David Gilmour's "Sound on Sound" technique

YouTube link.

Those interested in how Gilmour uses the foot pedals to modulate the sound might want to explore his webstie at Gilmourish, especially the article entitled Classic tone: Shine On You Crazy Diamond.

The Sound on Sound effect was something David introduced on the 2001/02 acoustic shows. He wanted to be able to play Shine On without using keyboards and developed this technique of sustaining a chord with delay and playing on top of it.

- A (very basic) graphic view of how the Sound on Sound effect works.

The Sound on Sound effect, isn’t an effect in form of a pedal but rather the effect achieved when splitting the signal in two with a long delay assigned to one channel. David strums a chord and makes a volume swell with the volume pedal assigned for the Sound on Sound channel. The signal travels to the Sound on Sound unit (basically a A/B router unit made by Pete Cornish) and into the Roland digital delay, which is set to 1500ms lasting about 20 seconds. The signal then travels into a Hiwatt and WEM cabinet used only for this effect. Gilmour lowers the volume pedal and plays a solo fed through the “normal” signal path, while the Sound on Sound pad is sustained by the long delay. The pattern is repeated for each chord. 

Much more very technical detail at the link.  For the more conventional studio version of the song, see the video at this old post.