eric@apollo.uucp (Eric Peters) (07/17/85)
I've been doing a lot of videotape editing and copying on my home equipment, and I find I'm being limited by one of the fundamental problems of VCRs: Time Base Error (TBE). TBE causes picture instability and color shifts. It's caused by the mechanical process of reading information encoded on a videotape, and by the stretching and shifting of the tape base material. When a tape is copied and the copy played back, the TBE is doubled. It's not a problem for most home videoists because they never copy a tape, or at most copy it one generation. The effect is there, but it isn't bad enough to bother with. By the second generation, it's easy to see that vertical lines in the picture jitter and wave, and solid colors have streaks in them. This second generation copy is unpleasant to watch. A third gen- eration tape is virtually unwatchable. All professional video production uses an expensive piece of equipment known as a Time Base Corrector (TBC) to solve the problem. A professional TBC runs about $20,000. A TBC works like a FIFO: It accepts a video signal with Time Base Error (the video signal is not evenly spaced in time) and records it in a buffer memory. On the other side of the memory, it doles out the video signal evenly. Most pro TBCs also have lots of features such as "genlock" that make them convenient to use in TV studios, but are generally unnecessary at home. It occurs to me that with recent advances in memories, and with flash A/D and D/A converters on chips, it should be possible to build a TBC with acceptable parameters for a few hundred dollars. Does anyone know of such a product? If such a thing doesn't exist, I'd like to try to build one. I'd appreciate it if the experts in the group could recommend any source materials for my project: reference texts, standards, particular A/D conv- erters, schematics, magazine articles, ideas, etc. Please send any thoughts and ideas to me, and I'll keep you informed of the state of my project. Eric Peters (...decvax!wanginst!apollo!eric) Apollo Computer Inc., Chelmsford, MA 01824