Dolby B, Dolby C and Dolby S Explained
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One question I get asked surprisingly often is, "What does the Dolby switch actually do?" If you've ever wondered whether you should have it switched on or off, this month's workshop article is for you.
Why Was Dolby Needed?
Cassette tapes are an incredible piece of engineering, but they have one unavoidable downside – tape hiss. This constant high-frequency background noise becomes more noticeable when listening to quieter passages of music.
To combat this, Dolby Laboratories developed a clever system that reduced tape hiss without significantly affecting the music.
The important thing to understand is that Dolby noise reduction works both when recording and when playing back. If the tape wasn't recorded with Dolby, you normally shouldn't use Dolby during playback.
Dolby B
Dolby B was introduced in the late 1960s and quickly became the world's most common cassette noise reduction system.
It works by gently boosting quieter high-frequency sounds during recording. During playback, the player reduces those same frequencies back to their original level, taking much of the tape hiss with them.
Advantages
- Compatible with almost every cassette deck and Walkman.
- Reduces hiss without dramatically changing the sound.
- Most pre-recorded commercial cassettes that mention Dolby were encoded using Dolby B.
Things to Remember
If you play a Dolby B recording with Dolby switched off, the music can sound a little bright or harsh. If you play a non-Dolby recording with Dolby B switched on, it will usually sound slightly dull because the high frequencies are reduced when they shouldn't be.
Dolby C
Dolby C arrived in the early 1980s and offered a much greater reduction in tape noise than Dolby B.
It provides roughly twice the noise reduction and works over a wider frequency range, producing noticeably cleaner recordings when used with good quality tape.
Advantages
- Much quieter background noise.
- Better suited to high-quality home recordings.
- Excellent when used with chrome or metal tapes.
Things to Remember
Dolby C recordings generally require Dolby C during playback. Playing them back without it often results in a sound that is far too bright and unnatural.
Because Dolby C requires much more accurate tape alignment and calibration, it was far more common on hi-fi cassette decks than on portable Walkmans.
Dolby S
Dolby S was the final evolution of analogue cassette noise reduction before digital formats took over.
Introduced in the 1990s, it borrowed technology from professional studio Dolby SR systems and could reduce tape noise by up to 24 dB under ideal conditions.
Advantages
- Outstanding noise reduction.
- Excellent preservation of musical detail.
- Makes high-quality cassette recordings surprisingly close to digital in quiet passages.
Things to Remember
Very few portable cassette players ever included Dolby S, and relatively few home cassette decks supported it. As a result, Dolby S remains quite rare today.
So Which Is Best?
Each Dolby system has its place:
- Dolby B – The most common and the one most collectors will encounter.
- Dolby C – Ideal for enthusiasts making high-quality recordings.
- Dolby S – The ultimate analogue cassette noise reduction system, but comparatively rare.
A Common Myth
Many people believe switching Dolby on automatically improves sound quality. In reality, Dolby is not an equaliser or sound enhancement feature. It only works correctly if the tape was originally recorded using the same Dolby system.
If the wrong Dolby setting is used, the music can sound either too bright or too dull.
From the RetroTechnical Workshop
When I refurbish a Walkman, I always test the Dolby circuitry if the model includes it. Dirty switches, ageing components or incorrect tape alignment can all affect how well the system performs.
A properly serviced Walkman with correctly functioning Dolby circuitry should reproduce Dolby-encoded tapes exactly as they were intended to be heard—clear, detailed and with significantly less background hiss.
If you've got an old cassette collection, take a look at the cassette inlay card. You may be surprised to find that many of your favourite albums were recorded with Dolby B, even if you've never noticed before.
Until next time,
Simon
RetroTechnical