WM8961 datasheet(31/116 Pages) WOLFSON

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WM8961

Pre-Production

PP, August 2009, Rev 3.1

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.

hold

time

decay

time

attack

time

input

signal

after

ALC

PGA

gain

ALC

target

level

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 (2

n) 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 (2

n) 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 (2

n) 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.

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