Re: RGB to sync on green

From: Malcolm Reeves (mreeves_at_fullcircuit.com)
Date: 02/04/05 

Date: Fri, 04 Feb 2005 18:16:07 +0000

On Fri, 04 Feb 2005 18:05:27 +0100, martin griffith
<martingriffith@yahoo.co.uk> wrote:

>On Fri, 04 Feb 2005 15:53:33 +0000, in sci.electronics.design Malcolm
>Reeves <mreeves@fullcircuit.com> wrote:
>
>>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?
>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 *** sucks and you will probably get a
>picture shift, since the H phase will have a time difference between
>the composite

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
 (mreeves@fullcircuit.com, mreeves@fullcircuit.co.uk or mreeves@iee.org).
 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
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