The early history of colour TV had several false starts, of which perhaps one of the most interesting might-have-beens was the CBS field-sequential system. This was a rival to the nascent system which would become NTSC, which instead of encoding red, green, and blue all at once for each pixel, made sequential frames carry them.
The Korean war stopped colour TV development for its duration in the early 1950s, and by the end of hostilities NTSC had matured into what we know today, so field-sequential colour became a historical footnote. But what if it had survived? [Nicole Express] takes into this alternative history, with a look at how a field-sequential 8-bit home computer might have worked.
The CBS system had a much higher line frequency in order to squeeze in those extra frames without lowering the overall frame rate, so given the clock speeds of the 8-bit era it rapidly becomes obvious that a field-sequential computer would be restricted to a lower pixel resolution than its NTSC cousin. The fantasy computer discussed leans heavily on the Apple II, and we explore in depth the clock scheme of that machine.
While it would have been possible with the faster memory chips of the day to achieve a higher resolution, the conclusion is that the processor itself wasn’t up to matching the required speed. So the field-sequential computer would end up with wide pixels. After a look at a Breakout clone and how a field-sequential Atari 2600 might have worked, there’s a conclusion that field-sequential 8-bit machines would not be as practical as their NTSC cousins. From where we’re sitting we’d expect them to have used dedicated field-sequential CRT controller chips to take away some of the heartache, but such fantasy silicon really is pushing the boundaries.
Meanwhile, while field-sequential broadcast TV never made it, we do have field-sequential TV here in 2026, in the form of DLP projectors. We’ve seen their spinning filter disks in a project or two.
1950 CBS color logo: Archive.org, CC0.
The most famous example of a field sequential system was the colour TV standard used on Apollo and early Shuttle flights. It was very similar to the CBS system but used the NTSC frame parameters (525-line, 30/1.001 fps) for easier conversion back to NTSC.
https://en.wikipedia.org/wiki/Apollo_TV_camera
https://www.tvtechnology.com/news/equipping-apollo-for-color-television
this is legit intresting, why did the Korean war slow down tv development? sick timing I just got my partner on board with watching my MASH collection. Im really into how TV was filmed back then so Im stoked to annoy her some more while zhe tries to get through 70s/80s era tv
*she, lol pronouns
The National Production Authority specifically banned the manufacture of colour television sets during the Korean War, claiming it was done to conserve materials like copper and steel. At the same time, CBS (the only game in town for colour television at the time) was experiencing huge issues with demand and broadcaster takeup (as it was incompatible with B&W sets) for their system so it’s not outlandish to suppose that the NPA might have been deliberately doing CBS a favour.
The breathing room this created allowed for the creation of compatible-colour, avoiding a format war.
Well JVC did come out with a field sequential monitor.
https://www.youtube.com/watch?v=sUFFqBhGUgY
Why would the processor need to be faster? Early video cards used hardware to turn a bitmap (or character map) into a video signal. A 6502 or Z80 wasn’t fast enough to do any real video generation directly.
Isn’t the issue how the Apple II (and most other 6502 systems) interleaves RAM access between the CPU and video hardware? I would assume that in this alternate reality, they would have done something different. Maybe use high speed RAM like the BBC micro did? I’ve done absolutely no investigation of how the timing might have worked, but we are making it up anyway, so why not!?
Woz is master of the TTL shift register.
Apple 2s video generator was amazingly simple.
Half the hirez colors would render half a pixel to the right…
On B&W screens, double rez!
Early floppy controller, same trick.
If the resolution and effective frame rate had stayed the same, the video output hardware/interrupts would have had to access ram 3 times as often.
Unless the RAM to screen map was rearranged and it was only hitting each color once.
It’s just a bandwidth issue, IIRC on the crapple refresh and video shared half the ram ‘bus’.
Been forever, video ram access might have been refresh.
As you say, interleaved.
You just might have been able to squeeze hirez in by ‘underclocking’ refresh and using overspec ram.
Also upthread to Bill.
Timex/Sinclair says ‘hi’, using CPU to generate video directly was exactly what it did.
It shut down video to ‘think hard’.
What is a field sequential computer?
What is a field sequential home computer?
What would it do?
instead of a screen being scanned RGB simultaneously a screen was scanned 3 times once for each color with a filter applied at each scan. This required a 3x faster scan rate, not an issue for a CRT based system, and resulted in a 3x higher perceived resolution.
While some higher end DLP projectors employ 3 chips one per color, Most DLP projectors use one chip and operate with this same sceme. An array of 1920×1080 mirrors switching at 3x framerate and color filter between sequential (color) fields, allowing a higher resolution than would result if the 1920×1080 field was divided. The chip can juggle this easily, so easily that 4k projectors operate with the same 1920×1080 chip shuffling at 6x framerate shifting projection slightly between frames to double their perceived resolution.
So basically, a field sequential computer, home or otherwise, would have given our old systems a much more respectable resolution. at least in the horizontal axis. Adding to the vertical would have been another can of worms of its own to sort out.
Thank you, I read the article.
“So basically, a field sequential computer, home or otherwise, would have given our old systems a much more respectable resolution. ” Is this a separate box from “our old systems” or are we now calling video cards “computers”?
Originally computers didn’t necessarily have video cards
old computers, or old systems, whether they had video cards or onboard video would have had higher resolution if their video feed scheme and monitors were both designed for field sequential operations.
Think of it like this, Raster scan and vector scan took two different approaches to producing images. Raster scan going line by line and vector tracing out the shapes. Field sequential color and RGB clustering are two different methods of implementing color image creation. Raster scan was the commercially successful system, as was RGB clustering.
Tech history is full of these sort of battles. Raster won its battle because its superior performance at forming images over raster which was really only good for wireframe style images. RGB won over field sequential color primarily because it was easier simpler/cheaper to manufacture. In DLP field sequential is the norm because a spinning color wheel is cheaper/easier than driving 3 separate chips.
Presumably field sequential TVs would have been backwards compatible with black and white. So the computers would have had high-resolution black and white modes and low-resolution color modes.
they would have had the same resolution. A DLP projector can project B&W images using field sequential red, green, and blue layers. A FSTV could have done the same. They also could have been equipped with an additional clear “filter” that would allow them to skip the 3 color mix and display B&W with either less data throughput or triple framerate. Either way there wouldnt be a reduction in resolution.
I mean, classic black and white TV has one third the framerate a field sequential color TV would have had. Thus the needed screen pixel frequency would be less when operating in black and white mode.
All this is assuming it would have had compatibility with the old black and white transmissions.
classic black and white would only have one third the framerate if the system was setup with a fourth null field filter. Otherwise it would have to mix red green and blue fields to create the white of the black and white, Just as CRT sets had to do to display shows broadcast in black and white. There is no reason for any color capable system to need to use a different scheme for black and white than it does color.
Asking the real question: How would a Computer interface to a mechanical TV like the original Baird Televisor? What would you do with it?
(If I had more time… tons of more time… I’d try developing a PONG console, ideally using vacuum tubes, that works with a Baird Televisor)