|Audio Tape Digitisation Workflow
It is possible to decode a recording made using a noise-reduction system at a later time, but the multiple variables in alignment can compound any errors and make it difficult to decode the signal accurately once the tape has been transferred.
Select the same noise reduction system that has been used when the recording was made, IF the decoding system is known. If in doubt, make a linear transfer. Perfect amplitude adjustment is very important with noise reduction systems in order to guarantee the symmetrical operation of both the recording and playback sytems.
IASA-TC04, p. 31.
Wikipedia: Dolby noise reduction system
Wikipedia: DBX noise reduction
Richard L. Hess: Noise Reduction
The Practitioners Guide - Dolby SP NR Frame Alignment Procedure
The Practitioners Guide - Dolby 361 NR Frame Alignment Procedure
Dolby B System Encoding