WM8750JL Datasheet, PDF (23/61 Pages) Wolfson Microelectronics plc – Stereo CODEC for Portable Audio Applications

Production Data
WM8750JL
AUTOMATIC LEVEL CONTROL (ALC)
The WM8750JL has an automatic level control that aims to keep a constant recording volume
irrespective of the input signal level. This is achieved by continuously adjusting the PGA gain so that
the signal level at the ADC input remains constant. A digital peak detector monitors the ADC output
and changes the PGA gain if necessary. Note that when the ALC function is enabled, the settings of
registers 0 and 1 (LINVOL, LIVU, LIZC, LINMUTE, RINVOL, RIVU, RIZC and RINMUTE) are ignored.
A selectable Zero-Cross function ensures that the ALC volume updates will be timed to coincide with
a zero-crossing of the audio signal.
input
signal
PGA
gain
signal
after
ALC
ALC
target
level
hold decay
time time
attack
time
Figure 10 ALC Operation
The ALC function is enabled using the ALCSEL control bits. When enabled, the recording volume can
be programmed between –6dB and –28.5dB (relative to ADC full scale) using the ALCL register bits.
An upper limit for the PGA gain can be imposed by setting the MAXGAIN control bits.
HLD, DCY and ATK control the hold, decay and attack times, respectively:
Hold time is the time delay between the peak level detected being below target and the PGA gain
beginning to ramp up. It can be programmed in power-of-two (2ⁿ) steps, e.g. 2.67ms, 5.33ms,
10.67ms etc. up to 43.7s. Alternatively, the hold time can also be set to zero. The hold time only
applies to gain ramp-up, there is no delay before ramping the gain down when the signal level is
above target.
Decay (Gain Ramp-Up) Time is the time that it takes for the PGA gain to ramp up across 90% of its
range (e.g. from –15B up to 27.75dB). The time it takes for the recording level to return to its target
value therefore depends on both the decay time and on the gain adjustment required. If the gain
adjustment is small, it will be shorter than the decay time. The decay time can be programmed in
power-of-two (2ⁿ) steps, from 24ms, 48ms, 96ms, etc. to 24.58s.
Attack (Gain Ramp-Down) Time is the time that it takes for the PGA gain to ramp down across 90%
of its range (e.g. from 27.75dB down to -15B gain). The time it takes for the recording level to return to
its target value therefore depends on both the attack time and on the gain adjustment required. If the
gain adjustment is small, it will be shorter than the attack time. The attack time can be programmed in
power-of-two (2ⁿ) steps, from 6ms, 12ms, 24ms, etc. to 6.14s.
When operating in stereo, the peak detector takes the maximum of left and right channel peak values,
and any new gain setting is applied to both left and right PGAs, so that the stereo image is preserved.
However, the ALC function can also be enabled on one channel only. In this case, only one PGA is
controlled by the ALC mechanism, while the other channel runs independently with its PGA gain set
through the control register.
When one ADC channel is unused or used for DC measurement, the peak detector disregards that
channel. The ALC function can also operate when the two ADC outputs are mixed to mono in the
digital domain, but not if they are mixed to mono in the analogue domain, before entering the ADCs.
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PD, April 2012, Rev 4.1
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