ADAU1361BCPZ-R7 datasheet(30/78 Pages) AD

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Data Sheet

ADAU1361

AUTOMATIC LEVEL CONTROL (ALC)

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Rev. E | 30 of 78

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 differen-

tially 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.

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 com-

press 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 register to a low value may

prevent the ALC from reaching 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.

Figure 37. Basic ALC Operation

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