Or: How not to build a Mellotron

What is it?

3.5" floppy disks hold information by storing them magnetically on 80 different tracks (effectively circles) across the media. A stepper motor moves a read/write head across the media and can select which track is being accessed.

In normal computer usage, the tracks are being formatted/modulated with modified frequency modulation (MFM) to store digital data onto these tracks.

In this project, we’re using the magnetic media as analog storage, more like an audio cassette or reel-to-reel tape. The electrical interface of a IBM PC-compatible 3.5" floppy drive is very simple and just has a number of pins to step the head motor into either direction, as well as the data pins, which will read/write raw flux transitions on the magnetic media. The floppy drive itself has no real “digital” electronics, it just contains some op-amps, motor drivers, etc.

How does it work?

By modulating analog audio as a pulse-width modulated (PWM) signal into a “digital” bitstream (not unlike a Class-D amplifier), we can send analog audio to the floppy drives write head.

In this project, an NE555 was used to modulate a 300kHz carrier wave with sample audio data. This sample audio data can be provided from a sound card (or in this case by an Arduino). The same Arduino also controls the STEP and DIRECTION pins of the floppy drive and can move the head across the magnetic media.

Each of the 80 tracks is then formatted with a different sample. Typically, each of the track represents a musical note. From the outer tracks to the inner tracks, the notes go up higher and higher. By stepping the head across the disk, the appropriate note/sample can be played back.

An Arduino was used to parse MIDI input (either from a PC or MIDI keyboard), it counts the current location of the head and steps the motor to the correct notes as fast as possible. This Arduino also controls the ENABLE line of the floppy drive to turn the sample playback on/off accordingly.

Giana Sisters (square wave sample disk)

23 (square wave sample disk)

Welle: Erdball’s 23 (originally written for the C64’s SID chip) has very fast arpeggios.
The head stepper motor has a hard time keeping up with the changing notes. Seek speeds are limited to about 4ms per step.

Mellotron “Flute” sample disk (🍓)

In this example, a flute sample is stored on the disk. You can hear the large DC offset from enabling/disabling the output driver in the floppy drive and also hear the index pulse on the media. With better electronics, both of these artifacts could be silenced.