[Discuss-gnuradio] PFB Clock Sync stops producing samples

[Andy Walls]

For a block that performs the identical function of the PFB clock synch
block with correct tag propagation, use the Symbol Synchronizer Block
available in this pull request:

https://github.com/gnuradio/gnuradio/pull/1294
https://github.com/awalls-cx18/gnuradio/tree/symbol_sync2

or this OOT module:

https://github.com/awalls-cx18/gr-nwr

Use the following settings:
Timing Error Detector:   y[n]y'[n] Maximum Likelyhood
Samples per Symbol:      whatever you're using now
Loop Bandwidth:          whatever you're using now (may need tweaking)
Damping Factor:          1.0/math.sqrt(2.0)
Maximum Deviation:       whatever you're using now
Output Samples/Symbol:   whatever you're using now
Interpolating Resampler: Polyphase Filterbank, MF
Filterbank Arms:         whatever you're using now for Filter Size
PFB MF Taps:             whatever prototype filter taps you're using now


Properly fixing the tag propagation in the PFB_clock_sync block is a
major change, which is one reason I wrote the new Symbol Synchronizer
blocks.

FWIW, I also do think get_tags_in_range() could stand to perform a
sanity check, but that won't fix the underlying problem.

-Andy


>                              From: 
> devin kelly
>                           Subject: 
> [Discuss-gnuradio] PFB Clock Sync
> stops producing samples
>                              Date: 
> Thu, 8 Jun 2017 12:08:12 -0400
> 
> ______________________________________________________________________
> I'm having a problem with the Polyphase Clock Sync block.  I believe
> what's happening is that the block is calling get_tags_in_range
> improperly and then causing an infiite loop in
> gnuradio-runtime/lib/buffer.cc, at this point the PFB Clock Sync block
> stops emitting samples.
> 
> 
> The problem occurs in the PFB clock sync block here:
> https://github.com/gnuradio/gnuradio/blob/master/gr-digital/lib/pfb_clock_sync_ccf_impl.cc#L473
> 
> 
> The PFB block calls get_tags_in_range where the start sample number is
> greater than then end sample number, here's the code in buffer.cc that
> may be getting stuck in an infiite loop.
> 
> 
> The entire file is here:
> https://github.com/gnuradio/gnuradio/blob/master/gnuradio-runtime/lib/buffer.cc#L354

[snip]
> When I run into this problem, my code seems to get stuck on itr++ (see
> debugger output below).  Though I'm not sure if the itr++ call just
> stalls or if itr just never equals itr_end.
> 
> 
> When the PFB Clock Sync block stalls, I attach with GDB, find the PFB
> thread and get a back trace, I then go to the PFB general_work frame
> and print the two inputs to get_tags_in_range

[snip]

>I've also found (separately, using breakpoints) that sometimes count is
>negative.  Is that the way it should be?

[snip]


> I'm not sure if we're putting bad data into the PFB block or if
> there's some bug in the PFB block.  Or should there be some checking
> in get_tags_in_range that checks if start > end.  What's the best way
> to proceed?
> 
> 
> Thanks,
> 
> Devin