WM8978 Wolfson Microelectronics Ltd., WM8978 Datasheet - Page 37

• Manufacturer Part Number: WM8978
• Description: The WM8978 is a low power, high quality stereo codec designed for portable applications such as Digital still camera or Digital Camcorde
• Manufacturer: Wolfson Microelectronics Ltd.

Preliminary Technical Data
w
PROGRAMMABLE NOTCH FILTER
Table 18 Notch Filter Function
DIGITAL ADC VOLUME CONTROL
A programmable notch filter is provided. This filter has a variable centre frequency and bandwidth,
programmable via two coefficients, a0 and a1. a0 and a1 are represented by the register bits
NFA0[13:0] and NFA1[13:0]. Because these coefficient values require four register writes to setup
there is an NFU (Notch Filter Update) flag which should be set only when all four registers are setup.
Where:
f
The actual register values can be determined from the coefficients as follows:
The output of the ADCs can be digitally attenuated over a range from –127dB to 0dB in 0.5dB steps.
The gain for a given eight-bit code X is given by:
0.5
The coefficients are calculated as follows:
a
w
w
c
R27
Notch Filter 1
R28
Notch Filter 2
R29
Notch Filter 3
R30
Notch Filter 4
1
= centre frequency in Hz, f
0
b
REGISTER
ADDRESS
(G-255) dB for 1
2
2
1 (
NFA0 = -a0 x 2
NFA1 = -a1 x 2
a
f
f
c
b
0
/
/
a
f
f
0
1
1
s
s
)
cos(
6:0
7
8
6:0
8
6:0
8
0-6
8
tan(
tan(
BIT
G
13
12
w
w
w
0
b
255;
)
b
b
= -3dB bandwidth in Hz, f
NFA0[13:7]
NFEN
NFU
NFA0[6:0]
NFU
NFA1[13:7]
NFU
NFA1[6:0]
NFU
/
/
) 2
) 2
LABEL
MUTE for G = 0
DEFAULT
0
0
0
0
0
0
0
0
0
s
= sample frequency in Hz
Notch Filter a0 coefficient, bits [13:7]
Notch filter enable:
0=Disabled
1=Enabled
Notch filter update. The notch filter
values used internally only update
when one of the NFU bits is set high.
Notch Filter a0 coefficient, bits [6:0]
Notch filter update. The notch filter
values used internally only update
when one of the NFU bits is set high.
Notch Filter a1 coefficient, bits [13:7]
Notch filter update. The notch filter
values used internally only update
when one of the NFU bits is set high.
Notch Filter a1 coefficient, bits [6:0]
Notch filter update. The notch filter
values used internally only update
when one of the NFU bits is set high.
PTD Rev 2.6 November 2005
DESCRIPTION
WM8978
37