WM8778 Datasheet, PDF (32/50 Pages) Wolfson Microelectronics plc – 24 BIT 192KHZ STEREO CODEC

WM8778
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LIMITER / AUTOMATIC LEVEL CONTROL (ALC)
The WM8778 has an automatic pga gain control circuit, which can function as a peak limiter or as an
automatic level control (ALC). In peak limiter mode, a digital peak detector detects when the input
signal goes above a predefined level and will ramp the pga gain down to prevent the signal becoming
too large for the input range of the ADC. When the signal returns to a level below the threshold, the
pga gain is slowly returned to its starting level. The peak limiter cannot increase the pga gain above
its static level.
input
signal
PGA
gain
signal
after
PGA
Limiter
threshold
attack
time
Figure 22 Limiter Operation
decay
time
In ALC mode, the circuit 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.
input
signal
PGA
gain
signal
after
ALC
ALC
target
level
hold
time
Figure 23 ALC Operation
decay
time
w
attack
time
PP Rev 1.7 June 2004
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