Production Data
WM8750L
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 WM8750L has a noise gate function that
prevents noise pumping by comparing the signal level at the LINPUT1/2/3 and/or RINPUT1/2/3 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 ADC output can then either be muted or alternatively, the PGA gain can be 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 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).
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R20 (14h)
NGTH
Noise gate threshold
7:3
00000
Noise Gate
Control
[4:0]
00000 -76.5dBfs
00001 -75dBfs
… 1.5 dB steps
11110 -31.5dBfs
11111 -30dBfs
NGG
[1:0]
Noise gate type
2:1
0
00
0
X0 = PGA gain held constant
01 = mute ADC output
11 = reserved (do not use this setting)
Noise gate function enable
1 = enable
NGAT
0 = disable
Table 14 Noise Gate Control
Note:
The performance of the ADC may degrade at high input signal levels if the monitor bypass mux is
selected with MIC boost and ALC enabled.
PD, Rev 4.4, August 2012
27
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
