WM8776 datasheet(33/55 Pages) WOLFSON

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WM8776

PP Rev 1.91 June 2004

The gain control circuit is enabled by setting the LCEN control bit.

The user can select

between Limiter mode and three different ALC modes using the LCSEL control bits.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R17 (11h)

0010001

ALC Control 2

LCEN

Enable the PGA gain control circuit.

0 = Disabled

1 = Enabled

R16 (10h)

0010000

ALC Control 1

8:7

LCSEL

LC function select

00 = Limiter

01 = ALC Right channel only

10 = ALC Left channel only

11 = ALC Stereo

The limiter function only operates in stereo, which means that 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 enabled, the threshold for the limiter or target level for the ALC is programmed using

the LCT control bits. This allows the threshold/target level to be programmed between -1dB

and -16dB in 1dB steps.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R16 (10h)

0010000

ALC Control 1

3:0

LCT[3:0]

1011

(-5dB)

Limiter Threshold/ALC target level in

1dB steps.

0000: -16dB FS

0001: -15dB FS

…

1101: -3dB FS

1110: -2dB FS

1111: -1dB FS

ATTACK AND DECAY TIMES

The limiter and ALC have different attack and decay times which determine their operation.

However, the attack and decay times are defined slightly differently for the limiter and for the

ALC. DCY and ATK control the decay and attack times, respectively.

Decay time (Gain Ramp-Up). When in ALC mode, this is defined as the time that it takes for

the PGA gain to ramp up across 90% of its range (e.g. from –21dB up to +20 dB). When in

limiter mode, it is defined as the time it takes for the gain to ramp up by 6dB.

The decay time can be programmed in power-of-two (2

n) steps. For the ALC this gives times

from 33.6ms, 67.2ms, 134.4ms etc. to 34.41s. For the limiter this gives times from 1.2ms,

2.4ms etc., up to 1.2288s.

Attack time (Gain Ramp-Down) When in ALC mode, this is defined as the time that it takes

for the PGA gain to ramp down across 90% of its range (e.g. from +20dB down to -21dB

gain). When in limiter mode, it is defined as the time it takes for the gain to ramp down by

6dB.

The attack time can be programmed in power-of-two (2

n) steps, from 8.4ms, 16.8ms, 33.6ms

etc. to 8.6s for the ALC and from 250us, 500us, etc. up to 256ms.

The time it takes for the recording level to return to its target value or static gain value

therefore depends on both the attack/decay time and on the gain adjustment required. If the

gain adjustment is small, it will be shorter than the attack/decay time.

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