The Mellotron is an electro-mechanical, polyphonic tape replay keyboard originally developed and built in Birmingham, England, in the early 1960s. It superseded the Chamberlin, which was the world's first sample-playback keyboard intended for music. The concept of the Chamberlin was itself modeled after the Laff Box invented by engineer Charlie Douglass in order to insert prerecorded laughs into TV and radio programs more easily in the then-developing field of post-production.

The heart of the instrument is a bank of parallel linear magnetic audio tape strips. Playback heads underneath each key enable the playing of pre-recorded sounds. Each of the tape strips has a playing time of approximately eight seconds, after which the tape comes to a dead stop and rewinds to the start position. A major advantage of using tape strips, as opposed to tape loops or cassettes (compare to the Birotron) is that the Mellotron can reproduce the "attack" transient of the instruments recorded on the tape. A drawback is the short "decay" time of the note.

A consequence of the eight second limit on the duration of each note is that if one wants to play chords that last longer than eight seconds, one must release different notes in sequence in a process that has been compared to a spider crawling across the keyboard.

The MKI, MKII, and MKV models contained two side-by-side keyboards: the right keyboard accessed 18 "lead/instrument" sounds such as strings, flutes, and brass; the left keyboard played pre-recorded musical rhythm tracks in various styles.

The tape banks for the lighter-weight M400 models contain only three selectable sounds including (typically) strings, cello, and an eight-voice choir. The sound on each individual tape piece was recorded at the pitch of the key to which it was assigned. To make up for the fewer sounds available, the M400 tapes came in a removable frame that allowed for relatively quick changes to new racks of sounds.

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