How Analogue Colour Televisions Works: the Coding and Decoding Process
well Cambridge is well known for being a high-tech center with lots of innovative electronics companies in fact Pi could be considered to be one of the founders of that industry the pine was named after William George Pye 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 color television was being demonstrated in 1949 and was actually used at the Queen's coronation in 1953 so it was a world leader in 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 pie items in their collection this exhibition covers all aspects of Pi with examples of the products and the history of the company this is the perfect location because we're on one side of the river and on the other side is where Pie's main Factory was so it's a very good place to be in the how color television Works video we explained how the eye perceives color how color cameras work and how the TV receiver displays the original picture however we did not explain in detail how the cuddle signal is encoded onto the video signal by the camera and how the television 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 wire looks like as the Electron Beam scans across the image sensors for one line dark portions of the image are a low voltage than bright portions the luminance 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 color on monochrome TV system the same signal needs to be able to be used for either black and white or color TV receivers without any detrimental effects compatible means that the color of stevia shows a colored picture of the scene and the monochrome receiver a black and white picture with the same scene using the same signal the monochrome receiver must receive and process them on the Chrome luminous content of the color signal and not respond to the color information the monochrome signal is the same as the luminant signal used in a compatible color TV system if you look at the frequencies contained within the Lumen 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 layer frequency two times FH 3 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 the spectrum between each of the harmonics and this is where the color information can be placed first compatible color TV system was the ntsc system introduced in the USA in 1953 ntsc stands for national television system committee in ntsc the color signal called chrominance was transmitted at reduced bandwidth since the eye does not see detail in color the black and white luminance signal carries a detail in the picture and is sent at the full bandwidth of about 4.2 megahertz the criminals was modulated onto a subcarrier at 3.579545 megahertz 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 color systems such as pal and secan the color subcarrier has a high frequency to minimize its visibility on the monochrome receiver the American 525 line monochrome system has a bandwidth of about 4.5 megahertz and the chrominance bandwidth is about one megahertz hence the subcarrier frequency should be about 3.5 megahertz
it must have a frequency that ensures the chroma nut signal occupies the frequency space between the luminance horizontal spectral lines and this could be achieved by making the subcarrier frequency an odd multiple of half the horizontal frequency so the subcarrier frequency FSC is 455 times 15.734 kilohertz divided by 2 which is equal to 3.579545 megahertz in conventional 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 the sub carrier 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 color information in the side bands and thus the interference to the lunar signal is minimized this diagram shows a luminance signal Y and the two color different signals R minus y and B minus y for a line of a color bar image these signals now need to be modulated onto the subcarrier to create the chrominant 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 color 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 chrominant signal the complete ntsc coder takes the red green and blue signals and adds them together in The Matrix circuit to create the luminant signal y it also produces the r minus y and B minus y signals which are fed to the balance modulators to create the chrominant signal as previously explained the chrominants and lumina signals are then combined to form the composite video signal this diagram shows the B minus y voltage for the color bars below this is the sub carrier and below this is the output from the balance modulator you can see that there's no output when there's no color information for example when the color bar is white or black you can also see the phase changes as the color bars change it is easier to understand the chronic signal than the vector diagram is shown the color sub carrier carries the color in two ways the vector diagram shows all possible positions of the color Vector representing an individual color any one of which is related to the phase is the unmodulated sub carrier the saturation of the color I.E the
amount of white in it is carried by the amplitude of the vector an example chromatous Vector is shown in the diagram also shown as a vector that represents the phase and amplitude of the subcarrier although the subcarrier is not transmitted with the chromium 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 chromolent signal spectrum is shown here with the sideband either side of the subcarrier frequency and the harmonics of the line frequency when the chromium signal is added to the luminance signal the color signal energy is interleave with the luminance interview as shown by this technique the color signal can be transmitted in the same bandwidth as the monochrome luminance signal without interfering with it short burst of unmodulated carrier signal is added between the line sink 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 chrominant vector for each of the colors relative to the reference burst signal White yellow cyan green magenta red blue and black this is a block diagram that the Philips ldk25 camera described in the previous video how color TV works coder 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 in a silogram as seen on an oscilloscope of the video signal for the color bars and on the right of the picture is a display from a vector scope showing the vectors for the color bar signal this diagram shows the basic functions of the decoder in the TV receiver to decode the ntsc color signal the video signal from the demodulator is processed in three ways it's passed through a filter circuit which removes the color reference burst signal and the chromium signal to recreate the luminance signal wire in parallel with this the signal is also passed through another filter which removes the luminance signal to recreate the chromat signal finally the color burst signal is used to synchronize a local oscillator which recreates the subcarrier the chrominant signal is demodulated by using two demodulators for the B minus y component the signal is demodulated using the reference subcarrier in the first demodulator for the r minus 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 RG and B as described in the previous video unfortunately a major problem with ntsc was that the Hue of the colors tended to vary in some situations for example in poor reception areas reflected signals could cause the phase of the chrominant signal to vary when compared to the reference signal and this manifests itself as an incorrect hue the picture in the center shows the original color 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 the control on the receiver called a hue or tint control to get a good color picture you may then have to adjust this Hue control each time you change the channel this diagram shows the vector for a red chrominant signal however if the phase of the Coronet signal is varying with respect to the subcarrier reference by just a few degrees than the color would vary with the introduction of the ntsc color television system in the USA the rest of the world's broadcasters were studying possible improvements the French developed the sea chem system and the Germans led by Walter Brock at Telefunken the power 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 seacam system and most of Europe chose the pal system the pal compatible color television system was developed 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 there's no need for a hue control on the television receiver engineers at the time jokingly refer to ntsc as never twice the same color ccam is something essentially contrary to the American method and pal as Peace at Last the now follows the description of how the power system works the pal color subcarrier must satisfy the following criteria it must have a frequency that ensures the chrominant signal occupies the frequency space between the Luminous horizontal Spectrum lines however if the subcarrier frequency for power is made an odd multiple of half the line frequency as in ntsc there is a problem due to the phase alternation or monochrome receiver produces vertical lines of dots on colored parts of the picture which become quite visible as the bandwidth of the European 65 line monochrome TV system was about 5.5 megahertz the color subcarrier frequency could be higher than ntsc at about 4.5 megahertz it was decided therefore to use a quarter line offset a further reduction in dot pattern visibility was achieved by a further 25 Hertz offset subcarrier frequency FSC equals 284 minus a quarter times FH plus FV so FSC equals 4.43361875 kilohertz
in pal the chroman suit all consists of two signals the 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 chromium 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 focused 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 a comparison of the panel and ntsc color signal Vector on the left of this picture you can see the ntsc vector display for a color 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 power 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 power coder takes the red green and blue signals and adds them together in a matrix circuit to create the luminum 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 subcarrier phase is Switched alternately by 180 degrees every other line at 7.8 kilohertz I.E half the line frequency the resulting v a new signals are then combined to create the chrominant signal the chrominants and luminance signals are then combined to form the composite video signal in this picture you can see the crew in its 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 chromium 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 color fade errors produced by the transmission train the picture on the left shows the effect of the 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 either brain combination actually sees the average of the two pictures and shows no color 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 the electronic solution to the averaging was required and this would be called pal D where d stands for delay line in pal D the chromat 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 u and v signals the delay was almost exactly the time taken to scan one line this was not made exactly online because subcarrier phase needs to be the same for each signal so it's actually set to a delay of 284 cycles of subcarrier which are mounted to a delay of 63.9 microseconds subtracting the input and output signals averages the result and removes the file zero to produce a clean new signal adding the input and output signals have reduced result and removes the phase error to boost the clean V signal note there's a slight loss of saturation which can be corrected but the color 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 power decoder the white block in the picture is the power delay line this works by converting the chrominant 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 chromat signal has to pass through the various circuit elements in the decoder it is slightly delayed compared to the Luminous signal therefore a delay line was also needed to delay the luminum signal so that it lines with the chrominant signal the long red coil at the bottom of the picture is the luminant delay line which is used for this function and has a delay of a few microseconds being a much simplified explanation of how color encoding Works viewers are interested in more detail particularly the mathematics describing the process are referred to the reference book color television with particular reference to the pal system by GN Patchett
2022-12-10 04:08
