WM8960CGEFLSLASHV datasheet(32/94 Pages) CIRRUS

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WM8960

Rev 4.4

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R27 (1Bh)

Additional

Control (3)

2:0

ADC_ALC_SR

[2:0]

000

ALC Sample Rate

000 = 44.1k / 48k

001 = 32k

010 = 22.05k / 24k

011 = 16k

100 = 11.25k / 12k

101 = 8k

110 and 111 = Reserved

Table 12 ALC Control

ALC SAMPLE RATE CONTROL

The register bits ADC_ALC_SR must be set correctly to ensure that the ALC attack, decay and hold

times are correct for the chosen sample rate as shown in Table 12.

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 WM8960 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 [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 will then be held constant (preventing it from ramping up as it normally would when the

signal is quiet).

Table 13 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).

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