Altera Corporation 7–19
September 2004 Stratix Device Handbook, Volume 2
Implementing High Performance DSP Functions in Stratix & Stratix GX Devices
Figure 7–11. Time & Frequency Domain Representations of Interpolation for I = 4
As an example, CD players use interpolation, where the nominal sample
rate of audio input is 44.1 kilosamples per second. A typical
implementation might have an interpolation (oversampling) factor of 4
generating 176.4 kilosamples per second of oversampled data stream.
Polyphase Interpolation Filters
A direct implementation of an interpolation filter, as shown in
Figure 7–10, imposes a high computational burden. For example, if the
filter is 16 taps long and a multiplication takes one cycle, then the number
of computations required per cycle is 16
× I. Depending on the
interpolation factor (I), this number can be quite big and may not be
achievable in hardware. A polyphase implementation of the low pass
filter can reduce the number of computations required per cycle, often by
a large factor, as will be evident later in this section.
The polyphase implementation “splits” the original filter into I
polyphase filters whose impulse responses are defined by the following
equation:
h
k
n() hk nI+()=
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