[AK4679]
(2) High Pass Filter (HPF)
This is composed 1st order HPF. The coefficient of HPF is set by NSHA13-0 bits and NSHB13-0 bits. NSHPF bit
controls ON/OFF of the HPF. When the HPF is OFF, the audio data passes this block by 0dB gain. The coefficient must
be set when NSHPF bit = “0” or PMDRC bit = “0”. The HPF starts operation 4/fs(max) after when NSHPF bit = “1” and
PMDRC bit = “1” are set.
fs: Sampling frequency
fc: Cut-off frequency
Register setting
HPF: NSHA[13:0] bits =A, NSHB[13:0] bits =B
(MSB=NSHA13, NSHB13; LSB=NSHA0, NSHB0)
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)
(3) Noise Suppression
The Noise Suppression is enabled when NSCE bit (Noise suppression enable bit) = “1” during DRC operation (PMDRC
bit = “1”). This function attenuates output signal level automatically when minute amount of the signal is input.
NSCE bit: Noise Suppression Enable
0: Disable (default)
1: Enable
(3-1) Noise Level Suppressing Operation
The output signal is suppressed when the input moving average level set by NSIAF1-0 bits (Table 42) is lower than
“Noise Suppression Threshold Low Level” set by NSTHL4-0 bits (Table 43) during the normal operation.
This operation attenuats the volume automatically to the reference level set by NSREF3-0 bits (Table 44) with the soft
transition of the attenuation speed set by NSATT2-0 bits (Table 45).
Moving Average Parameter
NSIAF1-0 bits
fs=8kHz
32ms
64ms
128ms
256ms
fs=16kHz fs=44.1kHz
00
01
10
11
256/fs
512/fs
1024/fs
2048/fs
16ms
32ms
64ms
128ms
5.8ms
11.6ms
23.2ms
46.4ms
(default)
Table 42. Moving Average Parameter Setting at Noise Suppression Off
MS1402-E-06
2013/02
- 85 -
AKM [ ASAHI KASEI MICROSYSTEMS ]
