333KS Image Parameters

Discussion about this major DATV Project. See also https://wiki.batc.org.uk/The_Portsdown_Transmitter
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gm8arv
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Joined: Fri Aug 17, 2018 1:03 pm

333KS Image Parameters

Post by gm8arv » Tue Feb 12, 2019 8:39 am

The URL:

http://www.batc.org.uk/club_stuff/rbtv.pdf

from the Introduction page:

viewtopic.php?t=4292

is giving a not found. I would like the specifications for 333ksps RBTV in terms of recommended image size, FEC, codec etc. (as might others seeing this free download for non-members - finding it through Google).

Thanks,
David

g8gtz
Posts: 1034
Joined: Wed Apr 07, 2010 6:26 pm

Re: 333KS Image Parameters

Post by g8gtz » Tue Feb 12, 2019 8:50 am

Hi David

I've updated the link on that post - it now points to the font of all knowledge - the BATC wiki!

https://wiki.batc.org.uk/Getting_Starte ... ndwidth_TV

Typically we run 333Ks (~500KHz) down to 65 Ks., 7/8 FEC and H264 / H 265.

73

Noel - G8GTZ

gm8arv
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Joined: Fri Aug 17, 2018 1:03 pm

Re: 333KS Image Parameters

Post by gm8arv » Tue Feb 12, 2019 9:59 am

Thanks, Noel. What image size? Presumably the smallest acceptable?

G8GKQ
Site Admin
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Re: 333KS Image Parameters

Post by G8GKQ » Tue Feb 12, 2019 10:42 am

Hi David

In Portsdown the transmitted image size and frame rate depends on the Video bitrate. Simplified code:

Code: Select all

  # Reduce video resolution at low bit rates
  if [ "$BITRATE_VIDEO" -lt 150000 ]; then
    VIDEO_WIDTH=160
    VIDEO_HEIGHT=140
  else
    if [ "$BITRATE_VIDEO" -lt 300000 ]; then
      VIDEO_WIDTH=352
      VIDEO_HEIGHT=288
    else
      VIDEO_WIDTH=720
      VIDEO_HEIGHT=$IMAGE_HEIGHT
    fi
  fi

  # Reduce frame rate at low bit rates
  if [ "$BITRATE_VIDEO" -lt 300000 ]; then
    VIDEO_FPS=15
   fi
We tend to use FEC 7/8. With LimeSDR Mini the Portsdown can transmit higher order DVB-S2 modes such as 32 APSK. This gives a video bitrate that will easily handle 1280x720 pictures. H264 gives higher quality within a given bitrate than MPEG-2, so is preferred. Some DATV Express (hardware) users have used the DATV Express software with an H265 encoder and achieved stunning results at 333KS.

The beauty of MiniTioune is that it is flexible enough to receive and display all these modes.

Dave

gm8arv
Posts: 4
Joined: Fri Aug 17, 2018 1:03 pm

Re: 333KS Image Parameters

Post by gm8arv » Tue Feb 12, 2019 1:47 pm

Dave,

OK, I see what you mean. So, the software (either DATV Express/Win-10/Pluto) will resample the input stream to those dimensions before transmission. I didn't appreciate it did that. But why the non-standard aspect ratios, or are those simply max-width and max-height limits?

I also note that on the DATV Express/Win-10 software, on the Video Codec dialog, there is a fixed video bitrate number displayed (e.g. 275495), but also an option setting at the bottom of the dialog currently set to 0.7 but also labelled video bitrate. Is this a k-factor setting or something else? It would help if these values and labels had mouse-over hints saying what they do. I find these can reduce support effort!

Unfortunately the SDRangel software I'm using for tests only has DVB-S, but I do have a MiniTiouner Express installed and working for later tests.

David

G8GKQ
Site Admin
Posts: 1045
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Re: 333KS Image Parameters

Post by G8GKQ » Tue Feb 12, 2019 3:03 pm

Hi David

I must admit to inheriting those image sizes with the original code and never checking or changing them. Some of the odd aspect ratios are because the hardware H264 encoder needs the horizontal image size to be divisible by 32 and the vertical to be divisible by 16.

I am not an expert in the DATV Express software - just an occasional user. Charles should be able to help.

There is a big push for some SDR software to support DVB-S2 for Es'hail-2, but there is a lot of work to be done. F4DAV is making some progress.

Dave

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