WM9714L datasheet(37/120 Pages) CIRRUS

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WM9714L

Rev 4.0

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 WM9714L 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.

Table 15 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

62h

ALC / Noise

Gate Control

NGAT

Noise gate function enable

1 = enable

0 = disable

NGG

Noise gate type

0 = PGA gain held constant

1 = mute ADC output

4:0

NGTH(4:0)

00000

Noise gate threshold

00000: -76.5dBFS

00001: -75dBFS

… 1.5 dB steps

11110: -31.5dBFS

11111: -30dBFS

Table 15 Noise Gate Control

WM9714L Datasheet(PDF) 37 Page - Cirrus Logic

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