Why no built-in TBC in 1980+ VCRs?

[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.