How to capture video frame over the air

Hello GR,

I tried to capture some TV broadcast video frames using

rx_samples_to_file --args “addr=myusrp” --file myfile.dat --time 120
–freq
450.75e6 --rate 5e6 --gain 30

Then I tried to play back that file using :
tx_samples_from_file --args “addr=myusrp” --file=myfile --type short
–rate
6.25e6 --freq 485e6 --gain 20 –repeat

However I was not able to get anything on the TV. Am I missing the
center
frequency for the capture (I tried to capture WUSA-HD)

I think the TV is already tuned to listen to 485e6 but I am not sure. Am
I
missing something or may be I should do automatic tuning for the TV to
be
sure to listen to 485e6 Hz.

Regards,
Ruecan


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Your capture frequency is way off. WUSA-TV transmits
on RF channel 9, which is 186 to 192 MHz. You want
to capture the center of the channel, so 189 MHz is
the correct number.

http://en.wikipedia.org/wiki/North_American_television_frequencies

The 5 MHz sample rate is too low. The signal is 6 MHz wide,
so you need a high enough sample rate to capture the entire
channel. So at least 6 MHz. A little more doesn’t hurt, so
a good value would be 6.4 Msps.

Also, use the same sample rate you chose for RX when
you TX the captured file.

Be aware that the TV has two channel plans. Over the air
and cable TV. Make sure you know which mode the TV
is in and select a frequency based on the center of the
channel. For example, over the air channel 14 would be
473 MHz and cable channel 58 would be 429 MHz.

Some TV’s won’t decode ATSC on cable frequencies
(they only decode QAM), so you may be forced to use over
the air frequencies (and make sure the TV is set up for that).

Ron

On 3/26/2014 10:18 AM, Ruecan wrote:
Hello GR,

I tried to capture some TV broadcast video frames using

rx_samples_to_file --args “addr=myusrp” --file myfile.dat --time 120
–freq
450.75e6 --rate 5e6 --gain 30

Then I tried to play back that file using :
tx_samples_from_file --args “addr=myusrp” --file=myfile --type short
–rate
6.25e6 --freq 485e6 --gain 20 –repeat

However I was not able to get anything on the TV. Am I missing the
center
frequency for the capture (I tried to capture WUSA-HD)

I think the TV is already tuned to listen to 485e6 but I am not sure. Am
I
missing something or may be I should do automatic tuning for the TV to
be
sure to listen to 485e6 Hz.

Regards,
Ruecan

Thank you Ron for this explanation.


View this message in context:
http://gnuradio.4.n7.nabble.com/How-to-capture-video-frame-over-the-air-tp47242p47252.html
Sent from the GnuRadio mailing list archive at Nabble.com.

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