adau1361 Analog Devices, Inc., adau1361 Datasheet - Page 30

• Manufacturer Part Number: adau1361
• Description: Stereo, Low Power, 96 Khz, 24-bit Audio Codec With Integrated Pll
• Manufacturer: Analog Devices, Inc.

ADAU1361
AUTOMATIC L
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 u
quality during gain changes.
To use the ALC function, the inputs must be applied either
differentially or pseu
LINP, for the left channel, and RINN and RINP
channel. The ALC function is not available for
input pins, LAUX and RAUX.
A block diagram of the ALC block is shown in Figure 35. T
PGA logic receives the ADC output signals and analyzes t
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.
ALC PARAMETERS
The ALC function is controlled with the ALC control registers
(Address 0x4011 through Address 0x4014) using the following
parameters:
•
•
IN RIGHT
ANALOG
ANALOG
CONTROL
IN LEFT
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.
I
2
C
–12dB TO +35.25dB
0.75dB STEP SIZE
DIGITAL
PGA
ALC
EVEL CONTROL (ALC)
do-differentially to input pins LINN and
Figure 35. ALC Architecture
RIGHT
ADC
LEFT
ADC
sed to ensure high audio
the auxiliary line
, for the right
MUTE
SERIAL
PORTS
hese
he
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•
•
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Figure 36 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 37.
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 a
gain-pumping effects, which sounds worse than using a
moderate value that allows brief periods of clipping on
transients. A typical setting
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 leve
gain is not inc
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 fa
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 seconds. A typical setting for
voice recording is 1.54 seconds.
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.
reased unless the output remains below the
for music recording is 384 ms.
st the ALC
udible
l, the