Page 191 - DSP Integrated Circuits

176 Chapter 4 Digital Filters

Figure 4.66 The first stage of the HSP43220 decimation FIR filter

are used to cancel the poles at z = 1 that are caused by accumulators. The com-
bined effect of the first stage is a coarse lowpass filtering with an attenuation of
aliasing into the passband of at least 96 dB.
A major problem with this approach is that the accumulators have poles at z = \,
i.e., they are marginally stable. Hence, an input signal with a nonzero mean will
eventually saturate the accumulators. In order to reduce the probability of over-
flow in the accumulators, special scaling techniques need to be employed. In this
case, word length in the first accumulator has been increased to 63 bits, the second
accumulator has 53 bits, and so on. This will reduce the overflow probability, but
the cost in terms of hardware is significant.

4.23 SAMPLING RATE CHANGE WITH A RATIO LIM

The sampling fre-
quency can be
changed by a ratio
LIM, where both L
and M are small inte-
gers, by combining
interpolation and dec-
imation as illustrated
in Figure 4.67[1, 3, 6, Figure 4.67 Sampling rate change by a factor L/M