Home
Studio OnLine
Studio400
What's New
Links
Sparky?
Feedback
Search

Time Base Correctors

Video images are made up of a number of scanned horizontal lines, formed into what is commonly known as a 'raster'. These lines are normally produced by electronic means, each comprising a synchronising pulse (not displayed), followed by a stream of video information that describes the intensity and colour of the image at that line's location.

When a video signal is recorded on tape its precise timing is disturbed by the essentially mechanical recording process, but the display device (usually a TV or Monitor) can compensate for this by locking on to the synchronising pulse that accompanies each line. The result is that the timing jitter caused by the VCR or camcorder is barely noticeable on our screens.

However, when a VCR recording is copied into a second VCR (another mechanical device), it is not possible to provide this compensation, due to the inertia of the head-drum, whereas in the TV tube the scanning of the electron beam can easily be controlled. The result is that the replayed image from the second VCR will have a degree of horizontal jitter, resulting in ragged vertical detail as illustrated here.

The more copies that are made, the worse the problem becomes until, in the end, the TV is incapable of holding on to the picture at all, and the whole image starts to tear apart.

This is not the only problem however. The video signal also contains a reference component for colour reproduction. This, too, will be disturbed by the timing jitter imposed by the VCRs, causing poor colour rendition or failure of the colour completely.

The solution to this problem is to provide a device between the VCRs that removes the jitter before the next copy is made. This device is known as a Time Base Corrector (TBC). There are basically two forms that these can take - a Line Buffer (cheap), and a Full Frame Buffer (moderately expensive).

Both of these contain electronic storage elements that hold either a couple of lines, or a whole frame's worth of image before passing them on to the next VCR. Because they are electronic, and therefore have no inertia, they are able to assemble jitter-free images, in much the same way that a TV can. When the lines, or complete frames, of the image are passed on they are provided with fresh, stable, synch pulses and colour references.

Time Base Correctors are manufactured in various qualities from simple 'black boxes' to units costing hundreds of pounds. The more expensive models have the ability to adjust the image quality, sharpness, and colour too.

Digital VCRs and Camcorders do not need TBCs. Their principle of operation naturally requires digital image buffers, both before and after the tape storage process.