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:
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: