adau1373 Analog Devices, Inc., adau1373 Datasheet - Page 68

• Manufacturer Part Number: adau1373
• Description: Low Power Codec With Speaker And Headphone Amplifier Adau1373
• Manufacturer: Analog Devices, Inc.

ADAU1373
Noise Gate Mode
When the signal is very quiet and consists mainly of noise, the
ALC function may cause a phenomenon called noise pumping.
The ALC, when disabled, treats the noise as a normal signal
without any processing; when the ALC is enabled, the noise gate
function is enabled. The noise gate prevents noise pumping by
comparing the signal level at the input against a noise gate
threshold. When the ALC noise gate function is enabled, there
are three optional modes, as follows:
•
•
•
The noise gate mode is set by the NGMODE bits (Register 0xC8,
Bits[5:4]). The ALCNGATT bits (Register 0xC7, Bits[7:4]) define
the gain reduction rate for Noise Gate Mode 2 and Noise Gate
Mode 3. In addition, when the ALC recovers from noise gate
mode, the ALCNGREC bits (Register 0xC7, Bits[3:0]) set the
PGA to increase to 0 dB at this rate at first; then the ALC enters
the attack or recovery phase.
Before the ALC enters into recovery mode, the ALC gain can be
held constant for some time. This hold time can be disabled or
enabled by the HLDRST bit (Register 0xC8, Bit 7). For Noise
Gate Mode 1, it is recommended that this bit be set to 1; for
Noise Gate Mode 2 and Noise Gate Mode 3, this bit should be
set to 0.
Noise Gate Attack Time
The noise gate attack time, which is set by the ALCNGATT bits,
is the time constant used when the ALC begins its noise gate phase.
This time constant is valid only in Noise Gate Mode 2 and Noise
Gate Mode 3. The attack time ranges from 6 ms to 24.576 sec
in 13 steps.
Noise Gate Recovery Time
The noise gate recovery time, which is set by the ALCNGREC
bits (Register 0xC7, Bits[3:0]), is the time constant used when
the ALC escapes from the noise gate. It is valid only in Noise
Gate Mode 3. The recovery time ranges from 1.5 ms to 6.144 sec
in 13 steps.
Noise Gate Hold Time
Noise gate hold time, which is set by the ALCNGHLD bits
(Register 0xC6, Bits[3:0]), is the time between the detection of
the signal level below the noise gate threshold and ALC entering
into noise gate mode. The hold time range is 0 ms to 5.460 sec
in 13 steps.
Noise Gate Threshold
The noise gate threshold, which is set by the ALCNGTH bits
(Register 0xC6, Bits[7:4]), defines the noise level below which
the signal is considered as noise. The noise gate threshold ranges
from −81 dBFS to −36 dBFS in 16 steps.
Noise Gate Mode 1. Keeps the PGA gain as a constant
when the ALC enters into noise gate.
Noise Gate Mode 2. Sets the PGA gain to 0.
Noise Gate Mode 3. Mutes the ALC output to −120 dB.
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–0.05
–0.05
–0.05
–0.05
0.05
–0.5
0.05
–0.5
0.05
0.05
–50
0.5
0.5
60
40
20
50
40
20
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
ALC OUTPUT
ALC GAIN
ALC OUTPUT
ALC GAIN
ALC GAIN
ALC OUTPUT
NOISE GATE HOLD
2000
20
20
20
20
20
20
50
50
NOISE GATE
HOLD TIME
4000
Figure 114. Noise Gate Mode 1
Figure 115. Noise Gate Mode 2
Figure 116. Noise Gate Mode 3
40
40
40
40
40
40
100
100
GAIN HOLD DURING NOISE GATE
6000
GAIN ATTACKS
TO 0dB
60
60
60
60
60
60
TIME (ms)
TIME (ms)
TIME (ms)
TIME (ms)
TIME (ms)
TIME (ms)
TIME (ms)
TIME (ms)
TIME (ms)
150
150
MUTE THE SIGNAL
TO –120dB
8000
80
80
80
80
80
80
200
200
10000
100
100
100
100
100
100
ESCAPE FROM
NOISE GATE RAMP
UP THE GAIN TO 0dB
250
250
12000
RECOVERY
ALC INPUT
RECOVERY
120
120
ALC INPUT
120
120
120
120
ALC INPUT
14000
300
300
140
140
140
140
140
140