Re: [Discuss-gnuradio] Bit shift sync error with DPSK

[Chris Biow]

Thanks, Josh and Tom. Those were the pointers I needed. It's fixed after
adding packet encode/decode, plus getting symbol lengths correct.

Tom's response from March
(http://permalink.gmane.org/gmane.comp.gnu.radio.general/38723) was helpful
in providing a working GRC example that reliably receives what it transmits.

-----Original Message-----
From: address@hidden
[mailto:address@hidden On Behalf Of Josh
Blum
Sent: Sunday, December 30, 2012 11:32 PM
To: address@hidden
Subject: Re: [Discuss-gnuradio] Bit shift sync error with DPSK



On 12/30/2012 05:04 PM, Chris Biow wrote:
> I'm trying to get a simple PSK loopback working in GRC, File Source -> 
> DPSK Mod -> DPSK Demod -> Unpacked to Packed -> File Sink.
> 
> On Ubuntu 12.10 x64 (build-gnuradio a few days ago), the sink file 
> winds up with leading 7 bytes x00, 3 bytes plus one bit noise, four 
> mangled bits, and then my correct bitstream. But the odd bit causes 
> the entire remainder of the bitstream to be right-shifted by one. 
> Throttling doesn't seem to have any effect.
> 
> Interestingly enough, using MSWin7, I get 6 bytes x00, 2 bytes noise, 
> 26 bytes mangle, and then a correctly aligned stream of bytes (other 
> than a mangled sequence from about bytes 100-107).
> 
> Am I missing something to provide symbol timing recovery?
> 
> The GRC file is at
> https://docs.google.com/open?id=0B56aEJNJrwBAX2NhMHczdGhUejA
> 
> 

This may not be 100% of your issue, but I cant say that with certainty that
the states of the various timing recovery are initialized or zeroed at
constructor time. So different runs or different OSs could yield with
different results.

Now, the real issue is there is no framing or frame recovery blocks in this
flow graph. So the recovery side doesn't know what symbol starts a byte, so
you could have bits sliding across byte boundaries in the unpacked to
packed, even in this simple example.

-josh


> TIA,
>  Chris
> 
> 
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