WM8972LEFLSLASHR datasheet(20/49 Pages) WOLFSON

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WM8972L

Preliminary Technical Data

PTD Rev 2.2 June 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 WM8972L has a noise gate function

that prevents noise pumping by comparing the signal level at the INPUT1+ and/or INPUT2+ 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 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.

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

11 = reserved (do not use this setting)

R20 (14h)

Noise Gate

Control

NGAT

Noise gate function enable

1 = enable

0 = disable

Table 9 Noise Gate Control

Note:

The performance of the ADC may degrade at high input signal levels if the monitor bypass mux is

selected with MIC boost and ALC enabled.

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