Early calculating devices and computers used mechanical digital to

analogue converters. This video describes one based on an arrangement

of metal bars called a "whiffletree" - also sometimes called a

"whippletree." It shows, briefly, the whiffletree used in IBM's

revolutionary selectric typewriter and then illustrates the principles

of a whiffletree converter by showing the simplest one - one that

encodes digital impulses into two bits of information. (This videos is

an appendix to Bill Hammack's video about the operation of the

Selectric Typewriter.)