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Re: [Discuss-gnuradio] signals/atsc question

From: Ilia Mirkin
Subject: Re: [Discuss-gnuradio] signals/atsc question
Date: Wed, 02 Mar 2005 01:41:49 -0500

> There are about 10.76 Msymbols/sec in an ATSC signal, so you are going
> to have Nyquist problems if you try to represent them in 8M complex
> samples/sec.  You'll need at least 10.76 complex MS/s.  (Indeed you
> may notice that we were skating inside the edge by using 20
> non-complex MS/s on the old hardware.)  If we had a faster USB bus,
> the USRP could sample at a nice high rate, like 32 Msamples/sec, then

So... this probably stems from my shoddy understanding of signals and/or
information theory, but here is the question:

I have 8M complex S/s. This means that I *should* be able to do some
sort of quadrature demodulation or something along those lines to get a
full 8MHz of spectrum, no? Is this logic wrong? The FFT displays the
signal from -4 to +4 MHz...  I was under the impression that the ATSC
signal was only 6MHz wide (if not a little less), so why do you need
21.52MS/s (real) to sample a 6MHz signal? Is this due to the
quantization of the measurements, and I would normally be right with
infinitely precise samples, but in the world of reality we live in, we
just have to oversample?

> The pcHDTV card uses a chip that's specialized and engineered to do
> this job very well in a variety of environments.  By contrast, you are
> only about the fourth person to ever run the ATSC input code.  It's a
> "dancing bear", which took more than a year of solid engineering just
> to get it dancing at all.  It hasn't been optimized, either for CPU
> usage or for non-perfect signal reception.

Point well taken. There's a lot of multipath going on where I'm
receiving this.... might have to try for something different. I do have
a different antenna source; I'll try playing with it.

> I'll attach my favorite point script below.  I thought it was in the 
> source trees, but now I can't find it except on my hard drive.

Yeah, the wiki referred to it, but it wasn't in the gnuradio-0.9

Thanks for the suggestions... I'll give them a whirl and see if I can
get anywhere.

Ilia Mirkin

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