 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. 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 in 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 television service in the UK started in 1936. Technology has steadily evolved since then into the large screen, ultra high definition digital colour TV system that we have today. This video explains how the early analog black and white television system worked. Television, or TV, is a system for converting visual images into electrical signals, then adding sound and transmitting them by radio or other means and displaying them electronically on a screen. Television literally means see at a distance. We're all familiar with TV in our homes today, but how does it work? The TV camera collects the light from the scene being televised with a lens in a similar way to the way a still camera works. The camera lens collects the light from the scene and projects this onto the image sensor. The sensor is then used to convert the image into an electrical signal. In order to capture a moving image, the camera takes a succession of still pictures every 25th of a second. Each of these pictures is called a frame. When these pictures are played back, the viewer does not see the separate frames, but sees a moving image due to the persistence and vision of the eye. Each frame is scanned in the camera to convert the image into an electrical signal that represents each frame. This is a very simple explanation of how scanning works. The picture shows a pattern of rectangles which represent the scene to be scanned. The camera works by scanning the scene from left to right or from top to bottom in the same way that the eye scans the page of a book when reading. The blue spot indicates the point on the scene that is being scanned at any one time. The spot scans the scene and flies back to scan the next line. When it finishes scanning the last line, it flies back to the top of the scene and starts the process again. The process then repeats for the next frame. The path followed by the spot is called a raster. The word raster comes from the Latin word rastrum, which means a rake. To recreate the scene at the receiver, the scanning spot draws the scene as the spot scans the raster. The process then repeats for the next frame. In practice, each picture was interlaced. This means that an image was scanned in a succession of odd lines shown in blue in the diagram from point A to point B. The scanning spot then moved to point C. And then the picture was scanned again in a succession of even lines shown in red from C to D. The scanning spot then returned to point A and started again for the next frame. The odd and even rasters, called fields, were interlaced so they did not scan the same part of the picture. The fields thus became odd and even fields. This was done to stop the picture from appearing to flicker to the viewer. In the British black and white TV system from 1936 to the 1960s, there were 405 lines per frame, 202.5 per field. In the 1960s, 625 lines per frame was introduced which gave a better quality picture. This is a picture of a camera tube that is used to convert the image into an electrical signal. It includes the image sensor. This tube was made by Cthodian, a pie company, and was called a staticon by pie, but is more generally known as a vidicon. It was 6 inches long by 1 inch in diameter. This is a simplified explanation of how a vidicon camera tube works. The image sensor referred to earlier is a light sensitive semiconductor layer mounted inside a glass camera tube in closing a vacuum as shown in the cross section in the diagram. The tube is similar to a thermionic valve. It contains a metal tube called a cathode. A filament similar to that used in a light bulb is placed inside the cathode. When a voltage VH is applied across the heater, a current passes through it and it closed red hot. This then heats the cathode so that it is also red hot. When the cathode becomes red hot, then electrons are emitted from the surface of the cathode into the vacuum close to the cathode. Electrons are subatomic particles and cannot actually be seen, but they are represented here by red dots. A metal electrode with a hole in the middle is mounted inside the tube and connected to a positive voltage of about 300 volts with respect to the cathode. This electrode was called the accelerator grid. A metal cylinder with a wire mesh at the end was fitted within the tube and this was called the wall anode and was connected to between 275 and 300 volts with respect to the cathode. A coil was wound round the tube and a current passed through it. When an electric current is passed through a coil it creates a magnetic field. The magnetic field runs along the axis of the tube. The electrons are negatively charged and are attracted towards the accelerator grid and pass through the hole. They are then attracted by the wall anode and pass through the wire mesh to strike the image sensor. By adjusting the magnetic field caused by the focus coil the electron beam is focused to a small spot by the time it arrives at the image sensor. Deflection coils are placed round the tube. As explained earlier when an electric current is passed through a coil it creates a magnetic field. However in this case the coils induce a magnetic field across the tube. This causes the electron beam to be deflected and the electrons hit a different part of the image sensor. The image sensor is connected to a voltage source of between 10 and 30 volts called the target bias via a resistor R. When the electron beam strikes the image sensor a current flows through the resistor in proportion to the amount of light that the image casts on the sensor at that point. If the image is dark at that point then only a small current will flow. If the image is light then a larger current will flow. The resulting video signal is then outputted via a capacitor C. A control grid is positioned between the cathode and the accelerated grid. When the electron beam scans back at the end of each line to start another line then a negative voltage is applied to the control grid. This repels the electron beam back towards the cathode so that no output voltage signal occurs. This diagram shows what the video signal looks like as the electron beam scans across the image sensor. Dark portions of the image are at a lower voltage than bright portions. At the end of each line the beam is scanned back very quickly to start the next line. This is called flyback. Because the control grid switches off the beam during the flyback then there is no output voltage during this time and the image on the sensor is not erased. In between each video signal line a synchronization pulse or sync pulse is added to trigger the horizontal flyback. When the odd field has been completely scanned a long pulse is added to trigger the vertical flyback. This is called a vertical sync pulse. In UK and European countries the total raster contains 625 lines but 49 of these contain no picture information. These were taken up by the vertical sync pulse which was used to enable the TV receiver to scan back vertically ready for the next field. So the odd field contains lines 1, 3, 5 etc up to line 575 and the even field contains lines 2, 4, 6 etc up to line 576. Each field is scanned 50 times per second which is fast enough for the human eye not to perceive the flicker as each image is scanned. Since two fields are required to create one frame this means that the frame rate is 25 frames per second. Prater 1964 the UK TV system used 405 lines. In France they use 819 lines. The US system used 525 lines but the frame rate was 30 frames per second. Pi built cameras for use with all of these systems. The television signal is then used to modulate a carrier wave as described in the how radio works video and is transmitted from the transmitting antenna along with the sound signal. There now follows a description of how the TV receiver recreates the image. The video and sound signal from the transmitter is received by the TV's antenna where the video signal is extracted to recreate the picture and the sound signal is converted to sound. The TV contains a tuner and amplifier which is used to select the incoming signal from the antenna to amplify it and demodulate the original TV picture signal and sound signal in the same way as described in the how radio works video. The received picture signal is then in the same form as it was transmitted with the video signal information and the horizontal and vertical sync pulses that described earlier. And along television receivers used a cathode ray tube to display the picture as shown here. The tube consists of a glass container enclosing a vacuum. The inside of the screen is coated with a phosphor. A phosphor is a substance that emits light when struck by an electron beam known as a cathode ray. The active emitting light is called fluorescence. The tube contains a heated cathode and other electrons called an electron gun which emits electrons which are then accelerated towards the screen to create the image. This is a simplified explanation of how a cathode ray tube works. The diagram shows the cross section of the glass tube which encloses a vacuum. A phosphor coating is applied to the inside of the screen. Just like the camera tube the cathode ray tube is similar to a thermionic valve. It contains a heated cathode. When the cathode becomes red hot then electrons are emitted from the surface of the cathode into the vacuum close to the cathode as described earlier. A metal disc with a hole in the center is fitted inside the tube as shown. This is called the first anode. A metal coating is also applied to the inside of the flared part of the glass tube and this is called the second anode. A connection to the first anode is made via a pin through the base of the tubes shown. A connection is also made to the second anode via a connection through the side of the tube. A positive voltage of about 400 volts is then applied between the first anode and the cathode. In addition a positive voltage of about 18,000 volts depending on the size of the tube is applied between the second anode and the cathode. The electrons are negatively charged and are attracted by the first anode where they pass through the hole in the center. They're then accelerated again by being attracted by the second anode. They then continue until they hit the phosphor screen. This causes a spot where they hit the screen to fluoresce and to give off light which can be seen by the viewer. After hitting the phosphor the electrons are then attracted back to the second anode. Finally another electrode called a grid is placed between the cathode and the first anode. If the voltage on the grid is varied then the number of electrons leaving the cathode can be varied and this varies the brightness of the screen. If it is adjusted so that all of the electrons leaving the cathode reach the screen it appears white. If a negative voltage is applied to the grid then the electrons are repelled and none reach the screen so this can be used to represent black. Deflection cars are placed around the tube as shown here. When an electric current is passed through a coil it creates a magnetic field. This causes the electron beam to be deflected and the electrons hit a different part of the screen. This shows how the electron beam can be deflected to a different part of the screen as the current is passed through the deflection coils in the same way as in the Vidicon tube. If the current is varied in the deflection coil then the beam can be made to scan across the screen. Vertical and horizontal coils are then used to scan the beam horizontally and vertically to create a raster. The scanning is synchronised to correspond to the original picture by using the synchronising pulses on the received signal to trigger the appropriate horizontal and vertical scans. By scanning the electron beam horizontally and vertically a raster can be made to appear on the screen. This simplified animation shows how the image is built up on the screen in slow motion. The video signal is used to vary the voltage on the grid of the cathode ray tube to vary the intensity of the electron beam and thus the light emitted at each point on the screen corresponding to the light or darkness of the image at each point. By this means the original image can be displayed. In reality there are a total of 576 displayed lines and the complete picture frame is replaced 25 times per second so that the eye sees the complete picture without perceiving the lines or the frame changes. To learn more about television cameras, tubes and receivers that Pi produced please visit Cambridge Museum of Technology.