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Revision as of 18:11, 2 April 2022
What is Medusa ?
Medusa is a device that allows connecting old computers to modern display devices (monitors, projectors etc.).
When first home computers appeared people didn't have special computer monitors in their homes, but almost everybody has standard TV set. This is why first home computers (such as ZX Spectrum, Atari, Commodore) were designed to be connected to standard TV set. In this times there were different color encoding standards - mainly NTSC, PAL and SECAM. Main differences between those standards were screen refresh rate (60fps for NTSC, 50fps for PAL and SECAM) and way of color signal encoding, but one thing was common - horizontal refresh rate (about 15kHz in all standards). These two values: horizontal refresh rate and screen or vertical refresh rate define number of lines per screen and therefore maximal vertical resolution. In 50fps systems there were about 312 lines per screen and in 60fps systems even less (about 262 lines). In times of 8-bit computers resolutions such as 320x200 was considered "hires", so it was absolutely enough. In 16-bit computers it was still ok but users started to want more. Atari for it's Atari ST model introduced special hires monitor (with 70fps and about 31.5kHz horizontal refresh rate - with resolution 640x400), in PC's they introduced EGA standard (21.8kHz and resolutions up to 640x350), and couple years later VGA with 31.5kHz horizontal refresh rate. At the beginning every video standard (computer or graphic card) needed dedicated monitor. The problem was that users didn't want to buy new monitors every time they change their graphics adapter. And in 1989 in America VESA organization was incorporated to introduce video standards. The only problem was that when they started defining their standards we already had Super VGA with resolution of 800x600 and nobody wanted to use old 15kHz resolutions anymore. Therefore the lowest available "standard" resolution defined by VESA was 640x350 with horizontal refresh rate 31.5kHz, so this was the lowest horizontal refresh rate accepted by almost all monitors. The other problem is also vertical (or screen) refresh rate. Minimal refresh rate defined by VESA is 60Hz (60fps). This is why most modern monitors didn't accept lower horizontal refresh rate than 31.5kHz. With vertical refresh rate is little better because with introduction of HDMI compatibility, with its TV standards, forced 50Hz, so majority of monitors that have HDMI port should accept 50Hz.
Problems with old computers
As we can see when we want to connect old computer to modern display we can meet such problems:
- color decoding in PAL/NTSC signal (in case of Composite Video or S-Video signal)
- double horizontal refresh rate (when we double 15.5kHz we are close enough to be accepted by modern display)
- some modern displays don't have analog input - only digital
- First problem to solve is to decode PAL or NTSC signal (some old computers don't have RGB output). To achieve that Medusa uses specialised video decoder. Integrated circuit created by Analog Devices that can sample and decode SDTV signal (PAL, NTSC or even SECAM). It outputs data in digital form YCrCB - one chroma per two luma pixels. The whole line has 720 pixels which is more than enough for old 8-bit computers
- Second problem is to double the horizontal refresh rate. To achieve that every line from input is put into small memory block inside FPGA chip and then emitted twice. This is why sometimes we call such devices "scan doublers".
- Third problem is to output signal in digital form. In Medusa we decided to use DVI-I standard. DVI standard is theoretically obsoleted by HDMI, but since HDMI is licensed and DVI not, we decided to use DVI. Also DVI in full version (which is in Medusa) also outputs all signals in analog format. This is very convenient because it allows using older 17" and 19" LCD panels that are very cheap and have better screen proportions for old computers than new TV's.
Why Medusa ?
This is actually very good question. There are a lot of devices that do similar things. The only problem is ... I've tried almost all of them (FrameMeister included). They all lacked something.
- Cheap Chinese devices uses chips designed for TV's with a lot of features that are good in case of movie but unacceptable in case of computers (few frames of latency). Also picture quality is horrible - RGB signal is sampled with constant ratio - not in sync with original pixels. They can't accept anything other than PAL or NTSC-like signals (forget about ST-mono or Amiga-AGA funny resolutions)
- FrameMeister - very good device, but expensive. It also accepted only PAL and NTSC-like resolution (no ST-mono etc.)
- OSSC - also very good device, but without composite video and s-video inputs. Only good for RGB signals. Touch to configure
So what can do Medusa ?
- Accept CVBS signal (Composite Video) on SCART (or JP21 - special JP21 version)
- Accept S-Video signal on SCART (or JP21)
- Accept RGB signal on SCART or VGA input (with more than 50 predefined settings - for popular old computers - it not only can work with ST-mono, but also Viking card emulated by MIST - yes 1280x1024 !!!)
- Encode audio can be switched on and (using DVI-HDMI adapter and HDMI capable display) audio signal from SCART (or 3.5mm jack) is digitised and mixed with video)
- OSD display with a lot of settings (such as contrast, brightness, X/Y picture shift etc.).