ISD91200 Series Technical Reference Manual
Release Date: Sep 16, 2019
- 421 -
Revision 2.4
In limiter mode, the PGA gain is constrained to be less than or equal to the gain setting at the time the
limiter mode is enabled. In addition, attack and decay times are faster in limiter mode than in normal
mode as indicated by the different lookup tables for these parameters for limiter mode. The following
waveform illustrates the behavior of the ALC in Limiter mode in response to changes in various ALC
parameters.
Limiter
Enabled
PGA Gain
PGA Input
PGA
Output
Figure 7-13: ALC Limiter Mode Operations
When the input signal exceeds 87.5% of full scale, the ALC block ramps down the PGA gain at the
maximum attack rate (ATKSEL=0000) regardless of the mode and attack rate settings until the SDADC
output level has been reduced below the threshold. This limits SDADC clipping if there is a sudden
increase in the input signal level.
7.5.1.4
Attack Time
When the absolute value of the SDADC output exceeds the level set by the ALC threshold, TARGETLV,
attack mode is initiated at a rate controlled by the attack rate register ATKSEL. The peak detector in the
ALC block loads the SDADC output value when the absolute value of the SDADC output exceeds the
current measured peak; otherwise, the peak decays towards zero, until a new peak has been identified.
This sequence is continuously running. If the peak is ever below the target threshold, then there is no
gain decrease at the next attack timer time; if it is ever above the target-1.5dB, then there is no gain
increase at the next decay timer time.
7.5.1.5
Decay Times
The decay time DECAYSEL is the time constant used when the gain is increasing. In limiter mode, the
time constants are faster than in ALC mode.
7.5.1.6
Noise gate (normal mode only)
A noise gate is used when there is no input signal or the noise level is below the noise gate threshold.
The noise gate is enabled by setting NGEN to HIGH. It does not remove noise from the signal. The noise
gate threshold NGTH is set to a desired level so when there is no signal or a very quiet signal (pause),
which is composed mostly of noise, the ALC holds the gain constant instead of amplifying the signal
towards the target threshold. The noise gate only operates in conjunction with the ALC (ALCEN HIGH)
and ONLY in Normal mode. The noise gate flag is asserted when
(Signal at SDADC – ALC gain) < NGTH (dB)
Levels at the extremes of the range may cause inappropriate operation, so care should be taken when
setting up the function.
