ark@rabbit.UUCP (Andrew Koenig) (01/15/84)
Consider a tape recorder. You put a signal in and later get a similar signal out. No matter how much information is in the input signal, the amount in the output is limited by the tape recorder. A noise reduction system such as Dolby or DBX works by redistributing the information in the input signal so that the losses will come in less objectionable places. However, these losses CANNOT be eliminated, because you can't create information that the tape recorder has thrown away. Noise reduction systems that work by compressing the dynamic range of the signal on tape pay for that in magnifying frequency response irregularities in the tape machine. No way to avoid it. The more compression you do, the more the irregularities will be magnified. Dolby B does the least compression, so it is least sensitive. Dolby C is somewhat more sensitive. DBX is the most sensitive. To fuel the flames a bit, I will point out that Dolby Labs makes the claim that while output noise from DBX-processed tape will be less than for Dolby C in the absence of any signal, Dolby C will have less output noise than DBX in the presence of, say, a single low note on a piano. Given the difference in their approach, this makes sense.