WM9714L_06 Datasheet, PDF (29/94 Pages) Wolfson Microelectronics plc – AC 97 Audio CODEC

Pre-Production
WM9714L
MAXIMUM GAIN
The MAXGAIN register sets the maximum gain value that the PGA can be set to whilst under the
control of the ALC. This has no effect on the PGA when ALC is not enabled.
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 WM9714L has a noise gate function
that prevents noise pumping by comparing the signal level at the input pins (i.e. before the record
PGA) against a noise gate threshold, NGTH. Provided that the noise gate function is enabled
(NGAT = 1), 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]
The PGA gain is then held constant (preventing it from ramping up as it normally would when the
signal is quiet). If the NGG bit is set, the ADC output is also muted when the noise gate cuts in.
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).
REGISTER
ADDRESS
62h
ALC / Noise
Gate Control
BIT
7
5
LABEL
NGAT
NGG
4:0
NGTH(4:0)
Table 15 Noise Gate Control
DEFAULT
0
0
00000
DESCRIPTION
Noise gate function enable
1 = enable
0 = disable
Noise gate type
0 = PGA gain held constant
1 = mute ADC output
Noise gate threshold
00000: -76.5dBFS
00001: -75dBFS
… 1.5 dB steps
11110: -31.5dBFS
11111: -30dBFS
w
PP Rev 3.0 June 2006
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