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Dolby B and C adjustments

technofile  by al fasoldt

Columns and commentaries in a life-long dance with technology
 

Simple gray rule


How to make sure your Dolby B or C recordings are done right
 

By Al Fasoldt

Copyright © 1987, The Syracuse Newspapers

The biggest news in high fidelity this year will be the long delayed introduction of digital audio tape recorders. They promise a new level of quality in home-brewed tapes.

But you don't have to wait for digital tape recorders to get a preview of the kind of fidelity they offer. By making the best possible use of the cassette deck you now own, you can achieve results that are close to digital in just about every way.

The key is the proper use of the Dolby noise reduction that is built into your cassette deck. If you have a deck that was built within the last two or three years, chances are it has two of Dolby's patented circuits, the "B" and the "C" systems.

Dolby B was invented decades ago and is used in nearly all prerecorded cassette tapes. Dolby C is relatively new, and is used mostly in home tapings; only a few recording companies issue their tapes in the Dolby C format.

Both circuits are named after Ray Dolby, who invented the first successful noise-reduction system for studio tape recorders in the 1960s. It was known as the Dolby A system, and is still used around the world.

All three Dolby circuits work in a similar fashion. They reduce tape hiss (a rushing noise that can be heard in all tapes) by a clever boost-and-cut method.

During recording, high-pitched sounds are made stronger. That helps to make them louder than the inherent tape hiss. Then, in playback, every high-pitched sound on the tape is made weaker. This means that the music's high frequencies are cut back in level to where they should have been to start with. It also means that the tape hiss is reduced by the same amount.

This simple explanation doesn't tell the full story of how the Dolby systems work. To keep from making a mess of the recording whenever the high-pitched musical sounds were loud to begin with, Ray Dolby made sure his circuit left all loud sounds alone. (After all, boosting loud notes in recording would just make them fuzzy sounding.) And to make sure that extra quiet musical tones had a fighting chance to beat the hiss, he made sure that they were boosted all the way.

Here's where the hard part comes in. When the tape is played back, the Dolby systems have to treat the music in the exact opposite way that they treated it in recording. Every boost has to be matched by an equal but opposite cut.

If one thing is off—if, say, the tape is dull-sounding to start with—the Dolby systems will exaggerate that dullness and make the music sound terrible.

The way to avoid problems is to make sure that the tape you use gives a flat sound, with no brightness or dullness when it is played back. Professionals make sure their tapes meet this requirement through the use of a few thousand dollars' worth of test equipment.

But there's a much easier and cheaper way of making sure your tape is matched to your cassette desk for proper Dolby operation. All it takes is your FM tuner or receiver.

Just tune the FM to a spot between two stations. If you don't hear anything, try turning off the "muting" switch on the FM. You should hear a lot of rough-sounding hiss.

Record this FM interstation hiss on your cassette deck at a low recording level, and then play it back. When you listen to the playback, switch between the actual hiss and the tape playback and compare how they sound. (One rule you must follow is to make sure the interstation hiss and the tape playback are the same volume. Otherwise, the louder sound will always seem better.)

Do this comparison first with the Dolby systems switched off, and then do it with Dolby B on and finally with Dolby C on. If the tape is matched correctly with your cassette deck, it will sound pretty much the same with the Dolby circuits on as with them off.

If you hear a big difference, try another type or brand of tape. Keep trying different tapes until you find one that matches your deck—and then stick with that tape. You'll find all your recordings will sound better when the machine and the tape are compatible.


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