Why is video inverted for transmission?

Discussion in 'Digital Photography' started by Green Xenon [Radium], Sep 20, 2007.

  1. Green Xenon [Radium]

    Jerry Avins Guest

    Randy, back when TV first went commercial, a frequency divider was a
    multivibrator with a little of the pilot signal introduced somewhere. It
    wasn't possible to make large divisions stably, so cascades of smaller
    divisions were used. RCA's original system used 441 lines -- I assume
    you know about odd line count and interleave -- and synced horizontal to
    vertical with two divisions by 7 and two by 3. The FCC, guessing that
    CRTs larger than ten inches would eventually come, insisted on 525. That
    factors to 5x5x7x3. Don't confuse what is easy today with what was
    practical using tubes and discrete components. The original TV sync
    circuits were designed when the Eccles-Jordan bistable was a novelty.

    Jerry Avins, Sep 24, 2007
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  2. Green Xenon [Radium]

    Jerry Avins Guest

    That's for horizontal sync. More area is used for vertical, so your
    original figure may be close to correct overall. (I haven't worked it out.)
    Strictly, "rolling" is loss of vertical sync. (If we don't watch out,
    we'll be discussing whether the equalizer notches during vertical sync
    are still needed.)

    Jerry Avins, Sep 24, 2007
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  3. Green Xenon [Radium]

    Don Pearce Guest

    Certainly lower, but probably not that close.
    Yebbut, vertical sync is just made of a load of horizontal syncs with
    variable mark-space ratios in the pulses.

    Don Pearce, Sep 24, 2007
  4. Green Xenon [Radium]

    Jerry Avins Guest

    The TV of a century ago -- Nipkow, 1884 patent -- has absolutely no
    influence on today's standard. There was one on-air broadcaster[1] and
    one maker of bare chassis and kit receivers[2] (build your own cabinet)
    in 1939.


    [1] RCA, experimental, 441 lines.
    [2] Andrea Radio, successor to the depression-bankrupt FADA Radio &
    Electric Co. FADA stood for Frank A. De Andrea. Frank was an old man
    when I worked for him in 1956, maybe even older than I am now.
    Jerry Avins, Sep 24, 2007
  5. Green Xenon [Radium]

    Jerry Avins Guest

    Randy Yates wrote:

    Pro: before. Gnosis: knowing. Literally, "knowing before". Practically,
    prediction. You need a better dictionary, but the context in which the
    word showed up here would have given me no hint either.

    Jerry Avins, Sep 24, 2007
  6. Green Xenon [Radium]

    Jerry Avins Guest

    Horizontal sync is gotten by differentiating. Vertical sync is gotten by
    integrating. During vertical retrace, we need to offset horizontal sync
    by half a line. Equalizer notches do that, but I won't discuss it. :)

    Jerry Avins, Sep 24, 2007
  7. Green Xenon [Radium]

    Don Pearce Guest

    Yup, that's what I was talking about.

    Don Pearce, Sep 24, 2007
  8. Green Xenon [Radium]

    Bob Myers Guest

    Nope; check the standards. The reference is always
    the blanking level; if the sync happens to be at "0V"
    with respect to the local reference at some point in the
    circuit, that's simply a coincidence. You never, ever
    reference the rest of the video signal to the sync tips
    if you're following the standards, so to speak of sync
    as being at "0V" is meaningless.

    Bob M.
    Bob Myers, Sep 24, 2007
  9. Green Xenon [Radium]

    Gary Tait Guest

    (Ray Fischer) wrote in $:
    It will still be very relavent even when broadcasters and cable turn off
    their analog. Stet-top boxes and displays will still output analog video
    for some time to come.
    Gary Tait, Sep 24, 2007
  10. Green Xenon [Radium]

    Don Pearce Guest

    Sure the standards use the blanking level as the reference, but you
    must apply a sync = 0 signal to a modulator.

    Don Pearce, Sep 24, 2007
  11. Green Xenon [Radium]

    Bob Myers Guest

    I just realized that we're talking similar but conflicting standards
    here. In the UK and other "PAL" countries, sync is indeed
    30% of the peak-to-peak signal, and the black-to-white range
    the other 70% - with no distinction made between the black
    and blank levels (i.e., "zero setup"). That is, however, NOT the
    case in the U.S. or other countries still adhering to the original
    NTSC and EIA video signal definitions. Those consistently
    described the peak-to-peak video signal in terms of "IRE
    units," with the blanking level as the reference and the blank-
    to-white range pegged at "100 IRE." Under that definition, the
    sync tips are 40 IRE "down" from blank, and black is 7.5 IRE
    "up". What you wind up with in terms of absolute voltages
    depends on the standard p-p swing of the signal. The older
    RS-170 standard established a 1.4 Vp-p video signal, so the
    sync tips were -0.400V with respect to blank, while white was
    + 1.000 V. Later, RS-343 revised this to a 1.000 Vp-p
    (including sync), which resulted in the currently-typical values
    of ROUGHLY 0.7V for white - but there is still a distinction
    between North American and European practice. Simply
    rescaling the 140 IRE range to a 1.000 Vp-p swing results
    in some odd numbers - sync is -0.286V with respect to blank,
    black is +0.054V, and white is +0.714V. When the European
    standards were established (Europe lagged a bit in producing
    TV standards, of course, due to the effects of WW II), a more
    sensible approach was taken, as the blank-to-black "setup" or
    "pedestal" was eliminated and the 1 V swing divided in the
    30/70 ratio you mentioned, between sync and video. We've
    also both been right about the reference - apparently the signal
    is "built up from the sync tips" in standard European practice,
    while we in North America always have considered the blanking
    level to be the "0V" reference.

    Bob M.
    Bob Myers, Sep 24, 2007
  12. Green Xenon [Radium]

    Bob Myers Guest

    Actually, see my later post on the same topic; Don and I
    were actually both right, as we were describing differing
    (European vs. North American) standards.

    Yes, the clamping diode is referenced to ground, but I was
    talking about what's used as the "internal" reference point for
    the video signal itself.

    Bob M.
    Bob Myers, Sep 24, 2007
  13. Green Xenon [Radium]

    Don Pearce Guest

    So we have! I've always lived with the PAL-I standard (the highest
    resolution and best colour in the world, one might say) and I'd
    forgotten that others do things differently.

    Unfortunately it is all moot because in a couple years we switch off
    our nice PAL in favour of digital which is, frankly, shit. The colours
    aren't right, there is no resolution to speak of and the sound is so
    far out of sync that we are lucky if it accompanies the right
    programme. So much for progress.

    Don Pearce, Sep 24, 2007
  14. On Mon, 24 Sep 2007 16:31:22 GMT, in sci.electronics.design
    And you didn't even mention DAB Vs FM radio <duck>

    Martin Griffith, Sep 24, 2007
  15. Green Xenon [Radium]

    Don Pearce Guest


    Ooh! Just for interest, my spell checker thought I was trying to say
    arteriosclerosis. That kind of sums up DAB.

    Don Pearce, Sep 24, 2007
  16. Green Xenon [Radium]

    Richard Fry Guest


    It is when video is transmitted in inverse polarity, using AM. The
    power radiated from the transmit antenna is maximum at the tip of
    sync, which means that the received field strength also is maximum at
    the tip of sync.

    If video were transmitted in normal polarity the output power of the
    transmitter would be considerably less at the tip of sync, and TV sets
    in weak signal locations would not lock up as reliably.

    Richard Fry, Sep 24, 2007
  17. Green Xenon [Radium]

    Bob Myers Guest

    I have to admit that I also haven't been keeping very
    current on what's going on re broadcast DTV in Europe,
    other than still being upset that we got stuck with 8-VSB and
    you didn't. :) What are you expecting now in terms of
    the standard resolutions that will be carried, etc.?

    Bob M.
    Bob Myers, Sep 24, 2007
  18. Green Xenon [Radium]

    Don Pearce Guest

    I just don't know. The ambition of the multiplex operators here
    appears to be simply to cram as many programmes on as the mux will
    carry. There are no sharp programmes on our digital service.

    Don Pearce, Sep 24, 2007
  19. It would have been in Randy Yates's dictionary if it had been spelled
    "prognostication". Perhaps that is what Randy meant, in his subtle way
    Gene E. Bloch, Sep 24, 2007
  20. Green Xenon [Radium]

    Randy Yates Guest

    I did find prognostication, but since its definition deals with
    predicting the future, and that definition didn't make sense, I
    decided to simply ask for clarification rather than attempting to
    second- and third-guess what he meant.

    Is that being subtle? Is it playing a game (i.e., "being subtle") to
    ask that wording be spelled correctly and make sense?
    % Randy Yates % "The dreamer, the unwoken fool -
    %% Fuquay-Varina, NC % in dreams, no pain will kiss the brow..."
    %%% 919-577-9882 %
    %%%% <> % 'Eldorado Overture', *Eldorado*, ELO
    Randy Yates, Sep 24, 2007
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