dbx is basically a 2:1 compression 1:2 expansion. Type II is designed for cassette and slower open reel where to compensate. Dolby uses a different method. eg Dolby A applies compression over four bands, SR is even more sophisticated. This manual contains a good explanation of dbx
http://www.dbxpro.com/product_downloads/Owner_Manuals/180 Owners Manual.pdf also check out the dolby site analog library:
http://www.dolby.com/DocLibTechLanding.aspx?taxid=174
The wiki articles are pretty good also took this from here
http://stason.org/TULARC/entertainm...e-difference-between-Dolby-A-B-C-S-and-S.html and here
http://www.audiotools.com/noise.html
The Dolby A, B, C, SR, and S noise reduction (NR) systems are
non-linear level-dependent companders (compressors/expanders). They
offer various amounts of noise reduction, as shown in the table
below.
Dolby HF NR LF NR Number Of Active Target
System Effect Effect Frequency Bands Market Year
------ ------ ------ ---------------------------- --------- ----
A 10 dB 10 dB 4 fixed Pro audio 1967
B 10 dB -- 1 sliding (HF) Domestic 1970
C 20 dB -- 1 sliding (HF) Domestic 1981
SR 24 dB 10 dB 1 sliding (HF), 1 fixed (LF) Pro audio 1986
S 24 dB 10 dB 1 sliding (HF), 1 fixed (LF) Domestic 1990
------ ------ ----- ---------------------------- --------- ----
DBX type I noise reduction
A system introduced shortly after the Dolby system and known today as the DBX type I, although intrinsically a simple 2:1:2 compander ...for consumer devices like open reel recorders running at 9,5 or 19cm ... it has to be said that the type II is a much better match, it's slightly less effective as a NR but the RMS circuit makes it close to being foolproof and it is much more tolerant of recording conditions than even the Dolby systems
