WM8971L Wolfson Microelectronics Ltd., WM8971L Datasheet - Page 27

• Manufacturer Part Number: WM8971L
• Description: Stereo Codec for Portable Audio Applications
• Manufacturer: Wolfson Microelectronics Ltd.

Production Data
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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 WM8971L 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:
•
This is equivalent to:
•
The ADC output can then either be muted or digitally attenuated by 18dB. 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 12 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.
Table 13 ADC and DAC 6dB Attenuation Select
R20 (14h)
Noise Gate
Control
R5 (05h)
ADC and DAC
control
REGISTER
REGISTER
ADDRESS
ADDRESS
Signal level at ADC [dB] < NGTH [dB] + PGA gain [dB] + Mic Boost gain [dB]
Signal level at input pin [dB] < NGTH [dB]
8
7
7:3
2:1
BIT
0
BIT
NGTH
[4:0]
NGG
[1:0]
NGAT
ADCDIV2
DACDIV2
LABEL
LABEL
00000
00
0
0
0
DEFAULT
DEFAULT
ADC 6dB attenuate enable
0 = disabled (0dB)
1 = -6dB enabled
DAC 6dB attenuate enable
0 = disabled (0dB)
1 = -6dB enabled
Noise gate threshold
00000
00001
… 1.5 dB steps
11110
11111
X0 = PGA gain held constant
01 = mute ADC output
10 = reserved (do not use this setting)
11 = reserved (do not use this setting)
Noise gate function enable
1 = enable
0 = disable
Noise gate type
-76.5dBfs
-75dBfs
-31.5dBfs
-30dBfs
DESCRIPTION
DESCRIPTION
PD Rev 4.1 August 2005
WM8971L
27