Re: [Discuss-gnuradio] Unable to find field sync for over the air HDTV signal

[Eric Blossom]

On Wed, Oct 10, 2007 at 03:23:51PM -0500, Nirali Patel wrote:
> Hello everyone,
> 
> I am trying to capture a few seconds of over the air HDTV on a file and then
> decode using the files in gr-atsc module according to the instructions in
> the README at gr-atsc/src/python. I am using the following command to
> capture the signals off the air: /usrp_rx_cfile.py -R B -d 10 -f 587M -g 75
> test_file, where 587MHz is the center of the 584-590 MHZ UHF band. I can
> successfully capture about 20 seconds of data without any overruns. After I
> set up the pipes and start the equalizer, it is unable to find field syncs.
> At some gain settings it initially detects field sync but loses it later on.
> Am I lacking enough gain/sensitivity in the front-end of the TV_RX? If so
> would an lna at the antenna help? Or could I be doing something else that
> prevents the equalizer to find field sync? I believe a number of folks must
> have decoded over the air hdtv signals using the TV_RX and USRP. Any
> suggestions and help appreciated.
> 
> Thanks,
> Nirali

What does the output of usrp_fft -d 8 -f 587 look like?
Set it to average, then post a link to a screenshot.  It should be
mostly flat, with the pilot tone visible at the bottom end.

[In looking at interp.py, it could be modified to run something like
4x faster by using gr.interp_fir_filter_ccf instead of what's in
there.  Also, decimating by 10 seems a bit agressive.  By 8 would give
you more usable signal, though would require changes in the interp.py
code.]

Eric