Date : Fri, 07 Apr 2006 12:10:59 +0100
From : Fragula <fragula@...>
Subject: Re: Using an AKF30 with a Beeb
Hi Kris!
Kris Adcock wrote:
> I'm guessing the refresh-rates it deals with are low enough
> for a BBC micro, and would give me a better picture quality than the
That was a good conversation killer.. Nothing in here for over 24 hours
now!
In leiu of anyone more knowledgable opening their traps, and with the
gotcha that I'm not sure about the specific monitor.. I'm talking about
old IBM ones. if yours is in fact CGA, then its probably at least based
on that.
CGA in general is not easy to adapt well. Its a bit of a pig of a
standard, based (if you like an analogy, and if my memory is right) on
525 line NTSC (but not NTSC encoded) at least if done in the IBM/Compaq
way. Frame rate is fixed at 60Hz, and has less lines than the BBCs
(essentially "625" unencoded PAL) output.
The CGA monitor (specifically the early 1st generation IBM one!) really
is quite sensitive about the line frequency. The line oscillator is part
of the power supply, and is *tuned*. So if you change the input
frequency too far, it (if you are lucky!) melts its scan coils. (and if
you are not, on early models, the insulation on the scan coils catches
fire!)
CGA also has split syncs, not insurmountable, but a PITA, and *if my
memory works at all* is TTL. Sounds good? Well not really.. it has a
seperate "brightness" TTL input. (at this point i'm not entirely sure
whether i'm confusing it with some even older standard).
Now this may well be different on an Acorn monitor. I'm not convinced
that Acorn would have badged a purely CGA monitor. Really on the grounds
that CGA is so crappy.
I wouldn't have expected it to have a 60Hz output for starters, or for
that matter, TTL inputs, as IIRC even the earliers arcs had a 256 colour
mode, and the compost out works with a standard TV set?
If its know to sync to 50Hz, and has analog inputs, you would probably
want an attenuator (resistor) in each line of the cable to tame the BBCs
rather hot outputs. the beeb normally puts out 5V on each pin, and an
analog monitor usually is happiest with just under a volt. You would
also probably need to split the H&V syncs somehow. Not terribly
difficult, but a small electronics project rather than just a cable.
> composite video I'm using at the moment. Does anyone have a wiring diagram
> for a suitable cable? (Assuming that the voltage levels of the Beeb's RGB-out
> are what the monitor expects.)
My advice would be, unless someone has better info, and this monitor can
be to find somebody nearby who has an arc with a cub, and will do you a
swap.
You both then have the authentic experience. :-)
Cheers!
M.