wm8988 Wolfson Microelectronics plc, wm8988 Datasheet - Page 24

• Manufacturer Part Number: wm8988
• Description: Stereo Codec For Portable Audio Applications
• Manufacturer: Wolfson Microelectronics plc

WM8988
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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 WM8988 has a noise gate function that
prevents noise pumping by comparing the signal level at the LINPUT1/2 and/or RINPUT1/2 pins
against a noise gate threshold, NGTH. The noise gate cuts in when:
•
This is equivalent to:
•
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).
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.
R20 (14h)
Noise Gate
Control
REGISTER
ADDRESS
Signal level at ADC [dB] < NGTH [dB] + PGA gain [dB] + Mic Boost gain [dB]
Signal level at input pin [dB] < NGTH [dB]
7:3
2:1
BIT
0
NGTH
[4:0]
NGG
[1:0]
NGAT
LABEL
00000
00
0
DEFAULT
Noise gate threshold
… 1.5 dB steps
11110
11111
X0 = PGA gain held constant
01 = mute ADC output
11 = reserved (do not use this setting)
Noise gate function enable
1 = enable
0 = disable
Noise gate type
13
13
-31.5dBfs
-30dBfs
DESCRIPTION
PD, Rev 4.0, October 2008
-76.5dBfs
-75dBfs
Production Data
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