CB2 microcomputer composite video out?

[BrianMiller]

Hi all,

I have built the cb2 vintage basic microcomputer presented here:-

http://cb2.qrp.gr/buildingguide/

And I have also built the composite video extension presented here:-

http://cb2.qrp.gr/extensions/#8_Colo...e_videoS-video

My problem is that whereas the SCART works perfectly OK, the composite video does not seem to work. I am a little confused, what crystal should I use on the composite video extension?

Thanks

[SiriusHardware]

I take it that when using SCART you use the RGB connections?

The crystal appears to depend on the TV system your display uses, NTSC or PAL - I note you are in Greece so which TV system do they use there?

Interesting project, by the way. Welcome to the forum.

[SiriusHardware]

Oh, did you also notice this in the circuit notes for the Composite extension?

Note, to use this circuit, you have to manually ground pin 25 of the ATMEGA644P, so that its pin 19 outputs an Hsync signal instead of the default Csync

[BrianMiller]

We are using PAL here. I have opened the J1 as suggested in the manual.
Hm... I have grounded pin 25 and it seems to work now!
Someone hasn't read the manual carefully...

The composite color seems good. I notice though that there are some "waves" in the screen, like tiny flickering that is slowly progressing like waves from one direction to the other, that is visible on the graphics on the screen. I am talking about tiny variations.

When I set the trimmer near the crystal accordingly, I am able to stop this "wave" from moving. But because the oscillator is never perfect, after a while, one may notice this again at a lower moving speed though.

Is this a normal behaviour?

[SiriusHardware]

It was normal on the Sinclair Spectrum, it even had a name, 'dot crawl'. Whether this is normal for this project I can not say. You'll need to ask around other people who have the kits.

I have to ask, if you were able to use RGB = much cleaner, better quality, avoids the artefacts which arise from mixing colour, luminance and sync all into one signal and then separating them again inside the TV / display - why then choose to use composite? I would always pick RGB+sync if that choice is available.

[BrianMiller]

Right I see...
It is just that the current TV I have available to use for the project, has not got a SCART input. Else SCART is way superior, in fact the best analogue signal is produced by SCART.
I thought many fellows in US might be interested in the composite part too.
By the way, in UK, do you have SCART as an option (in general) to the TV sets? I do not mean latest modern TVs but say 10 years old or so that people may still have in their houses.

Oh and yes, Dot Crawl is the "waves" effect I was describing, thanks for that!

[SiriusHardware]

In the UK / Europe SCART (and support for RGB-in on at least one SCART connector) were more or less standard for a couple of decades, but in the past few years SCART has now disappeared from most new sets.

Most modern TVs are tending to have only HDMI inputs now and if you are lucky a poorly implemented AV (Composite) input using a special three-way lead which plugs into a single 3.5mm socket on the TV rather than the traditional integral yellow / red / white RCA / Phono sockets.

Since these adaptor input leads are not physically bonded to the TV, most purchasers don't fit them, don't understand what they are for and instantly lose them.

There is another complication in that sets which can accept RGB input via SCART often don't have a manual / menu way to tell the SCART input to switch from Composite input mode to RGB input mode - instead this is done electronically by applying a voltage input to one of the SCART socket pins.

A project like yours may therefore have to provide this switching voltage down the SCART lead in order to force the SCART input into RGB mode.

[BrianMiller]

Originally Posted by SiriusHardware

There is another complication in that sets which can accept RGB input via SCART often don't have a manual / menu way to tell the SCART input to switch from Composite input mode to RGB input mode - instead this is done electronically by applying a voltage input to one of the SCART socket pins.

A project like yours may therefore have to provide this switching voltage down the SCART lead in order to force the SCART input into RGB mode.

Thanks!I think If I understand correctly what you said, the project website http://cb2.qrp.gr/ and specifically the schematic of the CB2 micro http://cb2.qrp.gr/buildingguide/#1.1_Schematic_diagram_ has that option. It is this R19 that tells the TV to use the scart.
On my other big TV there are 2 scart connectors and this works only in the first though.

Yes in an effort to reduce hardware/materials cost they ruin all the standards... But the prices stay the same or go higher! More money for them... This is what has been done to the analogue TV.

Yes, less bandwidth and better picture, but only on good signals. Try catching a digital tv signal on rural areas. At least in Greece, where there are a lot of mountains, this is a problem. With analogue TV we were watching with noise, but we were! With digital TV the signal is completely lost (cannot be decoded).
On the technical side, things are even worst! No technician can today repair a modern TV, unless there is a SMPSU problem. Even if they were to, it wouldn't worth it, cause it's better to trow it away and buy another one right? Anyone see the resources and environmental issue on this one, or it's just me? And they make things not to last. Consumerism at it's worst!

Anyway, that's a different topic...

[SiriusHardware]

It is this R19 that tells the TV to use the SCART. On my other big TV there are 2 SCART connectors and this works only in the first though.

It was quite common and quite annoying to find that where sets had more than one SCART socket, only the first / main one supported RGB input. I always had at least two, sometimes three sources which I wanted to connect via RGB.

I bought my current main TV - a medium sized supermarket cheapy - in a bit of a hurry because I came to the realisation that the big main manufacturers had already ceased to put most of the 'legacy' input connectors on their sets - as it happened the manufacturers of the lower end sets are always a bit behind the curve so I bought mine probably during the last days in which I could still buy a new TV with RGB SCART, Composite, Component, S-Video and PC-VGA inputs as well as HDMI, making it as past-and-future proof as possible.

As you can imagine on a forum like this, issues of 'improved' technology not really being much of an improvement and the difficulty of repairing modern sets vs. old sets have been gone over many times here, although the issues remain as valid as ever.