Getting TTL Sync from a Minigun Supergun

[evan1]

Hello everyone,

I'm using a CXA1645-based encoder to convert the output of my minigun to my tv. It requires a TTL sync while the RGB signals are 75ohm. Right now, I just have a jumper wire coming out from the jamma harness itself to get the sync from but I'd really like to make a cleaner setup and have switchable sync levels coming through the mini din connector. I've taken a part of the design from the picogun that outputs ttl sync with a 74hc14 buffer and used a DPDT switch to toggle between them. This is what I had envisioned. Excuse my use of MS paint to draw the schematic. Will this work? Are there any issues with this design? I'll have to cut some traces and tack a 74hc14 and switch onto the bottom of it, but it's much better than having a wire coming straight from the harness.

 

[evilsim]

the only way to guarantee TTL sync (since not ALL arcade boards use TTL sync(??) - afaik) is with a sync stripper - even the method you are using which will combine the H and V sync, you cannot be SURE its TTL sync without a sync stripper. you are also using a 470ohm resistor after the sync combiner, so that would lower the sync to level more compatible with SCART standard etc.

Just my thoughts, I think only way to know you have TTL sync is to use a stripper/cleaner like LM1881.

 

[Raph_friend]

Try a isl59885.

 

[evan1]

the only way to guarantee TTL sync (since not ALL arcade boards use TTL sync(??) - afaik) is with a sync stripper - even the method you are using which will combine the H and V sync, you cannot be SURE its TTL sync without a sync stripper. you are also using a 470ohm resistor after the sync combiner, so that would lower the sync to level more compatible with SCART standard etc.

Just my thoughts, I think only way to know you have TTL sync is to use a stripper/cleaner like LM1881.

Huh? The THS7374 assumes that the sync level is at TTL and so do all superguns. Why would we need to strip it then combine it again just to be sure it is at a TTL level?

 

[evilsim]

if you use the stripper you do not need the combiner. if you use the combiner you need to have H and V sync available in order to combine them.
i dont know if the ths assumes TTL, it doesnt care what sync or signal with sync in it which you throw at it eg (Y, CVBS, sync-on-green or TTL).
all i was saying is, if you give a sync stripper any sync signal in (almost) any condition it will output TTL sync for you.


@Raph_friend thanks for your message I think I will try one of those on a Ivan Stewart Super Off Road board I have which outputs a slightly off standard 15khz signal.

 

[evan1]

if you use the stripper you do not need the combiner. if you use the combiner you need to have H and V sync available in order to combine them.
i dont know if the ths assumes TTL, it doesnt care what sync or signal with sync in it which you throw at it eg (Y, CVBS, sync-on-green or TTL).
all i was saying is, if you give a sync stripper any sync signal in (almost) any condition it will output TTL sync for you.


@Raph_friend thanks for your message I think I will try one of those on a Ivan Stewart Super Off Road board I have which outputs a slightly off standard 15khz signal.

Ahh ok gotcha. But in most cases, and considering that the picogun uses the 74hc14 buffer for it's TTL sync buffer, I feel like the boards that have wonky video are very rare and probably not something I'm going to be playing with anyways. Shouldn't the solution in my OP be serviceable with most boards?

I'd also really like to use my existing part supply rather than ordering more stuff from digikey heh. I already have everything to modify my minigun into the design above.

 

[evan1]

if you use the stripper you do not need the combiner. if you use the combiner you need to have H and V sync available in order to combine them.
i dont know if the ths assumes TTL, it doesnt care what sync or signal with sync in it which you throw at it eg (Y, CVBS, sync-on-green or TTL).
all i was saying is, if you give a sync stripper any sync signal in (almost) any condition it will output TTL sync for you.


@Raph_friend thanks for your message I think I will try one of those on a Ivan Stewart Super Off Road board I have which outputs a slightly off standard 15khz signal.

Could I use an LM1881 in a design like this?
https://www.retrorgb.com/syncstripperchiponly.html

So instead of the dual 74hc14 schmidt triggers, you just put this in place of it and still have the DPDT switch in place? Would the switching of inputs/outputs cause any issues?