RCVDL56DPFL/SP, RCV56DPFL/SP, and RCV336DPFL/SP Designer’s Guide
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11-5
11.3 VOICE AGC
An optional AGC function in included in the voice receiver (compressed and uncompressed). Voice AGC is enabled or
disabled by setting or resetting the VAGC bit [04h:2 (shared with NRZIE)].
The AGC is updated on a 28-sample “voice frame” basis. The average input sample (AverageENG) is compared to a voice
threshold (VTHRESH) to determine if there is enough energy to perform the automatic gain control (AGC). When the
average energy is below this threshold, the input samples are not AGC’ed. When the energy is above the threshold, the
Gain is updated and the Gain is applied to the next voice frame. Gain is only updated when the energy is above VTHRESH
and only once per voice frame. Furthermore, Gain is limited to the maximum voice gain (VMAXG).
11.3.1 Voice AGC Parameters
The voice AGC parameters are listed in Table 11-2.
Table 11-2. Voice AGC Parameters
Name
Description
Address
(Hex)
Default* (Hex)
VMAXG
Maximum Gain
E63
4000
VAGCREF
AGC Reference Level
E62
0800
VTHRESH
AGC Energy Threshold
E65
0000
VSRATE0
Slew Rate 0
F62
4500, then 0300
Alpha
Low Pass Filter Gain
F61
0100
Pole
Low pass Filter Pole
D62
7EFF
VoiceSlewRateCtr
Counter Slew Rate Adjustment
3AE, 3AF
F10
* If bit 1 of register 04 (shared with TOD) is set, then the above voice AGC parameters are not initialized. This
allows the host to write in values without having to worry about contentions with the data pump initialization.
Voice AGC Reference (VAGCREF)
The AGC algorithm adjusts the Gain so that the squared output of the Voice LPF, (LPFOutput)
2
, tends to VAGCREF. The
default value of VAGCREF is 0100h corresponding to a recorded level of nearly -16 dBm. By lowering VAGCREF, the
recorded volume is lowered.
Slew Rate (VSRATE)
The slew rate parameter controls the rate at which the gain tracks the input signal. The larger the slew rate multiplier, the
faster the AGC Gain word tracks the input signal.
The VoiceSlewRateCtr is used to allow the AGC gain word to quickly adjust to the input level at the beginning of the
recording and then slow down to a “steady-state” rate thereafter. The counter counts down at the 16-bit data rate and is
disabled when the most significant byte is negative. The default VoiceSlewRateCtr value corresponds to two seconds for 4-
bit ADPCM recording. So for the first two seconds, the AGC slew rate is 4500h and then it changes to 0300h for the
remainder of the recording.
Maximum Gain (VMAXG)
The Gain value is limited to VMAXG. When VMAXG is small, the AGC process works to a lower level of input. Under default
conditions, the 4000h value of VMAXG cuts off at -40 dBm. That is, the Gain remains constant ( = VMAXG) for input levels
below -40 dBm. By lowering VMAXG, lower level signals will receive gain.
Voice Energy Threshold (VTHRESH)
When the average frame value is above VTHRESH, the AGC process runs, otherwise the received samples are recorded
without gain. This threshold comparison is intended to prevent the noise floor from being raised during silence periods. This
“silence” feature can introduce choppiness if not tuned correctly. As a result, the default value for VTHRESH is zero which,
effectively, disables this feature.
Low Pass Filter Output (LPFOutput)
The low pass filter output is a function of the input level and the LPF gain, Alpha. The default value of Alpha, together with
the LPF pole value, give a LPF gain of 0 dB. By increasing Alpha, the recorded volume is lowered.
AGC Gain Equation: Gain = Gain + [(LPFOutput)
2
- VAGCREF] VSRATE
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