getting (PAL) composite signal from the ORIC

mmu_man · Post by **mmu_man** » Tue Oct 10, 2006 4:13 pm

Supposedly the signal entering the UHF modulator in my ORIC Atmos is a PAL video composite signal. So I tried to extract it to send it to my parent's TV but it didn't seem to like it. Had some ghost-like image but it didn't seem to synchronize to it. I tried to amplify and/or invert (with a transistor in the way discussed in Theoric 1) it and tune RV1 but it didn't help. Anyone succeeded there ?
Forcing the format to PAL on the TV didn't help either.
I'll have to look closer to the form of the signal but my oscilloscope is quite old and not really reliable.

Silicebit. · Post by **Silicebit.** » Sun Oct 15, 2006 7:39 pm

You should amplify the signal to 1Vpp level. Time ago, I pull out the PAL video composite signal from UHF modulator input for feed, amplifying the signal by means of a video amplifier, a Commodore 1702 video monitor and work fine.

Cheers.

Alfonso.

mmu_man · Post by **mmu_man** » Mon Oct 16, 2006 4:44 pm

Yes, I tried with a transistor on a proto board but it was probably not enough.
I found later in the ORIC service manual that this signal is only 0.2 V pp.
I should probably try some single-chip video amp or a fast enough op. amp.

carlsson · Post by **carlsson** » Fri Aug 08, 2008 7:37 pm

Did you ever got anywhere with this hack?

I have just become the proud owner of my first (!) Oric-1, a 48K model in visually very nice condition. It was sold as spares/repair on eBay (some of you may have seen it) because it didn't do colour!

I have adjusted the two externally available knobs without getting any colours. Then I opened it and begun fiddling with the channel selector pot on the RF modulator. I found that if I push it down with a screwdriver, I get a colour display! The harder I push, the stronger the colours appear.

I've read that the RF modulators can be poor, and frankly I don't know if I can fix this in an easy way.

However I own a 48K ZX Spectrum which I performed the composite video hack on last year. It consists of desoldering the resistor to the RF connector, route the composite video in directly to the connector and ... I think that was it. To me it looks like the RF modulator in the Oric looks identical to the one in the Spectrum, so I had expected it work in the same way. Some web pages mention that you should amplify the signal from the Spectrum too to get a proper composite video signal, but I found it works out of the box.

Now, I read your message that you failed doing likewise on your Oric. I have for obvious reasons not yet pulled out the soldering iron to my brand new purchase, but if you think it would work - and more importantly give me a colour picture - I'd do it as soon as I have the time. For me a composite video signal is to prefer over RF anyway.

nobbysnuts · Post by **nobbysnuts** » Sat Aug 09, 2008 8:11 pm

why not just buy a rgb to scart lead from ebay this will give you excellent colour

carlsson · Post by **carlsson** » Sun Aug 10, 2008 3:21 pm

Well, true. Personally I'm rather mediocre at soldering, but believe I could manufacture a cable myself from the diagrams on Internet. But I didn't quite understand if the RGB is TTL, half-TTL or something else. In the mean time, composite video would be the simplest mod if it worked out.

Symoon · Post by **Symoon** » Sun Aug 10, 2008 4:36 pm

PAL signal would also be an easy way to connect to all video inupts, including Video capture cards, to display the Oric on a PC screen for instance.

Torlus · Post by **Torlus** » Sun Aug 10, 2008 8:44 pm

mmu_man wrote: I found later in the ORIC service manual that this signal is only 0.2 V pp.
I should probably try some single-chip video amp or a fast enough op. amp.

I used a OPA350 for a similar kind of composite mod for an Atari 2600 Secam, and it worked fine (video signal was about 0.2Vpp too at the entry of the RF modulator).

Twilighte · Post by **Twilighte** » Wed Dec 01, 2010 2:45 pm

I was recently reading up about the Sam Coupe and noticed they used a motorola MC1377P chip to convert between RGB and composite. The data sheet is here...

http://www.datasheetcatalog.org/datashe ... 1377DW.pdf

The only problem is it requires a minimum of 10volts to drive it. Internally one might use a voltage doubler to get it but externally it would require an external psu.
Interesting tho

Torlus · Post by **Torlus** » Wed Dec 01, 2010 5:57 pm

You can take a cheaper route with a more modern AD724 for instance.
http://www.analog.com/en/digital-to-ana ... oduct.html.
Runs on 5V.

telbee · Post by **telbee** » Tue May 31, 2011 12:29 am

Twilighte wrote:I was recently reading up about the Sam Coupe and noticed they used a motorola MC1377P chip to convert between RGB and composite. The data sheet is here...

http://www.datasheetcatalog.org/datashe ... 1377DW.pdf

The only problem is it requires a minimum of 10volts to drive it. Internally one might use a voltage doubler to get it but externally it would require an external psu.
Interesting tho

This actually sounds like a good option.. it could be implemented internally so long as the power unit provides at least 10v. the 7905 inside the oric should tolerate well over 12v input before throwing the toys out of the pram, and I'm guessing the quality would be fairly good if implemented well. Might have a go at this myself as my modulator has long since died, and my old Amstrad color RGB monitor which i use for my Oric is on its last legs!

T