In 1991, Sonic the Hedgehog burst onto the scene, forever changing the gaming landscape in the process. With its high-speed action and eye-popping visuals, Sonic helped rocket Sega's 16-bit console to the top of the charts - but something else was on the horizon... a sequel focused on the Mega CD add-on, shifting to shorter, exploration focused levels with a new time travel gimmick. Sonic's Mega CD outing remains an ambitious side-step in the series, and the history behind its development is fascinating. For a start, the reality is that it was created by a second Sonic Team in Japan, while a US-based Sonic Team produced the true series sequel in parallel.

The story of Sonic CD kicks off in the aftermath of Sonic the Hedgehog's enormously successful debut. Sega's first CD-ROM system - the Mega CD - had just launched in Japan and naturally, Sega wanted to capitalise on this by bringing the speedy blue hedgehog to its new cutting-edge CD-based hardware. There was just one problem: Yuji Naka, lead programmer and project manager on Sonic the Hedgehog, had grown unhappy with Sega management and had left the company.

Hearing the news, a certain Mark Cerny reached out to Naka and Sonic's chief game designer, Hirokazu Yasuhara, and convinced them to join his Sega Technical Institute in the States. It is here where they would build a new Sonic Team to create the stunning Sonic 2, leaving behind another Japan-based Sonic Team. The third key member of the original Sonic Team - character designer Naoto Ohshima - opted to stay behind in his homeland, where he was tasked with bringing Sonic to Mega CD. This project started with the intention of creating an enhanced port of the original game, but ultimately it became its own unique project with a new team. Ohshima was joined by Sega staffers with experience on such games as The Revenge of Shinobi, Golden Axe 2 and Streets of Rage. This was a whole new team with its own unique vision, which would come to fruition when Sonic CD was released in 1993 - two years after the original Sonic the Hedgehog.

Perhaps the most important element in Sonic CD's design lies in its progression system. At this point, the essence of what makes a Sonic game had yet to be fully defined and Sonic CD feels wildly different as a result. At first glance, it certainly looks the part but if you dive beneath the surface, this is a very different game. In many ways, Sonic CD is to Sonic as Zelda 2 and the American version of Super Mario Brothers 2 are to their respective series. While the original Sonic was sold on its high-speed gameplay, it was also packed with challenging platform sequences. With Sonic 2, Yuji Naka and his team leaned into the speed aspect of Sonic with faster, more focused levels: finish each stage, conquer Eggman and win the game. It's simple, but extremely effective. Sonic CD takes a different path, doubling down instead on platforming and exploration - and time travel.

Our brand new DF Retro episode gives you a complete guide to Sonic CD, the technology that powers it, and a quality assessment of every port.

That's right, each of the game's seven zones features multiple time periods - the past, the present and both a good and bad future. Sonic's end goal is to make a good future by travelling back to the past and destroying Eggman's future-polluting machines in each act. This is where things get interesting - rather than simply running straight to the end of a stage (which is still possible, of course) reaching the true ending requires a series of steps. When you first enter a stage, the first goal is to first locate a time-traveling sign plate - there are both future and past plates scattered around each stage both of which allow you to jump to the respective time. Doing so requires you to reach a certain speed and maintain it long enough to initiate a time warp - a concept directly influenced by Ohshima's love of Back to the Future.

Once you reach the past, it's time to find and destroy Eggman's robot machines. This is where exploration comes into play and where Sonic CD differs most from other games in the series. Finding these machines is challenging yet rewarding: these are large, vertical stages with some of the most complex, criss-crossing design the series would ever see. A good future is created once this machine is destroyed. On top of that, there are also Metal Sonic projectors to find which bring nature back to the present and future once they are destroyed. At this point you can finish the stage or take your time and warp all the way to the beautiful new future you created.

If you do this on the first two acts of the stage, the third act will take place in the good future. However, if you fail to destroy those generators, you end up with a bad future - so it follows that achieving the best possible ending requires making a good future in each zone. This potent mix of time travel, exploration and great control help create a memorable and rewarding experience - and that's not even touching on the 3D bonus stages.

Another key element that defines Sonic CD is its style - in designing the game, art director Hiroyuki Kawaguchi went all out. Each stage features a potent mix of vibrant colours and eye-popping designs. It's one of the most visually striking pixel art games of its era. This is paired with a remarkable soundtrack that perfectly embodies the sound of the 90s - at least in Japan. Infamously, Sega of America used a new Spencer Nilsen soundtrack created for the US release with its own distinct style. While I'm partial to the Japanese original myself, I've come to love both over time and of course, the US version gave us the iconic 'Sonic Boom'.

Developer Christian Whitehead played a key role in bringing Sonic CD to 21st century devices, and he would go on to produce the brilliant Sonic Mania, one of our favourite games of 2017.

From its controls to its game design to its presentation, Sonic CD is a gem of a game, but as a premiere game for a new platform, it also carried the responsibility off showing of the new hardware. Sonic CD's technology is as fascinating as its design and if you break it down, it also takes advantage of every major hardware feature introduced with the Mega CD. First of all there's the storage space offered by the medium: yes it has CD audio and video sequences, but Sonic CD is a big game in and of itself: Sonic 2 is squeezed in a one megabyte ROM, while Sonic CD's game data weighs in at 21MB. This enables a larger variety of bespoke artwork to appear throughout the game, including the multiple time periods, of course.

Then there's the audio. Sonic CD features two distinct forms of music playback. The bulk of the soundtrack is stored as streaming redbook audio which is played directly from the CD like many other games for the system, and it sounds great. This only applies to the present and future zones, however. Rather than sticking with redbook audio, perhaps as a matter of disc space conservation, the past stages make use of the Sega CD's Ricoh audio chip to play back PCM sampled tracks instead. While it's limited in terms of available memory - just 64KB - the chip is used to great effect here; it isn't using pre-recorded tracks, this is sample-based music.

While full-motion video was over-used and generally of poor quality on the Mega CD, Sonic CD is a happy exception. The developers made smart use of the STM format, providing uncompressed imagery to the video display processor. This is well suited to animation and produces superior results to the grainy Cinepak compression used in many Sega CD titles.

Lastly, Sonic CD also made use of another cool technology provided by Mega CD - scaling and rotation. The add-on's graphics processor offers hardware support for scaling of both tilemaps and sprites - a step beyond Mode 7 on the Super NES which could scale background tilemaps only. This is used during the 3D bonus stages where Sonic is tasked with taking out UFOs to earn a time stone. While the Mega CD supports these features, it isn't especially fast and unfortunately the playfield updates at a low frame-rate of just 20 frames per second. The 2D backgrounds, however, are standard Mega Drive layers and move at a full 60 frames per second, creating a strange juxtaposition.

Unfortunately, this also highlights an issue inherent to Sonic CD: performance. As beautiful as its visuals are, the frame-rate just isn't is where it needs to be, especially in comparison to the rest of the series. Yuji Naka was primarily responsible for crafting the code which powers Sonic the Hedgehog and he moved on to work on Sonic 2, leaving the Sonic CD team with the original game as a base. This code isn't as well optimised as later Sonic games and with all the new objects and features bolted onto Sonic CD, the frame-rate suffers with regular drops throughout the game. Considering the quality of the artwork and concepts on display, it's a shame that the programming side fell short.

The Sonic fanbase is extremely talented, with fanmade projects like the in-development Sonic 2 HD producing some remarkable results.

However, despite these issues, Sonic CD remains one of the more accomplished Mega CD games released for the platform. It's a great showpiece title but this wasn't its only home. The title actually received two different Windows 95 ports - one in 1995, the other following a year later. The first version is known as Sonic CD Pentium Processor Edition but it's often referred to as the Dino 2D version and it was only bundled with new PCs rather than sold in stores.

In the mid-90s, the PC platform wasn't exactly well suited to high-speed side scrolling games. Consoles were built with bespoke hardware designed to support smooth scrolling, but there was no equivalent tech in the standard gaming PC. Instead, most PC side scrollers would draw graphics directly into a framebuffer - a process heavily limited by CPU speed and graphics card bandwidth. Some programmers found ways to improve performance in software but even then, background graphics were generally simplified, and requirements were increased.

For Sonic CD, Sega worked with Intel to develop libraries designed to allow faster scrolling with more complex graphics. The Pentium processor and mid-90s graphics cards were starting to become fast enough to power through a side-scrolling game at an appropriate speed and so games like Sonic CD started to appear. Despite requiring a Pentium processor, however, the 1995 version of Sonic CD exhibits some noticeable flaws - like 30fps scrolling, compromised video sequences and 'bonus' unwanted loading screens. The 1996 DirectX version of Sonic CD is essentially the same, the difference being it could hit 60fps - or at least something reasonably close to it. However, all versions released in all regions lacked the original Japanese soundtrack, opting for the US alternative.

Still, for 1996, the PC version of Sonic CD got the job done. After all, the Sega CD was no longer a viable platform and wasn't widely adopted in the first place, so it offered more players an opportunity to enjoy it. Indeed, this version would also form the basis of the port featured in the Sonic Gems Collection for PS2 and GameCube - though this added further issues and compromises, including a blurry anti-flicker presentation on the Nintendo console.

It would take another 13 years for Sonic CD to return, and this time, the quality of the port was simply tremendous. In 2009, a developer named Christian Whitehead released a teaser video online showing Sonic CD running on an iOS device. He noted that this was not emulation - it was actually built using his proprietary Retro Engine. This video arguably defined the future of Sonic. Sega gave Christian the chance to work on an official iOS version of Sonic CD which would lead to so much more. You could say that this proof of concept laid the groundwork for Whitehead's later masterpiece, Sonic Mania. The Retro Engine-powered version of Sonic CD was released for multiple devices including iOS and Android phones, Apple TV, PlayStation 3, Xbox 360 and the PC and it's the best way to play Sonic CD today.

Sonic's transition to the 3D era has been troublesome to say the least. For our money, the best game of its type is Sonic Generations - and the PC release looks amazing at 4K resolution on today's mainstream gaming hardware.

The view has been expanded to fill the screens of its target platforms. Most versions deliver a full 16:9 view of the action - there's no stretching here, the visible playfield is actually expanded. Slowdown has been eliminated completely and Sonic CD delivers a perfect 60 frames per second across its supported platforms. Keep in mind that Sonic CD for iOS was first shown during the iPhone 3GS era - a time when high-performance gaming on mobile devices was still uncommon. On top of that, the bonus stages now operate at a full 60 frames per second, appearing more fluid than ever before.

Even better, both the American and Japanese soundtracks are included this time. The one change here is that the Japanese vocal tracks lost those vocals because of licensing issues but, other than that, it's all here. The video sequences are now presented at the highest quality yet and look fantastic. The faster Sonic 2 style spin-dash is now an option and Tails is unlockable as a playable character.

Beyond that, a huge number of changes and tweaks were made to every single stage. I couldn't possibly list them all here but all of it leads to an even better Sonic CD experience. The only quibble I have is that the recently introduced ultra-wide support for iPhone X is broken, making the game unplayable. Still, despite this oversight, most versions of this port are truly excellent and it's easily the best way to experience Sonic CD. It's just a shame we never received it on physical media - I'd love to see Sonic CD and Christian's other Retro Engine ports appear on modern consoles in physical form.

Sonic CD is well worth revisiting then - and thanks to Christian Whitehead, there are plenty of viable ways to play an excellent, polished version of the game that honours the original while supporting modern features. It plays well on mobile devices, but the PC and Xbox 360 versions are the ones to check out on current systems if you're not looking for handheld play. The 360 release is fully supported via backwards compatibility on Xbox One, and by default, this gives owners of the Microsoft platform the means to play the title not just on its console of today, but on future hardware too.

But what happened with the bizarre situation of two entirely independent Sonic Teams working simultaneously on separate projects? Well, primary Sonic production shifted entirely to the Sega Technical Institute, with Sonic 3 and its Knuckles passthrough-cartridge sequel developed before Yuji Naka finally returned to Japan, reuniting with Naoto Ohshima. While staff may have chopped and changed over the years (some Sonic CD developers ended up working with Yu Suzuki on Shenmue), the return of Naka meant that there was just one Sonic Team again - with many years of great game development to come.

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About the author

John Linneman

John Linneman

Staff Writer, Digital Foundry

An American living in Germany, John has been gaming and collecting games since the late 80s. His keen eye for and obsession with high frame-rates have earned him the nickname "The Human FRAPS" in some circles. He’s also responsible for the creation of DF Retro.

More articles by John Linneman

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