[Discuss-gnuradio] How to synchronize feedback between blocks?

[Nick Foster]

Hi all,

I'm struggling with what I hope isn't a naive problem regarding feedback 
between flowgraph blocks. I've written a simple frequency estimator for FSK 
signals based on the squared FFT method. Its input is an N-point FFT, created 
by an N-item stream-to-vector block feeding an N-bin FFT block. The output of 
the frequency estimator is a float value in Hz, and it is a sync_block. 
Therefore, the output produces samples at the original (pre-stream-to-vector) 
sample rate divided by N. So if my original raw sample rate is 50000, and I use 
a 4096-bin FFT, the output frequency estimate arrives at ~12.2 samples per 
second. That's fine.

The problem is that I want to use the output of the frequency estimator to 
center the input data at baseband. With continuous streaming data, this is easy 
-- just use a frequency-xlating filter, and call set_center_freq() when data 
comes in from the frequency estimator. However, the packets I'm interested in 
are short, and I'd like the frequency correction to be applied at or near the 
start of the packet so the preamble can be detected appropriately. In order to 
do this, I have to be able to determine the delay it takes for feedback to come 
back, and that delay has to be constant. For this reason, I can't use a 
frequency-xlating filter, because the delay in calling set_center_freq() from 
the main Python loop could take any amount of time. I could write a version of 
the frequency-xlating filter that accepted a second stream of filter offset 
data at the input sample rate -- my thinking is that would make the delay a 
fixed number of samples. Is that assumption valid?

Before I go reinventing the wheel, I'm wondering if there is a "usual" way of 
solving this feedback problem within the architecture of GNU Radio, or if I'm 
totally out in left field.

Thanks for any feedback (no pun intended),
Nick
                                          
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