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RGB to sync on green

Discussion in 'Electronic Design' started by Malcolm Reeves, Feb 4, 2005.

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  1. Hi,

    I'm looking at various new TVs etc. and it seems like the TV will take
    component/RGB and the DVR etc can output RGB (component on some but
    not all). However, scart rgb uses the composite for the sync and the
    TV expects sync on green. So I can either buy a hugely expensive
    converter or knock my own up.

    So I think all I need is a LM1881 to get the sync from the composite.
    Since the LM1881 is high input impedance and I'm not using the
    composite then a lash up should not feed back significant noise to the
    RGB lines. The LM1881 sync output is rail so it could be 5 or 12V. I
    only need 0.3V so a 1K2 or 3K for 12V from sync output to G should
    give me the right sync. The high impedance will attenuate any noise
    on signal as well as not disturbing the impedance.

    Sounds too simple - am I missing something?


    --

    Malcolm

    Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
    (, or ).
    Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
    electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

    http://www.fullcircuit.com or http://www.fullcircuit.co.uk

    NEW - www.CharteredConsultant.co.uk - The Consultant A-List
     
  2. Havent done this for years, but FWIW
    it should work, but video impedances are 75R, and the 1881 can only
    manage a couple of mA, so buffer with a inverter (74UC04 I think)and
    you might be able to get away with capacitively coupling to the G sig,
    there are too many funnies to be predictable. You could generate a -ve
    sync with a transisto current source with the correct current into
    75R.You may need to put a subcarrier LC trap to get rid of the burst,
    since the LPF in the data sheet sucks and you will probably get a
    picture shift, since the H phase will have a time difference between
    the composite



    martin

    Serious error.
    All shortcuts have disappeared.
    Screen. Mind. Both are blank.
     
  3. Good points. I need 4mA so too much for the LM1881. The 74HCU04
    sounds good. More than enough there. BTW did you mean the unbuffered
    or just 74HC? I would have thought the HC would be better as I want
    squarewave not digital.

    In terms of space transistors to do constant current is probably more
    than an 04 and resistor. Errors in the 75ohm will give similar errors
    in the sync level so I don't see there is any advantage to the
    constant current method.

    The info on the poor LPF in the LM1881 is good to know made me look at
    other chips. The EL4581 styles itself as an improved LM1881 and it
    has an inbuilt 3 pole filter. That sounds a better option. A bit
    more expensive but not that much. Similar output drive to the LM1881
    so I still need the 04.

    Why would I get a phase shift? I think sync on green just needs the
    composite sync so I was going to use that. I don't care about the
    error in the H output of the EL4581 (LM1881).


    --

    Malcolm

    Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
    (, or ).
    Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
    electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

    http://www.fullcircuit.com or http://www.fullcircuit.co.uk

    NEW - www.CharteredConsultant.co.uk - The Consultant A-List
     
  4. Have a look at the LC cct on
    http://www.techmind.org/vd/vidmk2.html
    half way down the site, best way of minimising delays

    With the const source you dont load the 75R, technically cleaner IMHO,
    as for HC04 or U04, use a socket, see if there is any difference!

    Phase shift, there is more processing in the signal source (DVD) after
    the RGB out to derive the composite, hence, generally some H shift.
    I always find it annoying, but then i'm used to looking for this sort
    of thing



    martin

    Serious error.
    All shortcuts have disappeared.
    Screen. Mind. Both are blank.
     
  5. I think I see what you mean but on the same circuit there is the
    EL4581 which composite straight in and sync straight out which is what
    I had progressed after find out the LM1881 was not so hot. The EL4581
    has a filter inside, and at 3 poles more then the 2 pole LC.
    I was going to argue there wasn't much difference but thinking about
    it although the source impedance is 1200 that's still a 3% error which
    means my green signal would be 3% less than the R and B. That doesn't
    sound very clever! So I'm with you that the constant current source
    sounds a better bet.
    OK, I follow, but not much can be done about it as CVBS is the only
    sync available :-(


    --

    Malcolm

    Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
    (, or ).
    Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
    electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

    http://www.fullcircuit.com or http://www.fullcircuit.co.uk

    NEW - www.CharteredConsultant.co.uk - The Consultant A-List
     
  6. The problem is not really a "gain" problem, its DC offsets, any DC
    offset will cause an unpleasant colour cast, so blacks will be
    slightly green, this is why YUV is prefered for component signal
    distibution. If there is a slight DC offset on the Y, blacks are
    slightly grey, and the highlights will be slightly hotter or colder
    which is far less objectionable.
    On the gain, side gain errors are again far less objectionable in YUV
    than on RGB, which will cause peak whites to have a colour cast.

    This all depends on how and where the TV actually does a black level
    clamp internally. Fortunately I dont "do" TV sets, so its a S.E.P (
    see Douglas Adams)




    martin

    Serious error.
    All shortcuts have disappeared.
    Screen. Mind. Both are blank.
     
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