The VTR cannot play back at a constant speed

Time Base Error

The tape transport system is a very important and complex mechanism that pulls the tape across the tape heads. It is also the source of a major problem in videotape recording. The problem is that it's impossible to build such a system that operates at a truly constant speed, and that means that it is impossible for the machine to play back tape at precisely the same speed at which it was recorded.You can come pretty close, but in some cases that's not good enough.

If you review the section on sync generators, you'll see that the video signal is composed of some very precise bits of timing information. Because all this video and sync timing information is coming from the sync generator and camera, it is being recorded very accurately. But since the VTR can't play back the tape at exactly the same speed at which it was recorded, the playback information won't be as precise as what was recorded. This inability of a VTR to play back at exactly the same speed is called time base error. Time base error is measured in lines. It takes 63.5 μsec to scan a horizontal video line. If the playback signal is 63.5 μsec off from where it should be, there is one line of error.

Dealing with time base error can thus be an integral part of videotape recording. If the amount of error is small or can be corrected, the resulting picture problems will be minimal. However, a large error or, in some cases, any error at all can produce terrible jitters, jumps, and rolls in the picture.

1.  Sync generator puts out very stable signals with each video line being exactly the same length and the horizontal sync pulses falling at the same intervals. This precise signal is then recorded on videotape.

2.  Because the videotape machine cannot play back at an absolutely precise speed, the video lines vary somewhat in length and the horizontal sync pulses are reproduced at varying intervals. The degradation of the sync pulses comes from generation drop, not from time base error.