wm9714l Wolfson Microelectronics plc, wm9714l Datasheet - Page 35

• Manufacturer Part Number: wm9714l
• Description: Ac?97 Audio Codec
• Manufacturer: Wolfson Microelectronics plc

Pre-Production
AUTOMATIC LEVEL CONTROL
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The WM9714L 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 19 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
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
Decay 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
from 24ms, 48ms, 96ms, etc. to 24.58s.
ATTACK (GAIN RAMP-DOWN) TIME
Attack 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
signal
signal
input
PGA
after
ALC
gain
n
) steps, from 6ms, 12ms, 24ms, etc. to 6.14s.
hold
time
decay
time
attack
time
target
level
ALC
n
) steps, e.g. 2.67ms, 5.33ms,
PP, Rev 3.2, October 2008
WM9714L
n
) steps,
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