WM8976 datasheet(39/108 Pages) WOLFSON

39 / 108 page

Pre-Production

WM8976

PP Rev 3.0 April 2006

PEAK LIMITER

To prevent clipping when a large signal occurs just after a period of quiet, the ALC circuit includes a

limiter function. If the ADC input signal exceeds 87.5% of full scale (–1.16dB), the PGA gain is

ramped down at the maximum attack rate (as when ALCATK = 0000), until the signal level falls

below 87.5% of full scale. This function is automatically enabled whenever the ALC is enabled.

Note: If ALCATK = 0000, then the limiter makes no difference to the operation of the ALC. It is

designed to prevent clipping when long attack times are used.

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 WM8976 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

2:0

NGTH

000

Noise gate threshold:

000 = -39dB

001 = -45dB

010 = -51db

011 = -57dB

100 = -63dB

101 = -69dB

110 = -75dB

111 = -81dB

R35 (23h)

ALC Noise Gate

Control

NGATEN

Noise gate function enable

1 = enable

0 = disable

Table 25 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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