ADAU1361BCPZ-R7 datasheet(32/80 Pages) AD

32 / 80 page

ADAU1361

Data Sheet

Rev. D | Page 32 of 80

AUTOMATIC LEVEL CONTROL (ALC)

The ADAU1361 contains a hardware automatic level control

(ALC). The ALC is designed to continuously adjust the PGA

gain to keep the recording volume constant as the input level

varies.

For optimal noise performance, the ALC uses the analog PGA

to adjust the gain instead of using a digital method. This ensures

that the ADC noise is not amplified at low signal levels.

Extremely small gain step sizes are used to ensure high audio

quality during gain changes.

To use the ALC function, the inputs must be applied either

differentially or pseudo-differentially to input pins LINN and

LINP, for the left channel, and RINN and RINP, for the right

channel. The ALC function is not available for the auxiliary line

input pins, LAUX and RAUX.

A block diagram of the ALC block is shown in Figure 36. The

ALC logic receives the ADC output signals and analyzes these

digital signals to set the PGA gain. The ALC control registers

are used to control the time constants and output levels, as

described in this section.

RIGHT

ADC

LEFT

ADC

MUTE

SERIAL

PORTS

ALC

DIGITAL

ANALOG

INPUT

LEFT

I2C

CONTROL

ANALOG

INPUT

RIGHT

PGA

–12dB TO +35.25dB

0.75dB STEP SIZE

Figure 36. ALC Architecture

ALC PARAMETERS

The ALC function is controlled with the ALC control registers

(Address 0x4011 through Address 0x4014) using the following

parameters:



ALCSEL[2:0]: The ALC select bits are used to enable the

ALC and set the mode to left only, right only, or stereo. In

stereo mode, the greater of the left or right inputs is used

to calculate the gain, and the same gain is then applied to

both the left and right channels.



ALCTARG[3:0]: The ALC target is the desired input

recording level that the ALC attempts to achieve.



ALCATCK[3:0]: The ALC attack time sets how fast the

ALC starts attenuating after a sudden increase in input

level above the ALC target. Although it may seem that

the attack time should be set as fast as possible to avoid

clipping on transients, using a moderate value results in

better overall sound quality. If the value is too fast, the

ALC overreacts to very short transients, causing audible

gain-pumping effects, which sounds worse than using a

moderate value that allows brief periods of clipping on

transients. A typical setting for music recording is 384 ms.

A typical setting for voice recording is 24 ms.



ALCHOLD[3:0]: These bits set the ALC hold time. When

the output signal falls below the target output level, the

gain is not increased unless the output remains below the

target level for the period of time set by the hold time bits.

The hold time is used to prevent the gain from modulating

on a steady low frequency sine wave signal, which would

cause distortion.



ALCDEC[3:0]: The ALC decay time sets how fast the ALC

increases the PGA gain after a sudden decrease in input level

below the ALC target. A very slow setting can be used if the

main function of the ALC is to set a music recording level.

A faster setting can be used if the function of the ALC is to

compress the dynamic range of a voice recording. Using a

very fast decay time can cause audible artifacts such as noise

pumping or distortion. A typical setting for music recording

is 24.58 sec. A typical setting for voice recording is 1.54 sec.



ALCMAX[2:0]: The maximum ALC gain bits are used to

limit the maximum gain that can be programmed into the

ALC. This can be used to prevent excessive noise in the

recording for small input signals. Note that setting this

its target output level, but this behavior is often desirable to

achieve the best overall sound.

Figure 37 shows the dynamic behavior of the PGA gain for a

tone-burst input. The target output is achieved for three differ-

ent input levels, with the effect of attack, hold, and decay shown

in the figure. Note that for very small signals, the maximum PGA

gain may prevent the ALC from achieving its target level; in the

same way, for very large inputs, the minimum PGA gain may

prevent the ALC from achieving its target level (assuming that

the target output level is set to a very low value). The effects of

the PGA gain limit are shown in the input/output graph of

Figure 38.

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