[AK4675]
■ Digital Programmable Filter
(1) High Pass Filter (HPF)
Normally, this HPF is used for 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 PMADL=PMADR=PMDAL=PMDAR bits =
“0”.
fs: Sampling frequency
fc: Cut-off frequency
Register setting (Note 79)
HPF: F1A[13:0] bits =A, F1B[13:0] bits =B
(MSB=F1A13, F1B13; LSB=F1A0, F1B0)
1 / tan (πfc/fs)
1 − 1 / tan (πfc/fs)
1 + 1 / tan (πfc/fs)
A =
,
B =
1 + 1 / tan (πfc/fs)
Transfer function
1 − z −
1
H(z) = A
1 + Bz −
1
The cut-off frequency should be set as below.
fc/fs ≥ 0.0001 (fc min = 4.41Hz at 44.1kHz)
(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 PMADL=PMADR=PMDAL=PMDAR bits = “0”.
fs: Sampling frequency
fc: Cut-off frequency
Register setting (Note 79)
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)
Transfer function
1 + z −
1
H(z) = A
1 + Bz −
1
The cut-off frequency should be set as below.
fc/fs ≥ 0.05 (fc min = 2205Hz at 44.1kHz)
MS0963-E-00
2008/05
- 72 -
AKM [ ASAHI KASEI MICROSYSTEMS ]
