Path: utzoo!attcan!uunet!snorkelwacker!usc!samsung!sdd.hp.com!hplabs!hpda!hpcuhc!edwardm From: edwardm@hpcuhc.HP.COM (Edward McClanahan) Newsgroups: comp.music Subject: Re: dbx vs. Dolby C Noise Reduction Message-ID: <1920003@hpcuhc.HP.COM> Date: 19 Jul 90 00:43:29 GMT References: <590@lexicon.com> Organization: Hewlett Packard, Cupertino Lines: 65 Frank Cunningham writes: > If I recall the term correctly, both dbx and dolby are homomorphic > filters, in that their control is derived from the signal itself. > They are both invertable - the expansion circuits do the inverse of > the compression circuits. They both are reasonably resistant to > corruption of the encoded signal by the channel, although Dolby is > more sensitive to high-frequency losses since it only works on them. > ... > > 1. How does Dolby C work? > By some double compression of the high end only, above some frequency, > below some threshold (where dbx is level independent for reasonable > setups). I've forgotten the details. It's basically Dolby B done twice. I haven't been following the discussion Frank is responding to, but I think I can shed some light on the comparison. The basic premise of both Dolby B and Dolby C is to PRE-emphasize the highs prior to recording so that on playback, both the highs from the original signal and the tape hiss can be DE-emphasized. This should have the effect of restoring the highs to their original signal strength (relative to the lows and mids) while lowering the tape noise. The idea of DBX is to "compress" the signal by a factor of 2-to-1 prior to recording. Then, when played back, the signal is "expanded". This strategy has two advantages. First, because the signal is compressed, the recording level (say of the quieter passages) can be higher relative to the tape noise than if the signal was not compressed. Second, the tape noise is more noticeable during the quiet passages anyway, so the DBX expansion circuitry will lower BOTH the original signal (back to its original level) AND the tape noise. Another VERY significant difference between Dolby B/C and DBX (and one where the "improved" Dolby C suffers even more than Dolby B) is in reaching the limits of frequency response. Have you ever noticed that most tape and tape recorder/players quote their S/N at 0 dB and their frequency response at -20 dB? Using DBX allows one to not have to record at such a high level to avoid tape noise thus improving frequency response. Try this experiment at home. You will notice that if you record at -20 dB the highs will be much clearer (not deadened) but the tape hiss will be higher than you may want. DBX avoids this problem. Still, DBX has one MAJOR drawback. While Dolby B/C imposes a fixed boost to the highs (although the amount varies with frequency), DBX varies between emphasis and de-emphasis based on signal strength. Older DBX units were subject to a condition known as "pumping", basically a result of not being able to accurately decide (based on the signal) when to emphasize and when to de-emphasize. This problem, along with higher cost, is why you don't see many DBX units out there. The other reason is that tapes recorded with DBX will not sound at all good when played back without it. Dolby B/C tapes do sound okay when played backk without it (in fact, some people prefer this even though they could achieve nearly an identical effect with their treble control or an equalizer). I hope this helps the discussion... =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Edward McClanahan Hewlett Packard Company Mail Stop 47UP -or- edwardm%hpda@hplabs.hp.com 19447 Pruneridge Avenue Cupertino, CA 95014 Phone: (408)447-5651