How do you make a Sync combiner for cheap? Need it for some VGA devices.

How do you make a Sync combiner for cheap? I want to use my old Windows 98/MS-DOS 6.22 rig and my Dreamcast via RGB SCART, but in higher resolutions. How do you combine the H-Sync and V-Sync signals into one C-Sync signal? Can someone give me a parts list and instructions? I've been having trouble find good info with a parts list online. I know there are Sync combiners available for purchase online, but I can't afford them right now. I would like to be able to build a simple circuit that does the job.

 

This is what i built into my Dreamcast VGA box, works fine with the framemeister at 480p.

Needs these parts:
1x 74LS86
2x 2.2k Resistor
2x 22uf Capacitor

 

Did you see this page?: https://web.archive.org/web/20070601235545/http://www.tkk.fi/Misc/Electronics/circuits/vga2rgbs.html

Looks easy enough to build...

 

I did not see this page. It is a little confusing with all of the different diagrams but I'll figure it out (I'm already starting to). Thank you!

I'll post back here in a couple weeks with my results.

 

I don't understand why you need a sync combiner. If your SCART display works with >15 kHz video, I'd think it would expect YPbPr, not RGB.

Anyways, you can't easily make a real C sync signal from discrete H and V syncs. It would take a rather complicated circuit containing a PLL, CPLD and passive parts.

If you don't need a true C sync signal (can only be used on displays with robust sync reconstruction circuitry inside--otherwise you get distortion at the top of the screen), you simply need an XNOR gate and passives. The circuit linked above is an example of such, but isn't all that well designed; even with a low impedance outputs it's still technically off spec.

First it'd be better to use a 74HCT86 chip because then you can ditch the silly transistor output stages in exchange for a simple series 500-1k ohm resistor. A better output driver would be a couple of smaller resistors forming an impedance matched voltage divider, but it would require another less-common 5 V logic family with more output current and some resistor math I don't feel like doing now.

 

I'm using a SCART/HDMI to HDMI converter and I'm not sure if it accepts 31 KHz RGBS. It can do S-Video over SCART and it recognizes DVI-D to HDMI cables as a DVI input and displays it as such. I haven't tried component video yet because I don't have an adapter to try it with. I might just make one with my spare SCART cables.

So where could I get a 74HCT86 chip? I would really like to build a sync combiner instead of buying one. How much cheaper would it be to just build one instead of buying one?

 

>15 kHz RGB is typically delivered over RGsB or "RGBHV", not RGBS (or RGB+CVBS in SCART's case).

Almost anywhere. Building an XNOR combiner should cost maybe US$5 with a piece of prototyping board. I have no idea about buying one, but I can almost guarantee any that you see for sale--especially hobbyist/enthusiast devices will use the same low-quality method.

For a sampling device like a HDMI converter, it'd really be best to use a real C sync signal, since the converter must cleanly separate H sync to generate the sampling clock. This cheap method of sync combining screws up the whole V sync portion of the signal, and will throw off the converter's sampling clock and HDMI output clock. The chip used in the converter will have a little resilience to this, but compatibility shouldn't be expected.

 

I wonder if my converter supports RGsB. I think I will cut apart the SCART cable I have lying around and make an adapter. My PS2 supports RGsB so I can test it out using component cables set to RGB mode.

As for using real C-Sync, can the Dreamcast output H-Sync, V-Sync, and C-Sync at the same time? If so, can this be used to get 480p over SCART or would it only be 480i or less?

Do you have a better schematic and parts list for building a sync combiner or would the link above work fine, assuming 31KHz over SCART is compatible with my converter. If the sync combiner fails but RGsB ends up working on my converter, do you have a good schematic and parts list for converting RGBHV to RGsB?

My HDTV lacks a VGA input so I can't use my Dreamcast and my childhood PC rig on it. That's why I was looking into converting RGBHV to RGBS or RGsB (if supported). The HDTV does not accept RGsB over Component video but does support 240p over Component video.

I'm sorry if I ask too many questions. Trying to find info on all of this was really hard so I came here for help.

 

I've read that the Dreamcast can't output C sync at the same time as H/V sync. This is probably because the C sync pattern is fixed in hardware and not derived or generated from the actual H/V frame timing.

Whether the above circuit will work entirely depends on your hardware and display--I have no idea.

I think it's safe to assume though that the DC uses only negative sync, so you can pare down the circuit to an XNOR gate made by an XOR, followed by another XOR with one input tied to VCC/VDD. Again, if you use a 74HCT86 chip you can swap out the transistor section for a single resistor.

Converting RGBHV to RGsB is requires a fast-ish video amplifier and comparator. I would venture to say you'd have to look in app notes for a schematic, but the design wouldn't be very beginner friendly to build and definitely not to troubleshoot; it's expected that application engineers can implement the designs themselves (or pick IC solutions) so discrete design schematics are pretty hard to come by at a professional level.

Also this is a lot of effort to test non-standard uses of a SCART input.

 

As far as I remember this is working too...
http://members.optusnet.com.au/eviltim/vga2arc/syncadd.png
A cheaper and and easier to build passive circuit...

 

Remember it working for what? That "combiner" is a pseudo AND gate and there's no regard for signal levels or output impedance. Whether it "works" or not is entirely dependent on how well your display/converter can recover from and compensate for malformed sync signals--a very difficult task that displays still struggle with today.

An AND gate will have extremely bad performance (worse than an XNOR gate) because H sync pulses will be gated/lost during the two lines of V sync each field/frame. In many cases a CRT can recover horizontal lock before visible video starts, but with a HDMI converter the sampling and output clocks may instantaneously change which will affect the HDMI receiver/display in some way.

 

Isn't the Toro DC VGA box essentially already doing this? : http://www.beharbros.com/#!toro/cfom

Does anyone have one? Anyone know how it tackles this problem?

 

This is the simplest circuit to build. It works well on my Trinitron TV.
Obviously you won't be feeding this into an upscaler or any other modern digital device

 

Wow this thread is old! I completely forgot about it. Anyhow I no longer need the sync combiner. I was given a Dell ST2210b 1080p monitor by my grandmother in November and it supports 240p over VGA. I also modded my Dreamcast VGA box for RGB/VGA mode selection instead of TV/VGA mode selection. I was given an NEC MultiSync A700 too and it supports C-Sync over VGA.

Thank you everyone for the help though.

 

This circuit didn't work for me but the first posted image works great and has been floating around the net for some time. I wish someone would sell a prepopulated board of it though, so I wouldn't have to keep making them (it still takes time).

-Segasonicfan

 

Oh, sorry for digging up an old thread. I didn't notice.
It's still valid though, as the easy method is really hard to find.
(Even if it doesn't work for everyone, apparently ;p )