AK4648 Datasheet, PDF (51/119 Pages) Asahi Kasei Microsystems – Stereo CODEC with MIC/HP/SPK-AMP

[AK4648]
2. ALC Recovery Operation
The ALC recovery operation waits for the WTM2-0 bits (Table 30) to be set after completing the ALC limiter operation.
If the input signal does not exceed “ALC recovery waiting counter reset level” (Table 27) during the wait time, the ALC
recovery operation is done. The IVL and IVR values are automatically incremented by RGAIN1-0 bits (Table 31) up to
the set reference level (Table 32) with zero crossing detection which timeout period is set by ZTM1-0 bits (Table 29).
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 signal 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 33).
WTM2
0
0
0
0
1
1
1
1
WTM1
0
0
1
1
0
0
1
1
WTM0
ALC Recovery Operation Waiting Period
8kHz
16kHz
44.1kHz
0
128/fs
16ms
8ms
2.9ms
1
256/fs
32ms
16ms
5.8ms
0
512/fs
64ms
32ms
11.6ms
1
1024/fs
128ms
64ms
23.2ms
0
2048/fs
256ms
128ms
46.4ms
1
4096/fs
512ms
256ms
92.9ms
0
8192/fs
1024ms
512ms
185.8ms
1
16384/fs
2048ms
1024ms
371.5ms
Table 30. ALC Recovery Operation Waiting Period
(default)
RGAIN1
0
0
1
1
RGAIN0
GAIN STEP
0
1 step
0.375dB
1
2 step
0.750dB
0
3 step
1.125dB
1
4 step
1.500dB
Table 31. ALC Recovery GAIN Step
(default)
MS0625-E-01
- 51 -
2007/06