WM8980 datasheet(31/81 Pages) WOLFSON

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WM8980

PP Rev 1.6 January 2005

ALC CLIP PROTECTION

To prevent clipping when a large signal occurs just after a period of quiet, the ALC circuit includes a

clip protection 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 ATK = 0000, then the clip protection circuit 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 [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).

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. Note that the noise gate only works in conjunction with the ALC function.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

2:0

NGTH

000

Noise gate threshold:

000=-39dB

001=-45dB

010=-51db

… (6dB steps)

111=-81dB

R35

ALC Noise Gate

Control

NGATEN

Noise gate function enable

1 = enable

0 = disable

Table 17 ALC Noise Gate Control

OUTPUT SIGNAL PATH

The WM8980 output signal paths consist of digital application filters, up-sampling filters, stereo Hi-Fi

DACs, analogue mixers, speaker, stereo headphone and stereo line/mono/midrail output drivers.

The digital filters and DAC are enabled by register bits DACENL And DACENR. The mixers and

output drivers can be separately enabled by individual control bits (see Analogue Outputs). Thus it is

possible to utilise the analogue mixing and amplification provided by the WM8980, irrespective of

whether the DACs are running or not.

The WM8980 DACs receive digital input data on the DACDAT pin. The digital filter block processes

the data to provide the following functions:

Digital volume control

Graphic equaliser

A digital peak limiter.

Sigma-Delta Modulation

High performance sigma-delta audio DAC converts the digital data into an analogue signal.

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