Adding RGB to a TV

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dj_johnnyg
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Adding RGB to a TV

Post by dj_johnnyg »

OK,
I have been given a **** no-name CRT which has a pretty good picture, but being in NZ it only has composite input. I have read on forums about feeding a TV an RGB signal, but wondered if anyone here had any input...

The chassis is a Hyundai one, and it has 2 main ICs on it (LC863320A & LA76810). I have the chassis schematic, and from what I can see RGB is output from the LC863320A via a couple of resistors (1st one 4.7k in series, the second (no value on schematic) tied to ground) to an RGB input on the LA76810. This is then output from the same IC direct to the neck-board.

So I think I have 3 potential options to feed an RGB signal to the TV, either into the Resistors before the LA76810, after the resistors but before the LA76810A, or direct into the neck-board. From previous reading if I feed the sync/video signal into one of the AV ports and set the TV to that input I should get something.

Does this sound like ****? Don't care if I kill the TV, but if this is going to be a total none-starter, I'll just put up with crappy composite picture & get the light-guns out.

Cheers

John
Snakes: They're like bits of rope, only angrier.
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Asayuki
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Re: Adding RGB to a TV

Post by Asayuki »

Composite to RGB translation occurs in every display that accepts PAL or NTSC (or SECAM) through CVBS.
The problem is not really that of identifying where exactly the conversion occurs.
Rather, the issue is amplitude, impedance and bias of the internally generated RGB and sync signals.
In order to replace the internal CVBS to RGB converter and obtain good results, one needs to mimick its output signals in terms of those parameters.

Now, the Sanyo LC863320A you mentioned is the microcontroller providing the OSD, while the Sanyo LA76810 does CVBS decoding, overlays the OSD signal coming in at pins 14, 15 and 16 onto it and then feeds the finished picture out to pins 19, 20 and 21. Fast blanking also needs to be provided at pin 17 if I'm not wrong, but that can very likely be just tied high or low.

If you want to feed the arcade RGB into the LA76810 as if it was the OSD, you need to keep into account that the LA76810 expects the OSD input to be already synchronous to the signal incoming from the CVBS; that is you have to feed your arcade sync to the LA76810 as well, carefully adapted from 5V TTL levels down to the 0.3V implied by CVBS. The LA76810 does require input RGB signals in the range of 1V max and probably 75ohm impedance, which again requires amplitude and impedance adaptation as arcade signals are 5V 1kohm. The LA76810 will try to clamp to your "CVBS input" and RGB signals, and internal luma gain might or might not create issues, so you really need to build a small circuit.

Bypassing the LA76810 completely would require you to separate your composite sync on your own in order to drive all the sync outputs it generates, which in turn end up driving the horizontal and vertical deflection and blah blah. Not recommended.

Driving the neck card directly imho would be even more complicated because of the drivers there which are probably interfacing to some 200Vdc rail and the base current you need to provide to them.
My 15kHz cabinet Peplos will never power up, with any item, and I am quite proud of that.
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nem
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Re: Adding RGB to a TV

Post by nem »

There's a really great thread on this on the Shmups forum:

http://shmups.system11.org/viewtopic.php?f=6&t=56155

Basically what's outlined in the first post will work in most cases.
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dj_johnnyg
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Re: Adding RGB to a TV

Post by dj_johnnyg »

Cheers Guys.

shmups post looks interesting - I'll have a play & see what happens.

(I predict a trip to the tip :awe:)
Snakes: They're like bits of rope, only angrier.
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