Limited batch of new S-Video Adapters.

[atomicburner]

Hello Bryce, item arrived, fantastic, you made me a very happy man.


Thank you very much

[Bryce]

Glad you like it. Have you tried it out yet?

Bryce.

[atomicburner]

Yes Bryce, its fantastic, my dream finally come true, because of you.


Once again, thank you very much.

[Bryce]

The Amiga has a very clean RGBS signal that strictly adheres to the standard, so the S-Video picture is really good/sharp with my adapter. The ST gives great results too. The Spectrums aren't good at all. The CPC sometimes gives a slight shadow on the text.

Bryce.

[Gryzor]

Ooh, how do you work it with the ST?

[Bryce]

By connecting it to the appropriate signals??

Bryce.

[Gryzor]

Reeeeeally? How didn't I think of that


Now, looking at the ST pinout I can see the R/G/B, H/VSync, but 12V (and, interestingly, the 520 has no GND?). Do you do it internally?

[atomicburner]

Hy Bryce, I've got a slight shadow on the text in the WorkBench , but nothing that really matters.



In the games we don't see it.


Cheers

[Bryce]

Yup, on the ST it's easier to do internally, but possible externally with a regulator. No GND? Really? How about Pin 13

Bryce.

[Gryzor]

Er... wtf, I swear when I was looking at them 13 wasn't there


And actually the 520STF also has GND on pin 8, too.

[Bryce]

Yup, but if you want your expansion to work on all STs use 13.

Bryce.

[atomicburner]

Hi Bryce, it is possible to connect my dreamcast to this video adapter? Or other consoles?


If so, can you make a diagram?


Cheers

[Bryce]

Anything that has a half decent RGBS output and a 5V source will work with it.

Bryce.

[TotO]

But... The Dreamcast already allow to out Composite. What is the interest? 

[Gryzor]

I was going to post this. Doesn't it come with SCART by default?

[TotO]

RGB Scart by default in France.
May be Composite RCA in other countries?
And VGA for me. 

[Bryce]

But... The Dreamcast already allow to out Composite. What is the interest? 

S-Video is slightly sharper than Composite, but I'm not sure I'd go to the bother of adding S-Video to a device that already has composite.

Bryce.

[TotO]

The Dreamcast can out S-Video too.
It natively output all the analog video standard from its AV connector, w/o need of any electronic.
Composite, S-Video, RGB, VGA (640x480p).

[Bryce]

Hi,
     sorry for digging up a relatively old thread, but I've been asked quite often why my S-Video adapter (or any RGB to S-Video / VGA etc) doesn't work on the Spectrum +2/+3 although they also offer RGBS just like the CPC. My answer up to now was "I dunno", so it's been on my to-do list for quite some time and today I finally got around to checking it out. For those who haven't tried it, this is what you usually get on the screen if you try to connect the Spectrum via RGB to an adapter:

[attach=2]

I was hoping that this was just a slightly off frequency sync signal, maybe just the negative pulse not going low enough or the pulse length too short/long. So first off, what does a proper sync signal look like. Here's the CPCs sync signal:

[attach=3]

Mmmmm, nice clean sharp signal there with all the correct parameters. Let's take a look at the Spectrums sync:

[attach=4]

Bloody hell, it's extrememly noisy, but the frequency, length and levels are near enough not to bother my adapter, which is quite forgiving on most parameters. For those who've noticed: The positive level is slightly higher than it should be in this screenshot because I was measuring on the ASIC side of R34. I could reduce that noise easily by adding a 1nf capacitor between Sync and GND, but that noise shouldn't cause a problem.
Then I took a look at "the big picture" and what the hell! There's a sync dropout every 20ms! ie: The sync is going low and staying there for 250µs (a normal sync pulse is around 4µs). This is what the dropout looks like:

[attach=5]

The normal sync signals can be seen on the left and you can see how long the dropout is in relation to them.
Unfortunately, this dropout is coming directly out of the ASIC, so there's no easy fix to this. The ASIC designer must have seriously messed something up on his timing. This is causing the mess on the screen, so without recreating a completely new sync signal from scratch, the Spectrum is just going to have to stay on CRTs for now.

A possible solution would be to create a sync signal from a seperate clock, but it would have to be pretty accurate and would have to be triggered from one of the "good" syncs from the ASIC, otherwise it would be out of phase. Any other suggestions are welcome?

Bryce.

[pelrun]

Every 20ms? That's 50Hz. Perhaps this is the old-style 'block sync' type signal with the VSync and HSync ORed together instead of XORed. That would explain why a CRT can handle it.


http://what-when-how.com/display-interfaces/standards-for-analog-video-part-ii-the-personal-computer-display-interfaces-part-2/
Chapter 3. Building Blocks of a Video Format

[chinnyhill10]

I've had a sod of a time trying to capture the RGB from any of my three 128k Spectrums (two +3's, one Plus 2). Either get a rolling picture or a black screen with a very very faint black and white picture on it. That's why with ChinnyVision I've been stuck on composite from a 48k Speccy.

[Bryce]

@Pelrun: No, it's not quite block-sync, if it was, it would return to 5V at the normal sync intervals, but it doesn't do that, it stays low completely:

[attach=2]

As you can see from the following screenshot, the TEA2000 used to produce the TV Out isn't happy with the signal either. The blue channel is the composite output taken where it goes into the modulator:

[attach=3]

Here as a comparison is the CPC blanking signal which is a proper block sync:

[attach=4]

@Chinnyhill: If I come up with a hack I'll send you one for testing.

Bryce.

[pelrun]

You've obviously got a very different idea of what 'block sync' means; the links I posted (look at Figure 9-7a in the first link, Figure 3-8 in the second) define it exactly the same as the spectrum signal - the hsync pulses are blocked by the vsync pulse.


The waveform you call 'block sync' in your image is what I would call 'serrated sync', and the SGI document calls 'commercial sync'.

[Bryce]

Ah sorry, first off, I mis-read your first post and yes, I have always considered serrated sync to be block sync (learn something new every day). I didn't realise that the was a sync that permanently stayed low (TV signals aren't my strongest point ). Either way, digital equipment and modern adapters don't seem to be happy with a block sync, but I haven't found a way inside the Spectrum to get it back to serrated as they are being ORed inside the ASIC.

Bryce.

[pelrun]

Yeah, all the video processor datasheets I just read had text similar to "some video sources have this non-standard sync pattern which will cause problems but we just send it through unaltered".


You'd need a PLL and some assorted logic to rebuild a good sync signal from the block sync, which could probably be implemented in a CPLD.