Advanced Information
WM8753L
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:
1. 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 WM8753L 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:
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 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).
REGISTER
ADDRESS
BIT
7:3
LABEL
DEFAULT
00000
DESCRIPTION
R15 (0Fh)
NGTH
Noise gate threshold
Noise Gate
Control
[4:0]
00000 -76.5dBfs
00001 -75dBfs
… 1.5 dB steps
11110 -31.5dBfs
11111 -30dBfs
NGG
0
0
Noise gate type
0 = PGA gain held constant
1 = mute ADC output
1
0
NGAT
Noise gate function enable
1 = enable
0 = disable
Table 22 Noise Gate Control
3D STEREO ENHANCEMENT
The WM8753L has a digital 3D enhancement option to artificially increase the separation between the
left and right channels. This effect can be used for recording or playback, but not for both
simultaneously. Selection of 3D for record or playback is controlled by register bit MODE3D.
Switching the 3D filter from record to playback or from playback to record may only be done when
ADC and DAC are disabled. The WM8753L control interface will only allow MODE3D to be changed
when ADC and DAC are disabled (ie ADCL = 0, ADCR = 0, DACL = 0 and DACR = 0).
The 3D enhancement function is activated by the 3DEN bit, and has two programmable parameters.
The DEPTH3D setting controls the degree of stereo expansion. Additionally, one of four filter
characteristics can be selected for the 3D processing, using the 3DFILT control bits.
AI Rev 3.1 June 2004
34
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
