WM8776 datasheet(36/55 Pages) WOLFSON

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WM8776

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PP Rev 1.91 June 2004

NOISE GATE (ALC ONLY)

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 WM8776 has a noise

gate function that prevents noise pumping by comparing the signal level at the AINL1/2/3/4/5

and/or AINR1/2/3/4/5 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]

When the noise gate is triggered, the PGA gain is held constant (preventing it from ramping

up as it would normally 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

6dB 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

NGAT

Noise gate function enable

1 = enable

0 = disable

R19 (13h)

0010011

Noise Gate

Control

4:2

NGTH[2:0]

000

Noise gate threshold (with respect to

ADC output level)

000: -78dBFS

001: -72dBfs

… 6 dB steps

110: -42dBFS

111: -36dBFS

WM8776 Datasheet(PDF) 36 Page - Wolfson Microelectronics plc

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