Date   : Tue, 18 Jan 2000 12:31:06 -0000
From   : "Tribick, Paul" <TribickP@...>
Subject: Re: RGB -> Scart cable details

To connect the RGB output of a BBC micro to a TV SCART input requires the
SCART plug to be wired in the following way. (This assumes that the TV has a
SCART AV input that is RGB capable.)

Connect the R, G, B & Sync outputs from the BBC RGB socket to Pins 15, 11, 7
& 20 respectively; the ground pins 13, 9, 5, 17 and common ground (shield)
connected together in the SCART socket and to the ground in the BBC RGB
socket.

This, in theory, is sufficient to get the TV to display BBC screen output on
the TV provided you select the appropriate SCART AV input on your TV _and
can select RGB mode_. Selecting RGB mode cannot usually be achieved
directly, but you can usually test if RGB is on the input by going into
Teletext mode when in the BBC's display should be seen, albeit rolling
behind the Teletext!

To get the TV to automatically select RGB input (once the AV input is
selected) Pin 16 of the SCART must be raised to 1-3 Volts. This is may be
conveniently achieved by connecting Pin 16 to the Sync output of the BBC (ie
connecting Pins 16 & 20). However, on one of my BBC Masters there is not
enough drive from the Sync output to cause RGB switching. In this case, the
5 Volt output on the BBC's RGB socket can be connected to Pin 16, but a
suitable series resistor should be incorporated to reduce the voltage to the
nominal 1 Volt/75 Ohm input of Pin 16, ie around 220/270 ohms, preferably
alter on test to the highest value that allows reliable RGB switching.

To get the TV to switch automatically to the AV input once the BBC is
powered up is more difficult since Function Select (Pin 8) requires a
nominal 9.5-12 Volts which is not readily available from BBC micros. Thus
you have to select the AV input manually on your TV.

However, it should be noted that on a widescreen TV or a 4:3 TV with
widescreen mode ("16:9", "vertical squeeze", etc), a nominal voltage of 5-8
Volts on the Function Select pin will cause the TV to switch to the AV
input, but in widescreen mode. Thus connecting the 5 Volt output of the BBC
micro to Pin 16 may cause a widescreen capable TVs to enter widescreen mode!
(As appears to be happening in John Simpson's case).

Hope this helps.
(E&OE!)

Paul
--
Paul Tribick
tribickp@...       


> ----------
> From:     John Simpson[SMTP:Simpson.J@...             ]
> Sent:     13 January 2000 08:55
> To:       BBC MICRO Mailing List POSTINGS
> Subject:  RE: [BBC-Micro] RGB -> Scart cable details
> 
> Now that Christmas etc is over I've finally finished soldering together my
> RGB to SCART cable. Perhaps predictably, it doesn't work - but I've got a
> fair idea why this might be the case. I wonder if anyone could confirm it?
> 
> Basically, I tested it my switching on my TV and plugging the (switched
> off)
> BBC into it. I then flicked on the BBC and the TV immediately detected the
> signal and switched to AV1 input. So far, so good. However, the screen was
> completely black. It was also letterboxed, in other words there was a
> strip
> at the top and bottom which was even blacker than the rest!
> 
> The only semblance of a picture was three horizontal lines at the top of
> the
> picture: one red, one green and one blue. They were pretty feint. 
> 
> With a multimeter I tested the outputs at the SCART plug. The signal
> select
> and sync select pins were +4.92v, Ov was, er... Ov, and the Sync pin was
> 3.92v. I put the high reading of the latter down to the high impedance of
> the voltmeter compared to the TV, and reckon that the TV is actually
> seeing
> about 0.8v, which is slightly too much. Of course, the sync signal must be
> rapidly fluctating and so a multimeter probably doesn't give a very
> accurate
> impression anyway.  
> 
> The Red, Green and Blue pins had 0.09v, 0.15v and 0.12v on them
> respectively. I couldn't really understand why they were different to each
> other. 
> 
> My current theory is that the voltage on the sync select pin is not high
> enough, as suggested might be possible by Mike Cook. 
> 
> Has anyone got any alternative ideas?
> 
> john
> 
>