Wednesday, 14 September 2011

SSTV on PMR446

Following on from my experiments in receiving slow-scan television (SSTV) from the ARISSat-1 satellite I was intrigued by the prospect of actually transmitting and receiving some images myself.

Being unlicensed the only legal equipment I can used is PMR446 these are low power (500mW) handheld transceivers with very limited range. I have several some older models kicking about from when I built a eQSO internet PMR446 gateway.

The SSTV image is transmitted by frequency modulation using a varying audio tone to indicate different brightnesses. The software I have been using to decode SSTV is MMSSTV by JE3HHT (Makoto Mori) It can also be used to transmit SSTV images by connecting the audio output of the PC to the transmitter audio in. It allows editing and customisation of the images and the encoding format to use.

Obviously I couldn't carry a PC around so instead I generated some images and recorded the generated sound to a WAV audio file and then used an old cheap MP3 player (which supported WAV files) connected to one of the Audiovox PMRS-838 handsets, configured to operate in VOX mode.

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The plan was to drive around to a number of local high spots and with the transceiver on the dashboard of the card output a series of images and see if they could be received and decoded back at base! - Each image taking around 2 minutes to transmit.

The base receiver was my Alinco DJ-X3 connected to the loft based discone. I had to use the DJ-X3 because it supports to 6.25kHz narrow band channel step required by the PMR446 channel allocations.

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Well the first set of results today have been pretty good, I went to three sites to transmit from and using the terrain profiler on HeyWhatsThat.com I could calculated any obstacles in the line of site..The base is on the left hand side, the transmission point on the right of the diagrams below.

Site 1, was 0.9 miles from home  This site is just across on the other side of the Trent Valley with no significant buildings in the transmission path.

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as you can see the five images all came across very well, just a slight sync error on one.

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Site 2, was 1.2 miles from home and is one of the local high spots. Unfortunately there is a housing estate on the top of the hill. It is possible to go by foot on some footpaths to get into some fields with a clear vantage point across the town and down the Trent Valley towards Nottingham. However today these were from the car parked up near to the edge of the estate. I just sent two images - and they are a lot nosier.

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Site 3, was 3.2 miles from home and is on a slight high spot across on the other side of the Trent Valley. I parked up in a layby and transmitted three images and really wasn't expecting much. So was very pleasantly surprised by the high quality of the received images, as you can see there isn't any geographical obstruction but there is a very big new power station at Staythorpe in the line of transmission which I expected to cause some issues. Some of the lines of noise on the images I suspect were caused when large lorries and vans were passing by on the road momentarily blocking the transmission path as it is quite a busy road.

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From these encouraging results definitely some more experiments to be done on this, and some tests further afield!

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