# How to capture video frame over the air **URL:** \ **Category:** GNU Radio\ **Created:** [March 26, 2014, 7:19pm UTC](https://www.ruby-forum.com/t/how-to-capture-video-frame-over-the-air/240084 "2014-03-26T19:19:29Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![Detlef\_R](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/58f4c7/32.png) [@Detlef\_R](https://www.ruby-forum.com/u/Detlef_R)\ **Post date:** [March 26, 2014, 7:19pm UTC](https://www.ruby-forum.com/t/how-to-capture-video-frame-over-the-air/240084/1 "2014-03-26T19:19:29Z")
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 – View this message in context: [http://gnuradio.4.n7.nabble.com/How-to-capture-video-frame-over-the-air-tp47242.html](http://gnuradio.4.n7.nabble.com/How-to-capture-video-frame-over-the-air-tp47242.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![naeo](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/57b2e6/32.png) [@naeo](https://www.ruby-forum.com/u/naeo)\ **Post date:** [March 27, 2014, 12:14am UTC](https://www.ruby-forum.com/t/how-to-capture-video-frame-over-the-air/240084/2 "2014-03-27T00:14:17Z")
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. > **[Pan-American television frequencies](https://en.wikipedia.org/wiki/North_American_television_frequencies)** > > The Pan-American television frequencies are different for terrestrial and cable television systems. Terrestrial television channels are divided into two bands: the VHF band which comprises channels 2 through 13 and occupies frequencies between 54 and 216 MHz, and the UHF band, which comprises channels 14 through 36 and occupies frequencies between 470 and 700 MHz. These bands are different enough in frequency that they often require separate antennas to receive (although many antennas cover both... 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 ---
**Author:** ![naeo](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/57b2e6/32.png) [@naeo](https://www.ruby-forum.com/u/naeo)\ **Post date:** [March 27, 2014, 4:01am UTC](https://www.ruby-forum.com/t/how-to-capture-video-frame-over-the-air/240084/3 "2014-03-27T04:01:17Z")
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](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](http://Nabble.com).