Trying to separate v-sync and h-sync from composite video in RGBs device

Thread Starter

asprinwizard

Joined Jan 1, 2019
26
Hi,

I have built an emulated Atari ST and am trying to get it working with contemporary hardware. To this end I am attempting to recreate the 13pin din monitor socket and the idea is to have a switch to go between the low/med and high modes. I have a VGA to CGA conversion board that outputs RGBs and composite video. I am trying to get this so it works with the SC1224 and compatible monitors which require RBGHV not RGBs. I don't have this monitor at the moment so I'm using an NEC Muyltisync E222W monitor which apparently supports 15Khz. My plan is to use either a GS4981 or an EL4583 sync splitter to extract the V&H sync for this purpose. The GS4981 is on order so until that arrives I'm testing with a LM1881. I've been told the csync output is similar enough that it could at least be used in place of the Hsync for testing at least. However I'm getting some odd results with both chips.

I have hooked them up as per the suggested circuits. Composite video through a 1uF capacitor to the video input. Another 1uF capacitor connected in parallel with a 680K resistor between Reset and Ground and a 470ohm resistor on the csync output. With this I can get a lovely stable RGBs image through the scart cable on my old CRT TV. So far so good, csync seems to be fine. But I'm getting very little on the vsync. Eventually I worked out there is a tiny voltage of around, either 0.7v or 0.07v (I forget which, sorry), which is far lower than the ~5v I have read should be expected. On the EL4583 the horizontal line is around 5v but it does an odd thing when used with the RGBs scart and TV. You will be aware that these cables use either the vsync or hsync pins to connect to pin 16 of the Scart which is the RGB Blanking signal, This pin needs to be high in order for the TV to switch to RGB mode. If I take a line from the 5v supply (higher that the 3v max suggested) the switch works fine but if I use the hsync from the EL4583 (or odd even on the LM1881) to do this switching nothing happens - and indeed the voltage across the cable drops to zero - so I'm guessing some kind of protection in either the TV or the chip? Obviously at 0.7v I can't test the vsync for this.

As for the image on the NEC monitor I get a blank screen - specifically no image BUT NOT no signal. If I remove the H and V sync lines it then shows no signal so there are signs that something is happening but not enough to display the image. As yet I have not tested to see if the H sync drops to 0v when connected to this monitor. I will do this in the next round of testing, but I'd really appreciate it if someone could try and explain what might be going on here.

Finally, just to say last night I went back to basics and connected up the LM1881 to a different composite source - no monitor connected just to read the voltages of the outputs and the results were the same. I don't have an oscilloscope to test these signals more thoroughly, although I think I will probably end up getting one before long.

Thanks for your help.
 

ronsimpson

Joined Oct 7, 2019
3,144
But I'm getting very little on the vsync. Eventually I worked out there is a tiny voltage of around, either 0.7v or 0.07v (I forget which, sorry), which is far lower than the ~5v I have read should be expected.
How are you measuring this voltage? Vsync is high about 1% of the time so a meter will read very low.
1713789343138.png
1uF capacitor connected in parallel with a 680K resistor between Reset and Ground
Why not a 0.1uF?
 

Thread Starter

asprinwizard

Joined Jan 1, 2019
26
Thanks for your reply. My apologies, just checked, of course I meant 0.1uF.

I've been reading the voltage on each pin (against ground) with a multimeter. I guess that explains why vsync is low, but it doesn't explain why it would be used to control the blanking signal on Atari to Scart cables: https://info-coach.fr/atari/hardware/interfaces.php (scroll to SCART/Peritel Cable). I have a number of these cables and some wire pin 9 (h-sync) to pin 16 and some wire pin 12 (v-sync) to pin 16.

Without an oscilloscope is there a way I can reliably read if the v-sync pin is outputting anything? Just had another look and getting no voltage off it at all. Also by way of an update, getting 5v from horizontal and that remains the case when connected to the NEC monitor. Still finding it drops to 0v when connected to the RGB scart. Apologies for the basic questions but I'm rather struggling with what might be going on. Any help appreciated.
 

schmitt trigger

Joined Jul 12, 2010
921
Back in the day of analog TV, I used the LM 1881 for several projects. It works the first time if you build the circuit exactly as the data sheet shows.

There are certain things in electronics that do require a scope. Viewing analog TV signals is one of those.
Beg, borrow or purchase a scope.
 

Thread Starter

asprinwizard

Joined Jan 1, 2019
26
Thanks. I have just ordered an oscilloscope. Can anyone explain the points in my last post re the use of sync signals as blanking signals, and why connecting the h-sync to pin 16 caused the voltage drop when connected up to the scart. Also should I have any resistors on the v-sync and h-sync outputs?

Thanks for your time.
 

Thread Starter

asprinwizard

Joined Jan 1, 2019
26
Fair enough. Hopefully the oscilloscope will shed some more light on this. Still wondering why I lose the 5v voltage reading when the h-sync is connected to pin 16 of the scart. can anyone shed any light? Thanks again for your help.
 

MisterBill2

Joined Jan 23, 2018
18,986
Now I am wondering if someplace in this AAC website there is a table listing the pinouts of all the different formats, and what those signals are. That resource would probably lead to a fairly quick solution for the TS, and it cold solve an issue for me, as well.
 

ronsimpson

Joined Oct 7, 2019
3,144
I have designed many monitors and TV sets. Some are very picky about the shape and timing of sync. Some sets want Hsync and some need sync inverted. Some sets don't care if there is an inversion or not. VGA monitors get information by testing which syncs are inverted.
I have a VGA to CGA conversion board
Some of the conversion boards send a distorted version of sync.
I can't tell if the output is VGA to interlaced CGA or non interlaced CGA. I have made both types.

If I have the project here I would try inverting Hsync just to see.
 

Thread Starter

asprinwizard

Joined Jan 1, 2019
26
Hi and thanks again for everyone who has taken time to post. You'll understand this is all a bit unfamiliar and therefore I will try and answer your points as best I can.

Firstly re the schematic, it's not really my strong point but here is what I was able to mock up. I hope it makes sense and let me know if you need more info:
Scheme-it-export-New-Project-2024-04-26-18-03.png

Also my Oscilloscope arrived today. It's only a cheap one and tbh I'm not sure if I'm using it right. I have attached one of the crocodile clips to a pin on the EL4583, and the other to ground. This is what I get:

CSYNC
20240426_194450.JPG

HSYNC
20240426_194525.JPG

VSYNC
20240426_194555.JPG

Apologies for the rotation, my phone doing odd things. I'm not sure if the settings are correct. They are:

5V, 500ms, DC

If I need to use other settings let me know and I'll redo the test.

Thanks again.
 

ronsimpson

Joined Oct 7, 2019
3,144
I don't see Vsync. It might be that the scope is having a hard time finding it. Try changing the time from 500mS to 50mS or 5mS.
Is it possible the input to the EL4583 is Hsync not Csync?
 

Thread Starter

asprinwizard

Joined Jan 1, 2019
26
Hi. Thank you for replying. This is essentially the crux of the issue I am having. Firstly the input is composite video, not csync. The EL4583 outputs csync as well as hsync and vsync. The output from csync seems ok, the hsync is a constant line so I guess ok (?), but the vsync registers nothing. I have tried changing the timing but the flatline is constant. I would put it down to a dodgy chip but the LM1881 does exactly the same. I will try a different source on the EL4583 but it made no real difference when I tried that on the LM1881, but then I didn't have the oscilloscope then.

As mentioned before the output from horizontal drops to nothing when connected to the blanking signal pin on a Atari to Scart TV cable, yet this is how the cable is wired and how the ST switched to RGB mode when using an original ST. But perhaps I should deal with one problem at a time!

I'll post the oscilloscope output from using the EL4583 with a different composite source. In the meantime let me know if there's anything wrong with the oscilloscope setup - I'm using DC mode, but not sure if that's right as composite video is AC I think (???). Would appreciate any advice anyhow. Thanks.
 

Thread Starter

asprinwizard

Joined Jan 1, 2019
26
Ok so following up that last message:

Here is everything set up:
20240427_113545.JPG

CSYNC:
20240427_112937.JPG

HSYNC:
20240427_113007.JPG

VSYNC:
20240427_113037.JPG

VSYNC at 5ms:
20240427_113051.JPG

VSYNC at 200mV increments:
20240427_113115.JPG

The composite output:
20240427_113617.JPG

(Once again apologies for the orientation).

So it looks pretty similar. Any thoughts?
 

MisterBill2

Joined Jan 23, 2018
18,986
Is it possible that the flat-line output requires an external pull-up resistor? Or pull-down resistor? or, like at least one synch IC that I recall, that the output is an internal transistor that requires both emitter and collector connections to provide an output?? I know I have cone across at least one of those in reading the application data sheets over the years.
I am not stating that IS the problem, just that it merits examination, and that it would cause a flat-line output if a connection was not made. So it will be worth examining the data sheet again.
 

Thread Starter

asprinwizard

Joined Jan 1, 2019
26
Hello again. Thanks for the replies. I've set up my circuits as per the 'schematic' I posted earlier:
Scheme-it-export-New-Project-2024-04-26-18-03.png
I have tried this with the LM1881 and the EL4583 and the VSYNC is a flatline for both. There are a load of small LM1881 devices you can buy that are sync strippers with this schematic setup. However they are generally used for getting CSYNC out of a composite video source and don't generally use the VSYNC output. However I would still expect there to be some output from this pin. Maybe it is worth waiting for the GS4981 to arrive before going any further with this but I am expecting similar results.

There is a more complex recommended circuit for the EL4583:
1714386699670.png

But I'm not sure I have all the components (13K and 82K resistors for example)! Also what is C2? What does OPEN refer to? I was told you only need to route the video in via the filter if you don't have a strong composite signal. But with no success so far I guess it's worth trying. Thanks.
 

Thread Starter

asprinwizard

Joined Jan 1, 2019
26
Hello again, a bit of further info from some googling suggests that some chips like the LM1881 output vsync at active low which would therefore require some kind of inverter to get a positive level which I think a VGA monitor would require. I'm not sure what you would use for an inverter. Could anyone offer any insight on this? Thanks.
 

ronsimpson

Joined Oct 7, 2019
3,144
74LS14, 74HC14, 74??04 Are good for 5V circuits.
CD4069, 40106 are good for 3 to 15V circuits.
Some inverters are Schmitt and some are not. Does not matter in your case.
Do not use Open Collector type.

It is true that VGA looks at the polarity of H & V sync to know what mode to run in. VGA has twice the Hsync frequency as CGA.
 

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