Okay, now let’s answer this challenge question. What kind of serial communications data rates do you need to be able to sustain a direct drive of a raster display? You pass a binary communication channel that is then processed by a very simple DAC to drive a VGA CRT display.
1280 * 960 * 32 * 60 = 2359296000 bits per second
= 2359296 kbps
= 288000 KiB/sec
= 281.25 MiB/sec
Insane! There is no way you could possibly cobble together simple serial communications electronics to do that when 115.2 kbps is considered “fast” for serial communications. By contrast, 10 Mbps is considered “slow” for Ethernet, in fact this was the speed of first commercially available variant of Ethernet. Even 100 Mbps (12 MiB/sec) Ethernet is still too slow for this uncompressed HD video.
Okay, okay, fine, let’s go down to standard definition, 256 color.
640 * 480 * 8 * 60 = 147456000 bits per second
= 147456 kbps
= 18000 KiB/sec
= 17.6 MiB/sec
Okay, wow, that’s still too fast for even 100 Mbps Ethernet to handle, we gotta go monochrome then, and decrease the frame rate while we’re at it.
640 * 480 * 1 * 30 = 9216000 bits per second
= 9216 kbps
= 1125 KiB/sec
= 1.01 MiB/sec
Okay, so now we’re finally in the range of 10 Mbps Ethernet, but still way too fast for even a fast serial connection. That means we have to decrease our resolution and frame rate even further.
320 * 240 * 1 * 10 = 768000 bits per second
= 768 kbps
= 93.75 KiB/sec
Even that low resolution is still too fast for serial communications. At this point, the only efficient way to dramatically decrease the necessary bit rate is by decreasing the spatial resolution. Let’s go down to 200 x 200.
200 * 200 * 1 * 10 = 400000 bits per second
= 400 kbps
= 48.8 KiB/sec
Still too fast for even fast serial communications. If we brought the frame rate down to 1 frame per second, that would be within the range of “fast” 56 kbps serial communications, but the user experience would be dismal for all but the most basic of still image display. Okay then, let’s try further decreasing the spatial resolution.
100 * 100 * 1 * 10 = 100000 bits per second
= 100 kbps
= 12.2 KiB/sec
Now it’s slow enough for a “fast” 115.2 kbps serial connection. However, that means you can only look through a 100 x 100 pixel window to get your graphical view into the digital world of your computer board under test.
What would the resolution need to be to work well under the slower serial connections like 9.6 kbps?
50 * 50 * 1 * 3 = 7500 bits per second
= 7.5 kbps
Okay, wow. So basically, you get to see only one medium-sized monochrome icon if you want to get framebuffer graphics out of a slow serial connection. Absolutely abysmal. What if we want to see a slightly larger icon?
64 * 64 * 1 * 3 = 12288 bits per second
= 12.3 kbps
Then you need a slightly faster serial connection, but at least it’s still not too fast. Alternatively, you could reduce the frame rate to 2 frames per second to get it working better in the slower speed ranges.
64 * 64 * 1 * 2 = 8192 bits per second
= 8.2 kbps
Okay, so I think 2 frames per second is livable… it means you have to wait half a second before you can see the results of your user input, but if it gives me that much more resolution, hey, I guess I’ll go with it. How high can our spatial resolution go at 2 frames per second?
128 * 128 * 1 * 2 = 32768 bits per second
= 32.8 kbps
Looks good, but hey, let’s assume I have a “fast” serial connection. Let’s go higher.
256 * 256 * 1 * 2 = 131072 bits per second
= 131 kbps
Oops, that’s a little too fast, maybe. If you have a really fast serial connection, yeah you could probably do it. Otherwise, 200 x 200 does the trick.
200 * 200 * 1 * 2 = 80000 bits per second
= 80 kbps
With 8 x 8 pixel characters, how many lines and columns can you see?
200 / 8 = 25 lines
25 lines, that’s good. But that’s not quite enough columns. Divide the width by 2 and you get 50 columns, that sounds pretty good. It’s like we’re talking early 8-bit home computers here. The Apple II gave you a 40 x 24 monochrome text screen in standard 40-column resolution mode, and that was basically the limits of readable text you could get on an NTSC color television.
Well, what can I say? 200 x 200 pixels was the resolution of my Flying Bunny movie graphics, and I guess if you can watch monochrome version of that movie, that’s pretty good. But at only 2 frames per second? Heck no. No, you need at least 10 frames per second to get an interesting movie. But still, you could get much more convincing ordered dither grayscale at 60 frames per second. 2.6 bit grayscale, then add error diffusion on top of that and you should be pretty good. You could also try watching at only 100 x 100 resolution, and that would be mostly okay.
Also, as it turns out, that neighborhood of resolutions is about the same resolution that the COSMAC ELF could get for driving a graphics display.
20190614/https://en.wikipedia.org/wiki/COSMAC_ELF
Okay, now after all of that, let’s take a look at the far-far end of modern technologies. We’re talking 16K Ultra-Ultra-HD resolution, high dynamic range wide-color gamut with a 32-bit floating point number per each color channel, 240 Hertz refresh rate. What is the data rate of this?
(8 * 1920) * (8 * 1080) * (32 * 3) * 240
= 3057647616000 bits per second
= 356 GiB/sec
Whoa, now that’s a dewsey. About ten million times the speed of our slow serial communications graphics.
But come on, that’s like you can only see one 8.5 x 11 inch page at 1200 DPI on your display. Clearly, that’s an obsolete printing technology. With modern displays, you want to be able to see at least two full 8.5 x 11 inch pages on your display, so we need to double the resolution for that. Plus, we want to also be able to see 2400 DPI pages with no aliasing, so lets do another (2 * 8 = 16) on the resolution for that. And seriously, it would be nice if we could get 1000 frames per second out of this monitor, again without aliasing, so let’s increase the refresh rate to 9600 Herz.
(32 * 8 * 1920) * (32 * 8 * 1080) * (32 * 3) * 9600 = 125241246351360000 bits per second = 14 PiB/sec
14 pebibytes per second, wow, that’s a lot of data to be flowing through your computer display of the future. Alas, we might never get there as it is apparent that the average consumer really doesn’t care for that much resolution, similar to the fact that commodify computer audio sampling rates have stopped at 48 kHz, although the professional equipment keeps getting higher in sampling rate.