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Re: [Discuss-gnuradio] Simple QAM mod/demod

From: Fabián Kozynski
Subject: Re: [Discuss-gnuradio] Simple QAM mod/demod
Date: Wed, 3 Oct 2012 13:31:42 -0400

I figured out the problem. There were two things I wasn't taking into account:


On Wed, Oct 3, 2012 at 10:12 AM, Fabián Kozynski <address@hidden> wrote:

It's not just an offset problem. Even though in the output file, there is a delayed sequence, it doesn't have the right bits. As it has exactly the same period it leads me to believe that it's just a problem of phase sync.

Attached is a minimal grc file.


On Wed, Oct 3, 2012 at 12:23 AM, Ben Reynwar <address@hidden> wrote:

On Mon, Oct 1, 2012 at 1:27 PM, Fabián Kozynski <address@hidden> wrote:
> As sanity check I'm trying a simple flowgraph in GRC with a QAM mod/demod:
> Vector Source ==> Throttle ==> QAM Mod ==> QAM Demod ==> Unpacked to Packed
> ==> File Sink
> The parameters in the mod/demod are as default. I also have a File Sink
> before the mod.
> In the output I see first a bunch of zeros, then a bunch of random numbers
> and after 20 or so bytes I get a sequence with the same period as the
> original one, but different numbers. I suppose this is a problem of phase
> sync.
> What's the right way to simulate a simple mod/demod? How do I add sync
> between mod and demod?
> Thanks,
> Fabián

It sounds like you're comparing bytes in the two files.  First thing
that I'd check is that the two files aren't offset from each other by
a few bits so that the bytes don't line up in the same way.

If that's not the problem then email me the grc file and I'll have a look.


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