[AK4634]
■ Digital Programmable Filter Circuit
The AK4634 has 2 steps of 1st order HPF, 1st order LPF and 5-band Equalizer built-in in a recording path and a playback
path.
(1) High Pass Filter (HPF)
Normally, this HPF is used as a Wind-Noise Reduction Filter. This is composed with 2 steps of 1st order HPF. The
coefficient of both HPF is the same and set by F1A13-0 bits and F1B13-0 bits. HPFAD bit controls ON/OFF of the 1st
step HPF and HPF bit controls ON/OFF of the 2nd step HPF. When the HPF is OFF, the audio data passes this block by
0dB gain. The coefficient should be set when HPFAD = HPF bits = “0” or PMADC = PMPFIL bits = “0”.
fs : Sampling frequency
fc : Cut-off frequency
Register setting (Note 32)
HPF: F1A[13:0] bits = A, F1B[13:0] bits = B
(MSB = F1A13, F1B13; LSB = F1A0, F1B0)
1
1− tan (πfc/fs)
1 + tan (πfc/fs)
A =
,
B =
1 + tan (πfc/fs)
The cut-off frequency should be set as below.
fc/fs ≥ 0.0001 (fc min = 1.6Hz at 16kHz)
(2) Low Pass Filter(LPF)
This is composed with 1st order LPF. F2A13-0 bits and F2B13-0 bits set the coefficient of LPF. LPF bit controls ON/OFF
of the LPF. When the LPF is OFF, the audio data passes this block by 0dB gain. The coefficient should be set when LPF
bit = “0” or PMPFIL bits = “0”.
fs : Sampling frequency
fc : Cut-off frequency
Register setting (Note 32)
LPF: F2A[13:0] bits =A, F2B[13:0] bits =B
(MSB=F2A13, F1B13; LSB=F2A0, F2B0)
1
1 − 1 / tan (πfc/fs)
1 + 1 / tan (πfc/fs)
A =
,
B =
1 + 1 / tan (πfc/fs)
The cut-off frequency should be set as below.
fc/fs ≥ 0.05 (fc min = 2205Hz at 44.1kHz)
Rev. 0.5
2007/10
- 39 -
AKM [ ASAHI KASEI MICROSYSTEMS ]
