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Author Topic: Dropping C-sync levels for SCART?  (Read 357 times)

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Paradroid

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Dropping C-sync levels for SCART?
« on: October 24, 2018, 08:25:35 pm »
I have to admit, I still haven't revisited C-sync since Calamity kindly updated his suite to deal with this internally (last time I experimented I used WinModelines to set C-sync). I'd like to give it a go for the next current project I'm working on though as I'm sick of building external sync combining circuits.

Since VGA outputs sync at +5v and SCART wants it at +1v, what are people doing to drop the level? Or do you just ignore the issue and feed your TVs +5 and hope for the best?

Adding an inline 300 ohm resistor to the C-sync signal sounds like the thing to do (creating a voltage divider once the signal hits the 75 ohm termination inside the TV).

Just wondering if anybody else has considered this issue and arrived at a solution?
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cyb

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Re: Dropping C-sync levels for SCART?
« Reply #1 on: October 24, 2018, 08:36:56 pm »
On my RGB modded Sony TV I use a 470 ohm resistor on the sync line just to be safe.  It's been working fine so far.

Paradroid

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Re: Dropping C-sync levels for SCART?
« Reply #2 on: October 24, 2018, 09:38:08 pm »
On my RGB modded Sony TV I use a 470 ohm resistor on the sync line just to be safe.

Good to know! So, that would get the sync down to around 0.7 volts. Wish I had an oscilloscope to be able to actually measure what's going on...

Are you feeding the sync in through the existing composite input or feeding direct to the Jungle IC? I've had success in the past with the 'direct to jungle' approach for TVs that don't automatically switch to the AV channel at power-on.
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cyb

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Re: Dropping C-sync levels for SCART?
« Reply #3 on: October 24, 2018, 10:00:06 pm »
I fed sync through the luma pin on the s-video input. I've read using composite for sync on this particular tv will cause the picture to be shifted way off center. So far off center, that it can't totally be corrected in the service menu.

Paradroid

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Re: Dropping C-sync levels for SCART?
« Reply #4 on: October 25, 2018, 05:56:57 am »
Worked a treat!

Used a 330R (no 300R resistors on hand) to cut down the sync level. Took the 5v line from the VGA output to use for blanking on the jungle chip. Trim pot to drop blanking signal to 1.5v.

Was so easy to get this going with the VMMaker switch. Way easier than my first experiments with C-sync. Thanks Calamity!

Chuffed! :)

Sent from my SM-G955F using Tapatalk

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Calamity

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Re: Dropping C-sync levels for SCART?
« Reply #5 on: October 25, 2018, 06:03:58 am »
Very clean job, congratulations!
Important note: posts reporting GM issues without a log will be IGNORED.
Steps to create a log:
 - From command line, run: groovymame.exe -v romname >romname.txt
 - Attach resulting romname.txt file to your post, instead or pasting it.

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Zebidee

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Re: Dropping C-sync levels for SCART?
« Reply #6 on: October 25, 2018, 01:28:59 pm »
I've made dozens of SCART cables for even more TVs, and never had a problem with just twisting H&V together and feeding that to SCART sync. No resistor on sync.

So long as the sync polarities are the same (ATI/AMD cards always put both out as negative by default) you don't actually need any sync conversion as the signals/waveforms do not interfere with each other.

I have often used a 75ohm resistor inline to SCART pin 16 (blanking) to create a voltage divider (with the existing 75 ohm termination) and cut the 5v blanking to about 2.5v, as some TVs are fussy when the blanking voltage is too high. With some VGA-SCART cables I even did away with 5v inputs altogether, just bridged pin 16 (blanking) to pin 20 (sync) in order to get blanking (this only works with some TVs, maybe 50%).

Note that 5v is also available from VGA pin 9 on cards that support it (ATI/AMD ones do afaik), although beware that many VGA cables don't even bother to connect pin 9.

I've used some of these various cables with different TVs (and some the same TV for a long time) for about 12 years now, all is good.

With more experience now, I would likely take a different, more conservative view about sync voltage levels and combining H+V for RGB modding (those inputs are very "raw") and VGA-component conversion purposes. ~300ohms inline on sync would bring sync down to about 1v, but I've seen people use up to 1k in other projects.

Note that SCART sync levels are actually only 0 - 0.3v peak-peak, so 1k resistors make a lot of sense (brings sync voltage to ~0.35v max). Pin 20 is "rated" to 1v on pinouts because it needs to be able to deal with both composite video & sync on the same pin
« Last Edit: October 25, 2018, 01:33:04 pm by Zebidee »
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Paradroid

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Re: Dropping C-sync levels for SCART?
« Reply #7 on: October 25, 2018, 03:47:50 pm »
So long as the sync polarities are the same (ATI/AMD cards always put both out as negative by default) you don't actually need any sync conversion as the signals/waveforms do not interfere with each other.

Hmm... so why do you think that Extron, UMSA, etc. bother using logic gates to combine the sync signals? I mean, in theory, that approach sounds more robust but I don't fully understand the reason for not using your approach (especially if it works).

I do know that direct insertion to the jungle chip can be fussier than a SCART input sync input (the old transistor sync combiner I used to use never worked).

Either way, no reason not to take a properly formed C-sync straight from the video card now that Calamity's software supports it.

Note that 5v is also available from VGA pin 9 on cards that support it (ATI/AMD ones do afaik), although beware that many VGA cables don't even bother to connect pin 9.

Yep. That's exactly what I used. Pretty handy.

Note that SCART sync levels are actually only 0 - 0.3v peak-peak, so 1k resistors make a lot of sense (brings sync voltage to ~0.35v max). Pin 20 is "rated" to 1v on pinouts because it needs to be able to deal with both composite video & sync on the same pin

Thanks. Will investigate this!

I realised last night that my Fluke DMM actually has a 'max' function. That should be able to tell me the voltage of the sync pulses after the voltage divider (without having to visualise with an oscilloscope).
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Zebidee

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Re: Dropping C-sync levels for SCART?
« Reply #8 on: October 25, 2018, 07:18:38 pm »
Hmm... so why do you think that Extron, UMSA, etc. bother using logic gates to combine the sync signals? I mean, in theory, that approach sounds more robust but I don't fully understand the reason for not using your approach (especially if it works).

Mostly because sync can also come with a positive/negative mix. Also, the incoming sync could be at various voltage levels, depending upon the source, yet still be converted/output at a consistent voltage etc. The creators of those devices would want to accommodate those variations in the interests of consistency, so that they can promote/sell their devices without caveats.

Maybe we're never meant to admit to such weaknesses, but I understand electronics sooooooooo much better than I did back when I started this hobby (which is a really good thing :D), so I now know that making a sync converter is not really that difficult so long as you have the parts available (as many electronics hobbyists & professionals probably would). But when I was a n00b, it was all a bit scary (and unnecessary) so I chose the much easier option (twist & combine H+V sync, negative polarity on both in video card), with due care.

Quote
Either way, no reason not to take a properly formed C-sync straight from the video card now that Calamity's software supports it.

Yes, quite true that. I probably should click the c-sync box in VMMaker or wherever it is, even though I don't expect to see a difference in my image output. Note that it'll still be at max 5v levels!

I realised last night that my Fluke DMM actually has a 'max' function. That should be able to tell me the voltage of the sync pulses after the voltage divider (without having to visualise with an oscilloscope).

You can get a rough impression of video/sync pulses voltage range by just using a $5 multimeter from Bunnings, although this does also require you to use patience (and a little imagination) and is clearly lacks the precise accuracy of your fluke DMM or esp a oscilloscope.

One of these days I'll find a cheap ossc, or even learn how to make one out of an old CRT.
 
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Paradroid

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Re: Dropping C-sync levels for SCART?
« Reply #9 on: October 25, 2018, 09:17:46 pm »
Note that it'll still be at max 5v levels!

Yep... original point of the thread. ;)

You're right about the sync levels.

Will try knocking the 5 v output down even further (closer to 0.3v). Fits with what you discussed here.

One of these days I'll find a cheap ossc, or even learn how to make one out of an old CRT.

Haha! Would be quite happy to defer to an LCD when it comes to test equipment! ;)
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Zebidee

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Re: Dropping C-sync levels for SCART?
« Reply #10 on: October 25, 2018, 09:40:31 pm »
One of these days I'll find a cheap ossc, or even learn how to make one out of an old CRT.

Haha! Would be quite happy to defer to an LCD when it comes to test equipment! ;)

I think the hidden value of a CRT ossc is the coolness factor  :afro:, but yes practicality & price wins over coolness.

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