Video Add-On Software for FreeDV Digital Voice

The place to discuss reduced bandwidth fast scan amateur television on 70cms and below
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wa6nut
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Location: Buena Vista, Colorado, U.S.A.
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Video Add-On Software for FreeDV Digital Voice

Post by wa6nut » Thu Nov 14, 2013 6:33 pm

Hopefully everyone has viewed the CAT 13 presentation by G4GUO at:

http://www.batc.tv/streams/cat1314

Charles' presentation posed a question about minimum frame rate for narrow band DATV. I would consider 4 frames per second an acceptable value. Video at this frame rate provides fairly smooth motion. Years ago, I worked with a version of ZL2AFP's NBTV at 2.9 frames per second. The motion wasn't smooth, but it wasn't objectionable. Amateur television programming is mostly "talking heads", so a high frame rate is less necessary. I'd be interested in seeing an update of Charles' soundcard-based modem (4 frames per second) he described in another post to this forum (June 13, 2013).

Digital voice on HF has gained in popularity since the introduction of FreeDV earlier this year. There has been some interest in making FreeDV a multimedia mode, by adding video. I've developed a soundcard-based scheme for adding video, using a modified version of ZL2AFP's "FFT-TV" software. Using 64 sub-carriers (including FreeDV), the frame rate is only 1 frame per second, with 36-line resolution. Bandwidth is 3.5 kHz, so it can be used with some HF transceivers. The scheme is compatible with FreeDV ("1400" mode). More details are at:

http://www.qsl.net/wa6nut/FreeDVplusVideo

One problem with this scheme is that IM products are mingled with the voice and video signals. The result is noisier video and reduced SNR for the FreeDV. This problem is discussed on my website, in a "white paper" on the "Technical" page, under the "FDMA" heading. One solution is to provide more separation between the video and voice signals, increasing the bandwidth to 5.7 kHz (however, not many HF transceivers can accommodate this bandwidth).

73, Rick, WA6NUT
Buena Vista, Colorado

G0DJA
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Location: Bolsover

Re: Video Add-On Software for FreeDV Digital Voice

Post by G0DJA » Tue Nov 26, 2013 2:01 pm

Unfortunately, I couldn't make the Finningley presentations in the end, so I missed the presentation.

However, my TS2000X will get up to about 5kHz bandwidth, would this be enough for a reasonable NBTV signal I wonder?

Dave (G0DJA)

G0DJA
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Joined: Fri Mar 28, 2008 2:35 pm
Location: Bolsover

Re: Video Add-On Software for FreeDV Digital Voice

Post by G0DJA » Tue Nov 26, 2013 2:18 pm

Just realised that the KX3 will do eSSB as well, so I could use that instead of the TS2000X

KenW6HHC
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Joined: Mon Jun 01, 2009 4:20 pm

Re: Video Add-On Software for FreeDV Digital Voice

Post by KenW6HHC » Tue Nov 26, 2013 3:06 pm

Hi Dave G0DJA,

Low grade video stream (meaning small amount of motion content from frame to frame) will require approximately 1,000,000 bits/sec after compression/encoding. Using a spectrum-compact digital modulation technology like 256-QAM will require a bandwidth of about 15 KHz to 20 KHz with audio (in theory). That modulation will also require a extremely good S/N.

If you reduce the video frame rate from 30 F/sec to 4 F/sec...the compressed video stream should be down to about 140,000 bits/sec. Now the audio stream component becomes even more significant portion of the final bandwidth. I have not seen the final video/audio quality...but a bandwidth smaller than 5 KHz should be possible.

This year, there was a posting on the ham-DATV-forums about government trials for low-band-width video, including HF bands. The companies conducting the video trials were using 24KHz bandwidth as one of the configurations. The success was marginal due to the high S/N propagation conditions required on HF. I was able to find the thread on the Yahoo DigitalATV Group forum, by searching for "HF".

BTW...the CAT13 presentations have been archived by the good folks at BATC. You can watch the streaming video of the G4GUO presentation in the CAT13 section, as Rick WA6NUT discussed (with URL) earlier in this thread.

73...de Ken W6HHC

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