Nintendo's GameCube is a fascinating design - and an under-appreciated masterpiece of console technology. First released in Japan in September 2001, it offered a vast leap in 3D power compared to its predecessor - the Nintendo 64 - while at the same time delivering the whole package in a tiny form-factor. But there are hints that Nintendo had further plans for its machine, ideas built into the design that were never fully utilised - until now. A series of HDMI adapters for the machine are now available, delivering crystal-clear 480p, derived from a lossless digital signal that was mysteriously built into the GameCube hardware.

PlayStation 3 would go on to be the first mainstream console to deliver a digital video output via HDMI, but the fact is that Nintendo built a similar pure digital output into the GameCube five years earlier. What's curious about this is that the only digital display interface with any kind of traction at the time would have been DVI, a standard for PC monitors and surely not Nintendo's target for the functionality built into the GameCube. Perhaps not surprisingly then, the digital AV output was barely utilised.

Indeed, the only use for the digital output - bizarrely - came from Nintendo's official component cable, which featured a built-in digital to analogue converter (DAC) for the most pristine image quality you could get from the hardware. Nintendo's component cable was never distributed that widely and now command prices of around $200-$300 on eBay. However, its existence has made the new wave of HDMI dongles for the Cube possible - the DAC itself has been reverse-engineered via an open source effort, and mapped onto an FPGA that now allows GameCube owners to hook up their consoles to modern screens via HDMI. These adapters range from $75 for the Insurrection Industries' Carby, up to the $150 for the Eon GCHD Mk2 I was sent for review, which offers a number of additional features.

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None of these options are cheap - the use of an FPGA precludes this - but they are worthwhile on two counts. First of all, the only real alternative with a decent quality level is the astonishingly expensive Nintendo component cable. Secondly, it's worth stressing that these devices shouldn't be compared with the cheapo HDMI dongles that shipped for the Wii - those devices converted analogue to digital and scaled the image, while the GameCube adapters deliver a pure end-to-end digital signal. On that second point, the Wii may have been based on similar hardware to the Cube, but the digital output was actually stripped out of its design. In fact, functionality was actually removed in later versions of the GameCube console too, which will not be compatible with the new wave of HDMI dongles.

All of which raises the question of why Nintendo added a digital output to the original GameCube design in an era before HDMI existed and before there was any digital interface on the kind of consumer-level displays the console was likely to be attached to. Clearly there was a cost significance to this as otherwise Nintendo would not have removed it later on in the console's lifespan. The most compelling explanation is that if there wasn't a display available that could work with the digital output, Nintendo was planning to make one itself.

This was actually confirmed by Nintendo's Satoru Iwata himself in an old Iwata Asks article dating back to the release of the 3DS. Iwata describes a 'special LCD' designed for the kind of stereoscopic 3D gaming that the 3DS handheld would eventually deliver, with a 'functional' 3D version of Luigi's Mansion developed in-house for the Cube. "Even without special glasses, the 3D looked pretty good," Iwata said. "But we considered how much the liquid crystal would cost, and it was just too expensive. We figured the market just wasn't there for it."

Iwata also talks about the GameCube having "3D circuitry built in" which again seems like a good fit for the digital AV functionality that ended up being used just for the official component cable. But today, that barely-used part of the GameCube tech is opening the door to pure HDMI support for the vintage 2001 console. Several commercial solutions are available, all based on the GCVideo open source project.

The Eon GCHD Mk2 features HDMI and analogue support (both can run simultaneously) along with a 3.5mm stereo sound output that also doubles up as a mini-Toslink port.

At the minimum, you get an HDMI implementation for the GameCube, but the new Eon GCHD Mark 2 goes further, with headphone and mini-Toslink support, plus analogue passthrough for Wii component and SCART support (meaning that high quality analogue support can be achieved using cheaper Wii cables). There's even an OSD with support for non-standard resolutions, boosted HDMI audio and scanlines. Combine it with the homebrew tool Swiss and you can effectively run any game at the resolution and aspect ratio of your choice - game compatibility permitting. So, for example, the original Resident Evil port is 480i only; Swiss allows you to force it to progressive 480p, with the GCHD delivering that improved presentation to your HDMI screen.

So, are there any drawbacks? Well, head to head with the expensive component cable solution, image clarity is cleaner, brightness is improved and there are fewer video artefacts. Curiously, the GameCube internally processes using the digital component YCbCr, and there is some evidence of chroma sub-sampling - or at least, something that looks very much like it. This presents as a strange artefact when primary colours collide. It's noticeable on the official component cable and while still there on the HDMI solutions, it does seem to be a touch cleaner. Most people won't notice, especially in the heat of the action, but it would be fascinating to know exactly what is causing this minor issue.

Also noteworthy is that owing to FPGA space restrictions, there is no internal scaler, meaning that it's entirely down to your set-up to blow up the 480p image to your display's native resolution. Some kind of nearest neighbour scaling solution would be our preference here, but it's really down to your kit to deliver the final presentation. On the plus side, the lack of processing on the FPGA does mean that there is no additional latency - the GCVideo-based products are indeed lag-free.

The bottom line is this - if you're still using a GameCube, products like the GCHD Mk2 offer an excellent, high quality solution for running the classic Nintendo console on a modern display with end-to-end digital quality. And it's a fascinating and satisfying use for forgotten technology built into the Cube. Stereo 3D support may never have made it to the console, but it speaks to the level of ambition Nintendo had for the machine during its development phase - which for our money, represents the biggest leap in gen-on-gen 3D performance that the company has ever delivered.