Pure JAMMA sync on AV3 (VGA)

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  • November 29, 2018 at 8:05 PM #24043
    bringbackyun
    Participant

    Community,

    I purchased an OSSC over the weekend and, now I’ve seen just how good 240p lx5 is, I’m looking to hook some of my arcade boards upto it.

    I built my own supergun a number of years ago which uses pots on the RGB lines and pure sync through a JAMMA connector. This worked with my previous TV for many years – a blissful ignorance. It is only during research about the OSSC that I’ve found out that this is a very bad idea, and people would typically use a resistor on the sync line to reduce it down to a safe level!

    Looking on the wiki I can see that AV3 using RGBS requires TTL-level sync, which as I understand it is what JAMMA puts out. I have the materials lying around to rig up a SCART to HD-15 lead. My googling has only seemed to take me so far, so I wanted to ask a few questions about this and get some definitive answers before I take any action:

    Will using JAMMA sync into AV3 without a resistor damage or reduce the life of the OSSC?

    If not, do people typically avoid going this route because of the absence of a LPF on AV3?

    Is there a resistor that I can use on the sync line which will play nice with both AV1 and AV3?

    Thanks in advance!

    November 29, 2018 at 8:47 PM #24049
    nmalinoski
    Participant

    If the sync off the JAMMA connector is 5V TTL or less, it should be fine; but the video output, depending on its fidelity, may benefit from the low-pass filtering provided on AV1/SCART, so don’t entirely discount a SCART lead with properly-attenuated sync.

    If 75Ohm sync really doesn’t work on AV3, then I’m not sure that you can have a one-cable-fits-both solution, since properly attenuating sync to bring it into spec with SCART will bring sync out of spec with VGA.

    Since it sounds like your homebuilt supergun has a SCART output that outputs TTL sync, I think it would be best to attenuate sync there, before it hits the SCART socket. SCART devices can’t handle TTL sync without risk (guarantee?) of damage, so it makes no sense to have a SCART socket with out-of-spec output. Do that, and you should be able to safely connect it to the OSSC or any other SCART equipment with an off-the-shelf male-to-male SCART cable.

    November 30, 2018 at 9:26 AM #24078
    bringbackyun
    Participant

    I think you’re right, I think it makes the most sense to attenuate sync at the SOCKET socket. I’ll look to do this when I iterate on the supergun in the future.

    In the short-term I’ve rigged up a SCART to HD-15 lead which I’m running into AV3. It works and the picture is fantastic.

    Thanks for the information.

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