Rainbow Electronics MAX9860 - Manual (Page 30)

MAX9860
Automatic Gain Control (AGC)
and Noise Gate
The MAX9860 includes AGC on both microphone
inputs. AGC is enabled by setting the hold time through
AGCHLD. AGC dynamically controls the analog PGA
microphone input gain to hold the level constant over a
20dB input range, enhancing the voice path operation
for various use conditions. When AGC is enabled, it
monitors the signal level at the output of the ADC and
then makes gain adjustments by controlling the analog
microphone PGA. When AGC is enabled, PGAM is not
user programmable.
Since AGC increases the level of all signals below a
user-defined threshold, the noise floor effectively is
increased by 20dB. To counteract this, a noise gate is
included to reduce the gain at low levels. Unlike typical
noise gates that completely silence the output below a
threshold, the noise gate in the MAX9860 reduces the
gain for signals below the defined level. As the signal
level becomes further below the threshold, the gain is
further reduced. The Automatic Gain Control
Thresholds and Noise Gate Thresholds graphs in the
Typical Operating Characteristics show the resulting
steady-state transfer curves when AGC and the noise
gate are enabled.
16-Bit Mono Audio Voice Codec
30 ______________________________________________________________________________________
Table 10. AGC and Noise Gate Registers
REGISTER ADDRESS B7 B6 B5 B4 B3 B2 B1 B0
0x0E AGCSRC AGCRLS AGCATK AGCHLD
0x0F ANTH AGCTH
BITS FUNCTION
AGCSRC
AGC/Noise Gate Signal Source Select
0 = The left ADC output is used by the AGC and noise gate.
1 = The sum of the left and right ADC outputs is used by the AGC and noise gate.
AGC Release Time
Time taken by the AGC circuit to increase the gain from minimum to maximum.
CODE TIME
000 78ms
001 156ms
010 312ms
011 625ms
100 1.25s
101 2.5s
110 5s
AGCRLS
111 10s
AGC Attack Time
The time constant of the AGC gain reduction curve.
CODE TIME (ms)
00 3
01 12
10 50
AGCATK
11 200
AGC Hold Time
Time the AGC circuit waits before beginning to increase gain when a signal below the threshold is
detected.
CODE TIME (ms)
00 AGC disabled
01 50
10 100
AGCHLD
11 400