The AGC_MAXGAIN can be programmed from 3 dB to 42 dB with steps of 3 dB and the default value is set to 24 dB. Table 43 lists the AGC_MAXGAIN configuration settings.
|P0_R112_D[3:0] : AGC_MAXGAIN[3:0]||AGC MAXIMUM GAIN ALLOWED|
|0000||The AGC maximum gain allowed is 3 dB|
|0001||The AGC maximum gain allowed is 6 dB|
|0010||The AGC maximum gain allowed is 9 dB|
|0111 (default)||The AGC maximum gain allowed is 24 dB|
|1110||The AGC maximum gain allowed is 39 dB|
|1111||The AGC maximum gain allowed is 42 dB|
For further details on the AGC various configurable parameter and application use, see the Using the Automatic Gain Controller in PCM6xx0-Q1 application report.