51动漫

Everyone is probably familiar with the white noise, or 鈥渟now鈥�, that can appear on an analogue television screen. I know this comes from electronic noise and stray electromagnetic signals, but television signals encode colour information, so how come this random noise always shows up as white?

鈥�Analogue television was originally designed to be monochrome 鈥� or black and white 鈥� with colour added as an afterthought. The colour signal is coded on a separate 鈥渟ubcarrier鈥� signal within the band of the broadcast TV channel. The colour-decoding circuitry in the TV is only activated when it detects this subcarrier. So a colour TV can still be used to watch black-and-white transmissions.

In the absence of a broadcast TV signal, the tuner circuitry picks up nothing but random background noise. Though this is at a lower level than the normal signal power, the amplifier in the receiver is capable of automatically compensating. This is why the snow has the full range of intensities of a normal picture.

However, nothing in that noise is going to be mistaken for the colour subcarrier signal, except perhaps by pure chance for the occasional fraction of a second. Even so, it will not be enough to properly engage the colour decoding system. Hence you see no colour.

Lawrence D鈥橭liveiro

By email, no address supplied

鈥�The white snow on an analogue television is a result of the need for 鈥渂ackwards compatibility鈥� of colour TVs with earlier black-and-white transmissions.

When colour TV was first introduced the colour information had to be added within the assigned 6-megahertz bandwidth already fully occupied by the black-and-white signal, without making earlier, monochrome TVs obsolete.

This colour information was modulated onto what鈥檚 called a colour subcarrier signal at about 3.6 MHz for the (4.4 MHz for ) and added into the black-and-white signal.

This subcarrier signal was needed by the TV set to recreate the colour but it could not be broadcast continually because that would have produced a continuous patterning of the picture. Instead, it is sent in short (nine-cycle) 鈥渃olour bursts鈥� that are sent before the start of each of the lines that make up the picture. The full subcarrier is then generated by a device inside the TV, which synchronises itself with the transmitted bursts.

In the event that the colour burst is missing, the TV assumes that it is a black-and-white picture and activates a 鈥渃olour killer鈥� circuit, which ties together the red, green and blue circuits so that no colour is displayed. If the received signal is so poor that no, or very little, recognisable picture exists then the colour killer activates and any noise appears as black and white.

鈥淚f the received signal is so poor that little picture exists, the 鈥榗olour killer鈥� circuit is activated鈥�

So, noise on a weak signal will not produce coloured snow but strong intermittent interference, such as from an engine鈥檚 ignition, superimposed on a normal colour programme, can.

R. J. Best, Fremont, California, US