The original delay lines were made by using a three-head tape recorder to record a signal while playing it back
on the same machine. : The distance between the record and replay heads causes a time delay which varies with tape speed; this technique is called tape delay. If some of the replay signal was mixed with the direct signal, a pseudo-reverb could be created. The record-replay idea was further developed with specific machines which used tape loops and multiple replay heads and the ability to adjust the contribution and feedback of each head. These machines were replaced with DDLs.