WM8961
Pre-Production
AUTOMATIC LEVEL CONTROL (ALC)
The WM8961 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 input 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, IPVU, LIZC, LINMUTE, RINVOL, RIZC and RINMUTE) are
ignored.
input
signal
PGA
gain
signal
after
ALC
ALC
target
level
hold decay
time time
attack
time
Figure 15 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. An lower limit
for the PGA gain can be imposed by setting the MINGAIN 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 (for example from –21B up to 21.75dB). 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 (for example from 21.75dB down to -21B 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.
The input PGA and MICBOOST gain settings should be identical when entering ALC stereo mode in
order for gain updates to be applied correctly.
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, the peak detector disregards that channel.
PP, August 2009, Rev 3.1
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
