Re: Farrow interpolation in QPSK synchro

Let me try to summarize Farrow's idea in three bullet points:
The numbers are from the reference.

* I can use a FIR filter to interpolate one particular point between
samples, for example at an offset of 1/32 sample time

* I keep 32 different banks of FIR coefficients at hand, so that I can
choose between interpolating at for example 0/32, 1/32, 2/32... 31/32
delay. That's a "polyphase" filter.

* I may still need a higher timing resolution. Instead of decreasing the
step size (and using more polyphase filter "banks") I calculate each FIR
coefficient through a polynomial (for example 4th order) with the delay
0..1 as variable.

Hope this is useful...

Now some wild speculation:
Your numbers (interpolation between 1/2 and 1/2 gives 1) might be correct
with regard to the first bullet point. But usually one uses a much longer
FIR filter to interpolate, not linear interpolation (which boils down to
two-tap FIR).

-mn



.

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