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Creating a composite sync — Parallax Forums

Creating a composite sync

Invent-O-DocInvent-O-Doc Posts: 768
edited 2012-06-10 07:01 in General Discussion
So, I've been hooking up all my old computers again - got the atari 800 running and having lots of fun playing star raiders, archon and jumpman.

My atari 1040ST outputs RGB and has separate HSYNC and VSYNC. (it still uses NTSC timings). I found out my dad threw out my monitors, so all I have are some nice Sony 1271 studio monitors. They take either composite or RGB with CSYNC. (A commodore amiga is going to be on the second monitor)

My question is this - how do I make a composite sync? Can I get away with just tying the HSYNC and VSYNC lines together?

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-06-09 08:29
    Invent-O-Doc wrote:
    Can I get away with just tying the HSYNC and VSYNC lines together?
    No, but you could try XORing them then inverting. HSync has to be present during VSync in the form of positive-going "serrations". This is to ensure that the PLL that controls the horizontal oscillator in the monitor has a constant signal.

    -Phil
  • Heater.Heater. Posts: 21,230
    edited 2012-06-09 09:01
    I did this with my ST years back with only a couple of resistors. I was from a circuit I found in some Atari magazine. Sorry but I don't recall the details.
  • Invent-O-DocInvent-O-Doc Posts: 768
    edited 2012-06-09 13:16
    OK, I did figure out a circuit that would use a 74LS86 as an XOR and then a pull up on a second gate to NOT the output.

    Lo and behold, the maintenance manual for the sony shows a 25 pin computer input (for a sony computer) - It has separate HSYNC and VSYNC there - I hooked everything up and - well, I can't lock vertical hold for some reason. Frustrating. If this bombs, I will end up making a sync combiner, but a DB-25 cable would be easier...

    Thanks for the advice.
  • ForrestForrest Posts: 1,341
    edited 2012-06-09 22:46
    Back in 1986, I was able to connect my Atari ST to a Sony XBR monitor, based on a Atari newsletter article. I expanded on the original article - and attached it to this message. As you can see from the article, 2 diodes and 2 resistors were needed to create csync from hsync and vsync.

    A good source of info on Atari computers is the forums at AtariAge http://www.atariage.com/forums/forum/20-atari-sttt-computers/

    Forrest
    pdf
    pdf
    Atari ST to Sony Monitor cable.pdf
    2M
  • Invent-O-DocInvent-O-Doc Posts: 768
    edited 2012-06-10 07:01
    Thanks for the article forrest! I may go that route. I'm currently hooking it up to the CMPTR connector that has hsync/vsync but the vsync doesn't seem to lock on. The service manual states the VSYNC is "Negative Polarity TLL"

    I'm thinking of hooking up the vsync to an 74LS04 inverter to see if that will work, if it doesn't, then I'll make a cable to hook up to independent RGB and CSYNC on the back. I've read that the diode/resistor combo works without having to make a logic-based combiner. (I have a 74LS86, but it requires a 5V source). I'm fearful that 'negative polarity' means negative voltage and not inverted logic states (and hence the 74LS04 wont work)

    Thanks again.
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