ark (12/13/82)
It is true that dbx provides more noise reduction than Dolby B or C. However, since information is conserved, this additional noise reduction comes at a cost -- in this case, two costs. First, the more you compress on recording (and expand on playback), the more any response irregularities and distortion will be exaggerated. Thus, if your record-playback chain is a bit peaky to begin with, it will be very peaky after going through dbx, more so than with Dolby B or C. Second, dbx is a so-called "broad-band" compression system. The problem with this is that noise is only masked by signals in substantially the same frequency range. Dbx does not take this into account. Consider a bass drum playing alone. Every time the drum sounds, there is a bunch of low-frequency signal. Dbx will say: "Aha, here's a signal, let's compress it." If it's a loud drum, the level will be reduced on record and increased on playback. Also increased on playback will be the high-frequency tape noise. Since the drum has no high- frequency content, the noise will become that much more perceptible. In other words, each beat of the drum will appear to be accompanied by a burst of tape hiss. This phenomenon is sometimes called "pumping" or "breathing." Dolby noise reduction has been carefully designed to avoid these problems by two means: (1) no modification is made to signals louder than a threshold (called the "Dolby level"). (2) Low-frequency signals are never touched by the system, because that's not where tape noise lies. In the bass drum situation, then, Dolby (B or C) will detect that there is low-frequency signal content only, and pass the drum signal through unchanged. It will continue to emphasize the highs on record (of which there are none) and de-emphasize them on playback (thus reducing the tape hiss), and there will be no breathing problems.