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[Discuss-gnuradio] signals/atsc question

From: Ilia Mirkin
Subject: [Discuss-gnuradio] signals/atsc question
Date: Sat, 26 Feb 2005 16:10:23 -0500

Hey everyone,

I've been mulling over thinking about considering trying to rewrite the
old atsc stuff over to the new architecture; it doesn't actually look
that hard... maybe a day's work or so. However before doing that, I
wanted to see that code work, since I don't want to port non-working
code... so I've been capturing signals with a USRP board.

The old code expects the signals to be in either 20MS/s shorts or in
21.52MS/s shorts (and while I do have access to an mc4020 board, it is
not in a location that can receive ATSC signals and I'm not about to run
coax all over the place). I am using a 4937 frontend, so that gets the
USRP a signal from 2.75 to 8.75MHz. The only way that I've been able to
capture the signal reliably is by doing a DDC by -5.75MHz with a
decimation factor of 8. This puts the signal nicely between -3 and +3
MHz, at 8MS/s with complex samples. In order to convert this to 20MS/s
real, I've tried two approaches:

(a) interpolate 5x, lowpass, freq_xlat, decimate 2x (tried with bandpass
and without), throw away complex

(b) interpolate 5x, lowpass, multiply by a 5.75MHz complex sine wave,
decimate 2x, throw away complex

Both of these approaches yield a resulting wave that looks right. It is
centered around 5.75, and if the output were averaged, I could imagine
that it would look like http://comsec.com/wiki?HdtvFFT with the
exception of the pilot which isn't very pronounced. (Note that it
*seems* as if the pilot is stronger on the original than on the
converted -- see http://web.mit.edu/imirkin/www/gnuradio)

Both of these methods yield similar results -- the gnuradio-0.9 atsc_rx
happily takes the input, and generates an output TS file, with an error
rate of 1. However, I have plugged this very same antenna pointed in the
same way into my pcHDTV 3000 card, and it has no problems with the
signal (maybe an occasional glitch).

Any suggestions? I'm not really a signals person, though I do have a
rather cursory signals experience. I feel that what I'm doing to convert
it is right -- is it? Did the old ATSC code work well? Is the pcHDTV3000
card just that much better of a decoder and to get this stuff to work
we'd need to make the atsc rx stuff a lot cleverer?

A note on the antenna: it is an el-cheapo Jensen atenna with a UHF loop
and VHF dipoles. It does amplification, but in order to get it to work
with the pcHDTV card I need it to be at the lowest setting (but still
turned on), so I am doing the same for the USRP.

Ilia Mirkin

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