WM8775

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PP Rev 1.8, June 2004

24

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.

REGISTER

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R17(11h)

0010001

ALC Control 2

8

LCEN

0

Enable the PGA gain control circuit.

0 = Disabled

1 = Enabled

R16(10h)

0010000

ALC Control 1

8:7

LCSEL

00

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.

Note that for the ALC, target levels of -1dB and -2dB give a threshold of -3dB. This is

because the ALC can give erroneous operation if the target level is set too high.

REGISTER

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

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

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.