I recently appeared on my local TV station. When I arrived at the studio I was asked to change my shirt because its pattern would look distorted on screen. What causes this effect and why, in this day and age, is it impossible for television to simply record what is in front of the cameras?
There are two possibilities. First, analogue television in the UK uses PAL colour coding, which transmits the colour information as very fine luminance patterning on a simple black-and-white signal. If you feed a colour TV signal into a high-resolution, black-and-white monitor, strong blues and yellows appear as fine backward-slanting stripes and strong reds and greens as forward-slanting stripes. Mixed colours, such as orange or cyan, make cross-hatch patterns. A colour TV receiver can mistakenly decode fine-patterned shirts as encoded colours which flicker and are generally annoying. The decoders in newer TVs are much better than those of the 1960s when the system was invented, but it is a fact of the coding standards that colour and luminance cannot be fully separated under all circumstances.
鈥淎 colour TV can mistakenly decode fine-patterned shirts, which flicker and are annoying鈥�
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The second possibility relates to the bandwidth of the broadcast signal. Studio-grade digital television operates at a data rate of 270 megabits per second, but for digital broadcast by satellite this signal is compressed to between 2 and 5 megabits per second. To achieve this, many of the 25 frames that are shown per second are not transmitted in their entirety; instead, the differences between one frame and the next are sent. Busy patterns on clothing can cause stress in the part of the coder that monitors changes from one frame to the next, resulting in shimmering of the cloth and degradation of the entire picture because too much data capacity has been used on the cloth.
Broadcasters now transmit four or five digital TV stations in the channel previously occupied by a single analogue programme, so it is hardly a surprise that something gets lost along the way!
A couple of images on my website show luminance/chrominance coding patterns: . You can also find the BBC鈥檚 research and development group鈥檚 white paper on digital TV compression issues at .
Andrew Steer, Horley, Surrey, UK
There are several reasons why your shirt may be unsuitable for TV.
If it has a complex pattern, it may shimmer annoyingly because of the phenomenon of 鈥渁liasing鈥�. Imagine stripes that, when appearing on the screen, are closer together than half the spacing of the screen pixels. The TV cannot resolve the stripes because it doesn鈥檛 have the resolution. This is also why wagon wheels appear to go backwards in movies. The problem can be reduced by higher-resolution TV but will never be completely eliminated.
Certain very bright colours, known as 鈥渉ot colours鈥�, can fall outside the range of colours that can be encoded by the TV signal. If your shirt is extremely loud, this could be the problem.
Finally, if the TV studio was using a blue or green screen and your shirt contained a colour similar to the screen, this would have the effect of making your body transparent, which would be embarrassing.
Jerry Huxtable