Bucket Brigade Delay Lines

Before digital delay lines were commonplace, delay effects were harder to produce. For fairly long delays loops of tape were used, like the famous WEM Copicat, but these could not produce very short delays without moving the tape at very high speeds. One solution to this was the bucket brigade delay line, which is a form of charge-coupled device (yes, a bit like the ones used in cameras). It is a kind of a cross between a shift register and a dynamic memory, for analogue signals. Imagine a row of capacitors, connected by switches. Every time you apply a pulse to the clock pin, the switch on the left of a capacitor opens and closes, as the one on the right closes and opens. This lets the charge in the capacitor move into its neighbour to the right. Rack up a few thousand of these and you can delay an analogue signal for up to a few hundred milliseconds.

The name comes from the idea of firemen passing buckets of water down a line. Each clock pulse makes the little hypothetical fireman pass his bucket with a tiny analogue sample in it to his neighbour.