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Plenty of early '80s microcomputer systems with 16-colour graphics offered digital RGBI output: four TTL (or at least digital on/off) signals for red, green, blue and intensity. These included many models from NEC, Hitachi, Fujitsu and Sharp, IBM PC CGA, some later MSX machines, and the Commodore 128. All of these were "NTSC compatible" in having 240 line progressive output (generally with 200 lines displayed) and using a horizontal sync frequency of around 15.7 kHz.

Were there any systems using RGBI that used a significantly different sync frequency in order to display more lines? I'm looking specifically for RGBI output at these higher frequencies; IBM's EGA, for example, does not fit this because it uses RrGgBb in its higher resolution modes.

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  • What kind of color monitor that's cheap enough for microcomputers in the 80s do you expect these systems to use? And if they use an expensive monitor, why should they restrict to RGBI, instead of using the full capabilities of the expensive monitor?
    – dirkt
    Jul 29, 2022 at 4:32
  • @dirkt I don't know particular models off-hand, but 24 kHz colour monitors were widely available for home/small business computers at least as early as 1981 in some markets. (The NEC PC-8801, with 640×400 resolution, was released in that year.) DRGB was used for many years after this (CBM introduced a new DRGB system in 1985); that 4+ year gap is plenty of time for higher-frequency monitors to get cheap.
    – cjs
    Jul 29, 2022 at 5:37
  • @dirkt FWIW, NEC's original price on the PC-8853K, a 14" DRGB 24.83 kHz colour monitor, was ¥168,000, which was about twice the price of a 15.7 kHz CVBS colour monitor of the time. So not that cheap, but keep in mind that the computers it was typically used with would be in the ¥168,000-¥228,000 price range, not including disk drives.
    – cjs
    Jul 29, 2022 at 7:41
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    The Tandy 2000 reportedly output digital RGBI at 26.4KHz, for a resolution of 640x400. en.wikipedia.org/wiki/Tandy_2000
    – DamageX
    Jul 30, 2022 at 21:59
  • The AT&T 6300 series used a 640x400 display, as did the display card on one of the lab machines at my college. The latter display card could be used with some programs to display 16-color high-resolution graphics, but I don't think the 6300 supported such ability. The latter, however, is interesting for the way it handled CGA compatibility. The CGA card used a 6845 display controller with somewhere between 16 and 32 write-only registers to control dispaly parameters. The 6300 used the same chip, but inserted an 8Kx8 ROM, organized as a 32x256 table, between the CPU bus and the 6485.
    – supercat
    Jul 31, 2022 at 16:12

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TL;DR As is often the case when discussing retro systems with very flexible video output capabilities, the Amiga is an obvious example.

It is often overlooked that the standard 23-pin Amiga video output port includes Digital/TTL, as well as Analog RGB. Pins 6-9 of the port provide the RGBI signal, which I believe is always active.

Most users obviously opted for an RGBA monitor to display the full Amiga OCS/ECS color palette (4096 colors). However, I personally used an RGBI monitor with a front-panel switch for selecting Composite for my Amiga 1000 in 1986-1988. This was a much more affordaable type of monitor at the time, and provided crisp, hi-res, RGBI output with easy switching to composite video for lo-res gaming.

Most Amigas shipped with, or were upgraded to, the Amiga ECS chipset. With ECS, the horizontal and vertical refresh rates are programmable to support roughly double the number of lines in progressive display mode, or double the number of horizontal pixels in interlaced modes. "Standard" display modes for the Amiga are defined/controlled by monitor drivers installed by Workbench.

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