ADAU1373BCBZ-R7 datasheet(67/296 Pages) AD

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ADAU1373

Rev. 0 | Page 67 of 296

DIGITAL AUTOMATIC LEVEL CONTROL (ALC)

The automatic level control (ALC) provides continuous

adjustment of the input PGA in response to the rms amplitude

of the input signal to maintain it at a constant level, which is

defined by the ALCREF bits (Register 0xC4, Bits[3:0]). A digital

peak detector monitors the input signal amplitude and limits it

to the register-defined threshold level, using the ALCLVL bits

(Register 0xC4, Bits[7:4]).

The ALC provides control over analog PGA and digital PGA,

which can be selected individually or together.

To reduce the gain during the silent portion of the speech or

music signal, the ALC provides a noise gate that can be oper-

ated in four different modes, as defined by the NGMODE bits

(Register 0xC8, Bits[5:4]).

•

Noise Gate Mode 1. Maintains the gain at the level it was

before the ALC entered into noise gate mode.

•

Noise Gate Mode 2. Sets the PGA gain to 0 dB.

•

Noise Gate Mode 3. Mutes the ALC output.

•

Noise Gate Mode 4. Noise gate function is disabled.

The ALC can be enabled or disabled for the left or right channel

or for both channels via the ALCEN bits (Register 0xC8, Bits[3:2]).

If the rms value of the signal falls below the ALC target threshold

(as defined by the ALCREF bits), the ALC increases the gain of

the PGA at the rate set by the ALCREC bits (Register 0xC3,

Bits[3:0]). If the signal is above the threshold, the ALC reduces

the gain of the PGA at a rate that is set by the ALCATT bits

(Register 0xC3, Bits[7:4]).

Because the dc offset introduced by the analog circuits greatly

influences ALC operation, it is recommended that the HPF

included in the ALC block be enabled when the ALC is enabled.

Peak Limiter Level

To prevent clipping during high level signals, the ALC circuit

includes a limiter function. If the ALC input signal exceeds the

peak level threshold (as defined by the ALCLVL bits), the PGA

gain is ramped down as per the attack time set by the ALCATT

bits until the signal level falls below that threshold. This function

is automatically enabled when the ALC is enabled. The peak

limiter level can be set using the ALCLVL bits. The level can be

set from −22.5 dBFs to 0 dBFS in 16 steps.

ALC Target level

When the signal level is below the peak limiter threshold, the

ALC attempts to maintain a constant signal level by increasing

or decreasing the gain of the PGA. That constant level is defined as

the ALC target level, which is set by the ALCREF bits. The level

can be set from −24 dBFS to −1.5 dBFS in 16 steps.

ALC Maximum Gain

The ALCMAX bits (Register 0xC5, Bits[3:0]) set the maximum

gain value that the PGA can reach while under the control of

the ALC. The available gain range is −12 dB to +60 dB in 13 steps.

RMS Average Time

The rms average time is the time taken for the rms value esti-

mation. It is set by the ALCTAV bits (Register 0xC2, Bits[7:4]).

The averaging time range is 1.5 ms to 6.144 sec in 13 steps.

Target Level Ripple Remove

The ALCRIP bits (Register 0xC5, Bits[7:4]) define the ALC

target level ripple range. When the input signal rms level is

within this range, the ALC gain does not change. The ripple

range can be set from 0 dB to −7.5 dB in 16 steps.

For example, if the ALC target level is −6 dB, the ripple is defined as

0.5 dB, and the detected rms value of the input signal is between

−6 dB and −6.5 dB, the ALC gain does not change.

Attack (Gain Ramp-Down) Time

Attack time, which is set by the ALCATT bits, is the time that is

required for the PGA gain to ramp down through 90% of its range.

Therefore, the time for the recording level to return to its target

value (limit operation threshold) depends on both the attack

time and the gain adjustment required. If the gain adjustment is

small, the real adjustment time is less than the attack time. The

attack time range is from 1.5 ms to 6.144 sec in 13 steps. In mute

condition, this is the mute attack time.

Decay/Recovery (Gain Ramp-Up) Time

Decay time, which is set by the ALCREC bits, is the time that

is required for the PGA gain to ramp up to 90% of its range.

Therefore, the time required for the recording level to return to

its target value (recovery threshold) depends on both the decay

time and the gain adjustment required. If the gain adjustment is

small, the real adjustment time is less than the decay time. The

recovery time ranges from 6 ms to 24.576 sec in 16 steps.

Recovery Hold Time

Recovery hold time, set by the ALCHLD bits (Register 0xC2,

Bits[3:0]), is the time delay between the detection of the signal

level below the recovery threshold and the PGA gain beginning

to ramp up. The hold time applies only to gain ramp-up; there is

no delay before the gain ramping down when the signal level is

above target. The hold time range is 0 ms to 5.468 sec in 13 steps.

INPUT

SIGNAL

PGA

GAIN

SIGNAL

AFTER

ALC

HOLD

TIME

DECAY

TIME

ATTACK

TIME

ALC

TARGET

LEVEL

Figure 113. Digital PGA and ALC Decay Time, Hold Time, and Attack Time

ADAU1373BCBZ-R7 Datasheet(PDF) 67 Page - Analog Devices

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