WM8971LEFLSLASHR datasheet(25/56 Pages) WOLFSON

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WM8971L

AI Rev 3.0 March 2004

NOISE GATE

When the signal is very quiet and consists mainly of noise, the ALC function may cause “noise

pumping”, i.e. loud hissing noise during silence periods. The WM8971L has a noise gate function

that prevents noise pumping by comparing the signal level at the LINPUT1/2/3 and/or RINPUT1/2/3

pins against a noise gate threshold, NGTH. The noise gate cuts in when:

•

Signal level at ADC [dB] < NGTH [dB] + PGA gain [dB] + Mic Boost gain [dB]

This is equivalent to:

•

Signal level at input pin [dB] < NGTH [dB]

The ADC output can then either be muted or digitally attenuated by 18dB. Alternatively, the PGA gain

can be held constant (preventing it from ramping up as it normally would when the signal is quiet).

The table below summarises the noise gate control register. The NGTH control bits set the noise

gate threshold with respect to the ADC full-scale range. The threshold is adjusted in 1.5dB steps.

Levels at the extremes of the range may cause inappropriate operation, so care should be taken with

set–up of the function. Note that the noise gate only works in conjunction with the ALC function, and

always operates on the same channel(s) as the ALC (left, right, both, or none).

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

7:3

NGTH

[4:0]

00000

Noise gate threshold

00000

-76.5dBfs

00001

-75dBfs

… 1.5 dB steps

11110

-31.5dBfs

11111

-30dBfs

2:1

NGG

[1:0]

Noise gate type

X0 = PGA gain held constant

01 = mute ADC output

10 = reserved (do not use this setting)

11 = reserved (do not use this setting)

R20 (14h)

Noise Gate

Control

NGAT

Noise gate function enable

1 = enable

0 = disable

Table 10 Noise Gate Control

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

ADCDIV2

ADC 6dB attenuate enable

0 = disabled (0dB)

1 = -6dB enabled

R5 (05h)

ADC and DAC

control

DACDIV2

DAC 6dB attenuate enable

0 = disabled (0dB)

1 = -6dB enabled

Table 11 ADC and DAC 6dB Attenuation Select

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