Motorola DSP56156 Manual page 143

ON-CHIP CODEC FREQUENCY RESPONSE AND GAIN ANALYSIS
In the present example, with D=128 and F=2.048 MHz, the frequency response of this
second order comb filter is:
The frequency response of the interpolation DSP filter can be shaped by 1/F(f) in order to
flatten the D/A section response.
Table 6-10 gives an example of a 4 biquad IIR low pass filter whose transfer function has
been shaped accordingly.Figure 6-14 shows the filter frequency response and the effects
on the overall D/A section response.
Table 6-10 Example of a Four Biquad IIR Interpolation and Compensation Filter
; Source filter file: "comp1282nd.IIR.Filter"
_filter_type
_NSTAGES
NOTE:
6 - 28
1
F f ( )
--------------- -
=
( )
2
128
equ BIQUAD_FILTER_TYPE
equ 4
dc $0270
; Biquad stage no. 1
dc $e722
dc $16e0
dc $329e
dc $43d3
; Biquad stage no. 2
dc $d947
dc $13eb
dc $1014
dc $1146
; Biquad stage no. 3
dc $f1a9
dc $30ad
dc $3603
dc $4614
; Biquad stage no. 4
dc $c72a
dc $0a66
dc $2f25
dc $1c8f
This filter, as well as all the figures representing filter re-
sponses, has been generated using ZOLA Technologies,
Inc., DSP Designer™ software package.
DSP56156 ON-CHIP SIGMA/DELTA CODEC
×
2
 
 2π 128 
×
sin --------------------- f
 
 
×
2 2048
 
------------------------------------------- -
 
 
2π
×
sin f
-------------------- -
 
 
×
2 2048
; number of stages
; gain = 0.03807147513/2
; -d2_1 = -0.3885309315/2
; -d1_1 = 0.3574372286/2
; n2_1 = 0.7908895273/2
; n1_1 = 1.059754603/2
; -d2_2 = -0.6050537737/2
; -d1_2 = 0.3112185033/2
; n2_2 = 0.2512476586/2
; n1_2 = 0.2698979411/2
; -d2_3 = -0.2240856408/2
; -d1_3 = 0.7605800752/2
; n2_3 = 0.8439490258/2
; n1_3 = 1.094956692/2
; -d2_4 = -0.8880624617/2
; -d1_4 = 0.162456827/2
; n2_4 = 0.7366593399/2
; n1_4 = 0.4462262322/2
MOTOROLA