Dolby ®
Like DBX, Dolby uses pre & de-emphasis. Unlike DBX, Dolby realized that in loud sections of the material, the signal to noise ratio was already more than acceptable... No sense having to do anything with the loud passages, as the noise was unable be be heard over the "din" anyways. In those sections of the material, the signal to noise ratio was already more than adequate... Where noise (tape hiss) became noticeable to the listener, was in the quiet sections.
Where Dolby differs, is that Dolby uses pre-emphasis/de-emphasis only for the quiet sections. Since only the quiet sections are processed and compression/expansion not used at all, the necessity of exactly replicating the exact de-emphasis correction on playback, is not nearly as critical. .
The end result: Dolby averages around 10dB in noise reduction. Still pretty good, but a far cry from DBX's 30 dB....
There are various "flavors" of Dolby such as Type B & C (the most popular to consumers) as well as a few others. Without delving into it, the main difference between B & C are the cutoff points below which nothing is processed (400 Hz for Type B and 100 Hz for Type C).