Beeb to SCART Aspect ratio switching?

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spark001uk
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Beeb to SCART Aspect ratio switching?

Postby spark001uk » Tue Nov 07, 2017 10:05 pm

Just wondered if anyone has experimented with this? That being the aspect ratio function of SCART pin 8 - where approx 5-8V selects 16:9 and 9-12V selects 4:3, the latter being the one we generally want for the beeb, and the former being (for me anyway) what I usually get, having to switch it manually back on the remote at every power up!
AIUI there is 12V available at the beeb's PSU, and so far I've been considering two proposals. One being to run 12V to the RGB DIN (and cut the 5V to it obviously), and the other, less obtrusive way would be to use a simple voltage doubler on pin 8 at the SCART end, to provide 10V (or indeed to just use an external source such as a 9V battery). Obviously with the former idea I would need to adjust the resistor to the RGB switch line (p16) to drop the extra voltage.

Any thoughts? Has anyone been toying with this idea already perhaps?

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tricky
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Re: Beeb to SCART Aspect ratio switching?

Postby tricky » Wed Nov 08, 2017 6:41 am

I was thinking about it as my first CRT requires changing every time and somewhere around here, three is a discussion between Mark, myself and maybe others, but the conclusion was to use one of the methods you state.

I haven't bothered to do anything as all my other CRTs are old enough to not know what 16:9 is :)

I did think about adding a transistor to the TV to raise the voltage internally as I doubt I will be connecting anything else to it, and if I did, I could use the menu or add a switch to the TV.

johnkenyon
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Re: Beeb to SCART Aspect ratio switching?

Postby johnkenyon » Wed Nov 08, 2017 7:41 am

A MT3608 board in a small box between the Beeb RGB plug and the scart plug would give you 12v from 5v for 99p without having to modify TV or computer. (from China via eBay)
/john

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tricky
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Re: Beeb to SCART Aspect ratio switching?

Postby tricky » Wed Nov 08, 2017 1:49 pm

Thanks, they are smaller and cheaper than last time I looked, but I am still tempted to fit it in the CRT ;)

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spark001uk
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Re: Beeb to SCART Aspect ratio switching?

Postby spark001uk » Wed Nov 08, 2017 6:45 pm

Thanks for the info. I was just working out the currents, and being Scart it's typically 75R terminations, which at 12V is asking for a not insignificant 160mA, more if boosting from 5V. Not sure what the efficiency of a simple discrete doubler is off top of my head, but in any case I'm thinking we should be a little wary of putting our poor old 1980s PSU under extra stress.!

I think for my own needs, and to use a simple solution, I'll probably tap into the Scart with a little 12V half amp mains adaptor I have laying around. Might put a cap across it at the Scart too in case it's a bit noisy.

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tricky
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Re: Beeb to SCART Aspect ratio switching?

Postby tricky » Wed Nov 08, 2017 9:37 pm

This says that the Impedance of pin 8 (aspect ratio) is > 10K ohms.

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1024MAK
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Re: Beeb to SCART Aspect ratio switching?

Postby 1024MAK » Wed Nov 08, 2017 11:09 pm

For testing (as in asking users to test their TV), you will see that I suggest using a PP3 9V battery. The aspect switching input should not be too current hungry.

What does have a 75 ohm impedance, is the fast blanking / Composite/ RGB input on pin 16. But for RGB, that only needs 1V to 3V.

Mark
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spark001uk
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Re: Beeb to SCART Aspect ratio switching?

Postby spark001uk » Wed Nov 08, 2017 11:23 pm

1024MAK wrote:For testing (as in asking users to test their TV), you will see that I suggest using a PP3 9V battery. The aspect switching input should not be too current hungry.

What does have a 75 ohm impedance, is the fast blanking / Composite/ RGB input on pin 16. But for RGB, that only needs 1V to 3V.

Mark


Ah OK, I knew one of the lines was 75R. I calculated if I send 12V up the cable, a 510R resistor to pin 16 would supply it with about 1.5V, so well into the RGB switching voltage.
Or as you say, a PP3 or equivalent should also do the trick.

johnkenyon
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Re: Beeb to SCART Aspect ratio switching?

Postby johnkenyon » Thu Nov 09, 2017 8:42 am

spark001uk wrote:Thanks for the info. I was just working out the currents, and being Scart it's typically 75R terminations, which at 12V is asking for a not insignificant 160mA, more if boosting from 5V. Not sure what the efficiency of a simple discrete doubler is off top of my head, but in any case I'm thinking we should be a little wary of putting our poor old 1980s PSU under extra stress.!

I think for my own needs, and to use a simple solution, I'll probably tap into the Scart with a little 12V half amp mains adaptor I have laying around. Might put a cap across it at the Scart too in case it's a bit noisy.


The aspect ratio input on the telly is drawing next to no current - it's just detecting the voltage level. You could probably get away with a one transistor "joule thief" with a 12v zener across the output (to clamp the voltage down), but by the time you have sorted out the generated noise, a chinese ready made boost regulator (with a 4k7 "load" resistor) is a lot easier.


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