Its a compromise of awkwardness v quality v price.

There's the RGB to HDMI approach. But that either requires a small computer, or a professional converter.

Incidentally I see Ebay suddenly has several of these....

https://www.ebay.co.uk/sch/i.html?_nkw=rgb-hdmi+300

....but they would require resistors to bring the TTL down to 1Vp-p.

There's composite video..... with its bleeding colours.

It occurred to me that to produce the composite and RF outputs from RGB, that YUV must exist somewhere inside the electron.

YUV to digital video converters are far more commonplace, easier to get hold of, and cheaper.

So, this would be a modification which would then require an external (yet cheap) adapter, but 'should' provide a very good image quality.

Before I proceed any further I must confess my knowledge of analogue circuitry is limited. Whilst at university most of my course concentrated on digital circuitry. I just about understand what is going on in this bit of circuitry, but precisely how. Furthermore, I don't have any suitable test kit nor means to check out my 'theory'.

Anyhow, cue the (badly scanned) circuit diagram. For both the RF and composite outputs the luminance/Y is derrived using a small network of resistors.

The composite video path uses different resistor values to the RF path for some reason. I don't know why.

Y=(R x 0.299)+(G x 0.587)+(B x 0.114)

I

*think*U and V are somehow created by six XOR and six NAND gates.

U = B - Y = B - ((R x 0.299)+(G x 0.587)+(B x 0.114))

V = R - Y = R - ((R x 0.299)+(G x 0.587)+(B x 0.114))

The clever bit is that V is 90 degrees behind U. Achieving this in binary logic is (

*I think*) the clever bit.

This network of gates has three extra inputs which originate from two places:

**J**- originates from a 17MHz crcrystal (which I guess generates the 4.4Mhz chroma-subcarrier when divided by 4) - Likely generates chroma-subcarrier

**K**- a function of !HS and a 17MHz crystal (Aplogies, no ideas what this bit does)

**L**- colour burst which occurs briefly at the start of each line, triggered by !HS.

I unpicked what the logic network does....

XOR gates:

X1 = R XOR G

X2 = B XOR G

X3 = B XOR J

X4 = G XOR J

X5 = G XOR K

X6 = B XOR K

Note, only one function of R so far.

Unpicking the NAND gates

N1 = X2 NAND X3 = (B XOR G) NAND (B XOR J)

N2 = X1 NAND X4 = (R XOR G) NAND (G XOR J)

N3 = X1 NAND X5 = (R XOR G) NAND (G XOR K)

N4 = X2 NAND X6 = (B XOR G) NAND (B XOR K)

N5 = X4 NAND L = (G XOR J) NAND L

N6 = X6 NAND L = (B XOR K) NAND L

The outputs of these go through various resistors before being mixed together along with the output of X1 (R XOR G) and X2 (B XOR G).

Would the calculation of the 'mixing' be made using a parallel resistor formula?

Again, U = B - Y , and V = R - Y

N1 and N4 are a function of B. Therefore, I reckon the sum of N1, N4 and X2 = U.

N2 and N3 are a function of R. Therefore, I reckon the sum of N2, N3 and X1 = V. Cleverly, somehow 90 degrees after U.

These signals then go through a coil and capacitor which I guess round the whole thing off to a nice (enough) sine wave, before being buffered by a capacitor biased at 2.5V. The output of which is then mixed to the luminance signal before the RF modulator.

I ask the more knowledgable Acorn hive mind, would my theory work?