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Re: Question regarding transmission of a tone using QPSK

From: Jeff Long
Subject: Re: Question regarding transmission of a tone using QPSK
Date: Tue, 14 Jul 2020 21:57:47 -0400

A better explanation of why that plot is correct: if you sample a tone twice per cycle, you see [-1,1,-1,1,...]. Four times per cycle, looks like [-1,0,1,0,...]. Even though it looks discontinuous, it will sound like a tone when played through your sound card due to filtering in the audio software and/or hardware.

That tutorial goes through the low level portions of the digital chain, including timing recovery. Framing, error correction and (optionally) an audio codec would all be in addition to the blocks shown in the tutorial.

On Tue, Jul 14, 2020 at 9:03 PM Jeff Long <willcode4@gmail.com> wrote:
Depending on your sample rate and tone frequency, that plot would be correct.

The analog signal needs to be encoded somehow as data before transmission. While you could feed an audio file 2 bits at a time into a QPSK modulator, it's pretty unlikely that you will be able to recover the audio. If you're thinking of "transmitting audio", look into audio codecs. If you're thinking of sending a wav file, you're really just sending packets. Either way, you will need a complete chain that includes error correction, clock recovery, etc.

On Tue, Jul 14, 2020 at 3:58 PM lannan jiang <jln925@live.com> wrote:
Hi all,
    I have been following the PSK guided tutorial https://wiki.gnuradio.org/index.php/Guided_Tutorial_PSK_Demodulation . I am on the mpsk_stage6.grc, but I want to transmit a simple tone instead of a random source, so I added a signal source which generates a sine wave. However, here are my questions:

   1.  I select the output of the signal source as bytes, and the time plot of it is attached. As you can see, the plot looks like bursts. But if I add an audio sink after signal source directly, I hear a constant tone. This does not make sense to me, as I thought I should hear discontinuous sound as the plot shows, could someone explain this?  

    2. With the first question being said,  I am using a constellation modulator (QPSK) that takes 2 bits/symbol.
    How can I feed the output  of signal source ( a 16-bit audio file later on) to the constellation modulator properly?  

Thanks in advance!

Lannan Jiang

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