Has anyone made this adapter?
Apparently it merely pulls the NTSC composite signal from the video port (as well as the audio signal from the same port)... is that it??
Word is that the output quality is noticeably improved over the regular composite... why would that be?
So many questions...
Looking at the schematic, the video from the different connectors comes from two different pins from the video chip. And the built in composite jack has an inductor inline, and a small 330pf bypass capacitor. Compared to the pin that goes to the RGB connector, it is untouched.
With out a data sheet of the video chip (which actually looks like some kind of SIP IC, but not an IC - more like multiple components in a ceramic shell), it may be hard to tell exactly what the difference is. Could have something to do with them expecting the composite signal from the TGB port to be modulated, as that's what 99% of the time that port was used for.
But maybe someone who understands the video circuit better than me has a general idea.
I built a similar connector. I pull composite video from the video port along with +12V to power my small "Night Owl" LCD monitor.
I have not noticed any difference in the video qulity of either output.
Someone prove me wrong, but I think that if there are claims made about video quality from one port over another I would be suspious of that seller peddling snake oil.
I agree. I also built the adapter to feed video, power, and also audio into my Night Owl display.
And PLEASE, let's stop referring to the //c Video Expansion Connector as an RGB Port.
Alexander Jacocks said a few times during his K-Fest 2020 presentation that the output from the adapter in question is "considerably" better than normal composite, but we don't get a side-by-side comparison in the video. I assume he's an impartial judge:
I suppose there could be a few variables that would impact perceived differences, like the display being used, but it is interesting that the signal going to the composite connector is on a a different line than the one going to the video expansion connector, as @nick3092 observes (I didn't expect to see that on the schematic).
Do we know much about the VID hybrid?
I'll wire up a quick adapter to see what I see.
I don't think the two outputs from the VID hybrid are different in any way (but it would be great to check if you can).
The RCA jack is fed through a low pass filter however and that could have an impact on the "perceived" image. It might remove ringing that would be undesirable, especially if feeding an RF modulator. While the filter provides a "cleaner" signal, one might argue that any ringing could mimic a peaking filter (otherwise known as a sharpness enhancer). Some images, for example 80 col text, might benefit from this effect. But on some displays it might actually look worse.
A first test on a CRT TV and on an older LCD TV and... no difference between the two. None. Not even worth showing a comparison.
So, either this thing is being oversold, or there's something more to this adapter than just pulling off the NTSC signal, OR the screens I've tested it on are not able to get the best out of the signal.
My test for screen adequacy and ability to render properly is to fill the screen with uppercase "A" characters.
] 10 FOR I = 1 TO 1920: PRINT "A";:NEXT I
If it can display repeating A characters in 80 column mode with no character distortion then the screen is good and syncs up properly to the video signal.
You can also repeat with uppercase "O". Many (I'd say the majority) LCD screens cannot pass this test. Especially cheap VGA LCD monitors with a VGA scaler.
With 40 column text and graphics, most screens are perfectly fine, though.
I'll try that out.
And I should clarify just a little: the testing I had done was with Lode Runner, taking reference photos of the load screen and demo. I did two tests on each display, composite and the adapter from the video expansion connector. So when I said there was no difference between the two, I was referring to the signals (not the display).
These are not 'monitors' properly speaking, I know. I actually don't have a color monitor that accepts composite signals here... I suppose I should get that sorted.
So what were you using for a "monitor?"
Were you looking at the signals on a scope? If so, what bandwidth? And was there any difference in the overshoot/undershoot of the signal?
I had my Apple //c+ on the bench, so I decided to take a couple scope pics. Mind you, the c+ has a slightly different filter on the composite out and this was not a perfect lab setup. But this is what I found:
Upper trace is composite out from the computer via coax to 50 ohm termination at the scope.
Lower trace is NTSC out from the Video Expansion connector (via 10X probe). Note the faster fall time and slight over/undershoot to the signal which could make for a "sharper" look.
However if this pin feeds a monitor with 50 ohm input, things change quite a bit. When I added a 50 ohm resistor to ground at the probe, I saw this:
So while not as "analog accurate", the faster slew rates and even the over/undershoots might actually make it look sharper on many monitors!
So in the case of this particular IIc+, the composite pin in the DB-15 video port actually has a worse signal than the round connector.
With a noisy and unfiltered video signal causing artifacts that some might perceive to be enhanced quality.
It would be a good exercise to view that signal comparison on a non-Plus IIc.
I suspect the regular //c would give similar results.
So what were you using for a "monitor?"
Well, as I said, a CRT TV and a LCD TV... which I don't define as 'monitors'... displays, sure, but to me a monitor in this contex is something designed to be used with a computer... I have several monochrome 'monitors' that will receive composite directly, but no color ones. Perhaps this is a distinction without a difference? It certainly seems to have caused some confusion, so I apologise for that.
So, I wasn't measuring the signal on a scope... but that would have been a great idea, and thank you for doing that as it's very interesting and relevant information. It was ust a visual comparison. When I was referring to the signal, I was merely trying to clarify that when I said I did two tests, I was testing from both the composite connector and the video expansion connector on each machine. Again, it seems to have caused some confusion. Le sigh.