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

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] Bit shift sync error with DPSK
Date: Mon, 31 Dec 2012 14:20:07 -0500

On Sun, Dec 30, 2012 at 6:04 PM, Chris Biow <address@hidden> 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



What you are seeing is the result of the group delays of the digital filters. Both the modulator and demodulator have filters in them that each add their own delay. The system also works on symbols that are converted to bits. It doesn't know about or care about byte alignment. Some of the packetizing/framing blocks we have use an access code to find the start of the frame and then know how to appropriately align for the byte boundaries.


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