Wendy Carlos reports:
...The sentient computer in Arthur C. Clarke's story is called HAL (and no, Clarke did not plan the name to be the three letters of IBM shifted one place to the left -- that myth overlooks an even lovelier example of serendipitous coincidence!). In the film the role was voiced by Canadian actor, Douglas Rain, who was able to give a cool, detached -- yet feelingful duality to the character. Here's a publicity photo of Rain taken from around that time.
During the scene in which Dave (Keir Dullea) "lobotomizes" HAL, you'll easily hear how the tempo of Rain's voice becomes slowly expanded and pitch-shifted gradually downwards. Actually, his entire performance as HAL has a mild amount of time stretching (no alteration of pitch) going on, as Stanley confided to me. I told him I hadn't noticed it before, and he smiled: "it was about 10-20%, rather subtle." But that was enough to enhance Rain's performance with a slightly more measured quality. It's in the final HAL scene that the Eltro effect is cranked way up. "We did that in two passes", Kubrick quietly explained. One pass gradually dropped HALs pitch down to almost zero, remaining at a constant speed. The other pass gradually stretched it out in time, but not as extreme, as HAL sang "Daisy, Daisy" (Bicycle Built For Two by Harry Dacre). And indeed, you couldn't do this simply by slowing down a regular tape recording, as many pundits have since wrongly guessed (to reach the final low pitch, the tempo would crawl to a near-stop).
http://www.wendycarlos.com/other/Eltro-1967/