The principle of the Mellotron® was simple - take a 35 note keyboard and have a piece of tape for each note containing real recordings of real players. In this way it was arguably the world's first sampler, with recordings varying from single notes (as in the Flutes sound used on the legendary Strawberry Fields Forever) or entire musical motifs (as in The Beatles' Bungalow Bill). Each sound lasted a maximum of eight seconds before they had to release the note, allowing the tape to rewind. This forced the player to adopt a different technique (often referred to as 'a spider crawling up the keys') and, in turn, this allowed the music to truly breathe.
However, while the sound was undoubtedly magnificent, because of the huge array of moving parts, the instrument's reliability was questionable and the price was expensive. Rick Wakeman regularly recounts a story of becoming so fed up with the unreliability of the instrument, he unceremoniously burned his at the end of a Yes tour.
Purchasing an original M400 now would cost several thousand dollars and would probably only come with the default three sounds, Cello, Flutes and Violins. The M-Tron, on the other hand, costs a fraction of this price and now comes with over 2Gb of sounds that don't require the instrument to be dismantled in order to hear them.
The M-Tron captures all of the character of the original instrument by sampling every note of every key and adhering to the eight second limit of each note. This way, not only do the resultant sounds ebb and flow, all the inherent and nostalgic magic of each sound is closely observed - minor imperfections and all.