NBTV colour? by Sam Hunt
About 6 months ago, I handed in for my Electronics course work a system which I had devised which gave colour to 405 line TV (I got an A). This system was very simple, and allowed you to get full colour on a connection with no bandwidth limitation. When the bandwidth started to become limited, it dropped first down to a 2 colour system, then to a mono system. It worked very well, and gave very good colours at 3 colour, and quite good colours on the 2 colour system. The mono signal was just the same as standard 405 line TV. If the colour signal was being played back on a standard 405 line TV (The school, amazingly, still had one of these!), then I could not notice any extra noise, or reduction of picture quality.
I went through my proposal for this system, which outlined all of the key points, then changed all the frequencies to more sensible frequencies for NBTV, then changed some of the content. I have put this explanation of the system onto the end of this message.
The only difference between a colour and a mono mechanical receiver would be that it has three light emitters, not one (One for red, one for green and one for blue), and it has a colour decoding circuit.
Anyway, here is the explanation. I thought you may be interested.
Explanation of frequency multiplexing colour drop down system
This system is that at 0-10KHZ there is a channel containing all three colours. At 10-20KHZ there is a channel containing both the green and the blue colours. At 20-30KHZ there is a channel containing just the blue colour. At the top of each frequency band, there is a pilot tone.
How it works is like this. If all frequencies are being received by the reliever: The reliever tests for a 30KHZ pilot tone, and detects one. It then decodes the green + blue channel (10-20KHZ), and subtracts this from the mixed channel (0-10KHZ). This leaves the red channel, and the blue +
green channel. Next the blue channel is decoded (20-30KHZ), and this is subtracted from the green + blue channel. This leaves the green and blue channel. You now have all three colours to feed into all three emitters.
If frequencies past 20KHZ are being cut off. The reliever tests for a 30KHZ pilot tone, and fails. It then tests for a 20KHZ pilot tone, and succeeds. It then decodes the green + blue channel (10-20KHZ), and subtracts it from the mixed channel (0-10KHZ). This leaves
the red and the green + blue channel. This is a fairly acceptable two colour system (Red + Cyan as the primaries). Using this system, flesh tones are very accurate. Sky tones are quite accurate, although you can tell something isn't quite right. Sky still looks pleasing to the eye. Grass and trees are quite inaccurate, but they are still acceptable. Any shade of gray is 100% accurate.
If frequencies past 10KHZ are being cut off (Or even less than 10KHZ) If this is happening, the reliever has no choice but to revert to the mono signal. It tests at 30KHZ and 20KHZ, and fails to detect a tone. It then feeds the signal directly to all three emitters. This results in a mono image. A mono signal would display in black and white, since there are no pilot tones.
Colour relievers can display mono signals because they would not detect any pilot tones, so could revert to the mono signal (0-10KHZ)
Mono relievers can display colour signals because they cannot display frequencies above about 10KHZ anyway. Newer sets may be "Colour enhanced" - They have a low - pass filter, which ensures that there is no chance of the colour signal interfering with the mono display.
The advantages over field sequential colour? This system gives much less flicker than field sequential colour, and it has backwards compatibility. It does not need any extra synchronization, and uses up the "spare" bandwidth which is not used for anything else at the moment.
This system could also have different bandwidth for different components - the mono (0-10KHZ) channel could have 10KHZ bandwidth, then the green + blue channel could have a lower bandwidth (About 7.5KHZ), since the eye is less sensitive to resolution in green and blue than it is to resolution in red. Finally, the blue channel could be about 5KHZ. This is because the eye is very insensitive to resolution in blue. This also would give a more realistic bandwidth of 22.5KHZ, which can be stored on CD, compared to 30KHZ, in which part of the blue channel would be ignored.
Your comments on this subject would be appreciated.
Thank you for taking time to read it.