THE PURPOSE, LEGEND, AND LEGACY OF DOLBY NOISE REDUCTION

In our modern world, Dolby is more associated with movie theaters and surround sound than it is with cassette decks. When you see those double Dolby Ds standing back to back, your mind probably goes to Dolby Atmos, maybe Dolby Vision. But Dolby’s start actually dates back sixty years ago and begins modestly with the little ol’ cassette tape.

In 1965, Ray Dolby founded the company. Their first product? The Dolby A301, a prototype noise reduction unit to help eliminate tape noise. This solution was sophisticated yet elegant, and it swept over the chest board in one fell swoop, crowning Dolby as a king of sound.

IT STARTED WITH A HISS…

So why is there a need for noise reduction in the first place? Well, it has to do with the structure of magnetic tape. The particles that make up the tape create a quiet but noticeable noise. This unwanted noise sounds like a non-stop “hiss,” which is quite annoying to listen to while trying to enjoy your music. And while it’s not as discernible during loud sections of a recording, it does become quite a problem during the quieter sections of a song.

Since cassettes use magnetic tape, this means that all recordings had that annoying tape hiss. Ray Dolby wanted to find a solution to help reduce that noise. And find a solution he did.

THE BEGINNINGS OF A SOLUTION

First, Dolby used spectral analysis of a blank tape to break down what tape noise was; in other words, he figured out what sound frequencies make up the tape hiss. Once he got the noise profile, he was able to use an equalizer to tune down those frequencies. Problem solved!

Well, not quite. Tuning the frequencies down to make the tape noise quieter worked. But this caused the tape’s audio to become muffled and dull. So to solve this new problem, the recording has to be processed by Dolby noise reduction as well. This means getting the cassette deck to do the opposite of what it does during playback.

TURNING RIGHT TO GO LEFT

Though it may sound counterintuitive, Dolby Noise Reduction works by boosting the frequencies during recording that it cuts during playback. See, it’s actually a two-way process. If you over-boost the frequencies during recording that lie within the spectrum of the tape noise, you eliminate the problem of a dull recording. Then, during playback, those frequencies are pulled down, which not only lowers the hiss and makes it quieter, it also brings down the boosted parts of the signal back to where they should be. They sort of cancel each other out, so everything sounds normal during playback.

This is successful due to “companding,” a process in which the dynamic range (the difference between loud and soft sounds) of the recording is compressed because some of the recorded frequencies are louder than they should be. Then during playback, the amplitude of those frequencies is lowered, and the dynamic range is expanded.

In other words, during recording, there’s the automatic “boost” of low-level audio signals within the noise profile so they survive the noise floor of the tape. Then during playback, the opposite happens where the tape’s low-level signals are “reduced” back down to a normal range while also rejecting the noise of the tape itself. So, the sounds we want during recording are boosted, then are brought back to normal during playback, while the sounds we don’t want (like the tape hiss) get brought down low enough that they’re not easily perceptible.

In short? Dolby noise reduction compresses and expands audio signals in a double-sided process.

THE A,B,C,S OF DOLBY NOISE REDUCTION

It’s important to note that there’s not just one type of Dolby noise reduction. First, there’s Dolby A, which is what Ray Dolby first invented; however, this was a professional system designed for recording studios and film production. This leads us to Dolby B noise reduction. Dolby B is a simpler, cheaper, consumer-oriented system. It is what was used on the vast majority of consumer cassette tapes. Plus, a Dolby B encoded tape could be played on a standard cassette deck, although you’d get a bright sound due to the over-emphasis on the treble. But this wasn’t much of a problem anyway, as most stereos have treble and bass adjustments anyhow, allowing you to slightly turn down the treble, though the noise reduction isn’t as precise that way.

Then Dolby C was rolled out in 1980. This was another consumer system, but it improved on the noise reduction process and worked about 60% better than Dolby B. Unfortunately, however, on non-compatible equipment, Dolby C NR tapes sound odd, making pre-recorded Dolby C cassettes pretty rare. Dolby S was the next consumer system. This one outshone Dolby C: it was more compatible with Dolby B equipment and had excellent sound quality. Some even argue that a Dolby S-encoded Chrome tape sounds just as good as a CD. The problem here, though, is that it did come out in 1989, which is a time when the CD was already taking over.

In the professional realm, there was Dolby SR, which used rather vigorous companding. So did the competing dbx noise reduction system (dbx being a company known for manufacturing professional audio equipment, founded by David E. Blackmer in 1971). Dolby SR and dbx sounded great, with tape hiss being nearly imperceptible. However, playing a Dolby SR or a dbx tape without a compatible decoder results in weird audio that’s unpleasant to listen to. That’s the main reason why these remained in the professional space, rather than leaking into the consumer realm.

SUMMARY

After his invention, recording studios quickly adopted Dolby’s system, and it became the norm for cassette tapes and tape players to use Dolby NR. Sony added a compact version of Dolby’s noise reduction to its Walkman Pro, so even when cassette tapes became the name of the game in portable music, Dolby still stood at the forefront of audio.

Ray Dolby’s electronic circuitry successfully removed cassette tapes' unwanted hissing noise, crowning his company as one of the kings of sound. While the digital world eliminated the need for noise reduction, Dolby has adapted to the times, so it still holds that title even today.

Thanks for reading! Next time you’re sitting in a big, dark theater when the lights go down and Dolby Atmos surround sound takes over, we hope you’ll remember how it all started with Ray Dolby and his humble noise reduction. Have a great day!