Software Support > GroovyMAME
Testing Composite Blending from PC – VideoAMP + Antonio Villena VGA → Composite/
Zebidee:
So, I went and did it. Then I took a video!
Using the VideoAMP v3.3, I took video outputs from the VGA passthru and SCART heads.
* The VGA output (via VGA M-M cable and GreenAntz v1.94 RGB-YPbPr transcoder) goes to Sony CRT TV with component inputs
* The SCART output (via home-brewed VGA-SCART cable) goes to a recently RGB-modded JVC
Even though VideoAMP v3.3 does not technically support 2x outputs, the picture looks great.
The Sony/component TV looks full, very sharp picture and colours clear and strong. Obviously the GreenAntz has its own internal video amp (THS7374) with the forementioned DC-bias voltage referencing of outputs, which means it will boost the signal even if the inputs are a bit low. The colours may actually be a little stronger than usual.
The JVC/RGB TV looks great too, colours clear and bright. The picture is perhaps slightly less sharp and bright, very slightly, but difficult to notice unless you hot-swap it back to single video output. EDIT: the sharpness and brightness came back fully after some minor adjustment to the color and brightness via normal TV menu.
Why? The JVC likes lower input voltage anyway, and I built 68R resistors into the RGB inputs to bring voltage down to the 0.5vpp it prefers (the JVC's jungle RGB input pins are internal, not standard for external input). These extra 68R resistors on the JVC's RGB may affect the "balance" of outputs from the VideoAMP, giving the GreenAntz/Sony a slightly larger "share" than the JVC. This explains why the Sony is slightly brighter. The two video outputs look great next to each other anyway. Any video output "balance" affects are small, very hard to discern, probably indistinguishable if the the JVC did not have those 68R resistors. The issue should disappear with VideoAMP "Pro" v.5 as it uses separate amps on the outputs.
I call this a success!
Some further explanation of the SCART:
I don't have a SCART-SCART cable, so improvised and used a VGA-SCART (now SCART-VGA) I already have, but in reverse. The VGA then connects to an alternate input I'd already prepared for the JVC TV.
The VGA-SCART cable is one I "rolled" myself years ago. It is extremely simple, all the connections are straight passthroughs, no resistors except a 150R on the blanking. It was designed for use as VGA to SCART, but works just as well for SCART to VGA! I had to swap the sync from pin 20 to pin 19 to match SCART-OUT pinout, but otherwise everything the same. Swapping the sync pins was super-easy because I build my SCART cables using female crimps and slide these onto the flat blades or pins used in SCART headers.
VideoAMP v3.3 does not support Csync input, which makes hot-swapping the unforgiving JVC back to single video output configuration potentially complicated. The JVC accepts pure csync input only, no "smushy" sync from twisting H&V together. I'd have to exit games, swap around displays and reset CRT_emudriver to Csync.
To completely bypass this potential sync-swap drama, I used one of Buttersoft's csync 15khz protection dongles. H+V IN, Csync OUT. Very useful device. Even though it is not used in the final setup, it helps occasionally for troubleshooting, so I included it in the pic below. The dongle is wrapped in paper tape as the 3D-printed enclosure falls apart after a while! Poked holes for the LEDs. I also needed two VGA gender-changers to match my cables on this occasion. Gender changers are attached in the pic.
Zebidee:
--- Quote from: Calamity on September 05, 2025, 03:33:27 am ---
--- Quote from: Zebidee on September 04, 2025, 10:43:35 pm ---This might just be a problem with my setup (Win7, HD6450), so interested if this problem (vertical sync still output when in csync mode) is universal or just affecting me. I posted about it here, but only heard crickets chirping. Maybe I should just ask Calamity directly.
--- End quote ---
Sorry, I probably missed that or got me in the middle of some extra busy period. AMD's native c-sync is barely used by a few users as far as I know. I found it problematic to setup.
--- End quote ---
Oh, truly sorry, I really don't expect you to read all my posts and respond personally. Rather I was surprised that nobody responded at all, lol! I'm sure you are busy enough already. My own bad, posted on the main GreenAntz thread, so it was a bit buried. I should put up a new topic on the GM forum branch. Thank you for responding.
Even so, I'd argue that the issue must affect a fairly reasonable number of people, at different times, even if they don't understand it. If you are outputting to a CRT TV then an option for native c-sync output is better than smushy sync (twisting H+V wires) or some external device, like an extron, or a csync circuit built into a SCART header (advanced!). JPACs invisibly solved the problem for arcade monitor users (you need the JPAC for the JAMMA interface anyway), but not for TV users.
--- Quote --- E.g. if things don't go well it's hard to tell if the lack of sync is caused by c-sync or something else. If things break eventually and you need to do some configuration, you'll be back to separated sync without the proper wires. If you want to dual boot on Linux, no c-sync there. Etc. That's why I find a much better solution to handle c-sync externally.
Anyway, it's interesting what you mention of vertical sync not being turn off. I didn't know that. That's probably because it was assumed that v-sync wouldn't be wired. In fact that's how I connect it to my BVM, leaving the vertical bnc disconnected.
--- End quote ---
Yes - when using Csync, disconnecting the Vsync, to avoid random interference, seems a sensible precaution. I've given such generic advice many times before. With your BVM & BNCs you'll probably never notice the vsync issue. For my own part, I was mostly using Buttersoft's VGA dongle (outputs csync with no vsync) when testing H+Vsync vs Csync autoswitching on GreenAntz, so I missed it for a long time too.
For me, the issue became obvious when I was using native csync from CRT_emulator for the JVC TV RGB mod, and swapping back to the GreenAntz/Sony TV for troubleshooting. The GreenAntz lost sync! At first I thought I'd done something wrong, or some extreme interference on the Vsync wire. Only after hooking up the oscilloscope did I realise that it was more than just random interference, which shouldn't normally bother GreenAntz, but rather a full Vsync signal.
The point I trying to highlight, and you've already mentioned it, is that there are many different circumstances: sometimes we want Vsync connected, sometimes not, but it is not always easy to disconnect like a BNC. You can't just pull the V wire out of the VGA cable, or clip off the pins, or haul out and plug-in a different monitor. At first my only solution was going back to an older GreenAntz unit with a sync switch, until I figured to just "blank" (pull high) the GreenAntz Vsync signal with a wire (really just another kind of switch). Not everyone has those options.
Take SCART CRT TVs, for example. Probably 90% of CRT TVs will work OK with raw smushy sync (twisted wires). Problem with smushy sync is that, during the vertical sync interval, it doesn't invert certain pulses and completely loses many/most of the timing and information pulses (this is also where data like teletext lives!). Even so, most CRT TVs are clever (a better word maybe is "forgiving") enough to deal with this and give you a clear picture anyway. Affects vary from model to model, but many TVs will show a telltale little zigzag near the top of the screen. Anyway, if it works for you, and you're OK with the limitations, twisting H+V together and running that through a resistor is OK. Heck, you don't even need the resistor much of the time.
However, if you are fussy, or have a TV that is fussy, you may need to run with "native csync" and physically disconnect that Vsync line. The only other alternative is to run it through some external device (Buttersoft's convenient dongle to the rescue!). Never say never, but it is something I prefer to avoid, at least for any kind of long-term arrangement.
With a SCART or VGA cable input, disconnecting Vsync can be a lot more difficult than a BNC terminal. If you have a soldered cable, it could be very messy or even impossible. This is one of the reasons I make different VGA-SCART cables for different occasions (with/without resistors, Vsync plugs, auto AV-switching, external power input, audio, for example).
It's also a reason why I prefer to use crimp terminals on the SCART pins - no soldering, more flexible.
Back to the current example, this JVC TV is one of those 10% of TVs that is very fussy about sync! It demands good clean csync. Right vpp levels and no vertical sync weirdness. So I enabled Csync with CRT_emulator and disconnected the Vsync wire. In this instance I just pulled it off and re-crimped the Csync wire. Not a big deal, but no way back without soldering or re-crimping. On some other occasions I've made connectors using dupont (arduino) wires to make reversal easier.
Many of these situational dramas could be avoided if Vsync switched off when Csync is enabled. Most of the time I'd be happy to leave the Vsync wire connected so long as there is no signal on it.
njz3:
Hi Zebidee,
--- Quote from: Zebidee on September 06, 2025, 01:57:11 am ---The "secret sauce" is to AC couple the inputs (put 0.1uF caps in series) and add "pullup" resistors from voltage supply (Vs).
.. snipped your precious explanations that for sure I will save ...
--- End quote ---
First an introduction: Bandicoot and I are not professional electronics engineers. Worth pointing out that what we came with is purely done based on example (manufacturer datasheets), trial and errors.
We are simply two hobbyists that were trying to solve minor issues we have seen with other existing solutions, specifically the fact that no solution provides VGA input monitor detection. For arcade cabinets, there are known solutions like jammaSD, jpac or ultimarc amplifier, and maybe a few other solutions that I don't know of. For TV with RGB Scart inputs, there are UMSA and many DIY solutions which offers more or less the same level of functionalities. The VideoAMP was only a way to "unify" all those functionalities in a single board.
Regarding AC operation mode of the THS: we actually use the DC input/output mode of operation with 5VDC power, as explained in the datasheet p.21, figure 52 p.24 and described on the schematics of the evaluation board Figure 55 p.29 when you set it in DC operation.
We tried the THS7374 first (which is what almost everybody in our community use), and also I remember asking myself whether to use decoupling input (AC input operation). We picked the DC mode because it was simple and does not require noise rejection or any EMC filtering (see p22 about "hum" 50/60Hz noise rejection in AC mode) that might filter the video signal, loosing the "quality" of the edges that you can get from a purely RGB signal. The result was so good compared to the THS7374 that we kept the solution we have tried.
--- Quote from: Zebidee on September 06, 2025, 01:57:11 am ---Please understand that I really don't know much about VideoAMP specifically either, except what I've been told and what I can physically see on the unit (v3.3) I have! I see the THS7375 has a couple of bypass caps for the Vs, but that is all. In fact I am just assuming this TSSOP-14 IC I've been looking at is a THS7375 (the numbers have been scratched off).
--- End quote ---
You are right, the caps are mainly for voltage input and filtering of sync signals.
--- Quote from: Zebidee on September 06, 2025, 01:57:11 am ---Hope this helps. Make a local copy if you think this may be useful. I am aware of sensitivities and may delete all this in a week or two to save it being "harvested" by unwelcome players.
--- End quote ---
Ok, will do.
--- Quote from: Zebidee on September 06, 2025, 01:57:11 am ---I imagine that the video output quality is improved as well, even for a single output - I'd pay a little extra for that alone. So I may just have to get one from you!
--- End quote ---
I can send one "Pro" board to you if you want (I have a few built available) at same price as a standard one.
If we could produce 1000 boards in a single batch, then the price decrease on the BOM would be so high that having 3x THS in DC mode, instead of 2x in AC mode would not even be a question ;-)
So far (and even if I try to make it more PCB assembly friendly), we only buy components by 10x or 20x and each board is still partially assembled by hand which takes time.
Zebidee:
--- Quote from: njz3 on September 09, 2025, 08:43:47 am ---First an introduction: Bandicoot and I are not professional electronics engineers. Worth pointing out that what we came with is purely done based on example (manufacturer datasheets), trial and errors.
We are simply two hobbyists that were trying to solve minor issues we have seen with other existing solutions, specifically the fact that no solution provides VGA input monitor detection. For arcade cabinets, there are known solutions like jammaSD, jpac or ultimarc amplifier, and maybe a few other solutions that I don't know of. For TV with RGB Scart inputs, there are UMSA and many DIY solutions which offers more or less the same level of functionalities. The VideoAMP was only a way to "unify" all those functionalities in a single board.
--- End quote ---
Thank you for that njz3. You and Bandicoot are much like me then. If there's no solution, then you'll make one! The best part about this hobby is learning as you go :D
I do have a general science & technology (including computers & gaming) background going back a long way, but that is mostly high-level stuff, some programming and data work, not hard technical or "real" electronics. More about understanding people, research, ideas and words.
So, much of my real electronics knowledge comes from what I've learned "hands on" with my arcade/retrogaming hobbies rather than years at school or in a technical role.
--- Quote ---... We picked the DC mode because it was simple and does not require noise rejection or any EMC filtering (see p22 about "hum" 50/60Hz noise rejection in AC mode) that might filter the video signal, loosing the "quality" of the edges that you can get from a purely RGB signal. ...
...
The result was so good compared to the THS7374 that we kept the solution we have tried.
--- End quote ---
The choice of AC or DC input mode for the amp is obviously whatever does the job, works best for you. FWIW A/V industry convention for devices is AC-coupled inputs and DC-coupled outputs. This is convention, not rule, and there are many exceptions (e.g. battery powered devices with AC-coupled outputs).
DC mode is simpler, and seen more commonly in our hobby, but AC coupling inputs only requires a 100nF (0.1uF) cap and a pullup resistor for each RGB line.
[REDACTED]
May every day be another wonderful secret!
Peace
PS - I'll PM you soon about the VideoAMP Pro v5!
Rion:
--- Quote from: Zebidee on September 04, 2025, 10:43:35 pm ---
--- Quote --- • Writing a wiki page for the VideoAMP when using CRT EmuDriver setup to make it easier for new users.
--- End quote ---
Great idea. Link?
Sorry for the late reply.
Here is the link https://github.com/Redemp/VideoAmp_wiki/wiki
--- End quote ---
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