WM8972L Datasheet, PDF (20/49 Pages) Wolfson Microelectronics plc – MONO CODEC FOR PORTABLE AUDIO APPLICATIONS

WM8972L
Preliminary Technical Data
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 WM8972L has a noise gate function
that prevents noise pumping by comparing the signal level at the INPUT1+ and/or INPUT2+ 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 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.
REGISTER
ADDRESS
R20 (14h)
Noise Gate
Control
BIT
LABEL
7:3 NGTH
[4:0]
2:1 NGG
[1:0]
0 NGAT
Table 9 Noise Gate Control
DEFAULT
DESCRIPTION
00000
00
0
Noise gate threshold
00000 -76.5dBfs
00001 -75dBfs
… 1.5 dB steps
11110 -31.5dBfs
11111 -30dBfs
Noise gate type
X0 = PGA gain held constant
01 = mute ADC output
11 = reserved (do not use this setting)
Noise gate function enable
1 = enable
0 = disable
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.
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PTD Rev 2.2 June 2004
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