ISD91200 Series Technical Reference Manual
Release Date: Sep 16, 2019
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Revision 2.4
PGA Input
PGA Output
PGA Gain
Figure 7-14: ALC Operation with Noise Gate disabled
PGA Input
PGA Output
PGA Gain
Noise Gate Threshold
Figure 7-15: ALC Operation with Noise Gate Enabled
7.5.1.7
Zero Crossing
The PGA gain comes from either the ALC block when it is enabled or from the PGA gain register setting
when the ALC is disabled. Zero crossing detection may be enabled to cause PGA gain changes to occur
only at an input zero crossing. Enabling zero crossing detection limits clicks and pops that may occur if
the gain changes while the input signal has a high volume.
There are two zero crossing detection enables:
•
Register ZCEN – is only relevant when the ALC is enabled.
If the zero crossing function is enabled (using either register), the zero cross timeout function may take
effect. If the zero crossing flag does not change polarity within 0.25 seconds of a PGA gain update (either
via ALC update or PGA gain register update), then the gain will update. This backup system prevents the
gain from locking up if the input signal has a small swing and a DC offset that prevents the zero crossing
flag from toggling.
