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.
The video drum speed is the same for all tape speeds, 2 fields per frame per rotation, 30 frames a second, 1800 rpm (1500 rpm for PAL/SECAM), Two speed VCR's have circuits to detect the sync pulses recorded at the bottom of the tape edge of the tape speed originally recorded at. It's timing not just slowing down and hope for the best. Those sync pulses are used to synchronize the capstan motor to lock on that speed, The final speed adjustment of the capstan motor is fine tuned by analyzing the video signal read by the video heads a.k.a automatic tracking, Pro decks are designed for one speed only with maybe 2% tolerance for manual or automatic tracking only, not half the speed.
So when the tape is running slow and the drum is at the same speed all the time, you get narrow video tracks vs spaced out video tracks in SP mode, With tracks close to each other you get what's called cross talk, The remedy is to design a separate two heads for low speeds and give them different azimuth angles so each head reads only the tracks that is intended to read and not the adjacent tracks, This is also found in audio tape recording that if you miss align the head azimuth you start loose high frequencies if you keep turning the head you loose the signal completely, It's the same principal. The head gap has to be parallel to the signal recorded or it won't detect it or at least not to the point of interference (VCR's use filters to filter out those weak signals).
The video drum has its own timing using the HBI, VBI and head switching signals recorded in the beginning and end of each video track (field) and between lines within the same track, If the video heads are not reading the video tracks correctly you get line timing issues, frame timing issues, head switch problems. VCR's that don't have separate heads for LP exhibit this issue more than the others.
One more thing you should know about capturing low speeds tapes, Not only the video tracks are close to each other, they are shorter than SP tracks, meaning that they use less space on the tape and that leads to low quality signal issues which again leads to poor horizontal and vertical timing signals and low visual video quality.
So if you want quality captures for a wide variety of tapes you want a VCR with 4 video heads, built in line TBC for better horizontal timing and some sort of frame TBC for better vertical frame timing, You want S-Video out for better chroma and luma separation.
Last edited by dellsam34; 9th May 2021 at 00:39.
Excellent info right there. Clears up everything.
Now I assume to find out whether your tape is EP/LP is to attempt playback, and if the machine spits it out you know it's not SP. Without damage?
*Beyond fast forwarding to the end.
An SP-only machine may or may not automatically eject a non-conforming EP or LP mode tape. Most I've used simply play the tape back at the default (wrong) speed, leading to no video image or HiFi and "chipmunks" sped-up linear audio. Similar to how loading a PAL format tape into an American NTSC vcr will play: no video and goofy audio. Most VCRs, even the elaborate once-$5000 pro models, are not "smart" enough to eject incompatible tapes: they will generally attempt to play any tape loaded into them with a readable control track (regardless of whether the actual video and audio tracks are playable by the deck).
No physical harm to the tapes should come from such misfired playback attempts: mechanically the tape should feed normally thru the transport until auto shutoff at the end. You just won't get a proper video or audio signal. Not to say damage is impossible: I have experienced a couple of decks that did not automatically try to make a best guess playback attempt (instead, the tape speed became unstable, loops formed and the tape ribbon got snagged on the guides) Such confused loading and playback would be far more likely from a DIY speed modification: it would be unusual with most un-tampered VCRs.
Last edited by orsetto; 9th May 2021 at 09:47.
[QUOTE=dellsam34;2618980] You want S-Video out for better chroma and luma separation. /QUOTE]
Would you still use s-video out if the source tape was dubbed from another vcr via composite cables?
Last edited by BenKlesc; 9th May 2021 at 11:14.
I mainly capture laserdiscs, which as you know is composite so I use an external 3D comb filter. I'm new to properly capturing vhs (haven't got all the equipment yet).
Everyone is always saying use s-video if you have a good SVHS player but what if source tape is a second or third gen from composite? If so should I use composite output with a comb filter? Can s-video still separate the chroma & luma if the dubbed tape was not done with s-video? or what if the tape was recorded on a standard non SHVS camcorder?
Pass thru would have been potentially problematic in the most common analog setups. The line TBC/DNR circuits common to consumer JVCs (and the Panasonic prosumer AG1980) are tightly coordinated with the video head outputs of the individual decks: tapping into them externally would be difficult or pointless. The frame sync TBCs built into some of the big studio SP-only VCRs were meant for some purposes and setups but not others. Pass thru (as we employ it for capture) was unheard of: if multiple decks needed to be synced they were connected to gen locked external TBCs.
Locking to racks of external TBCs was the most common approach, which is why comparatively few pro studio VCRs came with TBC fully built in as a standard feature. Most of them offered the 'built-in" TBC as an optional daughtercard at significant extra cost. The Sony 5600/5800, Panasonic 7xxx series, and a few JVC BRS models were exceptions with their standard integrated TBCs, arriving at the tail end of the pro VCR era. Ironically these final sophisticated studio Pannies and JVCs tend to be highly unreliable today, due to their unusually fragile video/TBC electronics. The two one-off Sonys somehow escaped the curse of late-90s thru early 2000s crap electronic parts: their atypical reliability is cosmically funny considering Sony's usual rep was none too good.
Last edited by orsetto; 9th May 2021 at 19:18.
reported on someone reputable on dfaq to work in pass-through. Personally I also found the TBC on the Sony PVW-2800 Betacam VCR also acted on video passed through that unit, tho unless used with a Panasonnic AG7350 with servo locked to the sync output the image would roll, and even then the image was rather wiggly (no idea if it was just bad or if it was due to bad electronics in the betacam deck.) Maybe one reason for it working on certain of these pro models is the use of component video on some of them, it makes sense to route video through digital/tbc unit as part of converting to/from component as the TBC unit decodes it's input to digital component internally anyhow. It would as you note not be all that ideal to hook the tbc to the inputs in a consumer vcr.
Another sort-of tbc-passthrough in VCRs are dvd-combo recorder decks as those essentially work similar to standalone dvd-recorders in that case. E.g as far as I know the Panasonic DMR-ES30V is essentially a DMR-ES10 with a VCR unit hooked up to it.
That would allow me to play consumer tapes while having the pro TBC controls. That is what I'm looking for. The 8700 has full frame TBC with video level and chroma level adjustments, separate TBC and DNR, and DNR has two separate stage luma and chroma NR settings. Hopefully that Pano also doesn't suffer from vertical line fatigue on the 7750's with the faulty TBC which was just a bad design.
Just for giggles next weekend I am going to see what happens when I run my VCR into an SVO-5800. You see now the best way to fund your hobby is to open up an archival business on the side. It ends up paying for itself at least hopefully. Thank you for the info folks!
Last edited by BenKlesc; 10th May 2021 at 16:29.
A Panasonic DMR-ES10 or 15 is the right tool for the job, Even with that, there is nothing produces better results than a S-VHS VCR with line TBC/DNR and S-Video out.