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WM8983
MINIMUM AND MAXIMUM GAIN
The ALCMINGAIN and ALCMAXGAIN register sets the minimum/maximum gain value that the PGA
can be set to whilst under the control of the ALC. This has no effect on the PGA when ALC is not
enabled.
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 WM8983 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
BIT
LABEL
DEFAULT
000
DESCRIPTION
R35
2:0 NGTH
Noise gate threshold:
ALC Noise Gate
Control
000 = -39dB
001 = -45dB
010 = -51db
… (6dB steps)
111 = -81dB
3
NGATEN
0
Noise gate function enable
1 = enable
0 = disable
Table 18 ALC Noise Gate Control
PP Rev 1.1 August 2005
37
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