AK4695 Datasheet, PDF (48/107 Pages) Asahi Kasei Microsystems – 24bit 4ch CODEC with MIC/HP/SPK/LINE-AMP

[AK4695]
(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” or PMDRC bit =
“1” is 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 (fc/fs)
A=
,
1 + 1 / tan (fc/fs)
1  1 / tan (fc/fs)
B=
1 + 1 / tan (fc/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 = 4.8Hz at 48kHz)
(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 40) is lower than “Noise
Suppression Threshold Low Level” set by NSTHL4-0 bits (Table 41) during the normal operation.
This operation attenuates the volume automatically to the reference level set by NSREF3-0 bits (Table 42) with the soft
transition of the attenuation speed set by NSATT2-0 bits (Table 43).
NSIAF1-0 bits
Moving Average Parameter
fs=8kHz fs=16kHz fs=48kHz
00
256/fs
32ms
16ms
5.3ms
01
512/fs
64ms
32ms
10.7ms
10
1024/fs 128ms
64ms
21.3ms (default)
11
2048/fs 256ms
128ms
42.7ms
Table 40. Input Signal moving Average Parameter Setting (Normal Operation )
MS1463-E-01
- 48 -
2014/12