D/A Section Frequency Response And Dc Gain - Motorola DSP56156 Manual

ON-CHIP CODEC FREQUENCY RESPONSE AND GAIN ANALYSIS
6.5.2

D/A Section Frequency Response and DC Gain

The D/A section DC gain and the frequency response depend on the interpolation rates
selected by programming CRS1-CRS0 in the COCR. In addition, a positive 5 dB DC gain
has been introduced between the third order analog comb filter and the analog low pass
filter (see Figure 6-2).
Table 6-9 shows the D/A section DC gain when the audio level is set to 0dB by the audio
level control bits VC3-VC0 in the COCR.
CRS1 CRS0 Decimation
0 0
0 1
1 0
1 1
The positive gain of the D/A section allows the interpolation digital filter performed by the
DSP core to compensate the D/A comb filter frequency response without risk of clipping
the higher frequency components in the pass band spectrum.
The on-chip codec D/A section is formed of three different filtering steps which are studied
in the next subsections. The frequency responses of the different D/A sections are illus-
trated in Figure 6-7.
MOTOROLA
Table 6-9 D/A Section DC Gain
DC gain of the D/A
Ratio Rate 2nd order Comb Filter
125 -0.206 dB
128 0.000
105 -1.720 dB
81 -3.974 dB
DSP56156 ON-CHIP SIGMA/DELTA CODEC
DC gain of the
D/A section
4.794 dB
dB 5.000 dB
3.280 dB
1.026 dB
1.7366
1.7783
1.4588
1.1254
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