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

[Josh Blum]

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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