ASAHI KASEI
[AK4644]
2. ALC Recovery Operation
The ALC recovery operation waits for the WTM2-0 bits (Table 31) to be set after completing the ALC limiter operation.
If the input signal does not exceed “ALC recovery waiting counter reset level” (Table 28) during the wait time, the ALC
recovery operation is done. The IVL and IVR values are automatically incremented by RGAIN1-0 bits (Table 32) up to
the set reference level (Table 33) with zero crossing detection which timeout period is set by ZTM1-0 bits (Table 30).
Then the IVL and IVR are set to the same value for both channels. The ALC recovery operation is done at a period set by
WTM2-0 bits. When zero cross is detected at both channels during the wait period set by WTM2-0 bits, the ALC recovery
operation waits until WTM2-0 period and the next recovery operation is done. If ZTM1-0 is longer than WTM2-0 and no
zero crossing occurs, the ALC recovery operation is done at a period set by ZTM1-0 bits.
For example, when the current IVOL value is 30H and RGAIN1-0 bits are set to “01”, IVOL is changed to 32H by the
auto limiter operation and then the input signal level is gained by 0.75dB (=0.375dB x 2). When the IVOL value exceeds
the reference level (REF7-0), the IVOL values are not increased.
When
“ALC recovery waiting counter reset level (LMTH1-0) ≤ Output Signal < ALC limiter detection level (LMTH1-0)”
during the ALC recovery operation, the waiting timer of ALC recovery operation is reset. When
“ALC recovery waiting counter reset level (LMTH1-0) > Output Signal”,
the waiting timer of ALC recovery operation starts.
The ALC operation corresponds to the impulse noise. When the impulse noise is input, the ALC recovery operation
becomes faster than a normal recovery operation (Fast Recovery Operation). When large noise is input to microphone
instantaneously, the quality of small level in the large noise can be improved by this fast recovery operation. The speed of
fast recovery operation is set by RFST1-0 bits (Table 34).
ALC Recovery Operation Waiting Period
WTM2
WTM1
WTM0
8kHz
16kHz
44.1kHz
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
128/fs
256/fs
512/fs
1024/fs
2048/fs
4096/fs
8192/fs
16384/fs
16ms
32ms
64ms
128ms
256ms
512ms
1024ms
2048ms
8ms
16ms
32ms
64ms
128ms
256ms
512ms
1024ms
2.9ms
5.8ms
Default
11.6ms
23.2ms
46.4ms
92.9ms
185.8ms
371.5ms
Table 31. ALC Recovery Operation Waiting Period
RGAIN1
RGAIN0
GAIN STEP
0
0
1
1
0
1
0
1
1 step
0.375dB
0.750dB
1.125dB
1.500dB
Default
2 step
3 step
4 step
Table 32. ALC Recovery GAIN Step
MS0477-E-01
2006/10
- 43 -
AKM [ ASAHI KASEI MICROSYSTEMS ]
