Thus I had to combine the HSYNCVSYNC from a CGA card, into single composite sync CSYNC, otherwise it would not work at all.Īnd this ought to be done properly with a sync combiner IC, but theres also a sloppy way of doing this: using a quad-XOR gate (see schematic below), that can work just as well.Īlso, the Intensity input is being handled, to account for three different shades: blackno video, normal text, high intensity text.Īll of these TTL levels are smacked down and mixed passively to one single composite video signal with sync. Notice the much better contrast and a lack of ghosting which is very prominent on the composite video.Īs an example, the GBS-8220 will not sync on an MDAHercules signal (18.43kHz HSYNC, -50Hz VSYNC), or the 640400 Hi-Res CGA from an Earthstation-I.Īlso, the upscaler provides both HSYNCVSYNC and CSYNC (composite sync) inputs however, the HSYNCVSYNC combo is only used when a VGA signal is being fed. This was connected directly from the circuit above, to the SCART connector of the TV, without an upscaler. If you do use it however, be sure to calibrate the colors with the color adjust trimpot, preferably by generating a 16-color test pattern, first. However, note that you might need to force the TV to use the SCART RGB video input by applying 1V to 3V onto pin 16, or find a setting in the Menu to do it for you. On display devices that accept an analog RGB input directly (TVs with a SCARTEuro-AV connector), the upscaler can be omitted, but only if the input video signal is conformant with the PAL (15.625kHz HSYNC, -50Hz VSYNC) or CGANTSC standard (15.75kHz HSYNC, 60Hz VSYNC), and the TV supports it. The fourth shade, which was not always supported, was setting the video line low with high intensity line high, yielding dark greenambergray, depending on the phosphor.Ĭonnecting this to a suitable HDMI or VGA upscaler, to scale up the resolution, yields a video signal that is compatible with more modern displays and does not require a vintage monitor, which relies on the slower horizontal sync frequencies (newer monitors dont go that low on HSYNC). On a monochrome display, like the IBM MDA or Hercules, three and sometimes four shades, could be used to display text or graphics.ĭepending on the monochrome monitor used, this was either green, amber or gray: video line low (black), video line high (greenambergray) and video line high with high-intensity line high (light greenambergray). Later, this limit was surpassed by switching the palettes andor varying the Intensity for four levels, instead of two (low and high intensity), to a maximum of 64 colors.
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