testing video on oscilloscope - RESULTS

[leonk]

I recently acquired a Rigol digital oscilloscope and I want to validate the RGB and sync output of an X68K. This will allow me to report back if any capacitors/resistors are needed on SCART RGB cables

But to do that, one requires a screen thats all white (255.255.255). Is there a 240p like test suite (or recommended way to get a solid white screen output? maybe game?

Please advise. Thanks.

--- RESULTS ---

I had a heck of a time getting solid white, so instead I used the trigger/1 shot feature of the oscilloscope on the insert disk menu.  It starts with white background so it works well.  I used a simple straight through SCART / X68K cable where the H and V were tied together.

Original X68K:
- R/G/B all about 1.3-1.5Vpp
- csync 31k freq - 5.7Vpp

Compact Red Zone:
- R/G/B all measured at about 1.6Vpp
- csync 31k freq - 2.64Vpp

What does all this mean??
- SCART RGB standard is only 0.7Vpp for RGB and about 0.3Vpp CSYNC.  5V CSYNC is TTL and must have resistors to reduce it or else you can damage your display device (XRGB/OSSC). 

Any suggestions on other tests I can do, or how I can modify the SCART cable to reduce the outputs to "safe levels".  Also, why would Red Zone CSYNC be almost 1/2 that of OG X68K?

[kamiboy]

Easiest way I can think of is to view a white image with a full screen image viewer. One of the x68k HDD images comes with one, and I think there are utilities to convert bmp images to x68k format.

[leonk]

Awesome idea!!!  I would do that.

[famiac]

Wouldn't you want to test the white screen at the x68k's different output modes?

[leonk]

resolution is different than Vpp max.

So the answer is no. Resolution only effects OSSC / XRGB / etc compatibility.

[famiac]

Is there a standard peak-to-peak voltage for video signals? 1V?
You would only need to use a cap if there was a DC component to the signal, right?
The stock X68k monitor cable has no resistors or caps inside, if that helps. It would be strange to have a DC-coupled video signal...

[leonk]

RGB/VGA standard is 0.7Vpp for R G and B. 0.3Vpp for sync.

[leonk]

Updated original post with my results..

[NFG]

5Vp-p on the SYNC lines is wildly out of spec, and there's no mention of anything like this anywhere in the world that I can find.

[leonk]

Here's the smoking gun.  Found in the X68000 schematics, right on our wiki here:

RGB signal:  Note the 75ohm resistor and DC blocking capacitor.  So it is impedance matched.
SYNC: pulses come as separate signals,  and pulled up to TTL level from 5V Vcc source via caps before going out.  The line above the VSYNC/HSYNC suggests that it's negative sync.

What does all this mean?  The X68000 output voltage levels is pretty much the same as a PC.  0.7Vpp 75ohm RGB and 5V TTL separate H/V sync.

One must treat it as such.  Making a SCART RGB cable (like for game consoles) and expecting it to work is dangerous.

[LowDefAl]

I'm not an expert on video signals, but is this display dependant? For example I use an eBay scart cable via a Scart->BNC adapter on my Sony BVM-2011p without issues but obviously this isn't a consumer tv.  The specs list 1-8v negative sync on the EXT sync port but I'm mixing the sync with the green (this seems to be required for my consoles).

I've also used it on the OSSC (largely because I killed the VGA port when I killed my first X68's video output) without issue but not necessarily long enough to monitor potential damage over time.

To go back to your comment comparing the OG and Red Zone, could this be due to the different RGB modules?

[leonk]

you got the numbers right.

OSSC and BVM accept TTL (5V) sync (as well as XPC-4). But xRGB mini does not. So don’t buy SCART RGB cables from ebay and connect to xrgb. you’ll cause damage. Also. not a fan of creating csync by combining H and V sync. You should use a proper sync combiner.

[famiac]

5Vp-p on the SYNC lines is wildly out of spec, and there's no mention of anything like this anywhere in the world that I can find.

TTL sync is often mentioned, and some monitors, such as the sony BVM series, accept TTL level (5vpp) sync without attenuation.

The problem is that logic devices aren’t designed to drive a 75ohm load, so a typical monitor would need/like a driver stage.

Good work leonk!

@lowdefAI how are you combining sync for your bvm’s scart to bnc input?

[LowDefAl]

@lowdefAI how are you combining sync for your bvm’s scart to bnc input?

I'm using a T adapter to connect green on one end and sync on the other and disabling ext sync (either through the menu or the control board, I don't recall).