On Dec 1, 15:19, Cameron Kaiser wrote: source? decoding There are three Dolby systems, all based on audio companders. Dolby A (the "professional" one, used by some recording studios for master tapes) works by splitting the audio into a number of frequency bands (like a graphic equaliser does). On recording, each band is then fed through a compressor to reduce the dynamic range, thus keeping the volume level fairly high. On playback, it's fed through an expander to recreate the dynamic range of the original signal. Dolby B is a simpler version, with a single filter for HF; it uses a similar compander for the upper frequencies only. Dolby C is a n "improved" Dolby B, which (IIRC) uses a different corner frequency and different compander. None of them use any form of frequency shifting. The reason Dolby B and C work so well (for tape, which is what they were designed for) is that tape hiss is largely made up of HF noise. You don't hear the hiss if the signal is at a sufficiently greater level than the hiss (which is more-or-less constant) -- in other words, if the signal-to-noise ratio is high enough. So Dolby B/C boost the level of quiet sections of the high frequency signal before recording, but leave the louder passages as-is, to avoid saturation. On playback, the amplitude of the quieter passages is attenuated again, and the hiss is attenuated with it. Unlikely to make much difference, unless the filter corner frequencies and rolloffs were quite different, which Dolby Labs were quite picky about before granting licences. prefab I'm but was Then it wasn't Dolby-compatible :-) -- Pete Peter Turnbull Dept. of Computer Science University of York