- #36
sophiecentaur
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The point about scanning, however you do it, is that it is a sampling process. Any sampling introduces artefacts which can make re-construction problematic. Conventional scanning was selected because it was convenient (a spinning disc, initially) and that led to a relatively simple sawtooth horizontal and vertical scan. The artefacts are 'predictable'. If you try to scan in other ways, the vertical and horizontal sample frequencies are no longer uniform so the nice, friendly 'comb' spectrum of a PAL signal is destroyed. The timing suddenly may need to be much better (the 'pixel accuracy' of @tech99 may come in).tech99 said:The pre War developments also included mechanical scan for larger screen projection, and this would not lend itself to zig-zag scan. For instance, at RadiOlympia in 1938 a large screen mechanical scan 405 line receiver made by Scophony was displayed. It utilised small, high speed mirror drums and used an opto-acoustic light modulator, called the Jeffree Cell. https://blog.scienceandmediamuseum....cophony-tv-receiver-high-speed-scanner-motor/
Another serious objection to zig-zag scan is that the scan must be perfectly linear to a precision of one pixel, or vertical edges will be ragged, and such linearity was impracticable.
The downside of the conventional scan for TV tubes is, as people have mentioned, the enormous power needed for sawtooth deflection, and the scan linearity with wide angle tubes. But that was more or less sorted out with large sweaty power circuits.
Repeat scanning is terrible value for bandwidth use, if the picture is actually transmitted in the same form that it's detected and displayed, as in conventional transmission.
Once you have modern digital signal processing, the same basic scanned picture can be compressed into a tiny channel compared with the 7MHz or whatever for old fashioned TV. In that situation, a picture is a picture and can be imaged or displayed in any way you choose. Transmission becomes a different issue.