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Featured Television Engineering History

  1. Mar 7, 2018 #1

    dlgoff

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  3. Mar 8, 2018 #2

    Averagesupernova

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  4. Mar 8, 2018 #3

    jim hardy

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    Wow . what the old timers did is remarkable.

    When was the last time you saw Vertical and Horizontal Hold knobs on a TV ? I think my RCA CTC19 was the last set i owned with them. Schematic starts page 3 here: http://www.americanradiohistory.com/Archive-Radio-Retailing/60s/Electronic-Technician-1966-02.pdf

    From Wiipedia
    ahhhh yes, i remember it well,,,
    ctc19Vert.jpg


    old jim
     
    Last edited: Mar 8, 2018
  5. Mar 8, 2018 #4

    jim hardy

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  6. Mar 8, 2018 #5

    CWatters

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    My father, sadly no longer alive, worked for the BBC in the early day of television. The BBC gave staff black and white rented televisions free - I think the idea was so they could monitor reception quality. I can't remember the make but ours had push button tuning and the buttons would sometimes fly out when another was pressed. TIt had horizontal and vertical hold controls on the back along with others I forget. The company was mean enough to ask for the TV back when he retired by by then we had our own colour TV. I'm sorry to say that as a child I drew on the back of Dr Who scripts.
     
  7. Mar 8, 2018 #6

    tech99

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    There is a treasure trove of interesting facts about how TV started.
    For instance, at the 1937 Radiolympia show, a large screen 405 line projection TV was for sale using an ultrasonic light modulator together with rotating mirrors.
    And the engineers who developed the EMI system, which was used for the World's first regular high definition service, in London, managed to make all the synchronising waveforms etc without having an oscilloscope. Only later did they have access to one, and saw that what they had done was correct, which must have included the interlace and front and back porches etc. and the stable black level.
     
  8. Mar 9, 2018 #7

    sophiecentaur

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    The use of "negative modulation" pretty well put an end to the need for H and V hold adjustment. Originally, the modulation of the RF carrier was proportional to the baseband signal voltage (this was for 405 line monochrome TV in the UK). The video signal voltage was (reasonably enough) proportional to the luminance of the regions of the image and synchronisation pulses were at a voltage negative with respect to black level. (One pulse at the beginning of each line and a string of pulses at the beginning of the field flyback interval. The AM signal had an envelope that had a maximum carrier level at peak white and minimum level at sync level. When the received signal was weak, the first thing to go was the sync pulses - in particular the string of field pulses. People with set-top antennae really suffered from this problem.
    When they went to higher definition (625 / 50 in UK and 525 / 60 in US) it was decided to reverse the sense of the modulation ("negative mod') and that put the sync pulses right on top of the envelope. A much more robust signal for any sync separator circuit and also not too fussy about linearity at high RF levels. So the Line and Field sync information still held up until the vision signal was not watchable. How clever was that? The other advantage was that actual signal to noise ratio was worse in the light areas of the picture where it is less visible.
     
  9. Mar 9, 2018 #8

    sophiecentaur

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    If you want an example of an even more whacky projection system, see the Eidophor Projector system (invented in 1939, even. It worked surprisingly well and gave a very bright (arc lamp) big picture. IT even worked in Colour! I believe they needed an engineer or two in constant attendance to get them to work but even so, pretty good stuff.
     
  10. Mar 9, 2018 #9

    Averagesupernova

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    I can't speak for anything but NTSC but H and V holds were on TV sets in the USA long after the NTSC standard was set in place. This standard places the sync pulse at max power on the envelope. This is the opposite of what you described @sophiecentaur.
     
  11. Mar 9, 2018 #10

    sophiecentaur

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    It is true that the early AM TV in the UK used 'positive' modulation, with the max envelope was at peak white and the minimum was at the bottom of the sync pulses. This system used bands I and III (VHF). I can't say how CATV was done because it was uncommon in the UK (Rediffusion was one of the very few companies in UK) but I believe it was compatible with the broadcast positive mod (it would have to have been as the sets were used for both. Vertical hold adjustment was necessary even when there was only one available broadcast channel (BBC) in Band I. A second (Commercial) channel was available in fewer locations and it used Band III. Transmitters were not co-sited, which meant the signal strengths were different.
    625 line mono was put in the UHF bands in the 60s and that used negative mod. Frame roll would only have been a problem in marginal areas with bad multi path conditions. The (initially) three UHF channels were always on co-sited transmitters and I seriously cannot remember any sets having dodgy sync adjustment. It may have been because there was a National Coverage Plan and reception was fairly uniform from channel to channel. People used to use rooftop antennae except in well served areas. A single, fixed wideband receiving antenna did the job. By the late 60s there was almost universal UHF coverage and the compatible PAL system appeared. There were a number of "Dual Standard (405/625) sets available in UK and they would have needed different front ends, IF and demodulators. Not a lot of use in most locations in UK except that 'rich' people could have been suckered into buying one to deal with the switch over phase.
    As for 'controls', I do remember that NTSC receivers needed colour / tint controls where the more modern PAL system largely eliminated the need for anything but saturation control. The demodulated colour subcarrier phase was much better behaved than in the NTSC system.
    Not the opposite. In my earlier post, I said that positive mod was for low definition 405 line TV and that negative mod was used for the 'HD" system. The whole organisation of TV signal delivery was different in the US, as I remember and, because of the commercial nature of the system and the population distributions, there were far more people in the US who had many more available channels to watch and they were in different sites. That may have meant that receivers needed to deal with more varied conditions.

    Edit: It is, I suppose, possible that early NTSC sets also used positive mod. I know that the US were is a great hurry to develop their TV service because of its commercial basis so it could have been brought out with a bad choice of modulation system. Surely your UHF system would have used negative mod. The UK were well behind you in TV development.
     
    Last edited: Mar 9, 2018
  12. Mar 9, 2018 #11

    jim hardy

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    I don't remember the nitty gritty details, never really got deep into TV.
    I was astonnished when i read how the color got basically just tacked on top of the B&W signal ...
    from https://www.extron.com/company/article.aspx?id=ntscdb1d

    NTSC_onehorizline.jpg

    3.58 mhz color carrier is actually 3.579545 mhz so as to not be a harmonic of 60 hz line frequency.
    It needs to be stable enough over duration of one horizontal sweep for phase comparison to convey the desired color. So every NTSC color TV had a 3.58 mhz crystal in it. They were prized for early digital clock time bases , several outfits made 17 stage divider IC's to produce accurate 60 hz for hobbyist clock kits..
    https://www.elmelectronics.com/wp-content/uploads/2016/07/ELM440DS.pdf


    and from https://www.tek.com/datasheet/mixed-domain-oscilloscopes

    ntsc_onehorizline2.jpg


    All that with vacuum tubes ! They were geniuses....

    Sure wish i knew enough about digital TV to detect commercials, turn down the volume and brightness to almost nothing. You can't in US legally delete them but attenuate ≠ delete.

    old jim
     
  13. Mar 9, 2018 #12

    Averagesupernova

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    I used to work on NTSC television service equipment (video test signal generators, monitors, etc.). I too was amazed to learn what they were able to do with color while maintaining compatibility with older black and white TV sets.
     
  14. Mar 9, 2018 #13

    dlgoff

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    This site has some nice pictures of early television test equipment.
     
  15. Mar 10, 2018 #14
    I have a copy of "Electronic Novelties For The Constructor" by E.N.Bradley, Norman Price (Publishers) London, from 1956 and it has the circuit drawings, materials list and instructions for building a 'Simple Home Television System" in 27 A5 pages.

    It also has the same for "Miniature Short-Range Model Control" in 7 pages and "A Light Beam Communicator" in 6 pages.
     
  16. Mar 10, 2018 #15

    sophiecentaur

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    What a Pandora's Box.
    The need for COMPATIBILITY was paramount and was responsible for a lot of effort to fit a quart into a pint pot in such a way that existing monochrome viewers wouldn't make a fuss. The sub carrier frequency was chosen to be locked to line frequency so that spectral products interleaved with the line frequency harmonics of all stationary images. They never solved the problem of moiree patterns on announcers' shirts though.
    NTSC did suffer greatly from subcarrier phase problems and the later move to PAL helped a lot. The Phase Alternation every other Line had the effect of reducing the effects of group delay by averaging out the error and made the colour much more stable. (Never The Same Colour Twice was used to describe the NTSC problem).
    Didja also know about SECAM? That system used FM instead of PM for the colour subcarrier and it was developed for use in mountainous regions where multi path propagation was worse. That system was used by France and there was a Russian version too. It died a death fairly soon, I believe because of the need for multi standard sets at a time when the electronics was still a major production cost.
     
  17. Mar 10, 2018 #16

    tech99

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    1) I think the introduction of colour did always degrade the monochrome picture; prior to that, I thought 625 reception on a monochrome CRT was really superb.
    2) The design of the NTSC system was an amazing leap of engineering - one can only admire its designers. For example, not only did it overcome the compatibilty problem, colour bandwidth was exploited by using low bandwidth DSB modulation for full colour reproduction of the lower resolution details, and hign bandwidth VSB for those fine details lying along the Orange-Cyan axis of the colour triangle, where the eye has maximum acuity.
     
  18. Mar 10, 2018 #17

    Baluncore

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    Legacy compatibility can only be stretched so far. Going from AM to VSB significantly narrowed the spectrum required. Using an FM sub-carrier for the sound. Later, monaural sound replaced with the sum of L and R stereo channels, with an extra FM sub-carrier for the stereo difference. Colour burst frequency, placed in a notch in the video spectrum created by the sync pulse frequency. Then line length delay lines making PAL colour correction economic. Not much more could have been squeezed into the TV signal, in either the time or frequency domain. It was time to go digital with data compression.
     
  19. Mar 10, 2018 #18

    sophiecentaur

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    Yes but the political decision to move to colour was a no brainer. Bigger audiences and big sales of all those new sets meant that the move was inevitable. Also, you would expect the wealthier viewers to go for colour and not to be using their top of the range monochrome sets so they wouldn't be bothered by the degradation of mono pictures. The same thing applied to high quality VHF mono FM sound when it went to stereo. Many enthusiasts with golden ears used to claim that they could 'hear' the degradation due to the switching between R and L channels in the coding.
    Except for the teletext signal in the blanking interval and so on and so on. :smile:
     
  20. Mar 10, 2018 #19

    sophiecentaur

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    Haha. I read this again and it made me think of the Animal Farm passage in which the animals were told that their tobacco ration had been 'increased' from four ounces to two ounces. It's all to do with how you look at it.
     
  21. Mar 10, 2018 #20
    Back in the early 1970's I took a correspondence course in color tv servicing from NRI. If you looked at Jim's link to the Radio Electronics magazine issue you may have noticed one of their advertisements, which was usually included in every issue. One of the things I remember learning about in the course was the way the addition of color to the NTSC standard took advantage of an effect of human vision. Our vision system tends to have a lower acuity for color differences than for luminance. The NTSC standard was the first to take advantage of this effect, which is now called Chroma Subsampling. That's the reason the bandwidth occupied by the chroma information is much less than that occupied by the luma information. So the development of color tv was ground breaking in this respect, since this system is still used today for image and video storage.
     
  22. Mar 11, 2018 #21

    tech99

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    I was involved with analogue HD in the late 80s, and I remember one of the compression techniques was to selectively reduce diagonal detail. This detail fills in the spectrum between the line frequency harmonics.
    Unrelated, I also remember seeing around 1990 a demo of 64kbit video, which was intended for a view phone using ISDN. The 30 second clip I watched had taken 24 hours processing time on a main frame computer.
     
  23. Mar 14, 2018 #22
    I use to do TV repair 1966 to about 1972 when they had tubes. Then TVs were solid state with plug in circuit boards. Now they are surface mount technology.
     
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