Why no built-in TBC in 1980+ VCRs?

[ppppenguin]

With the amount of noise comming off tape, 8 bits is entirely adequate for a TBC. The noise acts a dither signal which makes quantising errors invisible. This is true for any analogue to digital conversion of any signal, not just video.

I did some experiments many years ago with reduced numbers of bits. Even 1 bit (yes, one bit) with lots of random noise gave a picture without quantising effects, though I had to add a horrible amount of noise to get there.

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.

There was a lot of work done with sampling locked to colour subcarrier. This had some benefit but ultimately it's a lot better if the samples line up in a rectangular grid. For 625 and 525 systems the "601" standard settled on 13.5MHz to sample luminance. A lot of older systems sampled at 10 to 12 MHz.