WM8776
Production Data
NOISE GATE (ALC ONLY)
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 WM8776 has a noise gate function that
prevents noise pumping by comparing the signal level at the AINL1/2/3/4/5 and/or AINR1/2/3/4/5 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]
When the noise gate is triggered, the PGA gain is held constant (preventing it from ramping up as it
would normally 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. 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
R19 (13h)
0010011
0
NGAT
0
Noise gate function enable
1 = enable
Noise Gate
Control
0 = disable
4:2
NGTH[2:0]
000
Noise gate threshold (with respect to
ADC output level)
000: -78dBFS
001: -72dBfs
… 6 dB steps
110: -42dBFS
111: -36dBFS
PD Rev 4.0 April 2005
38
w
WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
