Source of Video Signal for a Modulator

[Sam]

I need a break from thinking about turbines, so my thoughts have wandered away from my project and on to the topic of my first 405-line TV.

I have looked at the circuits online for the 405-line moulators and had a thought. They all need a sound and a vision input. Sound is easy, but can the vision signal be fed from SCART lead? This is the same way my digibox feeds the TV in my room at home. As the digibox feeds a CRT, there still need to be the sync-pulses coming out of the box.

I know the line-frequencies are different for 405- and 625-lines, but the frame-rates are the same. This is what makes me think it may work. What signal would a SCART lead send out? Is it composite video? To be able to use this I would need to isolate the luminance signal, and remove the colour signal and sync pulses before the brightness signal is fed into the modulator. The transmitted frame-sync signals would have to be used to ensure the modulator frame is synchronised to the transmitted frame, but could the line-frequency run from a crystal-oscillator and be divided down and compared to the frame-sync signal to keep everything in step?

If the SCART sends out different signals (separate luminance, colour, and sync) I guess some of the stuff said above may not apply.

Sam

[ppppenguin]

A modulator doesn't do anything fancy with pulses or timings. It just modulates the signal on to the carrier. For +ve modulation, as used for 405 (and for 625 in France) the sync tips are at 0% carrier and peak white is 100% carrier. For -ve modulation as used in the UK, US etc the sync tips are at 100% carrier and peak white around 20%.

I'm not sure what you would use your 625 line +ve modulated signal for. If fed to a 405 set, the frame will lock but the picture will be a mess as there will be no attempt at line lock. In just a few sets, especially those with mains derived EHT, you may persuade the line timebase up to 15625Hz.

As for video sources and SCART leads:

The SCART lead always carries composite video. This can be fed directly to a modulator though the colour subcarrier could be a nuisance.

Virtually all equipment with a SCART output can also supply RGB which is not a lot of use to you. Some can also do YUV (more correctly YCbCr) or YC (also called S-video) which is ideal; just use the Y (monochrome) feed.

You can also use the Y signal from a 4 pin S-video connector.

Here's a useful URL for the pinout:

http://utopia.knoware.nl/users/eprebel/SoundAndVision/Engineering/SCART.html

[Panrock]

As an aside here, I did once feed 625-line PAL colour through David Looser's Ch1 modulator, picking the result on my little JVC CX-610GB tv.

This worked fine and in colour too, but I did need to put in a 3.5MHz trap on the vision input to prevent vision buzz on the sound channel.

Steve

[Sean Williams]

Hi Sam,

I use a video here that only has a scart output - I just got a scart to phono lead, and pick the video out that way - easy!

Well, unless you are going to look for a vintage video too.......

Then you just need to persuade some helpful forum members to assist with suitable 405 line video tape.......

To be honest, if you are as easily impressed as I am then just seeing a raster, and hum bars from a signal generator should be fun for a week or two....

Definatly have a go at Darius' Modulator - a doddle to build, can't fail to work really!

Then of course David Robinson has a superb multilegged fuse design needs a couple of specialist ICs (I have a couple of spares here if I can find the things) and a PIC (I thought builders used them ) - looks easy to build....

Cheers
Sean

Looks like your TV will arrive sometime early April.......

[oldeurope]

Originally Posted by Sam

I need a break from thinking about turbines, so my thoughts have wandered away from my project and on to the topic of my first 405-line TV.

I have looked at the circuits online for the 405-line moulators and had a thought. They all need a sound and a vision input. Sound is easy, but can the vision signal be fed from SCART lead? This is the same way my digibox feeds the TV in my room at home. As the digibox feeds a CRT, there still need to be the sync-pulses coming out of the box.

I know the line-frequencies are different for 405- and 625-lines, but the frame-rates are the same. This is what makes me think it may work. What signal would a SCART lead send out? Is it composite video? To be able to use this I would need to isolate the luminance signal, and remove the colour signal and sync pulses before the brightness signal is fed into the modulator. The transmitted frame-sync signals would have to be used to ensure the modulator frame is synchronised to the transmitted frame, but could the line-frequency run from a crystal-oscillator and be divided down and compared to the frame-sync signal to keep everything in step?

If the SCART sends out different signals (separate luminance, colour, and sync) I guess some of the stuff said above may not apply.

Sam

Hi Sam,
I tried this when I made my first steps generating a signal for my TV22.
First is doubling the sync frequency and divide it by three. This gives
the 405 sync pulses. Than seperate every third line of 625 and blank the
others. Move the phase of the new generated sync so that the seperated
line is in the center between the sync pulses. This gives a picture with
low H width but a picture.
Every second line is written on the screen this way (208 lines).
To get more lines more brightness and resolution, another line of the 625
must be delayed to get the same position as the preveous line between the
sync pulses. I tried a lot of things like a 100 meters of coaxcable to delay
the vision signal, but the result was ns. So I had a look how it is made in
the DOC circuit of VCRs. There I found these bucket chain ICs, delay ccds.
This was a success, but the picture was still small. Then I tryed to change the
clock frequency to change the length of a line, like the Doppler effect.
This was a great success and I came up to the standard converter I have
now. At the moment I am working on a kit for the clock generator.
The board is 80% routed and goes in production this month, I hope.

Making a modulator is very easy with the UVB. An english german website
with schematics pictures and examples is coming up soon (special thanks to Yagosaga).

Kind regards
Darius

[Danny]

Hi sam i could do you a tape for you experiments it wouldn be broadcast quality but would get you going once you have got your modulator built.
Danny