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| This remnant will be replaced soon. The FAQ is back, with a new design! Should I use Dolby when copying tapes? Why not? From Lee Silverman <ljs@cs.brown.edu> 5/7/93: I don't have time for a really intricate response. dbx, as you know, is a less common feature in tape decks but can be done on an outboard encoder/decoder. Basically, dbx compresses a "~22Khz" signal into something significantly less than that & puts that on the tape, so that noise at the high end doesn't affect the quality of the tape. For example, if you have a 16KHz tone at -10Db, it records a 10Khz tone at some other volume (no, these numbers are not accurate), probably less than -10Db. When you record this "compressed" signal on your tape deck, the deck adds noise to it, but it adds a lot more noise above ~15Khz than below, so frequencies recorded about 15Khz tend to be noisier. But dbx mapped your 16Khz frequency down to 10Khz, so the tape deck adds much less noise to that signal. When dbx goes to decode the signal, it takes the 10Khz signal that it recorded before & creates a 16Khz signal, probably a little louder than -10db because of the noise added at 10Khz on your tape deck, but not anything that you would notice without an oscilloscope. The advantage to this system is that you can buy a ~$200 dbx encoder & a ~$200 tape deck & get MUCH better sound than you could from a $400 tape deck. The disadvantage is that you can't play the tapes in your car, & it's tough to find people who trade/use dbx. S/N ratio: 90db (rumored). (BTW, I'm guessing at most of this, based on what little I know of dbx. Can someone clarify/correct me?) Dolby B/C boost the high end of your signal before they put the signal onto the tape, & cut it as it comes off, so that the net result is that noise added to the already increased high end is cut. For example, let's say you have a 16Khz tone at -10db. You record it with no dolby, & your machine adds -40db of noise. When you play it back, you have a -10db tone with -40db hiss in the background. If you record it Dolby B, the -10Khz tone is boosted up to 0Db. The tape deck adds -40 db of noise, as before. But on playback, Dolby B cuts the signals, so your tone goes back to being its original -10db but the noise is cut to -50Db instead of being -40Db, & -50 Db is 10 times quieter than -40db. No guarantees that that description is exactly correct, but the ideas are correct. The effective S/N ratio is about 50Db for dolby B, 60Db for Dolby C. Many tape deck manufacturers say that the ratios are 60 & 70 or something like that, but for real music I don't buy it, & particularly if you're copying a tape that's several generations old. Both dolby B & Dolby C are dynamic; they add & cut based on the existing signal stength & frequency, & never push a signal past 0db, to avoid saturation distortion. Dolby S also does essentially the same thing with the entire frequency spectrum. Dolby B is pretty insensitive to tape-deck parameters, so you can use it between decks. If you have really good ears you may hear some pumping that'll be a little disconcerting. Dolby C is MUCH more sensitive to tape deck parameters such as head alignment, bias adjustment, signal pickup, etc, etc, so that you can definetly tell when a dolby C tape is played on a tape deck that it wasn't recorded on. Hence, use dolby C for your own tapes only, and dolby B for trading. You can record a tape in dolby B and C and play it back without dolby, & often it sounds fine, or maybe a little hissy or tinny. That's because the signal itself hasn't really been changed, but the volume. Thus, you can play a dolby tape in your car stereo. With dbx, the tape sounds like your tape deck's motor is having a major fit; you can't listen to a dbx tape without decoding it. That's the reason why dbx never became very popular. Hope that's a reasonable summary of the differences. I know more about dolby than about dbx, so somebody please correct me or expand on what I've said if you think I'm wrong or unclear. Lee
From: Edwin Hurwitz (76420.1676@compuserve.com) 1993May8 Lee Silverman...has some things confused. Both (dolby & dbx) are compression formats that deal with decibels (db) not kilohertz (khz). dbx is full bandwidth compression & results in a peculiar sound when unencoded & dolby B & C are partial bandwidth , that is, the high end which is why it sounds like the treble is cranked. If you don't have dolby B on playback you can just reduce the treble control. Of course there is a large contingent of pholks who like it with dolby encoded but left out on playback for that extra crispness. If you have more questions, you know where to find me.......Edwin
From: Bill Press (press@lip.wustl.edu) 1993May8 | Both are compression formats that deal with decibels (db) | not kilohertz (khz). Although I did miss Lee's description, the above statement is not quite right (with all due respect, which is alot ;). Decibels refer to the magnitude (volume) of the signal, and kilohertz refers to the frequency. The two are not mutually exclusive. In fact, one can't describe a particular signal without including both measures. Dolby takes a certain range of frequencies (coincident with hiss noise) & boosts their intensity (decibels) before writing them to tape. This constitutes the encoded signal. Note that there is no compression involved here... the signal that's coincident with noise is being boosted, that's all. As the noise is relatively high frequency, playing back a dolby encoded tape with out decoding gives you a boosted treble. Decoding is merely lowering the intensity of the appropriate frequencies of the signal. Dbx, on the other hand, is a data compression scheme. It reorganizes the signal so that none of it is coincident with the tape hiss. The signal is then restored upon playback, while the hiss is left unplayed. Loosely speaking, Bill.
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