WM8980CGEFLSLASHV datasheet(45/123 Pages) WOLFSON

45 / 123 page

Pre-Production

WM8980

PP, Rev 3.8, May 2012

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

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 WM8980 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

ALC Noise Gate

Control

2:0

NGTH

000

Noise gate threshold:

000=-39dB

001=-45dB

010=-51db

… (6dB steps)

111=-81dB

NGEN

Noise gate function enable

1 = enable

0 = disable

Table 26 ALC Noise Gate Control

WM8980CGEFL/V Datasheet(PDF) 45 Page - Wolfson Microelectronics plc

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