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ISD91200 Series Technical Reference Manual
Release Date: Sep 16, 2019
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Revision 2.4
In Normal mode, the MAXGAIN bits set the maximum level for the PGA but in the Limiter mode MAXGAIN
has no effect because the maximum level is set by the initial PGA gain setting upon enabling of the ALC.
7.5.1.1
Normal Mode
Normal mode is selected when MODESEL is set LOW and the ALC is enabled by setting ALCEN HIGH.
This block adjusts the PGA gain setting up and down in response to the input level. A peak detector
circuit measures the envelope of the input signal and compares it to the target level set by TARGETLV.
The ALC increases the gain when the measured envelope is less than (target – 1.5dB) and decreases the
gain when the measured envelope is greater than the target. The following waveform illustrates the
behavior of the ALC.
PGA Input
PGA Output
PGA Gain
Figure 7-11: ALC Normal Mode Operation
7.5.1.2
ALC Hold Time (Normal mode Only)
The hold parameter HOLDTIME configures the time between detection of the input signal envelope being below the
target range and the actual gain increase.
Input signals with different characteristics (e.g., voice vs. music) may require different settings for this parameter for
optimal performance. Increasing the ALC hold time prevents the ALC from reacting too quickly to brief periods of
silence such as those that may appear in music recordings; having a shorter hold time, on the other hand, may be
useful in voice applications where a faster reaction time helps to adjust the volume setting for speakers with different
volumes. The waveform below shows the operation of the HOLDTIME parameter.
Hold Delay
Change
PGA Gain
PGA Input
PGA Output
Figure 7-12: ALC Hold Time
7.5.1.3
Peak Limiter Mode
Peak Limiter mode is selected when MODESEL is set to HIGH and the ALC is enabled by setting ALCEN.