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Newbie question: Passive Delay Circuits

Discussion in 'General Electronics Discussion' started by SparkyNZ, Dec 1, 2011.

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  1. SparkyNZ

    SparkyNZ

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    Dec 1, 2011
    Hi, I'm somewhat inexperienced in electronics (I've soldered wires, cables, made a few circuits with 555 timers etc). I have an old vintage computer that I've just rewired to with with S-video.

    The problem I have is that the luminance and chromaticity signals are slightly out of phase which results in a little ghosting on my monitor.

    Could somebody please recommend a way in which I could build a circuit that would introduce a delay of around 150 nanoseconds on the signal? I'd want to be able to try adjust the delay slightly too.

    I've been recommended a TZB78-7 IC form Rhombus but quite simply because I only would want one chip, they're not interested in me making an order.. so I'm wondering if there's a low cost way of building such a circuit from good old-fashioned everyday components?

    Thanks
    Paul
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,505
    2,852
    Jan 21, 2010
    About 30cm of coax should do it.

    Are you sure your cables are the same length? If one is longer than the other, or if they are different types of cable, the signal may take longer to get through one as compared with the other.
     
    Last edited: Dec 1, 2011
  3. SparkyNZ

    SparkyNZ

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    Dec 1, 2011
    Thanks Steve. The cable itself should be the same length for both signals. Its actually a modification on an old Commodore Vic20 computer. One signal comes straight out of the chip and passes through a 75Ohm resistor and a capacitor - the other signal comes out of the video socket on the board, so presumably that signal goes through a number of components first.

    So theoretically if were to add 30cm of coax to the one of the signal lines I should see either a further shift or reduction? Does resistance work as a delay or is there some other property of the cable that comes into effect?
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,505
    2,852
    Jan 21, 2010
    It essentially comes down to the speed of light.

    150ns is about 45 cm in a vacuum, but the dilectric constant of the cable slows down the signal pretty much the same way that an index of refraction equates to a slowing of the speed of light fir photons. Thus you need less than 45cm of cable. My guess is 30 cm :)
     
  5. SparkyNZ

    SparkyNZ

    3
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    Dec 1, 2011
    Thanks Steve - I like the explanation too! I'll give it a go tomorrow night once the kiddies go t bed. There shoudn't be a problem using the same existing wire for ground and inserting the coax into one of the signal lines should there? I have TV aerial coax - would I need to use the inner core or the shielding?

    Either way I'll let you know what happens.
     
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