Oli Charlesworth wrote:

> Hello all,
>
> This is a question which probably boils down to "what am I missing?".

This boils down to N versus log(N) operations, exactly like with the
FFTs. Look into the classic books such as "Multirate Signal Processing"
by Rabiner.


> It seems to be commonly stated that for interpolation or decimation,
> it's generally more computationally efficient to perform the operation
> in multiple stages, i.e. factorise the rate-change factor, and design a
> lower-order filter for each stage.

Athough this is generally true, this doesn't account for the overhead
associated with the setup of the each filtering stage. The overhead may
or may not be substantial.

> I completely accept that designing
> the individual filters in this way achieves a lower complexity, but I'm
> beginning to question whether one actually needs to implement the rate
> change in separate stages.

That entirely depends on the particularities of your application.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com