Datasheet

SSM2603 Data Sheet
Rev. C | Page 12 of 32
AUTOMATIC LEVEL CONTROL (ALC)
The SSM2603 codec has an automatic level control (ALC) that
can be activated to suppress clipping and improve dynamic
range even if a sudden, loud input signal is introduced. This is
achieved by continuously adjusting the PGA gain so that the
signal level at the ADC input remains constant.
Decay (Gain Ramp-Up) Time
Decay time is the time taken for the PGA gain to ramp up to
90% of its range. The time for the recording level to return to its
target value, therefore, depends on both the decay time and the
gain adjustment required. If the gain adjustment is small, the
time to return to the target value will be less than the decay time.
Attack (Gain Ramp-Down) Time
Attack time is the time taken for the PGA gain to ramp down
through 90% of its range. The time for the recording level to
return to its target value, therefore, depends on both the attack
time and the gain adjustment required. If the gain adjustment is
small, the time to return to the target value will be less than the
attack time.
Noise Gate
When the ALC function is enabled but the input signal is silent
for long periods, an audible hissing sound may be introduced by a
phenomenon called noise pumping. To prevent this occurrence, the
SSM2603 employs a noise gate function. A user-selected threshold
can be set by using the NGTH bits (Register R18, Bit D3 to Bit D7).
When the noise gate is enabled, the ADC output is either muted or
held at a constant gain to prevent the noise-pumping phenomenon.
For more information about the noise gate settings, see Table 41.
Figure 18. PGA and ALC Decay Time and Attack Time Definitions
07241-021
INPUT SIGNAL
PGA
SIGNAL
AFTER
ALC
DECAY TIME
ALC TARGET
VALUE
ATTACK TIME