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Re: [Discuss-gnuradio] HDTV

From: Ilia Mirkin
Subject: Re: [Discuss-gnuradio] HDTV
Date: Fri, 27 May 2005 01:08:29 -0400

On Thu, 2005-05-26 at 21:41 -0700, John E. Don Carlos wrote:
> What is the best way to get hdtv going?

I don't know if anyone's done anything on this topic lately, but this is
what I found from my experience on this:

There is code in gnuradio-0.9 (hint: you don't have to make install
gnuradio-0.9 -- the code will work out of the compiled source tree)
which will allegedly take a 20MS/s (short samples) file, where the
signal is centered around 5.75MHz (as is the case with the TVRX board
assuming it uses the Microtune 3x7702), and produce a file containing
the ATSC-encoded samples.

The problem is that the USRP can't send 20MS/s over the USB bus, nor can
it sample at 20MS/s without using an external reference. I think that
support for the USRP in gnuradio-0.9 is... lacking. Another problem is
that the code isn't particularly clever -- it needs a rather clean
signal in order to decode it properly -- all of my attempts ended up
with 100% errors while my pcHDTV card got the signal on that channel
just fine.

What I did was take the USRP, record the ATSC signal to a file centered
at 0MHz using DDC (make sure to downconvert in the right direction)
(compex floats), which allows you to use 8MS/s and the USB bus seems to
be able to keep up with that, mostly. Then you can write a script to
convert that to a file of shorts with the signal centered at 5.75MHz
(e.g. by multiplying it by a complex sine wave).

Now you have a file that the gnuradio-0.9 code is willing to deal with,
and if your signal is clean enough then you should get an mpeg file.
This is where I hit a dead end, and thus wasn't really willing to try to
convert the old code to new code since I had no way of knowing if it
would work or not.

Good luck :)

(If you're just interested in watching HDTV I'd personally advise you to
get a card like the pcHDTV 3000 which has good linux support and does
ATSC as well as QAM; today's computers don't have enough cpu power to do
this in real time)

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