Background: Television  
return to main hall

All moving picture systems - TV, film, cartoons etc - rely on a feature of the human brain called Persistance of Vision. This means that you will carry on "seeing" an image for a moment after it disappears, and if it is immediately replaced by another, identical picture you will not notice the interruption. If a sequence of almost-identical pictures is displayed in rapid succession, with suitably designed changes between them, your brain will interpret the changes as smooth movement. This technique was first used in Victorian toys such as the Zoetrope.

By the end of the nineteenth century, the principles exhibited by these early toys had been combined with the new technology of photography to produce moving picture cameras. These automatically took photographs one after the other, breaking the moving scene in front of them up into separate still images. This film could then be projected, in the same way as the popular Magic Lantern shows, to a mesmerised and startled audience.

In both TV and film each picture is called a frame, and the speed at which frames are shown is called the frame rate. Obviously, you want the fastest frame rate possible, for the smoothest movement, but for film fast frame rates means fast machinery which was delicate and expensive. The early film pioneers had to compromise between simplicity and smoothness, and eventually found that a rate of 24 frames per second gave acceptable movement without flickering too annoyingly - particularly when shown in darkened rooms.

The cinema quickly became a hugely popular form of entertainment. Every town had at least one "picture palace", and often several. Before too long sound was added, and newsreels were informing as well as entertaining. When colour finally arrived it seemed that the perfect moving picture was with us.

But there are drawbacks to film. Only a limited number of people can watch at a time, and each cinema needs its own copy of the film. For a feature film this is not a problem, but as newsreels became an important part of the cinema it started to be an issue: the events on the screen were days if not weeks old by the time the audience could watch them. With the advent of telegraph and then radio, transmitting sound over great distances, the possibility of transmitting moving pictures directly to the viewer was an obvious next step.

However, there is a fundamental difference between sound and vision. A sound can be represented by a single continuously varying quantity - the air pressure next to your ear, or the voltage produced by a microphone. This can be connected directly to a transmitter and sent over the air. Conversely, a picture is made up of many values, the brightnesses and colours of every point on its surface. So in order to transmit images, they must first be converted into a single varying value of some kind.

This applies whether it is to be transmitted using a radio wave or down a wire, and in fact the newspaper technique of sending pictures down telephone wires gives a clue as to how this is achieved. Like the newspaper pictures, the TV frame is scanned as a sequence of thin strips, each of which is considered to be a separate, varying value - the brightness at each point along the strip, in the case of a black and white picture.

The first method found to split an image up into strips was similar to the Victorian toys mentioned above. A device called a Nipkow disc was used, a spinning disc with a spiral pattern of holes. As each hole passed in front of the image, it scanned a single (curved) stripe down the screen, and as each hole reached the bottom of the screen the next was about to reach the top, slightly further along.

The scanning disc was used both as a camera, with a light-sensor behind it to pick up the signal, and as a TV set with a variable light source behind to reform the picture:

This technique was first demonstrated by Nipkow, but John Logie Baird is generally credited as being the first person to develop it into a practical TV system - although there is some debate about this. Certainly Baird's TV was the first to be sold to the public, and experimental transmissions from the BBC's Alexandra Palace began in 1936.

However, by this time other pioneers - notably Philo T. Farnsworth in America - had developed an alternate system, which did away with the spinning disc entirely. Instead, the picture was scanned by electronic means, using a beam of electrons. There were many benifits to this approach; in particular, having no moving parts meant that the system could be much faster and hence use more lines for a better picture. Bairds's system used 30 lines, while even the earliest experimental electronic systems gave several hundred, eventually settling on a standard of 405. By the time the BBC's experimental transmissions started, they were broadcasting both types on alternate nights, and the difference between the two was immediately apparent. The Baird system quickly fell out of favour, unable to compete with the picture quality of electronic TV. All modern TV uses this electronic system, and Farnsworth is the true father of television.

More information on Philo T. Farnsworth can be found in "The Farnsworth Chronicles". Ask at the enquiry desk.

Frames, Fields and Lines

As we have seen, in order to simulate movement, around 24 separate frames must be transmitted every second. In fact, TV uses 25 frames per second, in order not to clash with the frequency of the household mains electricity supply, which is 50Hz here in the UK. In the USA, where the mains is at 60Hz, the frame rate used for TV is 30 FPS.

The modern UK TV system, which is known as PAL, divides each frame into 625 lines. Hence each line must be transmitted in one 625th of one 25th of a second, or 1/15,625 of a second. This is 64 millionths of a second, or 64 microseconds (64µs). After all 625 lines of the frame have been transmitted, the first line of the next frame is sent.

However, the lines are not transmitted in the obvious order, starting at the top and working down each line in turn to the bottom; instead each frame is divided into two Fields, each of which consists of alternate lines - all the odd-numbered lines, or all the even-numbered ones:

The whole of field 1 is transmitted before field 2 is started, so that the picture is displayed by two passes down the screen, each taking 1/50 of a second. This is called interlacing.

Interlacing reduces flicker; the scanning speed is effectively doubled, without needing to transmit the extra information which would be required for a genuine 50 frames per second system.

The TV signal

So, a single frame of a TV signal consists of 625 lines, transmitted one after the other in interlaced order. The signal also contains synchronisation pulses, which are used by the TV set to help it to lock onto the signal, and to tell when the start of each frame, field and line occurs. The actual TV signal waveform looks like this:

Video signal, as voltage (vertical) against time (horizontal).

Extra sync pulses and blank lines (not shown) are used between fields and frames.

This diagram shows a black-and-white signal. A colour picture is formed by determining the proportion of the three primary colours, red green and blue, there is at each point along the line, as well as the overall brightness at that point. This colour information, or chrominance, is added to the monochrome information in such a way that the signal remains compatible with black-and-white TV sets.

previous exhibit go to enquiry desk return to main hall next exhibit
go to
previous
room
go to
previous
display
return to
current
room
go to
enquiry
desk
return to
main hall
(home)
go to
next
display
go to
next
room