WM9713 datasheet(34/111 Pages) WOLFSON

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WM9713L

Preliminary Technical Data

PTD, March 2004, Rev 2.2

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 WM9713L has a noise gate function

that prevents noise pumping by comparing the signal level at the input pins (i.e. before the record

PGA) against a noise gate threshold, NGTH. Provided that the noise gate function is enabled (NGAT

= 1), 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 PGA gain is then held constant (preventing it from ramping up as it normally would when the

signal is quiet). If the NGG bit is set, the ADC output is also muted when the noise gate cuts in.

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

NGAT

Noise gate function enable

1 = enable

0 = disable

NGG

Noise gate type

0 = PGA gain held constant

1 = mute ADC output

62h

ALC / Noise

Gate Control

4:0

NGTH(4:0)

00000

Noise gate threshold

00000: -76.5dBFS

00001: -75dBFS

… 1.5 dB steps

11110: -31.5dBFS

11111: -30dBFS

Table 14 Noise Gate Control

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