HFU HF Underground
General Category => General Radio Discussion => Topic started by: RadRoadrunner on March 13, 2017, 1320 UTC
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Has anyone ever considered making a digital SSTV mode that could yield a clearer picture than an analog signal? It seems like DX'ing would be made a whole lot easier if one were developed.
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There are digital SSTV modes, check out EasyPal.
The problem with digital modes in general is they tend to be very clear right up until they are not, then they fall completely apart. Sometimes analog modes can provide better reception in fringe areas, areas where you would have no decode in digital modes you can at least have something with analog.
Of course, you can have digital modes that are very good in weak signal application, working at or below the noise floor, but those tend to be very low data rate modes, not something that works well with SSTV or information containing a lot of data, as an image does. You absolutely could make an image mode that works better at lower SNRs, but it would require you to take longer to send each frame.
T!
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As above, easypal can work well for images and data. It uses a mod of DRM wich is a mod of STANAG4285 with some carriers added, as tuning a S4285 signal is hard enough without some automated help. That being said, S4285 demodulator is designed to lock on sigs that are at most 75Hz off and can ignore multiple interfering carriers.
Easypal has several modes just like its father S4285, some are for high data rate close range/v/uhf (aka qam64 mode) work where gobs of data can be sent quickly, or low data rate mode (aka qam4 mode) where it can take ages to send a small pic because of the redundancy of the emergency mode. However today in the HAMsphere, hybrid mode is all the rage, wich to me hardly qualifies as easypal. In hybrid mode all that the easypal sender is sending is a url that the receiving easypal then grabs the image from - so one needs to be online at each end, and have a image host with a valid url. This greatly speeds up the sending of pics but is less fun. Tune to 7173 lsb in the am to see some easypal sigs sent by HAMs. Tell Norm I said hi!
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As far as SSTV modes are concerned, for DX work I really like Scotty DX mode. It takes a very long time to send an image, but the advantage of that longer time is more integration of the audio signal per pixel, so better noise rejection.
Back when that NZ pirate was transmitting (TOTSE?), it used Scotty DX mode, and while I could rarely hear the audio/music, I could get recognizable images from the SSTV. A Scotty DX SSTV transmission would go a long way to IDing some of these weak pirate transmissions.