|Connecting a BBC micro to a Television|
John Simpson. Issue 2: August 2000
Written using original material from Mike Cook, and incorporating contributions from Colin Fraser, Tim Fardell, Paul Tribick and others.
Converted to HTML by J.G.Harston, June 2005
However, modern televisions typically have a number of input ports and these offer several new opportunities to the BBC user. This document therefore sets out to explain the various methods by which television sets can be interfaced with a BBC micro.
The author does not have an electronics background and has therefore tried to explain things in simple (and hopefully accurate) terms.
At the top left of the diagram it can be seen that the data is originally supplied along three lines - Red, Green and Blue. These are fed directly to the RGB output port along with 5v and 0v power lines and a "Sync" signal. The red, green and blue signals are TTL, which means that they are either on (5v) or off (0v). Consequently, the machine can output just eight colours: black, white, red, green, blue, yellow, magenta and cyan. More modern machines such as the Archimedes are able to produce more colours because they output an analogue value for each of the basic colours.
In the centre of the diagram the red, green and blue signals are superimposed to produce a single "luminance" value. This is termed "composite video" and is effectively a black and white signal. It can, however, have a "colour burst" (chrominance) component added to it to give a colour output, and the method for doing this is explained in section 6. Alternatively, the chrominance signal can be fed out of the BBC separately and used along with the luminance signal to provide an "S-video" output (section 5.4).
The composite video signal is processed further to produce an equivalent which is modulated to radio frequency (RF) by the UM1233 TV modulator. This unit feeds the TV port.
Hence the BBC micro can output the same picture via three ports:
|RGB||Red, Green and Blue digital signals to drive a monitor|
|Video out||PAL Composite video in black and white (colour if modified)|
Since all television sets have an aerial socket it follows that the TV port is the automatic choice for connecting a BBC micro to a television. However, this simple solution may not always be successful, because of the way in which the BBC's display circuitry works. Since the signal has to pass through so many processing stages (RGB -> PAL composite video -> RF) there are many opportunities for signal degradation to occur. The RGB output is therefore likely to be much better than the RF output, especially on old machines.
If two SCART sockets are fitted then they may not both offer the same functionality. All SCART sockets can accept composite video (monochrome or colour), but some can also accept RGB signals. To identify the capabilities of the SCART sockets on your television it will be necessary to read the manual or contact the supplier.
If two SCART sockets are fitted then typically only one will be RGB-capable. This socket could be used for either a games console (outputting RGB) or a video recorder (outputting composite video). A non-RGB capable socket could only accept a video recorder.
If no video in connector is present on the television then there is likely to be one on the video.
These are described in sections 5.1 to 5.4 below. In the case of option 3, the default picture displayed would be monochrome but it is possible to produce colour output if the motherboard alteration described in section 6 is carried out. Option 4 also requires part of the same modification to be carried out.
Six-way screened cable of the length required, of 0.7/2 gauge. (NB: if the cable is to be used with a TV set in a lounge, it may be wise to ensure a generous length of cable.)
The resistors are needed because whilst the SCART inputs are required to be 1v peak to peak, the BBC micro outputs 5v peak to peak. The resistors form a potential divider with the internal impedance of the TV set itself, reducing the voltage delivered to the SCART socket, as shown in the diagram overleaf:
This will be subject to the normal limitations of composite video, namely:
The connections to the SCART plug would be as follows for this option. It should be noted that this has not been tested and hence cannot be guaranteed to work. Pin 17 is "RGB inhibit" and connecting it to ground disables any input from the Scart RGB pins
S-Video is basically the same as composite video but with the luminance and chrominance signals separated out. This gives a better picture which does not suffer from the blurring which occurs when the BBC's composite video output is modified to give colour.
To use S-video, the chrominance signal must be located on the motherboard. Section 6 describes which components have to be added to condition the signal but in order to use S-video the actual link to the video circuits must not be made. Instead the chrominance signal should be fed separately to the mini DIN plug.
On a standard BBC B the signal is to be found on the East solder point of link S39. To this a 1kOhm resistor should be added. The overall arrangement should thus be as follows:
Ready-made cables with a mini-DIN plug at each end can be bought from electronics shops. These can easily be adapted to suit this application by cutting off the plug at one end, soldering the 1k resistor onto link 39 and soldering the ends of the cable to the appropriate positions.
Alternatively, for a neater solution the existing BNC "video out" port can be removed and replaced with a mini DIN socket. These are available from Maplin as part JX08. The pins of the socket should be wired as shown in the figure and a standard S-video cable used to connect the BBC to the TV.
The SCART socket has an 0v pin for sound plus a left and right hand input. Because the BBC outputs mono sound only, the two stereo inputs must be linked. In addition, a 10kOhm resistor must be used to attenuate the sound signals to a level acceptable to the TV. The wiring within the SCART plug is thus as shown:
The input to the speaker can be found on the motherboard by tracing the red and black wires which lead to the speaker. The speaker wire must be unplugged and tucked to one side, revealing the two pins which stick up from the motherboard. The signal should be on the left hand pin with ground on the right, but it would be wise to check this with a multimeter.
A replacement plug is needed to go onto the motherboard pins, and from here a two-core cable should be routed out through the cooling holes in the back of the case and up to the SCART plug.
However, no responsibility can be accepted for damage caused to computers, TV sets or persons as a result of following this advice.