Re: [Discuss-gnuradio] Getting really confused by PSK mod/demod blocks

[Kyeong Su Shin]

Dear Pierre:

In my experience, that is just how the PSK demod block is - it misses a few symbols at the beginning, because of the blind synchronization & equalization process. It receives fine after that, but you cannot simply pack the bits to bytes and get acceptable results. This is not really problematic in the real-world scenarios, since you will have to detect the beginning of the packets anyway..

The quickest fix will be adding a packet encoder and packet decoder to your flow graph. It may miss a bit of data, but at least you should get readable outputs.

(Also, I am not sure if you can actually suffer from the phase ambiguity in this configuration, but I believe that you have to set "Differental Encoding" to "yes" to make it a DBPSK modulator / demodulator).

Regards,

Kyeong Su Shin

On Thu, May 4, 2017 at 2:41 AM, Pierre <address@hidden> wrote:

Hello again,

I've tried comparing the unpacked bitstream, and it's utter nonsense :
the first half of it is only zeroes while I know none of the bytes
transfered to be null.

My guess is that there's some parameter for either the modulator,
demodulator, or both, that I haven't set correctly, but I can't seem to
find which or how to set it so that everything works properly.

Best regards,
Pierre

Le mardi 02 mai 2017 à 17:42 +0200, Marcus Müller a écrit :
> Hi Pierre, 
>
> really odd looking data – what's that? have you tried comparing your
> unpacked bit streams? it's possible you're just missing the first few
> bits at the beginning, but if you then reassemble the bits to octets,
> you'd get wrong data.
> Best regards,
> Marcus
>
>
> On 05/02/2017 05:28 PM, Pierre AVITAL wrote:
>
> > Hello,
> >
> > I've been trying to use the PSK mod/demod, as well as DPSK
> > mod/demod, blocks to try to listen to signals using DQPSK
> > modulation.
> > But I'm completely stumped by their behavior : as can be seen in
> > the attachement here, I've tried to simulate my demodulation by
> > having a byte-stream sent through either DPSK or GFSK modulation,
> > then demodulating it and repacking it before sending it to a block
> > I coded that just prints what's on input and signals whether or not
> > it could spot a sync word.
> >
> > This works fine with GFSK, where the output of my "Test Block" is
> > the same as what the file contains.
> > But with DPSK, I get really odd looking data. Is there something
> > I'm missing here ? I've been trying various repackings both before
> > and after mod/demoding the signal, and have yet to figure out how
> > to get my data demodulated (or even modulated) properly with the
> > PSK blocks. 
> >
> > Thanks in advance for any help you could provide.
> >
> >
> > _______________________________________________
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>  
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