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I'll give it a stab

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Posted by Silent Bob on January 29, 2000 at 04:11:14:

In Reply to: tape noise ... Silent Bob, you can answer the part about Dolby B & C posted by JR#97 on January 28, 2000 at 20:02:35:

to explain dolby B & C, I have to explain A.
A. passed out of use in the 70s. The technology used is called emphasis. It boosts the high frequencies up during recording and cuts it during playback.
B. finds the highest dominant frequency in the audio and boosts all the frequencies above it. On playback, it reverses the process. So it's basically an automatic parametric version of A.
C. is just 2 layers of B. in series.

A. cuts noise in the presence range where people hear noise the most (hence why JR said noise lives around 3.5KHz). The noise actually hogs more low frequencies than high but our ears just pick up the mid-highs better.
B. cuts high frequency noise by about 10dB. When there is no audio, this automatically slides down to the mid range to hide where people would notice it most but when there is very loud audio or very bright audio, it doesn't reduce noise at all.
C. works like B only can reduce noise by about 20dB in the high frequencies.

I suggest using dolby B if you must use noise reduction. Dolby C messes up the sound too much, I'd rather just deal with more noise. Dolby B usually doesn't have any audible effects unless either the record or playback deck is messed up.

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