SSM2603CPZ-R2 datasheet(12/32 Pages) AD

12 / 32 page

SSM2603

Rev. 0 | 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.

INPUT SIGNAL

PGA

SIGNAL

AFTER

ALC

DECAY TIME

ALC TARGET

VALUE

ATTACK TIME

Figure 18. PGA and ALC Decay Time and Attack Time Definitions

SSM2603CPZ-R2 Datasheet(PDF) 12 Page - Analog Devices