WM8971LEFLSLASHR datasheet(23/56 Pages) WOLFSON

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WM8971L

AI Rev 3.0 March 2004

AUTOMATIC LEVEL CONTROL (ALC)

The WM8971L 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, RINVOL, RIVU and RIZC, but not LINMUTE and RINMUTE)

are ignored.

hold

time

decay

time

attack

time

input

signal

after

ALC

PGA

gain

ALC

target

level

Figure 8 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 (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 (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.

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 (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. 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, 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.

WM8971LEFL/R 数据表(PDF) 23 Page - Wolfson Microelectronics plc

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