[Discuss-gnuradio] Obtaining the original (source) samples at the end of the chain (sink)

[Veljko Pejovic]

Hi,

I would like to process samples from a USRP (let's use FM
demodulation, for example) and then identify which of the original
samples correspond to a particular result at the end of the chain
(say, a beginning of a stereo transmission, a beginning of a song,
etc.).

The problem is, in the process of filtering, demodulation and
evaluation I lose the original samples. If, at the point of detection
I go back and fetch samples from USRP, those won't be the exact
samples corresponding to the generated result. Moreover, since the
delay the original samples take to go through the processing chain is
varying and unpredictable (I learnt that in my last experiment, and
thank you Macus for explaining this), I can't just build a parallel
chain that will merely delay the samples, and then fetch those delayed
samples.

So, what is the solution? One idea I have is to extend each block in
the processing chain with another input/output port and carry along
the original samples. Then, in the end, when I evaluate the result, I
can just take the piggybacked samples, and be sure that those are the
ones that generated the result. However, this means I have to modify
each block in the chain. Is there a more elegant solution to the
problem?

Thanks,


Veljko