WM9713 datasheet(31/111 Pages) WOLFSON

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Preliminary Technical Data

WM9713L

PTD, March 2004, Rev 2.2

AUTOMATIC LEVEL CONTROL

The WM9713L 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.

hold

time

decay

time

attack

time

input

signal

after

ALC

PGA

gain

ALC

target

level

Figure 11 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.

HLD, DCY and ATK control the hold, decay and attack times, respectively.

HOLD TIME

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

Decay 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 (2

n) steps,

from 24ms, 48ms, 96ms, etc. to 24.58s.

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