WM8980_12 Datasheet, PDF (45/123 Pages) Wolfson Microelectronics plc – Stereo CODEC with Speaker Driver and Video Buffer

Pre-Production
WM8980
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 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 ALCATK = 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 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 [dBFS] < NGTH [dBFS] + PGA gain [dB] + Mic Boost gain [dB]
This is equivalent to:
Signal level at input pin [dBFS] < NGTH [dBFS]
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. The noise gate only operates in conjunction with the ALC and cannot be used
in limiter mode.
REGISTER
ADDRESS
R35
ALC Noise Gate
Control
BIT
LABEL
2:0 NGTH
3 NGEN
Table 26 ALC Noise Gate Control
DEFAULT
DESCRIPTION
000
Noise gate threshold:
000=-39dB
001=-45dB
010=-51db
… (6dB steps)
111=-81dB
0
Noise gate function enable
1 = enable
0 = disable
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PP, Rev 3.8, May 2012
45