WM8940 datasheet(39/88 Pages) CIRRUS

39 / 88 page

WM8940

Rev 4.4

NOISE GATE (NORMAL MODE 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 WM8940 has a noise gate function that

prevents noise pumping by comparing the signal level at the input pins against a noise gate threshold,

NGTH. The noise gate cuts in when:

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

This is equivalent to:

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

The PGA gain is then 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 6dB steps. Levels at

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

–up of

the function. The noise gate only operates in conjunction with the ALC and cannot be used in limiter

mode.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R35 (23h)

ALC Noise Gate

Control

2:0

NGTH

000

Noise gate threshold:

000 = -39dB

001 = -45dB

010 = -51db

011 = -57dB

100 = -63dB

101 = -69dB

110 = -75dB

111 = -81dB

NGATEN

Noise gate function enable

1 = enable

0 = disable

Table 28 ALC Noise Gate Control

The diagrams below show the response of the system to the same signal with and without noise gate.

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