Re: [Discuss-gnuradio] MSK Loopback question

[Johnathan Corgan]

On Tue, 2009-03-24 at 11:59 -0400, Dan Tarr wrote:

> Thanks again for any help on this, I'm basically hitting my head against a 
> brick wall.

What is happening is that the data coming out of the demodulator no
longer has the same byte boundaries as the data going into the
modulator.  Your output files are bit-shifted. In your script using the
quadrature demodulator, just take out the channel model.  It will work
(I've verified this.)

The channel model contains a block (the fractional interpolator) that
can delay outputting samples during startup, that is, it might eat a few
bits on the input before it starts outputting samples.  So, when you
have the channel model in place, the slicer bits are missing the first
four bits, and the repacker is placing the next four bits at the start
of the byte.

So that will get your artificial example working, but also understand
that during "real" reception, your receiver will need to handle
arbitrary bit offsets like this.  Generally, in data transmission
systems, this is handled by having a frame sync word or flag, which,
when detected, indicates the next bit is the start of the data boundary.

By the way, I debugged this by attaching file sinks to each of the
blocks in your flowgraph and viewing them using Octave, where it became
obvious what was happening.

Johnathan