Modeline experiments

[murphyv310]

Hi.
I now have the required chips and multiturn pots so will commence building at some point this week.

[murphyv310]

Hi.
A question for those in the know.
I discover that the SN74LS00N chips I have are actually not SN74LS00N's they are SN74S00N's so a Schottky device which I think are more power hungry, I take it they are unsuitable for this application?

[Graham G3ZVT]

I used LS chips, but if I remember correctly Peter's origanal prototypes used "ordinary" TTL chips as that is what he had, and they worked.

[murphyv310]

Hi.
Well all work is now completed, set up the timings etc and working beautifully. Results are excellent on direct line sync sets and even the Murphy that would in no way interlace.
Thanks to all who have helped me with this.
Appreciate it.
https://youtu.be/cLCdFmYcXnI

[FERNSEH]

How does the device work? It's understood that the single frame sync pulse generated by the computer is replaced by a chain of eight 2H 40microsecond pulses.
Certain receivers such as properly designed models like the Murphy V310, V410 etc will only work with broadcast quality sync pulse chains. The eight 40microsecond frame pulses are integrated by a critical CR time constant circuit.
The Bush TV22. Recall the subject of too much integration in the frame sync circuit was discovered be Dr Hugo Holden? The simple matter of replacing a 100K resistor in the frame sync separator circuit with one of 10K improved interlace considerably.

DFWB.

[murphyv310]

Originally Posted by FERNSEH

How does the device work? It's understood that the single frame sync pulse generated by the computer is replaced by a chain of eight 2H 40microsecond pulses.
Certain receivers such as properly designed models like the Murphy V310, V410 etc will only work with broadcast quality sync pulse chains. The eight 40microsecond frame pulses are integrated by a critical CR time constant circuit.
The Bush TV22. Recall the subject of too much integration in the frame sync circuit was discovered be Dr Hugo Holden? The simple matter of replacing a 100K resistor in the frame sync separator circuit with one of 10K improved interlace considerably.

DFWB.

Best idea David is to ask Peter Scott as he came up with the circuit.
The odd thing is the V310 gives perfect interlace with 4 pulses or 10 and even the pulse length can be different, remove them and the set goes haywire and you also get tearing on line! This is the only set I have that is uncomfortable with "no" broad pulses.
Have to say Peters circuit is rock steady and once set has no drift whatsoever.

I've added the photo of the setup here for you David if you haven't seen it already

[Graham G3ZVT]

The roots of this project go back to 2010
Here's Kat Manton's explanation of the Modeline timings she worked out.

"8.10" sets the pixel clock to 8.10 MHz

"664 680 752 800" configures horizontal timing and breaks down as follows:
664 - number of active pixels (i.e., the horizontal resolution)
680 - sync start
752 - sync end
800 - total line length.
(8.10 MHz / 800 is 10.125 kHz. Which is spot-on.)

"377 378 385 405" configures vertical timing and breaks down as follows:
377 - number of active lines (i.e., the vertical resolution)
378 - sync start line
385 - sync end line
405 - total number of lines
"-hsync -vsync interlace" are fairly self-explanatory

(from https://www.vintage-radio.net/forum/...d.php?p=385733 post #18)



I presume Peter was working with the same figures when he designed his circuit,
but here's the thing,
I couldn't get an equal number of odd/even field broad pulses until I shortened the field sync interval by 1H, ie. 379-385

So my Modline became
Modeline "664x377_25 10.1kHz 50.0Hz" 8.100 664 680 752 800 377 379 385 405 interlace -hsync -vsync

I know Trevor was using this from the outset, but I'm wondering if Peter had to grapple with this.

It's something I've meant, to mention but haven't until now.

[peter_scott]

I know the monostables were a bit tricky to set up but I'm still using the modelines listed here.

Graham's modeline tweak is interesting and a subtle workaround to a good result.

Peter

[FERNSEH]

The circuit diagram of the 405 sync and blanking generator constructed for my 1985 standards converter project.

DFWB.

[peter_scott]

You were generating your mixed syncs digitally. I guess you could say that I generated them analoguely. Despite the rubbish technique I don't think I did too badly.

Peter

[FERNSEH]

Not bad at all. In fact excellent. Just different ways of solving a problem.

DFWB.

[Graham G3ZVT]

Peter, your field sync stats 1H before Kat's and ends 1H later. I must try that and see if it cures the permanently peak white line at the bottom of each field.

[murphyv310]

Originally Posted by rambo1152

Peter, your field sync stats 1H before Kat's and ends 1H later. I must try that and see if it cures the permanently peak white line at the bottom of each field.

It does!

[Graham G3ZVT]

Originally Posted by murphyv310

It does!

But not for me, In fact I came to the conclusion it was a Vidblaster artefact because I don't get it with media "slid over" so to speak.

Anyway I'll continue using Peters figures as the pulses still look correct on the scope without further adjustment, and so we are all singing from the same hymn sheet.

[murphyv310]

Hi.
In fact I'm wrong. The white line seems to be Vidblaster that's the culprit, it has though moved up slightly.
I still think that dropping across gives better resolution. I see these dell optiplex 780s are cheap on ebay, I might bag another and have it fully dedicated for 405.

[FERNSEH]

In 2012 the 405 sync and blanking board underwent a modification to simplify the circuits around the broad pulse and blanking pulse counters.
The removal of the load pulse generator.

DFWB.

[murphyv310]

Hi David.

I have to say I'd not know where to start on a project like that, I know my limitations and that is way beyond my skills.
It's an amazing feat what you did with those converters.

[FERNSEH]

Hi Trevor,
the prototype converter is still in existence.
It's thirty-seven years old now. Built entirely on veroboards. Uses Hitachi static RAMs in the line stores which are I believe now well and truly obsolete.
Type HM6164?

DFWB.

[FRANK.C]

Totally agree with you Trevor. I too would have no idea where to start.
What an incredible accomplishment, especially when it hadn't been done before.
Getting so many chips to behave couldn't have been easy.
David I am in awe each time I see your converters.

Frank

[Jac]

I second that!

Very, very impressive.

Jac