Note: This information has been incorporated into this page which contains a more in-depth discussion.
I received an urgent phone call yesterday from a man who had digitized several reels of 2″ 24-track analog recordings that he wished to re-mix.
The tapes were originally recorded in about 1978-1979 and he said that he needed them to have Dolby C noise-reduction processing applied to the files.
I did a bit of research, as that did not sound correct from an historic point of view.
Here is an approximate chronology of the major noise-reduction systems and their dates of introduction:
A – 1967 (pro)
B – 1971 (consumer)
C – 1983 (consumer/prosumer)
S – 1990 (consumer/prosumer)
I (pro) & II (consumer) – 1971
Telefunken (later ANT)
C4 – 1977
He later wrote me back saying the engineer was pretty sure it was Dolby A.
When I applied Dolby A, Dolby B, Dolby C, dbx I, and Telcom C4, only the dbx I sounded close to correct.
Fortunately, dbx I is less critical than the Dolby noise reduction systems for accurate level setting, since there are no test tones digitized along with the audio.
This work requires playing the digital files out through the D-A converter and then re-recording them via the A-D converter.