WM8777
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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 WM8777 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
(20h)
0
NGAT
0
Noise gate function enable
0 = Noise gate disabled
1 = Noise gate enabled
Noise Gate
Control
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 = -30dBFS
Table 79 Noise Gate Registers
Note: The Noise Gate should be set after the ALC to ensure correct operation.
SOFTWARE REGISTER RESET
Writing to register 1111111 will cause a register reset, resetting all register bits to their default
values. Note that the WM8777 is powered down by default so writing to this register will power down
the device.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
(7Fh)
Writing to this register will apply a
reset to the device registers.
Software reset
8:0
RESET
Table 80 Software Reset Register
PP Rev 1.94 November 2004
72
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
