[Discuss-gnuradio] filter taps and delays

[Miklos Maroti]

Hi Guys,

If I use interp_fir_filter_ccc(3, [1, 2, 3, 4, 5, 6]) on a vector
source containing [1, 0, 0, ...] then I get [1, 2, 3, 4, 5, 6, 0, 0,
...] as expected. However, if I use interp_fir_filter_ccc(3, [1, 2, 3,
4, 5]) then I get [0, 1, 2, 3, 4, 5, 0, 0, ...] so the samples are
shifted to the right by one. For taps of length divisible by 3 there
is no delay, and for others there is 1 or 2 sample delay introduced by
the interpolation filter. Why is this?

Also, maybe related to this, I see filter.declare_sample_delay(0) in
generated code, and I do not know what it does and if it is strictly
necessary.

Miklos