Can anyone solve my LM1881 problem?

[Computolio]

   I've got a combo monitor (Multisync 3D) set up with a variety of RGB devices. Out of all of them, it seems that the ones that use component sync have no problems, but all the machines I've set up with an LM1881 to convert composite video/sync into component sync are having some odd things happen to the picture.

   With the Genesis, when I have both the vertical sync pin on vertical sync and the composite sync pin on horizontal sync, usually what happens is that I get a picture that's too "tall". It's twice the height it should be and overscans nastily at the top and bottom of the screen no matter what I do to the geometry controls. Sometimes it'll start out at normal size, but it'll then jump to the wrong size. When I had my PS1 hooked up, I tried disconnecting the vertical sync and found that the picture would flicker erratically between normal and abnormal height. Sometimes it would stay in either position for as long as 20 minutes, then go back to going crazy.

   Meanwhile, my Amiga and Atari ST have never had a problem with the monitor as they both use component sync. My guess here is that my monitor has broken/shitty support for composite sync and doesn't like using composite sync as horizontal sync.

   With that in mind, how do I strip the composite sync signal so that it's just a horizontal sync signal?

[RARusk]

I've never had much of a problem using Composite Sync on my MultiSync 3D. However, I prefer to use Seperate Sync instead. You may want to get your hands on the EL4583 chip or, if you don't mind dealing with surface mount components, the EL1883. Both sync chips have Composite Sync, Horizontal Sync, and Vertical Sync lines. And Intersil (www.intersil.com (formerly Elantec)) will give you some free samples to play with. I am currently waiting on some samples from them (their newest sync chip the ISL59885). These chips should solve your problems.

[viletim]

Seperating Hsync from Csync from really quite simple, no need for esoteric sync chips. The undisirable parts of the Csync are the vertical sync pulses (obviously) and what arecalled 'equalisation' pulses. Equalisation pulses that are pulses that that appear during the vertical blanking interval. They are half the width of a H-sync pulse and run at twice the frequency.

The usual way to seperate out the H-sync is to use two non-retriggerable one-shots (such as a 74LS221). See the sync seperator circuit at http://www.ntsc-tv.com/ntsc-index-02.htm for an explanation.

Below is a practical sync separator circuit I've used in the past.

[viletim]

I should note that the above circuit delivers 'positive' horizontal sync. For negative polarity signal take the output from /Q insted of Q of the LS221.

[Computolio]

   OK! I somehow convinced Intersil to waste money on mailing me free chips. I have a bunch of EL1883s and they are uh, disturbingly tiny, even by surface-mount standards. Sucks that there's no DIP version. Any tips on dealing with these chips that I should hear?

[RARusk]

The way I solved it, as shown in the picture below, is by soldering 30-gauge wire to the bottom of each pin and bending them down. Then I used a protoboard from Radio Shack to mount the chip on placing the wires through the holes. Then I soldered each wire in place. Next I used regular wires on the adjacent holes. Then I cut the section out to save space.

There are also adapter boards that are much cleaner solutions than what I used here. I got a couple from my recent trip to Fry's Electronics in Austin that I plan to use with my newly acquired ISL59885 sync chips. They're made by Capitol Advanced (www.capitoladvanced.com) and the model number for their Surfboards brand is 9081.

[Computolio]

   All right- I've finally thrown an EL4583 in a project box and I'm getting a stable picture from my Genesis (but not my SNES) with the resulting cable. What happens with the SNES is that anywhere the screen has a scanline beginning with white (the brightest white mind you, not light gray), there's horrible and crazy distortion whenever something with a lot of black or other colors enters that area. It's most evident in the Mario Paint title screen- the screen freaks out, but that freaking out is localized within the area around Mario. None of this ever happened with the old (LM1881 based) setup.

   Here's the odd part- the freaking out stops when I short the capacitors in the SCART connector. Only problem is that if I do that, the screen loses all it's contrast and gets horribly washed out.