Andre Cassagnes the inventor of Etch A Sketch died at the weekend. Etch A Sketch is an ingenious analogue  device, that paradoxically makes sketching more difficult  yet has sold over 100 million copies since 1960.

Some of its charm must come from the pleasure to be had from acquiring the skills necessary to do something difficult; of mastering a device.

Etch A Sketch self consciously imitates a TV screen and in some ways was a precursor to the Visual Display units that were beginning to be used with early CAD systems.

These were specialist devices, used Cathode Ray Tubes (CRTs) and had a separate processor that continuously drove a  beam round the screen to create an image. One of the ones I first used in Edinburgh even had a round radar like monitor and a lightpen. The screen image could be easily changed, have parts added or removed, but would begin to flicker when the image became at all complicated.

This was initially overcome by the use of  Storage Tube devices. In these a steerable beam left a trace on the screen very like an Etch A Sketch. Adding to an image was straight forward but deleting was more difficult. The whole screen had to be erased, like shaking the Etch A Sketch, and the image recreated from scratch, often from a separately maintained display list.