9.3.7 Zero Crossing Detect

The PCM186x uses a zero crossing detector to make gain changes only when the incoming signal crosses the halfway point between negative and positive swing, reducing zipper noise.

There are two sources for the controller, the output of the ADC modulator and the output from the digital PGA. The analog PGA is sampled at four times the audio sampling rate to detect the zero crossing. The digital PGA is sampled at a similar rate.

The process for changing gain in the PCM186x is as follows:

1. Detect a zero crossing of the oversampled analog input channel.
2. Increment or decrement the gain toward the target PGA value step by 0.5 dB.
3. Repeat from (1) until arrival at the target PGA value.
4. If zero crossing does not occur for 8192 sample times (= time out), change the gain per sample.

This process does not require intervention by the user. This data serves as information only. Also, please note that DSPs must be running (clocked) for this functionality to work.