I'm not sure if this has ever been brought up before or if anyone has attempted it, but I have done thid with phonograph machines and my reel to reel machines when I wanted to slow down the motors to a speed slower than the lowest setting. I have used a variac in the past and ran at completely variable voltage and it worked.
My question is, would there be any way to modify the motors in a VCR to be at completely variable speed. I have a a few VCR's that only play SP only. I feel a few modifications would allow me to slow down the motors. It doesn't seem too complicated.
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The tapes use a helical scan. Meaning the spinning drum and it's two or four heads write slanted tracks across most of the tape width. If you only slow down the transport motor, the heads will no longer track the scan paths. You might be able to record and playback on the same machine but it wouldn't play back or record tapes that conform to normal standards. You will probably also run into problems with the drum synchronization circuits as they expect a narrow range of speeds and may 'hunt' back and forth when they don't see it.
And nothing will display the resulting video with the wrong timing.
Darn. Not as simple as slowing things down. I guess that's why decks that can play multiple speeds use separate heads.
You have two different points to unpack here: why "pro" decks only run at SP, and whats the story with LP/EP.
Pro decks run at SP-only because they were sold to a niche market that wouldn't be caught dead using LP or EP/SLP modes. The extended recording speeds were strictly a consumer feature: the video quality is subjectively much poorer, and technically so dismal under the hood that the signal would be too nasty and messy for professional users to lock in with their standardized studio-level hardware chain. Limiting the pro decks to SP-only allowed maximum optimization for best PQ and interfacing with special studio hardware never used in the home.
Your idea of slowing down the motors is in fact more-or-less exactly what is done in consumer VCRs to achieve the extended record/play modes of LP and EP/SLP. However as others have already told you, because the video is recorded helically (and not linearly as with a phonograph or audio tape deck), "slowing down the motors" becomes a bit more complex, to the degree it is not a feasible DIY modification. The linear speed of the reel and capstan motors is indeed slowed down, but this must be electronically synced to the spinning video heads to enable a coherent compatible video track to be recorded or played.
A VCR technically only needs one pair of spinning heads to make recordings at any speed, but those heads must be optimized for the slowest contemplated speed. This compromise approach was popular during the '80s for budget-priced consumer VCRs. But it diminishes the maximum achievable quality for the fastest SP speed (the narrow heads needed for slow speed tracks make terribly inefficient use of the tape at SP speed, some models wasting over 60% of the available tape surface). JVC, the inventor of VHS, did not want the format degraded by this method so came up with the idea of separate heads for SP and EP/SLP. The wide SP heads make proper use of tape area at that speed, and the narrow EP heads are optimized for slower recording tracks. Panasonic and other brands eventually devised a way to cleverly interlock the four heads during playback mode to improve speed search, freeze frame and slow motion at both SP and EP.
The super-wide spinning video heads of an SP-only pro VCR wouldn't make usable EP video recordings. Theoretically with a magic wand you could DIY a slowdown of the transport motors and sync them to the video head drum for EP playback purposes, but it probably wouldn't look too good. Head adaptability isn't bidirectional: EP heads can scale up to make a usable SP mode, but SP heads don't scale down well to EP mode.
Last edited by orsetto; 8th May 2021 at 13:43.
There is also the matter of FRAMES.
Unlike audio, whose payload analog signal is continuous and unaccompanied by any other signal, so lends itself well to time/tempo/pitch adjustment,
Video's analog signal includes not just image payload but also line and frame synchronization signals.
If you change the timing of those sync signals, you change the timebase of the framerate to well beyond what receiving devices (recorders, displays...) can handle. Then you would get either very skippy, very, very distorted, or very wobbly images, or some combination of those mixed, with nothing at all.