Date   : Thu, 16 Dec 1999 10:15:04 +0000
From   : Matthew Froggatt <Matthew@...>
Subject: Re: Video output on Master

I Have located the info on gaining a colour video output from the Acorn FTP
server.  I have pasted it below for any interested parties.

Now to work out where those resistors are (-:

Regards

Matthew

Modifications for the Master 128

On Master 128 issue 1 boards a 470 PicoFarad capacitor should be connected
between

the east leg of R137
and the east leg of R153 which are located directly to the east of the modulator.
Connections should be made on the component side of the PCB with the leads of the
470 PicoFarad capacitor being soldered directly to the leads of the two resistors.

On Master 128 issue 2 boards link LK11 should be made.


Modifications for the BBC Model B and B+

On Issue 1, 2 & 3 boards a 470 PicoFarad capacitor should be fitted between the
emitter of Q9 (identified by an "e" next to one leg) and the base of Q7 (the
central leg).
On Issue 4 and later, the capacitor is on board and can be connected simply by
fitting link 39 which may be
found next to the modulator in the far right hand corner of the PCB.
On the BBC Model B+ link S26 should be made.



16th June 1992
Support Group Application Note No. 001, Issue 1
Support Group Application Note No. 001, Issue 1


Tim Fardell wrote:

> I don't know how many replies you've already had to this, so please feel
> free to delete this one if you like!
>
> John Simpson wrote:
> >
> > Taking on board various people's suggestions on how to link a BBC to my
> > telly using either the RGB or Video ports, I had a good look at my TV last
> > night and at my Beeb. Perhaps inevitably I now have some further questions
> > (sorry about this), and I'm afraid they are of an electronically naive
> > nature. If they're off-topic for the mailing list I apologise, please email
> > me directly if this is the case.
>
> I don't think it's off-topic at all. I'm all ears...
>
> > My telly has 2 Scart sockets and a "video in" socket on the front. The
> > manual indicates that the 2 Scart sockets are not identical though: AV-1 has
> > RGB inputs whilst AV-2 does not. Presumably then, if I wanted to use RGB I'd
> > have to move my existing video-to-TV cable onto AV-2. I noticed that the
> > AV-2 port has luminance and chrominance pins as well - does this mean it is
> > equivalent to the "video in" port?
>
> Yes, if you want to use the BBC's RGB output, you will need to connect
> it to your AV-1 socket, which supports RGB signals. You shouldn't have
> any problems with connecting your VCR to AV-2, but (unless you have an
> S-VHS VCR) you may need to check that the input type for AV-2 is set to
> PAL Composite Video, rather than S-Video or S-VHS. This setting is
> probably accessible via your TV's menu system, if it is a relatively
> modern set. (no damage will occur if the setting is wrong - you will
> just see a black-and-white picture). The luminance and chrominance pins
> relate to S-Video signals, which will only be of use if you have a
> 'high-band' VCR (like S-VHS) or something like a DVD player that squirts
> out S-Video. I think that one of the S-video pins doubles-up as the
> composite video pin on the SCART plug.
>
> > The manual does not disclose whether the RGB signals can be TTL voltages, so
> > I'd be a bit nervous of trying this out - could the TV be damaged?
>
> It is possible that the TV could be damaged if it is not designed to
> accept TTL input, so I would check with the manufacturer before trying
> this out.
>
> > Finally I got out the BBC User Guide and had a look at the circuit diagram
> > for the display output hardware. As far as I could tell, the circuit
> > combines the Red, Green and Blue signals into one voltage (I assume that
> > this gives rise to the term "composite video"?) and outputs it to the
> > central part of the socket. What baffles me is previous posters'
> > explanations of how this port can give a colour signal. How this might work
> > is not obvious from the circuit diagram, because the Red Green and Blue
> > components have been combined.
>
> I'm no expert on video, but you are right that the composite video
> signal contains all the picture information in a single signal. My
> understanding is that a small section of the video waveform is reserved
> for a short pulse containing all the colour information for the current
> scan line. If this pulse is not present or suppressed, a black-and-white
> picture is seen. This 'colour-burst' signal is present in the BBC for
> the purpose of driving the RF modulator, but for some reason it is not
> fed to the video out socket. By adding the capacitor to the circuit, you
> are effectively adding the colour-burst signal to the existing
> monochrome video signal, effectively by joining the wires together (the
> capacitor filtering out other unwanted parts of the source signal).
>
> >
> > I realise that an understanding of this is not necessary in order to be able
> > to use it, but I'm curious.
>
> Hope this is of some help to you, John. Let me know how you get on!
>
> Tim.
>
> --
> This message represents the views of the author and does not
> necessarily accurately represent the views of BT.