REDUCING COLOUR BLEED PROBLEMS

Virtually all domestic analogue video tape recordings are afflicted by the dreaded 'colour bleed' problem. You will have seen recordings where a bright red jumper seems to be in a different place to the person wearing it! Well, this is 'colour bleed' and it is not just red that is in the wrong place but all colours. However, it is the saturated colours like red and blue that we notice the most.

The reason colour bleed causes such a problem with domestic formats is that the colour resolution is so low. Most manufacturers of camcorders and video recorders only quote black & white resolutions in their specifications. These would typically be 240 lines for standard VHS/8mm and 400 lines for SVHS/Hi8. The colour resolution is actually around 40 (yes forty) lines! (see Resolution).

There is no fine detail in the colour so you can see that the colours will never fit properly within the boundaries of the black and white part. In addition to smearing the colour, the low resolution also delays the colour so it becomes offset to the right and offset downwards on the screen. Both these effects, the smearing and the offset, are increased every time a video tape is copied. This is why it causes such a problem for amateur video editors who need to copy to 2 or 3 generations.

No matter what astounding claims you read in the ads, it is impossible to completely remove the effect of colour bleed. After all, if it was that easy the VCR manufacturers would have solved it long before the advent of digital! When using picture processing it is well worth doing some test recordings to try out the effect of each of the adjustments. It is not until you play back the copy that you can really see if it has made an improvement.

Try this:- Reduce colour saturation by 15 - 20% then adjust the contrast to compensate. In addition to timebase correction and picture processing, the VideoTech VMX410* digital mixer also had a colour re-registration facility, as did the Electronic-Design TBC-Enhancer*. This enabled you to move the colour part of the picture, horizontally and vertically, independently of the black and white part. The improvement this makes, especially when copying standard VHS tapes, is quite remarkable.

Adapted from a VideoTech Designs article.