Description

From Medusa

What is Medusa ?

Medusa is a device that allows connecting of old computers to modern display devices (monitors, projectors etc.).

Little History

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)
  • find exact pixel clock frequency.
  • 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

Solution

  • The 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
  • The second problem is to find exact pixel clock for input signal (for RGB signals). There are no ultimate solution for this. In Medusa you can always manually define number of pixels per line, but to achieve that more automatically we measure specific signal "fingerprint" (based on synchro signals) and pick one from tenths of available options.
  • The third 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". This is of course done only when necessary (input horizontal refresh rate is lower than 31kHz).
  • The last 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. At the same time using simple cable one can also connect it to HDMI device.

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 !!! - more modes will be added with future firmware updates)
  • 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)
  • Allows user to change a lot of settings on the fly using OSD or builtin OLED display (such as contrast, brightness, X/Y picture shift etc.).
  • Output signal analog and digital at the same time on DVI-I, so it can be connected both to old VGA monitors (also CRT ones) and as well as modern HDMI monitors or TV's.
  • Easy firmware update (no special device is needed - just connect Medusa via USB to computer and run updater binary - available for WIN,Linux,MacOS)

Why Medusa in not perfect ?

Because perfect solution doesn't exist. Medusa doesn't change vertical refresh rate, so if your monitor can't accept 50Hz then it will not display picture from Medusa. Most monitors with HDMI inputs are able to display 50Hz (because of TV standards), but old VGA LCD panels not always. I have good experience with LG's and NEC's, but can't guarantee that particular monitor will accept 50Hz. Probably at some point we will prepare list of monitors that can do that. With such approach to screen conversion (simple line doubling) any resolution change on input will also lead to resolution change on output. Even small change will usually cause black screen on displaying device (for about 0.5 - 1 second). This is normal - this is how displaying devices works. The only way to circumvent that would be to constantly display picture in particular resolution (f.e. 1920x1080@60) and sample pixels from input and put them in displayed framebuffer. But in such approach a lot of problems would occur. For example all 50Hz scrolls would be jagged. This is why we decided not to do that. Also device would be much more expensive.

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