Page 194 - DSP Integrated Circuits

4.25 Interpolator—Case Study 3 179

The group delay varies considerably in the passband 0 to 38.25°. The group
delay of the interpolator is therefore equalized by adding a seventh-order allpass fil-
ter in front of the interpolator. The resulting group delay is shown in Figure. 4.71.
The adaptor coefficients in the allpass filter are

If instead a ninth-order filter, which represents a significantly lower work
load, was used for the last stage, more of the processing capacity could be allocated
to the allpass filter to further reduce the variation in the group delay.
The complete interpolator with an allpass filter for correction of group delay is
shown in Figure 4.72. Alternatively, the allpass filter may be placed after the
interpolator where the sampling rate is higher. This results in a slightly higher
work load.
Also in this case the adaptor coefficients can be shortened to 9 bits. The arith-
metic work load is 35.2 • 106 adaptors/s corresponding to 35.2 • 106 multiplica-
tions/s and 105 • 106 additions/s and a significant number of operations for

Figure 4.71 Group delay, equalized in the band 0 to 38.25°