DOLBY NOISE REDUCTION- A Primer

 

Q: What is Dolby, and what is it good for?

Dolby NR stands for Dolby (Labs) Noise Reduction. You can use it if your cassette deck has a small "Dolby NR" switch on the panel. Most component tape decks costing over $100 have Dolby. Most car stereos and boom boxes and cheapo all-in-one stereos do not.

Dolby NR is used to reduce the amount of hiss on tapes that you record or play back. Hiss is most obvious between songs on a music tape.

Basically, the entire musical frequency range is divided into hz hertz). Bass tones occupy the lower end (20hz-500hz), the midrange is occupied by most vocals and guitar noises (500hz-5000hz) and the high end, or treble, (5000hz-20,000hz) is occupied with high-pitched things (Jon Anderson(lol) or cymbals or hiss.) Everything except the hiss is considered desirable, so the trick is to capture all of the frequencies on tape and keep the hiss to a minimum.

The problem is that hiss is a natural part of the analog recording process. Since hiss occupies the high end, what Dolby NR does is boost the volume of the low-level (quiet) music tones in the high end by 5 to 10 decibels. The high-level (loud) music tones in the high end do not need boosting, because they're loud enough to mask the hiss. The hiss is, of course not boosted, because the hiss is not part of the actual input signal that the recorder using Dolby NR is taking in. Hiss adds itself into the recording process separately from the (music) input signal.

During playback, Dolby NR reduces the decibel level of the same tones in the high end by 5 to 10 decibels, both the musical high end and the hiss. Since the musical parts were artificially boosted during the Dolby recording process, they will sound about right when played back using the Dolby process. Hiss, however, would be 5 to 10 decibels SOFTER during playback, because Dolby NR had cut back on the high end. So the net benefit is that we don't actually lose anything on the musical high end, but we do lose several decibels of hiss. Dolby NR won out mainly because it's cheap to build into recorders ($10-15 more than a non-Dolby deck), and it works relatively well.

 
Q: So, what happens when you mix and mismatch tapes and Dolby?
 

 
Q: So, what do I do now when I dub tapes for Dolby NR fans?

First off, Don't use a double-cassette deck! Most of the time, there is only ONE Dolby switch so you are stuck with the all-or-nothing situation: It will play the source tape in Deck 1 with Dolby, and record a copy in Deck 2 with Dolby. Or else, it will play the source tape in Deck 1 without Dolby and record the copy in Deck 2 without Dolby. I prefer the 2 or 3 separate decks, so I have different choices for different decks. I can play on Deck 1 with Dolby off, and record a copy in Deck 3 with Dolby on. If in doubt use Dolby on the recording deck, but leave it off on the playback deck.
 

Q: If you dub a Dolby tape without Dolby, won't the Dolby still be on the [copy] tape? If you dub on a double cassette deck without Dolby on, the highs will still be artificially boosted. Any hiss that may be added will still be knocked out when played back with Dolby on.

Yes, the Dolby encoding will still be on the copy tape, in some way. If you dub a Dolby tape on a double cassette deck with Dolby off, the artificial boost will still be there, but this boost possibly might not be 100% accurately recorded on the copy.

So, if the copy gets played on a Dolby deck, it's expecting true, direct Dolby encoding on the tape, and it gets, instead, a slightly hissier and somewhat degraded analog COPY of the Dolby encoding, minus some of the high frequencies that got clipped off during the dub. So Dolby NR trims off its normal 5-10 decibels of the high end during playback, and the copy tape would sound slightly dulled. On top of that, you would end up with the "twice as much hiss" factor.

 
Q: If cutting out hiss is so simple, why do people dub tapes using "NO DOLBY"?

It's part of the misperception that "Dolby makes your tapes sound duller". This was probably caused by trying to play a non-Dolby tape with the Dolby switch ON, and noticing that the music sounds duller. Some people jumped to the immediate conclusion that "Dolby Sucks" and never tried to use it properly.

Others expressed the concern that the Dolby calibrations vary from deck-to- deck, although I personally have not had a major problem with that. I think that non-matching tape-head alignments and speed differences are a bigger problem than Dolby calibrations.

 
Q: Is there any way to physically tell the difference between a Dolby-encoded tape and a non-Dolby tape?

 Unless the tape is a pre-recorded one (like YesYears on ATCO), there is no way to tell if a tape is Dolby or not, unless the person who recorded it tells you if they had it on or off. You may have to rely on your ears to make a judgement. Dolby is not like Chrome, Metal or Type II tapes, where the recorder can automatically sense what it is.

 
Q: What is dbx?

DBX is a different type of compression scheme. It is NOT compatible with Dolby, and tape decks that do not have dbx encoding will not be able to play back dbx tapes and make them sound pleasant.
 

Q: What is Dolby C?

Some decks have Dolby C, which is basically an enhanced version of Dolby B. Dolby C is able to cut hiss by 10-20 decibels, but there is a catch: You have to use better quality tape (dropouts sound WORSE using Dolby C, compared to Dolby B), and Dolby C tapes are not as compatible as Dolby B tapes when they are played in non-Dolby tape decks. By the time that you play Dolby C tapes on car stereos, what used to be "excessive brightness" is now "annoying shrillness". Also, playing with the "tone" and "treble" knobs will NOT result in a pleasant-sounding tape. Do not record with Dolby C unless you and the person receiving the tape have a pre-arranged agreement to use Dolby C.


This article is Copyright 1995, K.F. Louie. May not be reproduced without the written permission of the author.
This article has been previously published on Southside.org

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