Date : Thu, 16 Dec 1999 20:32:38 +0100
From : "Mark Usher" <mark.usher@...>
Subject: RGB -> Scart cable details
Helloooooo
I made up a RGB -> Scart cable some time ago and it has been a god send ever
since.
The details of the lead I made up can be found at the BBC Body Building
site, BB 86 (Oct. 90) Connect your computer to a TV set using its SCART plug
to be precise.
Ahh, just checked on the web and the site is down. soo... I hope Mike
doesn't mind....
I have also attached the piccie, it's only 3k so please I hope no one takes
offence at an attachment.
Mark
The Beeb Bodybuilding Course
Exercise 86
I've SCARTed so I'll finish
By Mike Cook
Last May I looked at how to use a IBM PC monitor with the BBC computer.
Since then my mail bag has been full of requests for information about how
to use the SCART connector found at the back of most modern TV sets.
It should be quite straightforward to connect up a computer to a SCART
socket. However things are a complicated by the nomenclature and
non-standard way this standard is implemented. It helps to realise why the
SCART connector was originally designed.
Its purpose was to connect a teletext unit to a TV set. In order to do this
we need to access the RGB video signals and to be able to take the
synchronising pulses from the internal TV receiver or the external video
source. In addition, in order to provide video overlays like headlines and
subtitles, there must be some way of rapidly switching between the internal
and external video signals many times over the period of a frame. It is
generally these requirements that lead to confusion when trying to use the
SCART with a computer.
Let's start with Table 1. This is a listing of all the signals on the SCART
socket. As well as the RGB video signals we want to use, there are also some
others we will need to deal with.
It often confuses beginners that each signal has its own earth or ground
connector, in fact only one ground connector need be wired up. If the video
were being sent a long distance (over five metres) then you would use
screened cable for each signal. When it came to wiring up the socket then
each screen would require its own terminal just to make the back of the plug
neat and manageable.
Now we need to connect the RGB signal from our computer to the RGB inputs of
the SCART socket. However the SCART's inputs specify that these signals be
one volt peak to peak, that is, a maximum of one volt. Now the range of
Acorn computers, except the Archimedes, produce digital video outputs. This
is nominally five volts but in practice is round about four volts. Can we
feed this straight in to the connector? Well it depends on the TV set, some
will cope with this excess voltage but others will not. Don't worry however
if a set won't cope with it, it will not cause any damage, what will happen
is that the display will look too bright, the colours will merge and in
general it will look out of focus or not sharp.
If this is the case then how do we condition our RGB signals? Well looking
at Table 1 you will see that the video inputs are terminated with 75 ohms.
That means in effect there is a resistance to earth of this value on each
input. We can use this resistance as part of a potential divider to cut down
our signal to the required size, in short we only need to add a series
resistor, this is shown in Figure 1. Note how we can't get the exact
resistance required but don't worry the one volt peak to peak requirement is
not that critical.
Having got the video into the TV set we need to get the synchronising pulses
in, note in Table 1 there is no mention of a sync input. The BBC computer
produces what is known as mixed or composite syncs, that is a mixture of
line and frame syncs on the same signal, this needs to be fed into the
composite video input. Now a composite video input is normally the mixed
syncs and the video signal, we could derive this from the video output of
our computer but better results are obtained from the RGB outputs.
Nevertheless we will achieve the desired results if we feed our video output
signal here. However doing this requires an extra connector so the best bet
is to cut our logic level sync pulses down to size just like we did with our
video signals. Some sets require this signal to be a maximum of 0.7 volts
peak to peak and so we need a larger resistor but again it is not critical.
Next comes the switching part, when we use the computer we want the display
to switch to the SCART socket's video inputs. This can normally be done by
some switch on the front of the TV set, however pin 16 acts as another
overriding way to switch over. This is not the whole tail because whilst it
switches the video signals it does not switch the synchronising signals.
This means the TV set would try to display the computer's output using its
own sync pulses, this results in a drifting display. The syncs are switched
independently using pin 8, so we need to connect this as well. Most sets
will allow switching to occur with the 5 volts available from the RGB socket
of the computer, however specifications I have seen require over 9 volts to
activate the switch. I think these will still switch off 5 volts but you
might have to connect a battery or 12 volt signal from the aux power supply
socket on the odd TV set. Again, on other sets this switching is not
implemented or not wired up to the SCART connector. This separate switching
of sync pulses makes sense in the context of a teletext decoder
superimposing on a TV picture but in the computer context it is an
unnecessary complication.
Now we come to the audio input, we can use this to channel the sound
produced from the computer through the TV set. This has two advantages,
first the better size speaker will result in a higher quality and secondly
you will have full use with the TV's volume control. As you can see the
SCART standard specifies stereo inputs even though most TV sets do not have
stereo sound systems. We simply need to connect our sound output to both the
left and right inputs.
Now the sensitivity of the audio input is sometimes different on different
sets but we can use the same technique to match the voltages as we did on
the video signal. However the termination on the audio line is normally
about 1KOhm and so we need to use a much higher series resistor. When you
remove the existing speaker socket the signal is on the left hand pin with
the ground on the right hand. The audio output can be used to record your
computers sound or to send the sound to your Hi-Fi system.
The whole interface is summarised in Figure 2 and all the components are
available as Body Build Pack No. 64.
Note that the speaker connection socket requires the wire to be crimped, the
kit will provide a ready crimped wire. When you get the SCART plug, the
individual plugs are supplied loose, all you have to do is to insert at the
front the pins which you want to use.
Incidental the D2B (EDITORS NOTE. this should be D squared B) connections
are serial input and output lines to allow communication between a teletext
decoder and a TV set so that such things as the screen brightness can be
controlled from the teletext remote control.
Well there you have it, having SCARTed I did finish ! There is just one
point what does SCART stand for? My guess is that the words Standard,
Connector and Television are in there somewhere but does anyone know the
rest? With that I will leave you until next month.
Table 1 The SCART connection list
Pin 1 - Audio output (right)
Pin 2 - Audio input (right)
Pin 3 - Audio output (left)
Pin 4 - Audio Ground
Pin 5 - Blue Ground
Pin 6 - Audio input (left)
Pin 7 - Blue video input 1V ptp 75ohm
Pin 8 - Sync source select
Pin 9 - Green ground
Pin 10 - Intercommunication D2B input (EDITORS NOTE. this should be D
squared B)
Pin 11 - Green video input 1V ptp 75ohm
Pin 12 - Intercommunication D2B output (EDITORS NOTE. this should be D
squared B)
Pin 13 - Red ground
Pin 14 - D2B ground
Pin 15 - Red video input 1V ptp 75ohm
Pin 16 - Fast blanking ( internal/external video switch )
Pin 17 - Sync source select ground
Pin 18 - Fast blanking ground
Pin 19 - Composite video signal output
Pin 20 - Composite video input 1V ptp 75ohm
Pin 21 - Socket ground
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