Dolby Noise Reduction Explained

dolbyb a 150x150 Dolby Noise Reduction Explained

Dolby B NR Logo

You know (or knew) the feeling…

You’re cruising down the highway listening to your favorite Hall & Oates jam, and just when Daryl and John’s most quiet, introspective lyric comes up, you hear that familiar sound: Cassette tape hiss. But not to worry! Just hit the famous “Double D” button and all the irritating hiss (well, most of it) disappears.

That’s the magic of Dolby Noise Reduction. And it really is some kind of magic too. Not only did Dolby B, C, and S cut the hiss in your favorite cassettes, they also made the magnetic tape—a technology originally invented for lo-fi voice recordings—the last true high fidelity medium for analog audio.

Amplitude and Frequency

Before we go any further, two concepts are worth reviewing: Amplitude and frequency. Amplitude is the relative loudness of individual parts within a recording. A soft vocal passage, for example, has a relatively low amplitude. A sharp bass drum rap, on the other hand, has a relatively high amplitude. The drum, put simply, is louder than the vocal. Frequency is the pitch of a sound. A bass solo has a much lower pitch than a guitar solo, to put things in extreme terms.

Tape hiss is high frequency and low amplitude—high pitch and relatively quiet. This means when the amplitude of a recording is relatively high, the hiss is sufficiently masked, but when the amplitude of a recording is relatively low, tape hiss reappears. Quiet passages like the one mentioned in the introduction are the most negatively affected by tape hiss. The challenge of noise reduction standards like Dolby is to boost the loudness of the high-frequency, low-amplitude portions of the recording other than tape hiss without also boosting the similarly high-frequency, low-amplitude tape hiss.

Zeroing In On Tape Hiss

Several technologies emerged from the ’60s well into the ’90s that sought to do this, but only Dolby was able to “zero in” on problematic passages alone without affecting others. The magic of Dolby NR is it allowed anyone to make a recording—without the need for sophisticated studio equipment—that boosts a precise band of low-amplitude audio frequencies upon recording.

The “precise” part is key. Dolby circuitry actively discriminates against high-frequency, low-amplitude tape hiss as well any high amplitude signal of any frequency, as Ray Dolby himself explained. Pretty amazing stuff considering the first commercially available deck with Dolby B came out in 1968!

Put another way, Dolby is smart enough to isolate the tape hiss from the recording and the loudest parts of the recording (which, by nature, mask any tape hiss) to work solely on the low-middle band of the recording. It then isolates the highest frequencies within that middle band and boosts their amplitude higher than any tape hiss.

Upon playback, Dolby circuitry reverses the process. The amplitude of any passages below a certain frequency is significantly reduced. In effect this means the passages boosted in the recording process are restored to their normal levels. Tape hiss, however, it so quiet it is hardly audible.

The entire noise reduction process, from recording to playback, is called companding—a portmanteau of compression and expansion. Dolby compresses the signal during recording and expands it upon playback.

For the visually-inclined, here’s a helpful graphic from Hyperphysics:

dolby noise reduction Dolby Noise Reduction Explained

Dolby NR “Companding” Process

Practical Applications

So is there any practical application for all this fanboy worship of Ray Dolby? Yes there is. Especially now that cassette tapes are almost exclusively reserved for collectors (and luddites), too many people use Dolby incorrectly. I’m even guilty of this myself. Let’s dispel some incorrect assumptions:

  • Dolby does not eliminate pre-existing tape hiss. It only avoids introducing additional tape hiss when dubbing from a master recording. In other words, re-dubbing a recording with Dolby will not mask the faults of a poor master recording.
  • Less is more. Using a Dolby setting during playback of a non-Dolby encoded tape results in a dull recording with significantly decreased high range. Conversely, playback of a Dolby-encoded tape without using the corresponding Dolby setting results in a sound with over-accentuated or “bright” high notes.

Oh, and one more thing. It’s pronounced DOLE-bee, not DUBB-lee:


  1. Colin Luke says:

    Thank you for an excellent reminder of the “modis operandi” of the Dolby System. I am old old timer who is resurrecting and restoring his “old” equipment with a view to enjoying many hundreds of accumulated cassettes, vinyls and other genre.

  2. I wish someone would release a Dolby emulator app for the iPhone/iPod. When I want to hear good old Dolby in my music, I use one of those car cassette adapters with a cassette player that has Dolby NR. The weaker the signal of the original source (cd player, iPod, etc.), the stronger the noise reduction got. I grew up on cassettes, and I always liked switching Dolby NR on and off.

    • I’ve never heard of using Dolby NR for anything other than tapes. It seems to me that it would only serve to cut out some of the high-end. Sounds to me like you prefer a flatter sound than I do.

      You could probably achieve what you’re looking for by cutting down the treble on your car stereo or by using a flat equalization setting on your iPod.

  3. David Johnson says:

    I was never a huge fan of Dolby Type B noise reduction in cassette formats. Cassette tape has an inherent limitation in its ability to record high frequencies without distortion. Other tape formats such as reel-to-reel do not have this limitation. The small tape size and slower tape speed of the cassette format made reproduction of high frequency content a challenge without saturating the tape and causing distortion. Higher resolution tapes, such as “metal” formulations were engineered in an attempt to overcome this problem. Dolby B in effect added 10dB to the high frequency content during the recording process which forced recording levels to be lower to avoid high frequency saturation in all but the highest quality tape recorder/tape combinations. Lower recording levels meant less masking of the noise by the actual signal effectively minimizing any effect Dolby B has on the recording. Even if you were so lucky as to be able to afford such a high quality tape recorder and the necessary high quality tape, mismatches in the Dolby calibration levels from tape deck to tape deck means that playing your high quality recorded tape in another tape deck caused an unpleasant “breathing” phenomenon in the sound quality and/or a lower perceived high frequency level. That is why most people turned off the Dolby during playback. In fact, the increase in high frequency content with the Dolby “off” was deemed good sounding to most people who were used to the lower frequency response of cassettes in general. It wasn’t until later on that the dbx noise reduction system was introduced that the cassette format had a viable noise reduction system. Unfortunately, it was too late on the scene to really catch on before the advent of the Compact Disc. The dbx system works by compressing the dynamics of the signal during recording and then expanding the dynamics during playback by the same amount. Because the background noise has virtually zero dynamics, It was played back at the compressed level while the music signal is restored. This resulted in a very significant reduction in playback noise of around 40dB. This made possible the ability to make recordings on cassette that are virtually indistinguishable from the source. It never really caught on because you couldn’t play a cassette recorded using dbx on a system that didn’t have a dbx decoder, such as a car stereo. That is the one advantage the Dolby B has. There were some outboard dbx decoders made for the car audio market by Concord, but once again the advent of CDs made this irrelevant.

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