ADAU1373BCBZ-R7 datasheet(68/296 Pages) AD

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ADAU1373

Rev. 0 | Page 68 of 296

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:

•

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.

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.

0.05

–0.05

0.5

–0.5

2000

4000

6000

8000

10000

12000

14000

100

150

200

250

300

100

150

200

250

300

GAIN HOLD DURING NOISE GATE

TIME (ms)

ALC GAIN

ALC OUTPUT

ALC INPUT

Figure 114. Noise Gate Mode 1

0.05

–0.05

0.05

–0.05

100

120

140

TIME (ms)

040

100

120

140

040

100

120

140

TIME (ms)

GAIN ATTACKS

TO 0dB

RECOVERY

NOISE GATE

HOLD TIME

ALC OUTPUT

ALC INPUT

ALC GAIN

Figure 115. Noise Gate Mode 2

0.05

–0.05

0.5

–0.5

–50

100

120

140

TIME (ms)

040

100

120

140

040

100

120

140

TIME (ms)

RECOVERY

ESCAPE FROM

NOISE GATE RAMP

UP THE GAIN TO 0dB

MUTE THE SIGNAL

TO –120dB

NOISE GATE HOLD

ALC INPUT

ALC OUTPUT

ALC GAIN

Figure 116. Noise Gate Mode 3

ADAU1373BCBZ-R7 Datasheet(PDF) 68 Page - Analog Devices

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