 Cambridge is well known for being a high-tech centre with lots of innovative electronics companies. In fact, Pi could be considered to be one of the founders of that industry. The Pi was named after William George Pi, who founded the company in 1896. It started as a small scientific instrument company, but it expanded into radio and all manner of electronics, the first British transistor radio. Colour television was being demonstrated in 1949 and was actually used at the Queen's Coronation in 1953. So it was a world leader in a great many of these technologies. So it was particularly important for us to create this exhibition to show people what Pi did. And it was very appropriate that it should be at this museum. The museum already had a number of Pi items in their collection. This exhibition covers all aspects of Pi with examples of the products and the history of the company. This is a perfect location because we're on one side of the river and on the other side is where Pi's main factory was. So it's a very good place to be. The BBC's colour television service in the UK started in 1967. The previous video in the series explained how early black and white television worked. This video is a simplified explanation of how analogue colour television systems of that time worked. Visible light is a form of electromagnetic radiation. It exists in various wavelengths from about 400 nanometers to 700 nanometers. A nanometer is a millionth of a millimetre. Radies at the longer wavelength enter the spectrum and violet is at the short end as shown in the diagram. White light consists of a collection of different wavelengths. When white visible light shines onto different objects then depending upon its chemical composition the object absorbs some of the light. What isn't absorbed is reflected back and this is called colour. If something is perceived as green it means that the object has absorbed every other wavelength of light and has reflected back only that green wavelength. When something is white it means that all of the wavelengths were reflected and when something is black all of the wavelengths were absorbed. Light from the sun for example contains all of the colours of the spectrum. However when it strikes an object such as a tree then only the green part of the spectrum is reflected and the object is perceived by the eye as green. This diagram shares a cross section of the human eye. Light from an object being viewed enters the eye through the iris. It is then focused onto the light sensitive lining of the eye called the retina. The retina contains cells called photoreceptors which are shown magnified here. There are two types of photoreceptors rods and cones. Rods see shades of grey and the cones deal with seeing colour. There are about 90 million rods and 4.5 million cones in each human eye. The rods enable the eye to see the fine detail in the image in black and white while the cones fill in the colour of the image. The cones fall into one of three categories. One kind is sensitive to red light, another green and another blue. Finally the optic nerve carries signals to the brain. This graph shows relatively how much light is absorbed by the rods and cones across the spectrum of visible light. Each cone absorbs light by different amounts depending upon the wavelength of the light. The brain then perceives the colour by combining the responses from the different cones. So if the eye sees light at one wavelength, for example at 485 nanometers, as shown in the diagram, then each of the three types of cone will absorb different amounts of this light. The brain then combines these responses in order to perceive the colour. The rods only respond to the scene brightness, called the luminance, and represent the brightness from black to white. As can be seen the rods response is mainly in the green area of the spectrum and from this can be calculated a formula relating luminance to the absorption of the three primary colours, namely y equals 0.59g plus 0.3r plus 0.11b. We will use this formula throughout this video. As the eye sees colour only in terms of its red, green and blue components, it's possible to make the eye believe that it's seeing different amounts of colours by adding suitable amounts of red, green and blue light. If the relative amounts of red, green and blue light are adjusted so that they produce the same response in the cones as the original, then the eye will perceive the added colours as the same colour as the original. This shows an experiment to illustrate this. A mirror is positioned so that the eye sees a spectrum of light and the eye focuses on a specific colour. Another mirror is positioned at 90 degrees to the first, and then three lamps for each of the primary colours are positioned to reflect off the mirrors into the eye. The brightness of each lamp can be adjusted and the resulting colour reflected to the eye can be made to be the same colour as the light reflected from this first mirror. To create the colours of the spectrum, it's possible to combine different amounts of red, green and blue at different intensities. This light shows three blocks of light, red, blue and green. If we add red light to green, we create yellow. Similarly, if we add red and blue light, we create magenta. If we add blue and green light, we create cyan. Finally, if we add red, blue and green, we can create white light. By varying the depth of colour, known as the saturation, then a wide range of colours can be produced. In order to capture the scene to be televised, the TV camera collects the light from the scene as was described in the previous video, how black and white television works. We need red, green and blue picture content to make a complete colour picture as we explained earlier. Early experimental colour television systems, for example a system proposed by CBS in the USA, captured the red, blue and green content by capturing the colours in each field in sequence one after the other. They did this by mechanically rotating a disc in front of the lens with red, green and blue colour filters that only pass one of the free colours. These filters are like the transparent coloured papers in which chocolates are wrapped. For example, if you look through a red one, everything looks red. This is because it only lets the red light through. Blue or green objects will look black. Colours which contain some red, such as purple, look dark red. This shows a simplified view of what the rotating disc in front of the camera would have looked like. The inset picture shows a pie engineer in about 1950 looking at the rotating camera disc. The red, green and blue content of the images would then be transmitted in rapid succession. When received at the TV, the prototype TV sets also had a rotating colour disc in front of the cathode ray tube screen. The red, green and blue content of the images would then be shown in rapid succession. In the same sequence as in the camera, and the persistence of vision of the eye would see the resulting picture in colour. Pie demonstrated sequential colour TV based on the CVS system at the Radio Olympia Show in London in 1949, and later at Queen Elizabeth II's Coronation in 1953, by which time they were using cathode ray tubes that could show colour without the need for rotating disc. However, the sequential colour process was not compatible with the black and white TV system. This would have meant separate channels for black and white televisions and colour televisions. What was needed was a compatible system whereby the same signal could be viewed in black and white on a black and white television and in colour on a colour television. The first compatible colour television system was the NTSC system launched in the USA in 1954. NTSC stands for National Television System Committee. Compatible means that the colour receiver shows a colour picture of the scene, and the monochrome receiver a black and white picture of the same scene. This system used a single TV signal that contained the entire colour information. The process for encoding the colour signal was very ingenious and complex and will not be described here. However, for those that are interested please see the separate video entitled How Colour Television Works Part 3, The Colour Encoding Process. A major problem with NTSC was that the hue of the colours tended to vary as you change channel. This meant that you had to adjust a control on the receiver each time you changed channel to get a good colour picture. The picture in the centre shows the original colour of the picture, and the two pictures on either side of the original show the effect of hue errors. The PAL compatible colour television system was developed in Europe to overcome the hue error problem with the NTSC system. PAL stands for Phase Alternating Line. The PAL system automatically corrected the hue of the picture so that there was no need for a hue control. The detail of how PAL works is described in the separate video that describes the colour encoding process in more technical detail. By 1970, Phillips had developed a world-booting camera called the LDK5. It used the Phillips Plumicon camera tube which was to become a world standard. Pie built many studios and outside broadcast vehicles using this camera which were sold worldwide. It was also used extensively by the BBC. The camera used a very thin cable called a tri-axe to connect to its base station in the studio or outside broadcast vehicle. The cable length was up to 2.8 kilometres, so it was ideal for sports events. It had many knobbled circuit innovations. The LDK5 camera optical system was state-of-the-art technology at the time it was introduced. On the left is the zoom lens and on the right is the optical splitter block which separates the incoming light into the three primary colours, red, green and blue. Between the zoom lens and the optical block are a number of light filters. The ND or Neutral Density filter is used to reduce excessive light levels and can be switched in by the camera operator. Also included is a colour correction filter and an infrared filter. The quarter wave plate is part of the zoom lens. The optical block optics is based on the characteristics of prisms and this is used to split the light into its primary components and direct them to the three pickup devices indicated by the red, green and blue images. Various filters are used in the optical block to prevent transmission of colours using what are called dichroic coatings. In addition, trimming filters are used to match the three colour responses to that of the eye. The optical block called the ice block by camera operators is a very expensive and precision made part of the colour camera. The diagram shows a basic colour camera circuit. The three pickup tubes are mechanically fitted to the optical block with high precision. The three tubes are identical but respond to the colour parsed them from the optical block. The tubes are scanned as I've described in the previous video. The scanning waveforms are generated in the scanning coil driver circuit which is triggered by a pulse from the pulse processor which in itself is fed from the synchronising pulse generator based in the studio and via the camera cable. The signals from the pickup tubes are very small and are therefore amplified as shown. In order to create the PAL signal in the PAL coder, three new signals are required which are generated in an electronic matrix circuit. The signals are the lumenant signal which represents the brightness of each element in the picture by convention this is called y not to be confused with yellow. r minus y the red colour different signal and b minus y the blue colour different signal. Finally the composite PAL signal is fed down the camera cable to be used in the studio or outside broadcast. We don't need to transmit a g minus y colour different signal because it can be calculated in the receiver. The lumenant signal y is 0.59 g plus 0.3 r plus 0.11 b. If the camera is pointed at a set of colour bars as shown in the picture from white through yellow, cyan, green, magenta, red, blue and black then the signals outputted from the colour matrix are as follows. The lumenant signal y is a set of grayscale bars from white to black. This diagram shows what the lumenant video signal looks like as the electron beams scanned across the image sensors for one line. Dark portions of the image are a lower voltage than bright portions. The lumen signal is the same as the video signal in the black and white system and contains the detailed information in the picture. This diagram shows the colour different signals r minus y and b minus y The television signal is then used to modulate a carrier wave as described in the how radio works video and it's transmitted from the transmitting antenna along with the sound signal. They now follow the description of how the colour TV receiver recreates the image. The video and sound signal from the transmitter is received by the TV's antenna and the sound signal is converted to the original sound in the same way that the radio receiver works as described in the video how radio works. The video signal is passed through a decoder which extracts the lumenant signal and the colour different signals r minus y and b minus y. This diagram shows the lumenant signal y and the two colour different signals r minus y and b minus y. If this signal is received by a monochrome TV receiver then the colour different signals are disregarded and the lumenant signal is used to create a black and white picture as described in the previous video. In the colour receiver the red green and blue video signals are obtained by adding and subtracting the various signals. Red is obtained by adding the lumenant signal y to the r minus y signal. Blue is obtained by adding the lumenant signal y to the b minus y signal. Finally green is obtained by subtracting r and b from y. So you can see that the r, g and b signal to the colour bars are as shown from white to yellow to cyan to green to magenta to red to blue and finally to black. In the previous video we explained how the cathode ray tube was used to produce a black and white picture. The electrodes within the tube that produced the electron beam were called the electron gun. However there needed to be substantial changes to make a tube that could produce a colour picture. Firstly there would need to be three electron guns one for each of the three primary colours red green and blue. Secondly there would need to be screened phosphors that could emit each of the three colours and finally there needed to be a way to ensure that the red electron gun would only cause the red phosphor to emit light and similarly for the blue and green colours. The solution was the Shadow Mars cathode ray tube. This diagram shows such a tube. The tube contains three electron guns as shown side by side in an electron gun assembly. There is a phosphor screen similar to that in the black and white cathode ray tube but in this case it's made of a collection of dots made of different types of phosphor which glow one of the three colours red green and blue. The light characteristics of the colour phosphor dots are carefully selected since that affects the accuracy of the colour rendition for the whole colour television system. When magnified the phosphor dots are as shown. A metal sheet called a shadow mask with a large number of very small holes in it was placed inside the tube just behind the phosphor screen as shown in the diagram. The phosphor screen has been cut away to reveal the shadow mask. This picture shows a magnified section of the shadow mask and phosphor dots. The holes in the shadow mask are made small enough so that the appropriate electron beam passes through to the phosphor screen to excite the correct phosphor dot. So the electron beam carrying the blue video signal only excites the blue phosphors. The electron beam carrying the green video signal only excites the green phosphors and the electron beam carrying the red video signal only excites the red phosphors. In order to correct the slightly different geometry caused by the fact that each of the electron gun is not on the axis of the tube coils and magnets were fitted around the tube in addition to the deflection coils described in the previous video. These were used to deflect the different colour electron beams by differing amounts to ensure that they convert to the correct points across the full area of the screen. This picture shows the rear view of a pie colour TV receiver. A variety of coils are mounted around the tube neck as shown. Some are used to deflect the beams to scan the raster as explained in the previous video and some are used to ensure that the three electron beams all converge onto the same hole in the shadow mask as the beams scan across the screen. The end result of the system described is then viewed by the viewer. To learn more about the television cameras, tubes and receivers that Pie produced please visit Cambridge Museum of Technology.