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[Discuss-gnuradio] simple mod-demod combinations doesn't work

From: MarkO
Subject: [Discuss-gnuradio] simple mod-demod combinations doesn't work
Date: Fri, 7 Oct 2016 18:16:12 -0700 (MST)

Hi everyone,

I currently use GRC and only its In-Tree-Modules.
PC:  Lenovo Laptop W530
CPU: Intel i7-3610QM with 8GB RAM
OS:  Ubuntu 16.04 64 bit

I tried to create some easy examples of different modems. My first goal is
to modulate ASCII- or WAV-files, add some noise, demodulate the data stream
and save or hear the expected result. Maybe I am wrong, but i expect an
lossless connection in GRC if I directly connect an modulator to its
appropriate demodulator - particularly when i use the default values.

The issue is, even without added noise it's not possible to get an lossless
connection in software.

My Flow-Graph looks like this:

Wav File Source --> Float to Char --> QAM Mod --> Throttle --> QAM Demod -->
Pack K Bits --> Char to Float --> Audio Sink

I use [Float to Char] to reduce data and get a byte-stream. Of course this
loss of data isn't reconstructable.
But the incoming signal at the [Audio Sink] sounds extreme noisy and clips
It is also a little bit random how bad it is - at one start i can hear what
song it is, at the next start it's just noise.

I think i can say that the Problem is the mod-demod part:
  --> QAM Mod --> Throttle --> QAM Demod --> Pack K Bits -->
because this:
  Wav File Source --> Float to Char --> Char to Float --> Audio Sink  
works pretty well.

I attached the GRC file with the different modems to this thread. Of course
only one Mod-Demod string can be active at a time.
The results are commented with [Notes] in the GRC file. I also tried every
modulation with reduced data rate (usage of down- and upsampling).

I would be deeply greatful if someone may explain me my fallacy or how it
would just work - especially for the QAM modulation mentioned at the



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