WM8775

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PP Rev 1.8, June 2004

26

REGISTER

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R16(10h)

0010000

ALC Control 1

6:4

MAXGAIN

111 (+24dB)

Set maximum gain for the PGA (ALC

only)

111 : +24dB

110 : +20dB

…..(-4dB steps)

010 : +4dB

001 : 0dB

000 : 0dB

Maximum attenuation of PGA

R20(14h)

0010100

Limiter Control

3:0

MAXATTEN

0110

Limiter

(attenuation

below static)

0011 or lower:

-3dB

0100: -4dB

…. (-1dB

steps)

1100: -12dB

ALC (lower PGA

gain limit)

1010 or lower

: -1dB

1011 : -5dB

….. (-4dB steps)

1110 : -17dB

1111 : -21dB

HOLD TIME (ALC ONLY)

The ALC also has a hold time, which 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

2.67ms, 5.33ms, 10.67ms etc. up to 43.7ms. 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.

REGISTER

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R17(11h)

0010001

ALC Control 2

3:0

HLD[3:0]

0000

ALC hold time before gain is

increased.

0000: 0ms

0001: 2.67ms

0010: 5.33ms

… (time doubles with every step)

1111: 43.691s

OVERLOAD DETECTOR (ALC ONLY)

To prevent clipping when a large signal occurs just after a period of quiet, the ALC circuit includes an

overload detector. If the ADC input signal exceeds 87.5% of full scale (–1.16dB), the PGA gain is

ramped down at the maximum attack rate (as when ATK = 0000), until the signal level falls below

87.5% of full scale. This function is automatically enabled whenever the ALC is enabled.

(Note: If ATK = 0000, then the overload detector makes no difference to the operation of the ALC. It

is designed to prevent clipping when long attack times are used).

NOISE GATE (ALC ONLY)

When the signal is very quiet and consists mainly of noise, the ALC function may cause “noise

pumping”, i.e. loud hissing noise during silence periods. The WM8775 has a noise gate function that

prevents noise pumping by comparing the signal level at the AINL1/2/3/4 and/or AINR1/2/3/4 pins

against a noise gate threshold, NGTH. The noise gate cuts in when:

•

Signal level at ADC [dB] < NGTH [dB] + PGA gain [dB] + Mic Boost gain [dB]

This is equivalent to:

•

Signal level at input pin [dB] < NGTH [dB]