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From: "Dan/W4NTI" on Thurs, Feb 24 2005 10:08 pm
Hams do contribute to the state of the art. Where do you think SSTV came from ? Just one example. Bell Labs and the "PicturePhone"? :-) Went into service on the Bell System over four decades ago, got deleted for lack of interest/use some years ago. Worked on a very limited bandwidth. The genesis of Slow-Scan TV began in several places. One could say its start was the first Facsimile...very slow data rate. A very close cousin was early television, also done at a very slow data rate considering their dependency on mechanical scanning. A few amateurs tried to advance into professional ranks with the mechanical scanning TV but none were successful. All that was before the USA got into WW2. During WW2, TV was rather limited but "wirephoto" facsimile got popular on wired communications circuits. A medium-scan-rate TV system was used on some experimental guided bombs late in WW2. "FAX" got its acronym-name during that war and was used for graphics such as weather maps sent out over HF radio circuits. With the end of WW2 began the virtual explosion of broadcast television and the availability of TV camera tubes, TV picture tubes, newer circuit technology (DuMont "flying spot" system, a sort of reversed light-subject-camera arrangement) and wide- band modulation (6 MHz in the USA, included audio). There began lots of research into Information Theory and Bandwidth in the late 1940s which resulted in insight to necessary bandwidths to maintain low error rates ("Shannon's Law" of 1948). Many different experiments began to send "live" TV at reduced bandwidths and the Bell "PicturePhone" was just such a system which did go into service in the NYC area. Information Theory got a few boosts from greater efforts of cryptologists during the Cold War trying to devise better codes for sensitive communications. Information Theory eventually morphed into the Motion Picture Experts Group (MPEG) which had formed about the same time as the explosion of the desktop personal computer during the 1980s. MPEG was based on earlier work using "blocks" of picture elements and their examination of redundancies plus the availability of new and better digital logic circuits to process the image blocks. Nearly all amateur "slow-scan TV" is little more than high-rate facsimile...on the order of the "PicturePhone" imaging. None of it is the moving picture quality found on the modern enhanced cellular telephones using MPEG compression-expansion of image data. "Modern television" (defined as all-electronic scanning) is wide bandwidth to preserve image quality. What is broadcast, even with old-style black-and-white "original" NTSC standards, is quite good. Any "NEMO" watcher (NEtwork MOnitor) viewing the direct input from a microwave relay link can tell you that. The same with the "air monitor" checking transmitted video. Early domestic-production TV receivers deliberately limited bandwidth to reduce costs, resulting in receiver picture quality being awful to poor compared with what was transmitted. The picture quality on amateur SSTV is approximately the same as early domestic-production TV receivers, but SSTV cannot handle motion nearly as well. Modern FAX standards use some data compression but that is limited. Picture quality there is reduced compared to what was possible with older, uncompressed facsimile. Amateur SSTV is NOT an "original" thing from hams but rather an adaptation to stay within shrinking RF spectrum on VHF and above available to amateurs. If it were ever standardized as to scan rates and bandwidth, there would be a chance for improvement. As it is, it remains a novelty, something good for the editors of QST to crow about. "PicturePhone" went into the dumpster long ago and SSTV will probably wind up there. |
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