15Khz -> 30Khz adaptor, or 30Hkz generator.

[Gherkin]

Hi there,
I have a rather antique computer of which the Video Synch signal is 15Khz way below regular monitor ranges. I'm looking to try feeding either converting or plain out replacing the 15Khz signal with 30Khz and try passing the rest of the Analogue RBG signals straight though.

About 20 years ago I saw a sample of this done.. have to admit the results weren't startling, but it was only an old MultiSync that I tried it on, I'm interested in trying this on an LCD screen though... seeing if its a better result.

G

 

[ronsimpson]

Some LCD TVs have a 15,750 video input. I just looked at several TVs in my house and they should sync to your 15khz signal.
Problem: Your computer probably has H, V, R, G, B while the TV has R, G, B. The sync is inside the green signal.
There is a chance that your Green has composite sync. If not there is a way of combining H+V+G.

I have built 15khz to 30khz converters but many TVs do that so just use a TV.

 

[Gherkin]

Erm well that wasn't what I was expecting, let me check the details, heres the signals I have to play with.

1 gnd
2 red
3 composite sync
5 Green
6 Gnd
7 -5v
8 +12
11 sound <-- never tried or seen anything using that
12 NTSC/PAL Composite Output
13 GND

I'm surprised that a TV will sync that low. But obviously I'm no expert, back in the old days you couldn't get a decent picture out of a TV but obviously things have changed in that department. For what its worth the resolution is 640x200 its a weird not quite EGA resolution with a sync of about CGA... it comes out of an Apple IIgs. My available inputs are VGA or Component.

G

 

[Gherkin]

Sorry 9 has blue on it

 

[ronsimpson]

My "old VCR" has NTSC out on one wire. The TV has a input for that. It works well.
Both the VCR and the TV have several different formats (in hardware) for NTSC. (example: R,G,B)
Now we are talking LCD, HDTV. Not a CRT TV.
The TV has a video scalding function. So a single pixel in "old video" will light maybe 4 pixels on a HDTV.
A TV should do "CGA". I don't know if it will do 640x200. It probably will see 640x200-P as 640x400-I so it might work.

 

[Gherkin]

The composite output leaves a lot to be desired if used on a TV... 80col text is bad, and the graphics are nigh on unrecognisable, hence the RGB... However I will try composite again on an LCD and report back.

 

[Gherkin]

Well better and worse than I expected to be truthful. The text is perfectly usable at 80Col but the graphics don't pass, they're horribly fuzzy.. So I'd prefer to try and use the RGB signals, but the only sync signals I have available are as above. I was considering as i said just considering trying the analogue RGB signals straight into a digital RGB input..While changing the sync signal up a bit..

I wonder if rather than originally considering VGA input as my primary option, I should try component with sync on composite.... I'll have to get my lead collection out and see what I can make.

 

[Nigel Goodwin]

Your computer (presumably a 'home' computer?) is designed to feed to a TV, and if it's at 15KHz (actually 15.625KHz) then it's a PAL output.

Pretty well any European TV with a SCART socket on will accept RGB directly from that socket, and give the best possible results.

What computer are we actually talking about?

 

[Gherkin]

AS per above, its an Apple IIgs, it had a pretty weird spec monitor that came with it, they're nigh on unreplaceable now. And although it has a composite output it wasn't really designed to use it, unless you were only using the thing as an older model. It wouldn't feed into an original nec multisync very well... Unfortunately I'm not in Europe, but the wonderful land of Aus. Others have reported using some kind of adapter initially, feeding it through a scart adapter, and ultimately into an hdmi converter, I wasn't going quite that crazy yet. If I recall correctly and I'm not 100% sure I have to admit, this thing is about 30years old now. Some video modes do not actually contain a composite signal.

The closest native resolution for comparison would be EGA... but that sync rate is more like CGA.

 

[Nigel Goodwin]

AS per above, its an Apple IIgs, it had a pretty weird spec monitor that came with it, they're nigh on unreplaceable now. And although it has a composite output it wasn't really designed to use it, unless you were only using the thing as an older model. It wouldn't feed into an original nec multisync very well... Unfortunately I'm not in Europe, but the wonderful land of Aus. Others have reported using some kind of adapter initially, feeding it through a scart adapter, and ultimately into an hdmi converter, I wasn't going quite that crazy yet. If I recall correctly and I'm not 100% sure I have to admit, this thing is about 30years old now. Some video modes do not actually contain a composite signal.

The closest native resolution for comparison would be EGA... but that sync rate is more like CGA.

Multi-sync monitors only rarely went low enough for normal TV signals, I remember back in the Amiga days (which I still have) they had a (very small) list of multi-sync monitors which would work.

As you're in Aus at least you're 625 line

However, presumably you don't have SCART sockets?, so no RGB inputs on TV's.

 

[AnalogKid]

Since Nigel started it, I won't seem as much of a pedantic twit--that 15,750 Hz actually is 15,734 Hz for NTSC. That is the color subcarrier frequency (3.579545 MHz) divided by 227.5 (the number of cycles of subcarrier in one horizontal line).

ak

 

[Gherkin]

Scart is pretty rare here... probably more likely to find component or svideo. Anyway none of this really helps me very much... I don't have a spare TV, I have monitors coming out of my wahzoo. Which is why I was hoping to insert a higher sync signal into the existing analog data.

The thing I saw back in the dark ages, used a timer of some sort to produce a 30Khhz square wave signal and pop that into whichever pin is the correct one on a VGA socket. Unfortunately my knowledge of design is... well non existant so I was hoping someone would have an idea how to do it. Then I can break out the soldering iron and some vero board and have a crack at it

Oh and when it comes to 15Khz its more like the speed of my overclocked chimp brain. Like everything, I guess they're just handy numbers to round out to. Ahh its 15.659.. its manual doesn't say to much more that looks useful. video bandwidth?

 

[ronsimpson]

I have monitors coming out of my wahzoo.

I bet that hurts.

I designed several monitors for Apple. Back then it was common for a monitor to be non standard.
If a monitor runs at 15xxxhz then it is single frequency! You can probably play with vertical adjustments some but not anything horizontally.
If a monitor runs at 31.5xxhz ( and it is old it most likely is 31.5khz only, but has three vertical modes) OR (it might be milti-sync and can run from 30 to some unknown frequency. 30-40, 30 50khz??)

I was going to suggest a scan converter board but all the ones I worked on are not in production years ago in part because that function was added to many TV sets.
scan converter board

Try this link:
Apple IIgs video

I you are looking for VGS monitors that work at 15khz try these:
Benq BL702A,
Samsung SyncMaster 510N/512N, 710N/712N, 910N/912N (I know the SyncMaster(s) well)
Dell 2001FP
Flatron M1721ATBC, M1921A

 

[Gherkin]

https://www.maximintegrated.com/en/app-notes/index.mvp/id/3327
this was the kind of thing I had in mind to try..... how well it works is another question.

I've seen various mention of GBS-8XXX boards around with associated horror stories, and excellent results I'm unclear what the difference between getting one that works well, and one that barely does the job is. It looks a bit like 8220 is probably the better one.... but I've hung off on that.

The first solution here, looks like the simplest thing to put together that may do the job. I found it from the link you posted, so thanks Fingers crossed, I'll come back with some results.

 

[ronsimpson]

this was the kind of thing I had in mind to try.

This only doubles the frequency of H. Sync. You need to double the frequency of the video also! (this circuit only works with square waves which H. Sync is not)

I don't have graphics on this computer so I will try with text:
Apple video out:
black,sync, black, .....video for line one....., black,sync
black,...... video for line two......,black......

With the 2X circuit"
black,sync, black, ..first half of line one,sync,
send half of line one, black, sync
black,sync, black, ..first half of line two,sync,
send half of line two, black, sync

Probably not clear, You will insert a new sync in the middle of each line. This will cause the monitor to start the next line in the middle of the line. So 1/2 of line one will be on the first line and the 2nd 1/2 of line 1 will be on line 2.
There are other problems but not time .....

------EDITED-----
The way the frequency double works is that on each edge (up or down) it makes a pulse of a certain time. T=RC
Hsync is something like the blue trace. The output (green) is 2X in frequency but nothing a monitor will like.

Video with sync. Time A is sync. Time "52uS" is the video. 15khz sync

VGA video is twice as fast. 32khz sync

 

[Gherkin]

Hmm ok, never as easy as it could be eh... starting to sound like the gonzo boards probably a better bet, if you double the hsync rate but the leave the vsync alone wouldn't you effectively just draw the same screen on two passes instead of just one? Which at the end of the day wouldn't matter because there won't be anything new in between...

 

[Gherkin]

Erm doubled is to much? Sorry I'm not good with this stuff. It would require an accurate length to literally double the data signals? To make the 64US boundaries align?