Maurice Blackburn worked closely with Norman McLaren, perfecting the technique of sound on film. Here, he demonstrates the production of synthetic music, i.e. without the use of musical instruments, microphones or recording devices. The non-magnetic, optical technique uses cards representing notes, and a 35 mm camera identical to those used to record animation films. Maurice Blackburn would press a button to make the camera photograph the card-note on the soundtrack. Because the sound is filmed one image at a time, it is also called "animated sound." A small label is attached to each card with its note in musical notation. The cards are boxed in five rows of twelve, one row per octave.
For more information on techniques used in the production of synthetic music with sound cards, the following documents are especially useful: Technical Notes on the Card method of Optical Animated Sound (IN Technical notes by Norman McLaren) and A Brief Summary of the Early History of Animated Sound on Film.