Date   : Wed, 23 Nov 2011 14:35:44 -0000
From   : jumbos.bazzar@... (Mark Haysman)
Subject: Beeb to HDMI unit

> Don't forget that the beeb's RGB output is 5V peak to peak designed to 
> drive
> into a single TTL logic gate load, whereas SCART is 1V peak to peak 
> designed
> to drive into a 75 ohm load.

The Beebs RGB output is from pull up transistors (The sync is direct out 
from an LS86), so is perfectly capable of driving 75 ohms, but as you say it 
does require resistors to drop the voltage to a suitable level. I've found 
that 220 Ohm tends to give the best results:

220R post fitted, plus 68R inline in the beeb, with 75R termination gives 
26% of input voltage
26% of 4.3v (allowing 0.7v junction drop of the transistors) gives 1.1v p-p.

Whilst it probably does overdrive slightly, it gives a good contrasty 
picture on most TVs and I've not had any complaints from the SCART cables 
I've made up that way. Using higher values seems to give a more washed out 
picture.

I've not tried that exact Maplin converter, but I have got it's predecessor 
which CPC also sold, and that doesn't work with the Beeb, just give a fuzzy 
mess on the screen unfortunately, but I have to admit I only tried it with a 
HDMI to DVI cable on a DVI monitor, and I didn't spend any extended time 
checking it was actually selecting RGB automatically, so with work and the 
"right" monitor, it may very well work. Would be worth a try, especially as 
Maplin have a no quibble return policy.

Only thing I've found so far that does a "good" job of scan converting is an 
Extron DVS204 - with V1.06 firmware if you want it use it with a Master. The 
later DVS304 does "too good" a job of converting the image, and also 
converts the interlace, making it very annoying in Mode 7 and other modes 
where it's turned on.

Mark.