WM8778SEDSSLASHR datasheet(36/50 Pages) WOLFSON

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WM8778

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PP Rev 1.7 June 2004

OVERLOAD DETECTOR (ALC ONLY)

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

overload detector. 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 overload detector makes no difference to the operation of the ALC. It

is designed to prevent clipping when long attack times are used).

NOISE GATE (ALC ONLY)

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 WM8778 has a noise gate function that

prevents noise pumping by comparing the signal level at the AINL1/2/3/4/5 and/or AINR1/2/3/4/5 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]

When the noise gate is triggered, the PGA gain is held constant (preventing it from ramping up as it

would normally 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, 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

R19 (13h)

0010011

Noise Gate

Control

4:2

NGTH[2:0]

000

Noise Gate Threshold (with respect to

ADC output level):

000: -78dBFS

001: -72dBfs

… 6 dB steps

110: -42dBFS

111: -36dBFS

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