This is a 24 channel Dolby A unit which can be used to reduce analogue tape noise by approximately 10 db. In the 1970's and 1980's all professional recording studios had an analogue multi-track recorder many of these used Dolby A, and later on Dolby SR, to reduce tape noise the alternative was to record at 30 inches per second which also reduced tape noise but doubled the tape costs and halved the amount of recording that could be stored on a reel.
For those looking for the ultimate recording quality - 16 track on 2" tape running at 15 ips using a Dolby A noise reduction system is considered by many as the best as far as multi-track recording is concerned, mastering on 1/2" ( or in some cases on 1" ) is even better but 1/4" per track is not practical for multi-tracking.
Although not designed for the purpose, Dolby can also be used to reduce noise made by some outboard equipment, if used on reverb or similar devices Dolby will tend to extend the decay time but this can be preferable to noise. Back in the day noise and distortion on recordings were unacceptable, however these days noise and distortion seems to be a feature on some modern recordings.
We also have some Dolby SR/A - SR & Cat 22 Dolby A cards available look in the Miscellaneous section for these