The first thing to note is the picture quality. TVs display reasonable graphics but poor text because the latter require sharp edges and fine detail, as anybody trying to read credits from a TV has noticed. The modulation and demodulation processes limit the bandwidth, and inevitably add some distortion. Most serious users bought a monitor with composite video input, or ideally RGB input. The Philips CM8833 monitor family were popular choices. If you could only afford a monochrome monitor, this was used for text work and colour TV was used for games.
After home-computers were decimated by the arrival of the PC with its dedicated monitors, more TVs started providing inputs that bypassed the RF modulation. This allowed better picture quality from devices such as VCRs.
The BBC Micro has composite video output which is easily connected to a modern TV. The former normally has a BNC connector, while most home video leads use phono plugs. So the only inconvenience is buying a BNC to phono adapter plug.
The BBC's composite video is normally monochrome. The first thought is to enable composite colour by making a link inside the BBC. I found that on my machine the display shows slight but noticeable colour-crawl. Perhaps from age, but I suspect the colour modulation inherently causes this.
The next step was to look for a way to carry the RGB signals to the TV, bypassing the composite video modulation process.
First thing to note here is that this is a TTL level output, which is not the analogue level signal that common PC monitors expect. TTL level monitors assume colour is either on or off, so TTL signals can be up to 5V. Analogue monitors expect about 0.7 or 1V analogue signals. Also, they expect seperate horizontal and vertical sync signals, while the BBC monitor output combines these into a composite sync signal.
Not having a BBC-specific monitor, I do have an Archimedes monitor. The Archimedes has a monitor output socket that is physically identical to PC monitor outputs (15-way high-density D-type) and has the RGB signals in similar positions, but uses a composite sync signal.
Having made a simple adapter to route the signals from the 6-pin DIN to the 15-way D-type connectors, I plugged it in and a picture appeared. However, the Archimedes monitor is optimised for its highest resolution (as are most monitors) and the fineness of the electron beam meant that the horizontal scan lines had small but noticeable gaps between them. So although I had one monitor that could service two machines, it wasn't quite the ideal solution.
TVs are optimised for their intended vertical resolution, so the beam is just the right size so that horizontal scan lines just touch each other. They are also brighter, generally bigger for the same price, and most homes already have one.
Connecting the BBC to the SCART socket of a TV requires that the signal levels be reduced from 5V down to 0.7V. The crudest ways is to use a resistor divider chain. Since the BBC has 75R output impedance and the TV has 75R input impedance, one could simply insert 385R in the signal path.
This is not the "proper" solution because the signals now don't "see" the ideal 75R impedance in their paths, which leads to signals being partially reflected and therefore "echoing up and down the cables. Instead, they each see 460R.
The "proper" solution is to use a divider chain followed by a unity-gain amplifier with 75R output impedance. Inside the BBC micro, this is done by single-transistor amplifiers in simple emitter-follower configuration.
I built similar circuitry on a piece of Veroboard with a SCART socket mounted on it. Small-signal high-frequency NPN transistors (e.g. 2N2222) with collectors at 5V and emitters loaded with 1K to 0V. Emitters connected to the RGB signals of the SCART socket via 75R resistors. Bases connected to the centre of resistor divider chains driven by the BBC micro's TTL RGB signals.
The divider chain has to present the bases with 1.4V + Vbe when 5V is applied. The actual divider chain resistors depend on the distance between the BBC and the Veroboard. If fairly long, then the cable is a transmission line of 75R impedance and therefore the terminating impedance must match. If the distance is short, then the impedance is not critical.
Next, TV must be told to accept input from the RGB signals instead of the composite video. This is done by driving the Fast Blanking signal to about 2V. There are two methods for doing this. (1) Tie it to 5V via a 100R resistor. (2) tie it to composite sync (which is high during the picture line display time) and hope it will be enough. I used method (1) because the BBC monitor output socket has a 5V output which I used to power the transistor amplifiers. Method (2) looks a bit dodgy to me, since the sync pulse is nominally only 1V, but maybe TVs generally consider anything above the "select composite input" level (i.e. 0.4V) as the "select RGB" level.
Next, the TV requires a signal to sync to. This is presented on the pin normally used for composite video input. There are two methods for doing this. Either the composite sync or composite video can be used. If the latter, any composite picture information is ignored because the TV will be using the RGB inputs.
Eventually the level shifter circuitry could be housed inside the BBC micro, in which case the connections are indeed short. I experimented with resistors and found that 2K+3K resistors (Vbase = 60% of TTL input) gave a slightly saturated picture where white pixels were slightly wider than black pixels, which in turn meant the text was slightly blurred. I replaced the 3K resistors with 2K and got better results. Similar experiment may be needed to find the best values for other TVs.
This page is a bit wordier than other pages that just say "here's a circuit". I've explained what I've done so you can understand what is going on and thus be able to design your own circuits.
Standard disclaimers apply: all I'm claiming is that this worked with my TV. If you want to mess around with your own electrical goods it's your own decision. Electronics is my living so I'm confident I know what I'm doing. If you aren't confident, leave it alone!