Analogue ghosting

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  • Tom41
    Badcaps Veteran
    • Oct 2005
    • 336
    • England

    #1

    Analogue ghosting

    This is kinda redundant now that the digital switchover is nearly complete, but I did want to ask about the causes of 'ghosting' on my picture.

    Attached you'll see an image I took with my capture card a couple of months before analogue was switched off. From what I understand, the ghosting on the right hand side is caused by the signal reflecting off something like a building (or even the ionosphere) and arriving at the aerial a split second later than the main signal. The ghosting on the left, on the other hand, is called pre-echo and is usually caused by the wires themselves acting as aerials. This signal arrives at the TV a split second BEFORE the signal from the aerial, and since it scans left to right, appears on the left of the image.

    If any of you are knowledgeable about this kind of stuff, what do you think? Is it pre-echo, or a VERY late post-echo from the ionosphere? If it is pre-echo, the precise nature of the ghosts seems to suggest it's leaking in at one or two specific points on the system, rather than the whole wire…
    Maybe the signal is 'ringing' in the wire?

    The reason I post this is because it's MUCH easier to spot problems like this on an analogue signal than it is with a digital signal. The first you know about it on digital is when the picture starts to go blocky and flash 'no signal'. So far my digital TV has been working ok (in fact the signal is a little on the strong side), but could these issues cause problems for me later on?
    Attached Files
    You know there's something wrong when you open your PC and it has vented Rubycons...
  • popeye
    Senior Member
    • May 2011
    • 155

    #2
    Re: Analogue ghosting

    hi
    If you have checked the signal strenth and it is good on digital then forget it the two systems are totaly differant and you should not find you have any problems.

    If it is not broke don't fix it.

    Tony

    Comment

    • tom66
      EVs Rule
      • Apr 2011
      • 32560
      • UK

      #3
      Re: Analogue ghosting

      It's not necessarily the ionosphere or a building... This problem can be caused by the signals from two transmitters interfering (one weak, and far away - and the other strong and close).

      Actually let's do some calculations. An NTSC or PAL scanline is about 64µs (52µs visible). It looks to be shifted across at least half of the display, so let's say 26µs, but it's difficult to say exactly how much.

      That makes for a distance of ~7,800 metres (assuming a speed of light in air is the same as in a vacuum). 7.8km is just about far enough to be atmospheric, but too far to be a building, and too close to be a transmitter... that's assuming of course they transmit at the same time, which is probably not the case...
      Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
      For service manual, schematic, boardview (board view), datasheet, cad - use our search.

      Comment

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