A time base corrector turns unstable video into stable video

Time Base Correctors (1)

What a Time Base Corrector Does

Total time base error in a typical video can amount to more than 20 or 30 lines and needs to be brought down to the range of 5 to 10 nsec. A modern time base corrector (TBC) can do this job for us. Unstable video goes from the VTR into the TBC and stable video comes out of the TBC, which is then fed into the system.

How a TBC Works

Of course, things are considerably more complex than this description would suggest. The biggest problem is that we need a very large memory to store the video picture in, so that it can be fed out at an even, synchronized rate. Memories that will hold a lot of analog video are very expensive and difficult to make. However, computer memories are relatively inexpensive and common, so if we could get our analog video into computer form, we could make good use of those memories. That's just what happens. As you learned earlier, however, computers can only deal with numbers. The first thing that happens after unstable video enters the time base corrector from the videotape machine is that it goes through an analog to digital (A to D) converter. From there the information goes to a large computer memory

Horizontal Sync as a Clock

A method needs to be devised to release this digital video information in sync with the rest of the system. As you might suspect, we go back to the sync generator, which is hooked up to the TBC. The horizontal sync pulses from the sync generator act as a clock. Each time a horizontal sync pulse reaches a special gate circuit, it lets one line of digital video information out of the memory.

1.  Because the videotape machine cannot play back at an absolutely precise speed, the video lines vary somewhat in length and the horizontal sync pulses fall at varying intervals.

2.  Time base corrector.