gbr@mb2c.UUCP (Jerry Ruhno) (06/22/84)
Could someone tell me the difference between Dolby B and C and also
how it differs from the Dolby on my old Pioneer CT-F9191. I am planning
to buy a new cassette deck for my car and I want to know what effect
B or C will have on my older recorded tapes. Thanks.
Jerry Ruhno epsilon!mb2c!gbrzzz@mit-eddie.UUCP (Mike Konopik) (06/24/84)
The way I heard it, Dolby C is basically 2 sequential Dolby B circuits. Is this correct? -- -Mike genrad!mit-eddie!zzz (UUCP) ZZZ%MIT-OZ@MIT-MC (ARPA)
ark@rabbit.UUCP (Andrew Koenig) (06/24/84)
Several people have recently expressed some confusion over the various Dolby noise reduction circuits. (Dolby is a trademark of Dolby Labs, Inc.) There are three systems presently available: A, B, and C. All three systems are frequency-dependent companders. In other words, they work by compressing the dynamic range of the signal during recording and expand it during playback. The amount of compression varies with frequency and amplitude, and is also different between the A, B, and C systems. All three systems share the characteristic that above a well-defined threshold (called the "Dolby level"), the signal is not affected AT ALL, thus guaranteeing that high-level transients (drums, etc.) will pass through unscathed (modulo overload of amplifier stages, etc.) You can pretty much forget about Dolby A. It is designed for studio use, particularly by multi-track recorders. If I remember right, it adjusts compression independently in four different, but overlapping, frequency bands, so it tends not only to get rid of tape hiss, but also hum, crosstalk, and so on. Dolby B was designed for home use. Its only purpose is to remove tape hiss, so it has essentially no effect below about 2 khZ. Above those frequencies, its compression is tailored to give about 10 dB total compression, hence about 10 dB overall noise reduction. As far as I know, Dolby B is the only form of Dolby noise reduction used in commercial pre-recorded cassettes. Dolby C is like Dolby B, only more so. It works over a wider band, and is more aggressive about compression, so it picks up about another 10 dB over Dolby B. Its most obvious disadvantage over Dolby B is that not many people are using it yet. A more subtle disadvantage of Dolby C is this: there is no such thing as a free lunch. Noise reduction cannot increase the amount of information stored on the tape. Thus, the information gained by lowering the noise floor must be taken from somewhere. In general, noise reduction systems of this type will get their information by exaggerating any coloration that already exists in the tape deck. The more you reduce noise, the more the coloration will be exaggerated. Thus there is quite a premium on getting the bias just right, aligning your tape heads perfectly, and so on. The premium becomes greater as the noise reduction becomes greater. Finally, it should be pointed out that proponents of dbx noise reduction claim it is much more effective than Dolby noise reduction. This may be true, but two caveats: (1) again, there's no such thing as a free lunch. The more compression you have, the more critical it is to get everything adjusted just right. (2) Because dbx is a broad-band system, it is possible for a low-frequency signal to be accompanied, on playback, by a burst of high-frequency noise. I have not auditioned a dbx system carefully, so I do not know if this is a serious problem in practice or not. I have seen several articles in various audio publications that take both sides of the issue.
andrew@inmet.UUCP (06/25/84)
#R:mb2c:-25700:inmet:2600087:000:875 inmet!andrew Jun 25 00:40:00 1984 > Could someone tell me the difference between Dolby B and C... "Dolby B and C differ in the amount of boost/cut they provide and in its frequency distribution. Dolby B operates mainly at high frequencies and suppresses hiss by a maximum of 10 dB. Dolby C operates at both middle and high frequencies and reduces hiss by as much as 20 dB." - NAD 6150C owner's manual > and also how it differs from the Dolby on my old Pioneer CT-F9191. Anything called just "Dolby" is Dolby B... a throwback to the days when B was the only system used in consumer audio. > I am planning to buy a new cassette deck for my car and I want to know > what effect B or C will have on my older recorded tapes. You've been using B all along (see above); all C decks incorporate B capability so that older tapes can be played. Andrew W. Rogers ...{harpo|ihnp4|ima|esquire}!inmet!andrew
prk@charm.UUCP (prk) (02/22/85)
A question: How does Dolby work?