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While early microcomputers used analog video outputs (often to use a television as a display), higher end machines such as the BBC Micro or Commodore 128 supported a digital RGB (or RGBI) video output. In the IBM-compatible world, the CGA and EGA display adapters likewise had digital outputs.

When the VGA standard landed in 1987, it was analog, rather than digital. VGA and analog output held sway for well over a decade or two, but then DVI, DisplayPort and HDMI took video outputs back to digital.

Why did video standards abandon digital in the first place, and then return to it several years later?

Early digital video outputs, like CGA, were not really akin to the later standards such as DVI and its follow-on's. The reason for using multiple lines to carry the different analog portions of the signal to the monitor was to prevent crosstalk interference of these signals.

You can see this in the very early computers like the Commodore 64 and Atari 800 separating the luminance and chrominance signals to produce a cleaner display. Similary, CGA (RGBI) separated each color component from the sync components in order to deliver a cleaner version of all those signals to the CRT, minimizing crosstalk. VGA, which was not limited to only 4 bits for representing color information, continued this approach of separate signal lines for color and sync.

Remember, within the CRT, there are only 3 color signals possible for controlling the 3 electron guns that will produce RGB on the display phosphors. So when VGA took the technology from CGA's 2^4 possible colors to VGA's 2^18 possible colors, it was still only necessary to separate the R from the G and the B signals. There would be no point to using 18 separate lines to carry the 18 bits of digital color data to only 3 electron guns in the CRT.

So, the more essential technology change was the move from CRT's, which are analog peripherals, to LCD screens, which incorporate more digital processing in controlling their individual pixel intensities. With LCD's, obviously there are no purely analog electron guns assigned to the primary colors. This fact, coupled with excellent advances in the bandwidth of serial communications channels, allows modern displays to carry the full digital display signal in its computer-native form to the display's processor.

Older monitors had analog timing and, with one notable exception, were designed so that the signals presented on their inputs at any moment would fully describe the color to be displayed at that moment. The only monitors that used any sort of time multiplexing on their inputs were those that used the same analog composite color encoding methods used in broadcast television receivers (and typically used much of the same circuitry to decode the signals).

Older "digital" monitors would need to send video signals over enough wires to identify every possible color as a binary number. For a 64-color monitor like an EGA display, that required using six signals, and for a 262,144-color monitor like the VGA it would have required 18--too many to be practical. Using an analog RGB or YUV (today called "component video") signal reduced the number of signals required to three without imposing any constraints on pixel timing.

Newer standards like HDMI require sending high-speed serial bit streams with many bits of data per pixel. That in turn requires higher data rates and thus faster switching rates than could be accommodated with older technologies.

Before VGA was invented, CGA RGBI used 4 wires to get 16 colors and EGA used 6 wires to get 64 colors on the cable. Add 3 more for sync signals and signal ground, and this won't be an issue, simple cables and connectors exist for getting EGA's about 16.3 million pixels per second digital signals over to the monitor easily.

Amiga was released. It supports 4 bits per color component, or 12 bits of color for 4096 colors. It can be connected to a standard color TV directly (via SCART analog RGB), and the RGB is also used to generate color composite and RF outputs. It also has digital RGBI interface, but only 16 colors so why use it, analog RGB is much better, and even composite with single RCA connection is tolerable.

Enter VGA. You have up to 8 bits per pixel at about 28.3 million pixels per second. Digital video links such as LVDS or SDI have not been invented yet, because there has been no need for them yet. But RAMDACs exist. What any sane designer would do is to add one of these to offer a color look-up table as well, and to generate 6 bit output per component, or 18 bits per pixel, directly converted to analog RGB. This means no huge amount of digital wires are needed, just 3 coaxial cables for analog video, plus 2 wires for sync. Plus the monitor can just eat analog signals as digital signals have to be converted to analog anyway for driving CRT guns. This just made no sense to connect VGA digitally, or the number of colors or color palette lookup feature would have to be discarded.

Besides VGA, others use analog connections too (SUN/SGI, Apple..). Resolutions increase, refresh rates increase, color depths increase. Transmitting high bandwidth analog video needs more precision and engineering to have acceptable quality. Meanwhile, LVDS was invented, finds its use in LCD panels as a digital video link. Laptops need LCD panels so they have LVDS chips after color look-up table, so LVDS gets eventually integrated to chipsets. Finally CRT monitors start to be replaced with LCD displays. As the source and display device are digital, the high quality analog link between them could be eliminated cheaply with digital interface to avoid extra conversions. First using the laptop-based LVDS technology in various forms, but those were just external versions of internal laptop display links, so very early on TMDS-based DVI link replaced these. So more bandwith and quality is much more easily and cheaply gotten over digital link than doing analog conversions on both ends, since they are not even necessary. The rest is history.

I believe it was cost and terminal emulation. The RGB monitors were very expensive at the time and would not work (easily) as dumb terminals. IBM wanted a "low" cost display that was double the resolution of CGA and that could be used to emulate their mainframe 3270 terminals. EGA was a temporary stop-gap design while work continued on VGA - 640x480 graphics and sharp 80 col x 25 row display in "B&W" and color. Lotus 123 never looked so good as did the 3270 display on a VGA screen.

Business PCs sales really took off when VGA hit the streets. You could now use one display to do PC work and emulate an IBM mainframe 3270 terminal. Dec VT100/200 emulation and Burroughs (Unisys) B6000/A10 TDI emulation soon followed. VGA was the key to having only one terminal at your desk. I was in banking in Chicago at the time and we couldn't get enough IBM XTs with VGA and 3270 cards. We made a lot of money using those 640x480 VGA monitors for PC work and dumb terminals. Networks based on Ethernet were not mainstream yet - Mainframes paid the bills.

That question is build on somewhat weak ground. After all, most of the video formats you list as 'digital' aren't such - or at least not more as any of the analogue ones are. Just because an output like RGBI uses two level per channel doesn't make it digital. They only feature a restricted number of signal levels.

Digital signalling only started with formats like DVI/HDMI/etc., were signal data was not transferred as levels but a data stream. Everything before isn't digital.

Now, the decision to use RGBI is one about saving components to encode the signals by some complex modulation (like FBAS or SVIDEO) just to add even more components to the display to decode it again.

And yes, as snips-n-snails suspects, it's all about amount of data - just not so much about memory than transfer bandwidth. Using multiple lines increases bandwidth without increasing data rate. Increasing values per step (and lane) as well increases bandwidth without increasing signaling steps.

Here also hides the reason why it took so long to switch for real digital transfer - analogue already delivered an awesome amount of bandwidth. But with increasing bandwidth even further, hard to compensate signal distortion grew as well.

Going digital offers the solution by separating transfer from content. Distortion on transfer level are compensated or corrected) without influence to the data (and thus the image) transferred.