Why no built-in TBC in 1980+ VCRs?

[DVtyro]

Originally Posted by ARC Tech-109

In retrospect it's too bad the Digital-8 format didn't make it past the consumer market before the other DV formats took center stage.

I have been asking this question for a while: why develop a completely new cassette family for DV instead of using existing ones, like Betamax for Betacam or VHS for M, and I could not find a definitive answer. Versions I considered and rumors I came across from other people so far:

• DV cassettes could be scaled easier than, say, 8-mm cassette.
• DV tape is narrower (6.35 mm or 1/4-inch) than 8-mm, so tape savings and thinner cassette.
• MiniDV cassette is much smaller than 8-mm cassette, which means more compact recorders, but then again, could 8-mm cassette be scaled down?
• 8-mm cassette would not have enough recording time (in practice, 40 minutes on 8-mm cassette vs 60 minutes on MiniDV is comparable, and in the olden days ENG cassettes were 20 minutes anyway).
• Probably the biggest one: JVC and Panasonic agreed to join DV if it would be a new format for everyone including Sony, so everyone would be in the same position. Panasonic made 8-mm machines for other brands, but not under its own name. JVC, I believe, made none. So, they did not want Sony to have competitive advantage with 8-mm cassette, otherwise Sony could take, say, the CCD-VX3 (VX1 in Europe), replace analog encoder with digital, and voila! Instead, Sony had to rip out the 8-mm transport and install MiniDV transport to create the VX1000. If this is a true reason, I wish JVC and Panasonic were less vengeful.

Originally Posted by ARC Tech-109

I can't speak for the VHS camcorders as I have always gone to great lengths to avoid the format in general but Panasonic did have some high end "prosumer" decks in the NV model line that had a very good TBC. As the format itself was geared for the home market very little was developed and refined for the more demanding broadcast and production arenas, unfortunately it takes more than a good TBC to bring the format to a level required for prime use.

JVC and to lesser extent Panasonic aggressively marketed SVHS towards pro market when M and then MII fizzled. In some aspects SVHS is better than Umatic, all it needed were pro connectors to integrate it into a plant. Pro-looking VTRs, consoles, TBCs were made, SVideo as well as Dub connectors, including provisions for timecode and I believe there were even claims of 1-frame precision editing.

[ARC Tech-109]

There is still the issue of the hetrodyne color under used by S/VHS, Betamax and Umatic that limits the color resolution to about 30 lines give or take so while the actual luma detail may be higher in the S-format there were still logistical challenges in the broadcast world wired for composite. The only real reason the M format fizzled is Sony had already established the Betacam format in the broadcast arena, sort of like a beta revenge despite the M format being technically superior in several aspects. That being said you're right on the mark regarding the modern DV technology as both Panasonic and the founder of VHS JVC didn't want another format monopoly. My biggest bitch is with tape compatibility. The Sony DVCAM decks will accept both a large and small DV tape without an adapter but Panasonic requires a difficult to find adapter to run both consumer and professional small DV tapes in their DVCPRO decks... thought we learned a lesson with VHS-C. Sony runs the DVCAM tapes faster than DVCPRO but does not have the linear cue track. I have a Sony DSR-50 portable that I use as both a 1394 feeder and 4-channel DAT for capturing older quadraphonic material plus two DSR-1 docks on D-55WS cameras for DVCAM and Panasonic AJ series for DVCPRO, one SD-755 and HD-150 along with two Sony DVW-A500/1 Betacam decks. they're from my "classroom in a can" days of technical training production that has been taken over by amateurs on YouTube.


I do have a Panasonic AG-7500 SVHS editor with "most" of the pro connectors, RCA audio rather than XLR and while its been a good deck I found U-Matic to "look" better on the screen than an S-tape running side-by-side.

[kf4rca]

The first one I ever saw was a CVS-504. It used digitally switched glass delay lines. It had 2 modes of operation-direct and heterodyne. The heterodyne mode looked awful but the direct mode looked pretty good. CVS won an emmy for it!
Stations loved them. They were able to get out from the expensive yoke of quadraplex.
The one I really liked was the DPS Phaser 1 (and later Phaser 2). It was made by Digital Phaser Systems out of Toronto who was later bought by Scientific Atlanta.
It would take any old crappy signal and make it broadcast-able. I remember live shots, that were falling apart, and we put them on the air.
It would make chicken salad out of chicken$#!+.

[DVtyro]

Originally Posted by ARC Tech-109

The only real reason the M format fizzled is Sony had already established the Betacam format in the broadcast arena, sort of like a beta revenge despite the M format being technically superior in several aspects.

M and Betacam were shown at 1981 NAB simultaneously, but M was production-ready, while Betacam still was a work in progress. M had about half a year lead time. It just was not as good as Betacam.

Originally Posted by ARC Tech-109

they're from my "classroom in a can" days of technical training production that has been taken over by amateurs on YouTube.

Do not discount amateurs, they have a desire to learn, show and tell. Also, many of them are quite knowledgeable. Dave 12voltvids is a former video engineer, so not an amateur. But Colin (video99.co.uk) did not work in the industry, AFAIK, but I don't know for sure. Also this Vietnamese guy is very good: https://www.youtube.com/@misu4602 (his videos have English subtitles).

[DVtyro]

Originally Posted by kf4rca

The first one I ever saw was a CVS-504. It used digitally switched glass delay lines. It had 2 modes of operation-direct and heterodyne. The heterodyne mode looked awful but the direct mode looked pretty good. CVS won an emmy for it! Stations loved them. They were able to get out from the expensive yoke of quadraplex.

What do you mean by "direct mode"? AFAIK, 2-inch had their own built-in mini-TBC, controlled with knobs and dials.

I thought that fixing crappy Umatic (heterodyne color) and earlier B/W portapaks (no color, so cannot be called heterodyne, can they?) was the primary reason for TBC. If Umatic looked awful, how CVS could win an Emmy for it? Could you clarify, please.

[ARC Tech-109]

Do keep in mind a TBC corrects the entire timebase of the video signal not just the chroma irregularities regardless of the format be it an EIAJ Sony AV-3450 B&W PortaPack or Sony BVW-75 BetacamSP even the Type-C format had a timebase corrector. Think of the scan lines as a deck of cards all stacked up, the TBC just centers all the cards so all the edges are nice & neat. Color correction is more of a Proc Amp that can manipulate the chroma & phase along with the positive & negative levels of the actual signal.


Going back to your previous post I'm not discounting the amateurs for a lack of knowledge, they just put me out of a job. Some of them are quite talented and others are learning as they go, the cool thing about knowledge is everyone can learn and expand themselves. My dad and I were technical writers and we produced countless training videos together, he wrote and I captured and we both edited everything together to make the final production.

The M-II format was actually far superior than the BetacamSP format in many aspects but Sony had already established the "oxide" Betacam format leaving Panasonic in the dust, M-II was the M format after puberty.

[kf4rca]

Quads had 3 timebase correction systems. The first was automatic guide servo which corrected gross errors at the headwheel. On early machines this was a manual adjustment. The second was MATC mono automatic timing corrector. The third was CATC color automatic timing corrector. Some machines had CAVEC which corrected hue banding across a head.
The later machines like AVR3 and TR600 had digital TBCs.

As I recall M2 format was a full bandwidth analog system. I saw a demo once and it was outstanding!
Some stations even bought the M2 automation system for commercial playback! No stations in this market but there was one in Augusta GA. that had it!

[ARC Tech-109]

There were some PBS stations and I believe the BBC dabbled in the M-II format for a number of years but eventually dropped the format in favor of BetacamSP and D2. My quad experience is limited to an AVR-1 during my Studio North days... what a monster!

[DVtyro]

I am re-reading this thread. I guess this was important to why TBC were not commonplace until the late 1980s:

Originally Posted by ppppenguin

The problem in early TBCs was the ADC rather than memory. Until the TDC1007 was invented, 8 bit video ADCs were very complex and expensive. That chip won an Emmy aawrd in 1988. It was still several hundred $$$. Then the price of ADCs rapidly came down.

An article from 1987 reads:

The chips that store the digital information are known as DRAM, for Dynamic Random Access Memory. Most of the current VCRs with digital effects use DRAM chips that each hold 256 kilobytes of binary data.

RCA's VMT-400 has nine, arranged to provide sufficient memory to hold two separate fields of color video data. Two of the DRAM's are dedicated to the picture-in-picture feature, while six more are used as the main memory for the freeze, mozaic and posterization special effects. The last chip is used to process synchronizing signals.

I don't think that in 1987 256 KByte chips were readily available. I think the author meant 256 Kbit modules. Looking at John C. McCallum memory prices, I see 256 Kbit modules. If six modules are used to store a complete frame, then it is 192 KB for the whole frame. How? I guess they did not use the full Rec 601 frame, but a low-res one that would match VHS. Say, 480 * 240 = 115200 elements. With full 8-bit color it would be 115200 * 3 = 345600, this would not fit. But with halved color it would be about 170 KB, good enough for a freeze frame, but I suppose they figired it was not good enough for a proper TBC.

An interesting observation from the memory price data: in 1987 RAM price price was about $160/MB, but next year it jumped to $500/MB! It fell back to $120-180 by the end of 1989, but I think this spike spooked VCR manufacturers, so they scaled down on digital features. They probably also figured that PiP and clean freeze frame were not really popular.

By 1990 RAM price dropped below $100/MB. An article from 1990 was hoping for TBCs to become more common, claiming that "top decks adopt a picture-fixing feature".

[ppppenguin]

Early DRAM was rather slow. In any case you need both write and read at video rate. The solution was in two parts. The first is to demultiplex the video. So 8 bit video at 13.5MHz (the 601 rate) might be demuxed on to a 64 bit bus at an eighth of the speed, under 2MHz.

The other need was to alternate read and write cycles (It's rather more complex than this. I know as I've designed video framestores with multiple video read and write channels) so time slots have to be allocated.

There were also special memory chips for video line and frame stores. Typically these worked as shift registers so no complex addressing needed. For example NEC had a range of linestores including the upd42102. Averlogic had the AL422 series of framestores. Nice chips, easy to use, but much less flexible than DRAM. In my own designs for clients I did a lot with first generation SDRAM. All the addressing, multiplexing, access arbitration etc was in Xiinx FPGAs.

Some early TBCs used fewer than eight bits. I think the CVS517 from the late 1970s used 7 bits https://worldradiohistory.com/hd2/ID...-Page-0232.pdf There may even have been a 6 bit TBC from a small UK company whose name I forget. If there's any noise on the input video it acts as a dither signal which makes the number of bits less important. I even once built an experimental 1 bit framestore. The monochrome picture was, of course, awful. In coloured areas, the high amplitude PAL subcarrier acted as a dither signal and gave surprisingly good pictures.

[DVtyro]

@ppppenguin, thanks a lot, I appreciate the answer! So, if not full-frame TBC, then at least TBCs that would hold several lines could be built relatively cheaply?

I've read that at least one late-1980s VCR had TBC with a window of just one line I think (is it possible? Should not it be longer than a line, at least a line and a half?) which was implemented purely in analog form, no digitizing. I don't know how they would sync up this analog line. I don't think they would fix velocity.

[ppppenguin]

Until you had one-chip line stores and affordable ADCs/DACs even a one line TBC was quite expensive. There was at least one general purpose TBC that used analogue methods. Charge coupled delay chips ISTR. I don't think it was very good.

The very first TBCs used variable analogue delay lines to get a microsecond or so of delay. This was enough to correct monochrome replay of quadruplex VTRs. Imagine a lumped LC delay line with varicap diodes instead of the fixed caps. Plus a compensator to change the termination resistance as you varied the characteristic impedance. Very complex, very expensive, very difficult. Ampex called it AMTEC. The very fine vernier TBC for NTSC/PAL colour replay was called COLORTEC. The Ampex AVR1 quadruplex introduced a one line delay using switched quartz delay lines. All analogue. Again very complex and expensive.

I think only quadruplex VTRs really needed velocity compensation. You only really needed it with direct colour and that was only used in full broadcast grade VTRs such as quadruplex. The delays here were small and implemented with analogue methods.