wm8987 Wolfson Microelectronics plc, wm8987 Datasheet - Page 26

• Manufacturer Part Number: wm8987
• Description: Stereo Codec For Portable Audio Applications
• Manufacturer: Wolfson Microelectronics plc

WM8987L
w
PEAK LIMITER
To prevent clipping when a large signal occurs just after a period of quiet, the ALC circuit includes a
limiter function. If the ADC input signal exceeds 87.5% of full scale (–1.16dB), the PGA gain is
ramped down at the maximum attack rate (as when ATK = 0000), until the signal level falls below
87.5% of full scale. This function is automatically enabled whenever the ALC is enabled.
Note:
If ATK = 0000, then the limiter makes no difference to the operation of the ALC. It is designed to
prevent clipping when long attack times are used.
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 WM8987L has a noise gate function
that prevents noise pumping by comparing the signal level at the L/RINPUT1 and/or L/RINPUT2 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
00000
00001
… 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
-76.5dBfs
-75dBfs
-31.5dBfs
-30dBfs
DESCRIPTION
PD Rev 4.0 August 2008
Production Data
26