Why no built-in TBC in 1980+ VCRs?

[DVtyro]

Originally Posted by Electronic M

In the consumer space it was typically only higher end S-VHS decks that got TBCs in the 80s and 90s, because it was an expensive feature that typically you would only go to after (or along with) the move to S-VHS. The people who bought S-VHS were the only ones who cared enough about picture quality to spend the extra on a TBC, and the number of S-VHS decks without TBC speaks to how many of the S-VHS buyers thought it wasn't necessary/worth the cost.

Sure, and these are the models I looked at. Again: how is it possible to get a clear freeze frame without basic TBC functionality? Did models with digital freeze frame had a TBC of sorts which was not advertised?

[ChrisW6ATV]

Originally Posted by DVtyro

how is it possible to get a clear freeze frame without basic TBC functionality?

A clear freeze frame does not explicitly require digital memory at all. Sony did it in 1981 with their SL-5800, using dual-azimuth video heads.

Pioneer sold its first Laser Disc player with digital video memory in 1988-89, the CLD-3030. That machine only stored one field of NTSC video, so that is an indication of the cost of memory at that time.

It also may be that just storing frames/fields of video is a lot simpler than actually manipulating that memory as needed for time-base correction. That level of information is beyond my knowledge.

[Electronic M]

Originally Posted by ChrisW6ATV

A clear freeze frame does not explicitly require digital memory at all. Sony did it in 1981 with their SL-5800, using dual-azimuth video heads.

Pioneer sold its first Laser Disc player with digital video memory in 1988-89, the CLD-3030. That machine only stored one field of NTSC video, so that is an indication of the cost of memory at that time.

It also may be that just storing frames/fields of video is a lot simpler than actually manipulating that memory as needed for time-base correction. That level of information is beyond my knowledge.

Of course on half the Laser discs made that player didn't need it's frame store. On CAV (constant angular velocity) discs where disc speed was constant and one rotation= one frame older LD players would simply skip the laser back one track on IIRC the vertical sync interval of the frame, effectively playing the same frame over and over the way many lower end VCRs do. On later CLV (constant linear velocity) discs where the disc speed changed depending on how far from the center you were reading and the sync pulses of adjacent frames were NOT in the same rotational location on the disc, a frame store was necessary for stills since on CLV there was no effective way to move the laser instantly from the end sync pulses of a frame to the beginning sync pulses of the same frame.

[ChrisW6ATV]

Originally Posted by Electronic M

Of course on half the Laser discs made that player didn't need it's frame store.

After about mid-1981 or so, probably 99% or more of Laser Discs were Extended Play (CLV), so that feature was a real treat to have.

[DVtyro]

Originally Posted by ChrisW6ATV

A clear freeze frame does not explicitly require digital memory at all. Sony did it in 1981 with their SL-5800, using dual-azimuth video heads.

The late 1980s magazines talked specifically about digital features, so it was a digital freeze frame.

Flagging/skewing/hooking is a defect that can originate from mechanical issues, but this is exactly what TBC should be able to fix. This is from 1985. One would expect if not late 1980s then 1990s models to take advantage of digital processing. OTOH, it looks that memory price was decreasing slowly and only dropped significantly by mid-late 1990s.

[ppppenguin]

The whole history of VTRs is a battle against mechanical imperfections.

Quadruplex, with its small drum, could, under ideal conditions, keep the jitter down to 100ns or so, good enough for synchronous monochrome replay. It wasn't easy to avoid banding and other visible errors. The first TBC was Amtec, an analogue delay line with varicaps instead of fixed capacitors. Correction range less than 1uS (I think) but it was enough. Followed by Colortec to get good enough stability for direct NTSC playback.

The frequency spectrum of timing errors on helical scan VTRs was very different. Much more low frequency stuff due to the big drum. Hence hooking. Still all sorts of other errors caused by tape flutter and heaven knows what else. If the errors are bad you get velocity errors along a line which aren't easy to correct.

Colour under was a brilliant kludge to record colour on low-cost VTRs, at the expense of wrecking the delicate relationship between colour subcarrier and H. Didn't matter in domestic environment.

General purpose TBCs had a tough job following all these errors on all sorts of VTRs and tapes. When memory was expensive enough that you only afford a few lines of storage a few early TBCs had a feedback output to drive the capstan motor of the VTR via a power amplifier in order to get true synchronous replay. No idea if this was used very much in practice. I once did it manually with an audio oscillator, public address amplifer with 100V output and a step-up transformer. I could just about keep an Ampex 7003 1" VTR within the correction window of an early CVS517 TBC. But not for very long.

From the early 1990s I designed framestore TBCs for a small company called G2 Systems in the UK. They also did standards conversion, though not to full professional quality. The first versions used dedicated framestore FIFO chips. Later I used first generation SDRAM with an FPGA to give multiport video framestore memory. There were great "get out of jail" boxes in their time. You could stick just about any SD video signal into them (including S video, SDI and in some cases analogue component) on PAL/SECAM/NTSC (and oddballs like NTSC443, PAL-M, PAL-N) and have the output perfectly timed into your vision mixer.

[DVtyro]

Originally Posted by ppppenguin

General purpose TBCs had a tough job following all these errors on all sorts of VTRs and tapes.

Which is why it seems to me that it made sense to build a TBC into VCRs themselves, at least into expensive top level models, as manufacturers shoudl have known better what sorts of errors their machine produced. Ah, well.