 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. In the How Colour Television Works video, we explained how the eye perceives colour, how colour cameras work and how the TV receiver displays the original picture. However, we did not explain in detail how the colour signal is encoded onto the video signal by the camera and how the TV receiver decodes this signal. So this video explains this process. It's technically quite challenging to understand this. In the previous video, we showed what the luminous video signal Y looks like as the electron beam scans across the image sensors for one line. Dark portions of the image are a lower voltage than bright portions. The luminous signal is the same as the video signal in the black and white or monochrome system and contains the detailed information in the picture. For a compatible colour and monochrome TV system, the same signal needs to be able to be used for either black and white or colour TV receivers without any detrimental effects. Compatible means that a colour receiver shows a colour picture of the scene and the monochrome receiver of a black and white picture of the same scene using the same signal. The monochrome receiver must receive and process the monochrome luminous content of the colour signal and not respond to the colour information. The monochrome signal is the same as the luminant signal used in a compatible colour TV system. If you look at the frequencies contained within a luminant signal spectrum as a graph of voltage versus frequency, then this looks like the graph shown here. It consists of an initial peak at the horizontal line frequency and then a succession of harmonics of the line frequency. Two times FH, three times FH, etc. The energy in the luminance of the picture is spread either side of the harmonics of the horizontal line frequency. This means there are empty spaces within a spectrum between each of the harmonics and this is where the colour information can be placed. The first compatible colour TV system was the NTSC system introduced in the USA in 1953. NTSC stands for National Television System Committee. In NTSC, the colour signal called chromonance was transmitted at reduced bandwidth and NTSC does not see detail in colour. The black and white luminance signal carries the detail in the picture and is sent at the full bandwidth of about 4.2 MHz. The chromonance was modulated onto a subcarrier at 3.579545 MHz using a technique called quadrature amplitude modulation. We will explain what that is meant by later in this video. The subcarrier frequency was carefully chosen to limit its visibility on the monochrome receiver. NTSC forms the basis of all future colour systems such as PAL and C-KEN. The colour subcarrier has a high frequency to minimise its visibility on the monochrome receiver. The American 525 line monochrome system has a bandwidth of about 4.5 MHz and the chromonance bandwidth is about 1 MHz. Hence, the subcarrier frequency should be about 3.5 MHz. It must have a frequency that ensures the chromonance signal occupies the frequency space between the luminance horizontal spectral lines. This could be achieved by making the subcarrier frequency an odd multiple of half the horizontal frequency. The subcarrier frequency FSC is 455 x 15.734 kHz divided by 2 which is equal to 3.579545 MHz. Inconventual amplitude modulation, the signal modulates the subcarrier as shown. The frequency spectrum of the amplitude modulated signal is then shown on the right-hand side of the picture. The spectrum consists of the subcarrier frequency and the sidebands that contain signal information. The problem with using this is that the subcarrier frequency could appear as a pattern on the luminance signal. NTSC uses suppressed carrier quadrature modulation. In this technique, the subcarrier is suppressed so that the spectrum just contains the colour information in the sidebands and thus the interference to the luminance signal is minimised. This diagram shows the luminance signal y and the two colour different signals r minus y and b minus y for a line of a colour bar image. These signals now need to be modulated onto the subcarrier to create the chromonance signal. In the NTSC system, the r minus y and b minus y signals are used to modulate the subcarrier using a circuit known as a balanced modulator which suppresses the subcarrier and so the resulting output from each modulator consists of the sidebands that contain the colour information. For the b minus y signal, the subcarrier is shifted in phase by 90 degrees compared to the r minus y signal. The two signals are then combined and the result is called the chromonance signal. The complete NTSC coder takes the red, green and blue signals and adds them together in a matrix circuit to create the luminance signal y. It also produces the r minus y and b minus y signals which are fed to the balanced modulators to create the chromonance signal as previously explained. The chromonance and luminance signals are then combined to form the composite video signal. This diagram shows the b minus y voltage for the colour bars. Below this is the subcarrier and below this is the output from the balanced modulator. You can see that there's no output when there's no colour information, for example when the colour bar is white or black. You can also see the phase changes as the colour bars change. It is easier to understand the chromonance signal than the vector diagram has shown. The colour subcarrier carries the colour in two ways. The vector diagram shows all possible positions of the colour vector representing an individual colour, any one of which is related to the phase of the unmodulated subcarrier. The saturation of the colour, i.e. the amount of white in it, is carried by the amplitude of the vector. An example chromonance vector is shown in the diagram. Also shown is a vector that represents the phase and amplitude of the subcarrier. Although the subcarrier is not transmitted with the chromonance signal, a small burst is transmitted at the beginning of each line as a reference. Strictly speaking, for NTSC the r minus y signal was called i and the b minus y signal was called q. And they were rotated by 30 degrees with respect to the x-axis. But for simplicity we will just refer to r minus y and b minus y for the rest of the video. The chromonance signal spectrum is shown here with the sideband either side of the subcarrier frequency and the harmonics of the line frequency. When the chromonance signal is added to the luminance signal, the colour signal energy is interleaved with the luminance energy shown. By this technique the colour signal can be transmitted in the same bandwidth as the monochromeed luminance signal without interfering with it. A short burst of unmodulated carrier signal is added between the line sync pulse and the beginning of the video signal. The purpose of this is to provide a reference carrier signal so that the carrier can later be reconstituted in the TV receiver. The vector diagram shows the chromonance factor for each of the colours relative to the reference burst signal. White, yellow, cyan, green, magenta, red, blue and black. This is a block diagram of the Philips LDK25 camera described in the previous video how colour TV works. The decoder has been added to the diagram and its output is fed down the camera cable to the control unit as a composite signal. This shows an oscillogram as seen on an oscilloscope of the video signal for the colour bars. On the right of the picture is a display from a vectorscope showing the vectors for the colour bar signal. This diagram shows the basic functions of the decoder in the TV receiver to decode the NTSC colour signal. The video signal from the demodulator is processed in three ways. It's passed through a filter circuit which removes the colour reference burst signal and the chromonance signal to recreate the luminance signal Y. In parallel with this, the signal is also passed through another filter which removes the luminance signal to recreate the chromonance signal. Finally, the colour burst signal is used to synchronise a local oscillator which recreates the subcarrier. The chromonance signal is demodulated by using two demodulators. For the B-Y component, the signal is demodulated using the reference subcarrier in the first demodulator. For the R-Y component, the signal is demodulated using the reference subcarrier phase shifted by 90 degrees corresponding to the phase difference when the signal was modulated. These signals are then converted to R, G and B as described in the previous video. Unfortunately, a major problem with NTSC was that the hue of the colours tended to vary in some situations. For example, in poor reception areas, reflected signals could cause the phase of the chromonance signal to vary when compared to the reference signal and this manifests itself as an incorrect hue. The picture in the centre shows the original colour of the picture. The two pictures on either side of the original show the effect of hue errors. This means that you had to adjust a control on the receiver called a hue or tint control to get a good colour picture. You may then have to adjust this hue control each time you change channel. This diagram shows the vector for a red chromonance signal. However, if the phase of the chromonance signal is varied with respect to the subcarrier reference, by just a few degrees then the colour would vary. With the introduction of the NTSC colour television system in the USA, the rest of the world's broadcasters were studying possible improvements. The French developed the CCAM system and the Germans led by Walter Brock at Telefunca, the PAL system. Eventually the choice for each country would become a political one and this resulted in the countries associated with the USA, such as Japan, choosing the NTSC system. The countries associated with France and Russia chose the CCAM system and most of Europe chose the PAL system. The PAL compatible colour television system was developed to overcome a hue error problem with the NTSC system. PAL stands for Phase Alternating Line. The PAL system automatically corrected the hue of the picture so there was no need for a hue control on the television receiver. Engineers at the time jokingly referred to NTSC as never twice the same colour. CCAM is something essentially contrary to the American method and PAL is peace at last. There now follows a description of how the PAL system works. The PAL colour subcarrier must satisfy the following criteria. It must have a frequency that ensures the chrominance signal occupies the frequency space between the luminance horizontal spectral lines. However, if the subcarrier frequency for PAL is made an odd multiple of half the line frequency as in NTSC, there is a problem. Due to the phase alternation, a monochrome receiver produces vertical lines of dots on coloured parts of the picture which become quite visible. As the bandwidth of the European 65 line monochrome TV system was about 5.5 MHz, the colour subcarrier frequency could be higher than NTSC at about 4.5 MHz. It was decided therefore to use a quarter line offset, a further reduction in dot pattern visibility was achieved by a further 25 Hz offset. Subcarrier frequency FSC equals 284 minus a quarter times FH plus FV, so FSC equals 4.43361875 kHz. In PAL, the chrominance signal consists of two signals. V, which is 0.877 times R minus Y, and U, which is 0.493 times B minus Y. Imagine that this diagram shows part of the chrominance signal for line one. On the right of the waveform, you can see the vector representation of the U and V components. On the subsequent line, the V signal is reversed in phase by 180 degrees. By switching the V modulation axis by 180 degrees from line to line, this allows averaging techniques to be used to reduce the visibility of any hue errors. Phase alternating lines are achieved by switching the V modulation axis by 180 degrees from line to line. This shows the comparison of the PAL and NTSC colour signal vector. On the left of this picture, you can see the NTSC vector display for a colour bar signal. Note the position of the vector for magenta. The burst is also shown. On the right, you can see the vector display for a PAL signal. The PAL display shows the chrominance vector as the phase of the V signal is alternated by 180 degrees. In PAL, the burst signal also switches phase by plus or minus 45 degrees for each alternate line. This is called the swinging burst. The PAL coder takes the red, green and blue signals and adds them together in a matrix circuit to create the luminance signal Y. It also produces the V 0.877 times R minus Y and the U 0.493 B minus Y signals, which are fed to the balanced modulators. The sub-carrier phase is switched alternately by 180 degrees every other line at 7.8 kHz, i.e. half the line frequency. The resulting V and U signals are then combined to create the chrominance signal. The chrominance and luminance signals are then combined to form the composite video signal. In this picture, you can see the chrominance vectors for successive lines showing the phase reversal of the V signal every other line. If a phase delay occurs, then the vector will be delayed equally on both lines as shown. When the successive chrominance signals are combined, then the average of the two vectors produces the originally transmitted correct phase. The first version of PAL to be developed was called Simple PAL, and was based on the idea that you could use the combination of the eye and the brain to average out colour phase errors produced by the transmission train. The picture on the left shows the effect of a phase error, and the picture on the right has the phase error in the opposite direction. If the eye sees the two pictures in rapid succession, then the eye and the brain combination actually sees the average of the two pictures and shows no colour errors. The system will work with phase errors of less than 15 degrees, but is subjective and varies with different observers. Also, observations with large phase errors produced an annoying horizontal line pattern called Hanover bars. It was decided that an electronic solution to the averaging was required, and this would be called PAL-D, where D stands for delay line. In PAL-D, the chrominance signal was averaged electronically. This was done by passing the signal through a delay line, and then combining it with the undelayed signal to obtain the UNV signals. The delay was almost exactly the time taken to scan one line. This was not made exactly one line because sub-carrier phase needs to be the same for each signal, so it is actually set to a delay of 284 cycles of sub-carrier, which amounted to a delay of 63.9 microseconds. Subtracting the input and output signals averages the result and removes the phase error to produce a clean new signal. Adding the input and output signals averages the result and removes the phase error to produce a clean V signal. Now, there's a slight loss of saturation which can be corrected, but the colour vertical resolution is now half that of NTSC. However, this reduction is not noticeable in practice. This is an example of the circuit board of a typical 1970s PAL decoder. The white block in the picture is the PAL delay line. This works by converting the chrominance signal into an ultrasonic mechanical vibration. The ultrasonic vibration passes through a block of quartz and then meets another transducer, which converts the ultrasonic signal back to an electrical signal. The delay was 63.9 microseconds. Since the chrominance signal has to pass through the various circuit elements in the decoder, it is slightly delayed compared to the luminance signal. Therefore, a delay line was also needed to delay the luminance signal so that it lines with the chrominance signal. The long red coil at the bottom of the picture is the luminance delay line, which is used for this function and has a delay of a few microseconds. This has been a much simplified explanation of how colour encoding works. Viewers who are interested in more detail, particularly the mathematics describing the process, are referred to the reference book, Colour Television with particular reference to the PAL system by GN Patchit.