Wednesday, August 23, 2017

Digital Slow Scan TV with gstreamer

I had a look at radio amateur Slow Scan TV and noticed that it is analogue, so I wondered whether I can create a digital version using gstreamer.

Here is an example script that will play one video frame every ten seconds.  This could very well form the basis for a digital SSTV system.

The first part of the script are commented tests that show how I worked my way up to the super slow method that uses the videorate plugin to drop most of the frames.

Note that due to the long time between SSTV frames, everything will always be different, so motion jpeg will likely work better than h.264 compression.

#! /bin/bash
echo Slow Scan TV example with a laptop PC camera and gstreamer

# Install all the plugins including v4l2src:
# dnf install gst*plugin*

# Play the laptop PC camera the simplest possible way using the system defaults:
# gst-launch-0.10 v4l2src device=/dev/video0 ! autovideosink

# Play the camera and scale it smaller at 30 frames per second
# gst-launch-0.10 v4l2src device=/dev/video0 ! video/x-raw-yuv,framerate=30/1,width=640,height=480 ! autovideosink

# Play the camera and scale it smaller at 1 frame per 10 seconds:
gst-launch-0.10 v4l2src device=/dev/video0 ! video/x-raw-yuv,framerate=30/1,width=640,height=480 ! videorate ! video/x-raw-yuv,framerate=1/10,width=640,height=480 ! autovideosink

# Eventually, one can use a motion jpeg encoder and replace autovideosink with udpsink to stream it:
# ! udpsink host= port=5000 sync=false

What it means is that whereas amateur radio SSTV sends low definition 320p video at 1 frame per 2 minutes using 1960s analogue techniques, one could possibly send 720p HD video at 1 frame per 2 minutes using present day digital techniques, over the same shortwave SSB radio.  It is worth exploring further.

I think all one needs to add to the above is a simple phase continuous dual tone FSK modem with some FEC.  Maybe I'll resurrect some ancient code I did for a modem some time late last century - if the disk drive lying in my junk box is still readable!

Bunnel's Horse With No Name sort of applies to where I am living now:
You see I've been through the desert on a horse with no name
It felt good to be out of the rain

La, la ... voila!


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