PC Generated 405 lines question

[murphyv310]

Hi.
I've noticed some sync disturbances on a couple of my remaining collection of 405 sets when using PC generated 405. A quick look at the waveform shows that the video is encroaching below black level and into the sync area. Can't remember if I noticed this before or not.
Any ideas why and how to cure it?

[FERNSEH]

Hi Trevor,
was the video waveform from the computer generated video DC restored before inserting the sync pulses? The black level has to be set to a specific level and then the syncs are introduced into the video signal.
Syncs should be 30% of the total P - P waveform.

DFWB.

[murphyv310]

Hi David.
I'm looking at the waveform directly from the PC after the broad pulses are added. To be fair I haven't used the PC for 405 for a couple of months as I tend not to do any vintage at all between late October and the end of March due to the sets being cold. I fired up a couple yesterday and had really bad bent verticals. A look on the baseband signal and low and behold video below black level. There is no additional DC restoration, unless the Modeline is wrong but it shouldn't be as nothing has changed. I'll open up the board for broad pulses insertion and see if there is an issue there. Other than that I'll need to add in DC restoration.

[FRANK.C]

Hi Trevor
I am not familiar with how that circuit works but I think the 20uF electrolytic capacitor might be worth looking at.
Because if it goes low in value I would expect the sync pulses to suffer.

Frank

[Jac]

Trevor,

I think I put in a 220µF - 25V capacitor.

Jac

[Graham G3ZVT]

I built the interface and Peter's pulse generator onto one board

as you can see the fixed resistors in Jeroni Paul's interface have been replaced with pots, giving me total control of the R, G, B, and mixed syncs components.

I don't think I ever sketched out a schematic! I'll try to resolve that for you later today, although I think it was literally swapping fixed with variable resistors. The sync coupler remained 10uF




Here are the odd & even field waveforms from the board. (only showing the last 1 or 2 lines of peak white video before the v sync train)

[FERNSEH]

A sync and blanking circuit I've used for several projects.
For best results from the driven clamps a low impedance source is required, otherwise a degree of "droop" will be introduced into the video waveform.
The clamp, blanking and mixed syncs are supplied from a TTL source.
A much simpler alternative to do the all the functions is employ an LM1486 modulator IC. In fact the BBC digital 625 to 405 line converter used the metal can version of the LM1486, the LM1496.

DFWB.

[FRANK.C]

Maybe I am missing something here. But why not get rid of the capacitor altogether?
Then you will be working purely with DC. Which should give improved black level.

Frank

[murphyv310]

Sorted.
Of all things it was water in the little box, now to fix the leaking roof!!!
Once dried out all was back to normal. I'll look at the circuit and see if I can dispense with the coupling capacitor.

[murphyv310]

Originally Posted by FRANK.C

Maybe I am missing something here. But why not get rid of the capacitor altogether?
Then you will be working purely with DC. Which should give improved black level.

Frank

Hi
The coupling capacitor only couples syncs. It's a good idea though to have a higher value or possibly directly couple it.

[FRANK.C]

Hi Trevor
Great you got it sorted
Moisture is not something you would think of.
As the syncs are negative going the capacitor along with the 470R resistor will also be responsible for setting blanking/black level.

Frank

[murphyv310]

Hi
I have dispensed with the coupling cap and this has improved the DC level no end, picture is far better on the V4. I'd suggest anyone running this method do likewise, it's not so noticeable on sets with no AGC but the sets with mean level AGC can "take off" but the mod is a 100% cure.

[FRANK.C]

Hi Trevor
Great result.
Did you have to adjust the resistor values or were they OK as they were.

Frank

[murphyv310]

Resistor values appears to be okay Frank.
Next would be some way of encoding the RGB to give NTSC 405, Way above my capabilities to be honest, perhaps you'd like the project?

Further info on this thread:. https://www.vintage-radio.net/forum/...ad.php?t=62417

[murphyv310]

Also from the same thread this!!!
https://www.vintage-radio.net/forum/...1&d=1291363156

[FRANK.C]

Hi Trevor
Maybe someday. But not anytime in the near future. I am busy at the moment with my current project and other things.

The only way I would be able to try doing it is by using a FPGA.
Right now I know nothing about encoding colour and the non availability of decent sized FPGA's don't help either.

Frank