"3.2 Multistage
3.2.1 Can I interpolate in multiple stages?
Yes, so long as the interpolation ratio, L, is not a prime number.
For example, to interpolate by a factor of 15, you could interpolate
by 3 then interpolate by 5. The more factors L has, the more choices
you have. For example you could interpolate by 16 in:
- one stage: 16
- two stages: 4 and 4
- three stages: 2, 2, and 4
- four stages: 2, 2, 2, and 2
3.2.2 Cool. But why bother with all that?
Just as with decimation, the computational and memory requirements
of interpolation filtering can often be reduced by using multiple
stages.
3.2.3 OK, so how do I figure out the optimum number of stages, and the
interpolation ratio at each stage?
There isn't a simple answer to this one: the answer varies depending
on many things. However, here are a couple of rules of thumb:
- Using two or three stages is usually optimal or near-optimal.
- Interpolate in order of the smallest to largest factors. For
example, when interpolating by a factor of 60 in three stages,
interpolate by 3, then by 4, then by 5. (Use the largest ratio on the
highest rate.)"
http://dspguru.com/dsp/faqs/multirate/interpolation
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