wm9712l Wolfson Microelectronics plc, wm9712l Datasheet - Page 22

• Manufacturer Part Number: wm9712l
• Description: Ac?97 Audio And Touchpanel Codec
• Manufacturer: Wolfson Microelectronics plc

WM9712L
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AUTOMATIC LEVEL CONTROL
The WM9712L has an automatic level control that aims to keep a constant recording volume
irrespective of the input signal level. This is achieved by continuously adjusting the PGA gain so that
the signal level at the ADC input remains constant. A digital peak detector monitors the ADC output
and changes the PGA gain if necessary.
Figure 3 ALC Operation
The ALC function is enabled using the ALCSEL control bits. When enabled, the recording volume
can be programmed between –6dB and –28.5dB (relative to ADC full scale) using the ALCL register
bits.
HLD, DCY and ATK control the hold, decay and attack times, respectively:
Hold time 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
10.67ms etc. up to 43.7s. 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.
Decay (Gain Ramp-Up) Time is the time that it takes for the PGA gain to ramp up across 90% of its
range (e.g. from –15B up to 27.75dB). The time it takes for the recording level to return to its target
value therefore depends on both the decay time and on the gain adjustment required. If the gain
adjustment is small, it will be shorter than the decay time. The decay time can be programmed in
power-of-two (2
Attack (Gain Ramp-Down) Time is the time that it takes for the PGA gain to ramp down across 90%
of its range (e.g. from 27.75dB down to –15B gain). The time it takes for the recording level to return
to its target value therefore depends on both the attack time and on the gain adjustment required. If
the gain adjustment is small, it will be shorter than the attack time. The attack time can be
programmed in power-of-two (2
When operating in stereo, the peak detector takes the maximum of left and right channel peak
values, and any new gain setting is applied to both left and right PGAs, so that the stereo image is
preserved. However, the ALC function can also be enabled on one channel only. In this case, only
one PGA is controlled by the ALC mechanism, while the other channel runs independently with its
PGA gain set through the control register.
signal
signal
input
PGA
after
ALC
gain
n
) steps, from 24ms, 48ms, 96ms, etc. to 24.58s.
hold
time
decay
time
n
) steps, from 6ms, 12ms, 24ms, etc. to 6.14s.
attack
time
target
level
ALC
n
) steps, e.g. 2.67ms, 5.33ms,
PD Rev 4.5 August 2006
Production Data
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