WM8987LGECOSLASHRV datasheet(25/58 Pages) WOLFSON

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Pre-Production

WM8987L

PP Rev 3.0 January 2007

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 ATK = 0000), until the signal level falls below

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

Note:

If ATK = 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 WM8987L has a noise gate function

that prevents noise pumping by comparing the signal level at the L/RINPUT1 and/or L/RINPUT2 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 alternatively, 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, and

always operates on the same channel(s) as the ALC (left, right, both, or none).

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 14 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.

WM8987LGECO/RV Datasheet(PDF) 25 Page - Wolfson Microelectronics plc

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