Harmonix' task in succeeding FreQuency must have been an exciting and punishing prospect. Sequels seldom justify their existence by re-treading old ground, yet how do you tune up an engine already running at 99% without something breaking in the process? Despite the risks, Amplitude had to happen - its devotees were amassed before it was even conceived, banging their knives and forks on the table.

Amplitude provides a series of music tracks deconstructed and spread across an undulating 'wave' of adjacent lanes. Each lane represents a musical component of the track - drums or synths for example. Divided into bars, each lane can be reconstructed (captured) by accurately striking the notes which flow in various sequences beneath your 'ship'. Whilst notes are mapped to three face buttons of the PS2 controller, it's the recommended trio of shoulder buttons which provide the more intuitive (and as it happens, vital) alternative. Capturing concurrent sequences in a chain is the key to high scoring and the best means by which to unlock the full rendition of each song.

Speaking of which, the 26-song track list for Amplitude is a wisely selected bunch, though not quite equalling the quality and range of FreQuency. Harmonix have largely resisted pressure to "go mainstream" though there are, admittedly, contributions by Garbage, Blink 182, Slipknot and Pink. Only the former two feel particularly awkward, but the trend is a dangerous one for the series' long-term credibility. Regardless, the game's power stems not from this, but from an immaculate integration of each song into the mechanics of the series. The featured music disassembles perfectly, each track becoming a consistently harmonious jigsaw with infinitely interchangeable pieces. Though the developers' selection occasionally falters (thank market demands for that one), the vital ingredients never come within a mile of failure.

Difficulty in Amplitude is another strong point which has been successfully carried over from its predecessor. Most games regard this area with a sense of obligation, gently modifying a game's vital statistics into ineffective tiers which no one will ever play. Halo bucked this trend, FreQuency even more so; its sequel, impressively, follows suit. Acknowledging a hardcore audience of self-proclaimed "master-Freqs", Amplitude's 'Insane' setting is an appropriately named bludgeoning of the senses, offering rich rewards on a par with its demands. Still welcoming newcomers, the game has sharpened its teeth to escape redundancy, its continued genius being to provide a challenge that defies over-familiarity.

FreQuency's purposefully restricted range of power-ups make a welcome return, the 'Auto-Capture' and 'Score Doubler' being fundamental components of the original design. There were fears that the game would be bastardised by novelty and glitz - that the lack of room for improvement would spawn a glut of unnecessary additions. It hasn't. The only addition to the arsenal is the 'Slo-Mo' which can be deployed at any time, slowing the music to a more manageable speed. Lasting a few seconds, this makes for an interesting, if not entirely justified upgrade; Amplitude forces you into what uninspired sportsmen call 'the zone', a trance-like state of sublime hand-eye co-ordination that doesn't take kindly to being slowed down. There is use for the power-up, but when the game's firing on all cylinders it becomes more trouble than it's worth.

One area where FreQuency could safely be improved was its visuals, and Harmonix have revelled in that opportunity. The original game was almost skeletal in its base detail, fulfilling the minimum requirement for its challenge to be delivered. Comparatively, Amplitude is a psychedelic revelation akin to the 1960s succumbing to Flower Power. Each track in Amplitude now has its own specific city-like arena (the original had a predefined set which could be applied to all) which has allowed an infinitely greater audio-visual synergy. Video clips of each artist (albeit short, looped ones) are stacked into great pillars while hundreds of varied objects cast perfectly timed beams of light across the screen. Mellow dub tracks now flow with an appropriate sense of calm while frenzied metal provides a far angrier environment; the most logical improvement to the design has therefore been made with gusto.

The discrepancies in this design, it must be stressed, are of miniscule importance. In the context of such an intelligent overall package, mere nit-picks are significant nowhere outside of a review - so here they are. Action in FreQuency transpired purely within the confines of a hexagonal tube, the last lane being one D-Pad press from the first. By opening this out into a linear wave with no wraparound the ramifications for navigation are clear - the immediacy has been diminished. This may trouble those at the very apex of the game's learning curve but only during a particular couple of tracks and certainly not to a significant degree. There are also a few technical difficulties which have crept into the visual overhaul: the occasional frame-rate hit is one (luckily very rare considering that this is a reflex-intensive game), the obscuring of action by occasionally extravagant special effects is another.

Amplitude's feature-list deserves extensive coverage, but not in an overview such as this; the following features therefore possess more depth than might be suggested here. Mutliplayer, first of all, is much the same as before (apply the upgrades made to single player for a quick impression) - it can be great fun, but lacks the polish and awesome challenge of solo play. Remix mode has been well implemented, and provides the same set of tools as FreQuency; rather than dictate the rules, here a player can produce endless versions of the game's tracks at their leisure. It's an acquired taste which many will simply ignore, but will conversely become a religion to some. Custom mixes can be played online, though the other significant online component - that of status, competition and score-determined rankings - is a somewhat ugly scene. Basically, there are exploits. Though a server patch may one day fix the problem, the attitudes and antics of many online players have all but ruined this particular annex of the Amplitude community.

Faults are mentioned here thanks to the structural demands of a video game review - do not, under any circumstances, let them prevent you adding Amplitude (and FreQuency, if required) to your PS2 collection. Unless the very thought of a game built specifically around music makes you physically sick, this is an imperative, required purchase. Harmonix - a developer whose portfolio singularly includes this series - represent collective genius that ranks alongside any Peter Molyneux, Shigeru Miyamoto or other elevated industry figure. Only slightly harmed by a few too many big-name acts and micro-glitches, Amplitude is nonetheless a supreme masterpiece; barring its predecessor no music-centred game will ever come close. No, not even Rez.