
P95020 / Preliminary Datasheet
Revision 0.7.10
28
2010 Integrated Device Technology, Inc.
includes an analog amplifier (0-22.5dB gain in 1.5dB steps) and a multiplexer to select between the line input path or the
analog mixer output.
Note: there is only 1 L/R clock per IS I/O port. Therefore the input and output rates for that port match.
2.7 AUDIO - AUTOMATIC GAIN CONTROL
The P95020 incorporates digital automatic gain control in the ADC1 record path to help maintain a constant record level
for voice recordings. The AGC maintains the recording level by monitoring the output of the ADC and adjusting the Boost
(analog for analog microphone path or digital for digital microphone path) and digital record gain to compensate for
varying input levels. While the AGC is enabled, the digital record gain and boost register values are ignored.
The AGC target level may be set from -1.5 dB to -22.5
dB relative to the ADC full scale output code in 1.5 dB
steps. The maximum gain allowed may be
programmed to prevent the AGC from using the entire
gain range. The AGC may be applied to either both
channels or only the right or left channel. The AGC
uses both channels to determine proper record level
unless only one channel is selected. When only one
channel is enabled, the other channel is ignored and
that channels gain is controlled by its record gain and
boost register values.
Delay time is the amount of delay between when the
peak record level falls below the target level and when
the AGC starts to adjust gain. The delay time may be
set from 0 ms to 5.9 seconds in 16 steps. Each step is
twice as long as the previous step where 0 is the first
step.
Each additional step may be calculated by:
((8*2
n)/44100) seconds
where n is the register value from 1 to 15
Decay time is the time that the AGC takes to ramp up
across its gain range. The time needed to adjust the
recording level depends on the decay time and the
amount of gain adjustment needed. If the input level is
close to the target level then a relatively small gain adjustment will be needed and will take much less than the
programmed decay time. Decay time is adjustable from 23.2 ms to 23.8 seconds and may be calculated as (2
n+10/44100)
where n is the register value from 0 to 10. Register values above 10 set the decay to 23.8 seconds.
Attack time is the time that it takes the AGC to ramp down across its gain range. As with the decay time, the actual time
needed to reach the target recording level depends on the attack time and the gain adjustment needed. The attack time
is adjustable from 5.8 ms to 5.9 seconds and may be calculated as (2
n+8/44100) where n is the register value from 0 to 10.
Register values above 10 set the decay to 5.9 seconds.
The P95020 also provides a peak limiter function. When the AGC is on, quiet passages will cause the gain to be set to
the maximum level allowed. When a large input signal follows a quiet passage, many samples will become clipped as the
AGC adjusts the gain to reach the target record level. Long attack times aggravate this situation. To reduce the number
of clipped samples the peak limiter will force the attack rate to be as fast as possible (equivalent to zero (0) value in the
attack register) until the record level is 87.5% of full scale or less.
To prevent excessive hiss during quiet periods, a signal threshold level may be programmed to prevent the AGC circuit
from increasing the gain in the absence of audio. This is often referred to as a noise gate or squelch function. The
signal threshold may be programmed from -72 dB FS to -24 dB FS in 1.5 dB increments.
Under some circumstances, it is desirable to force a minimum amount of gain in the record path. When the AGC is in
use, the minimum gain may be set from 0 to 30 dB to compensate for microphone sensitivity or other needs.
2.8 AUDIO - ANALOG MIXER BLOCK
The Audio subsection implements an analog mixing block for use as an input or output mixer.
The Audio Mixer Block consists of:
Delay
Decay
Attack
AGC Target
Level
Figure 8
– Automatic Gain Control