Dolby C: The Black Sheep Of The Dolby Noise Reduction Alphabet?

It may be a relatively unknown variant of the Dolby noise reduction system during cassette’s heyday, but is the Dolby C noise reduction system the “black sheep” of the Dolby noise reduction alphabet?

By: Ringo Bones 

From my own perspective, it seemed more like a “dark horse” than a black sheep of the then existing Dolby noise reduction variants used to reduce cassette tape hiss. But most veteran audio enthusiasts’ low opinion on Dolby C primarily stems from the fact that those few commercially produced prerecorded music cassette tapes that are Dolby C encoded tend to sound harsh when played on tape decks that aren’t equipped with the Dolby C noise reduction system. But is it really like that in practice during the heyday of the cassette tape? But first, here’s a primer for those who are unfamiliar on what the Dolby C noise reduction system is all about.

With Sony’s Elcaset failing to dethrone the reign of cassette tape near the end of the 1970s and other music media that offer potentially better sound quality – like quarter-track open reel tapes, 8-Track tapes and the vinyl LP – seems to be relegated to the more “extremist” audiophiles and therefore deemed not a threat to the cassette tape’s profitability, the creators and manufacturers of Philip’s compact cassette tape tasked themselves to improve its then existing sound quality further by formulating a “improved” version of the Dolby B noise reduction system. Thus Dolby C was born.

Dolby C was introduced around 1980 and it managed to provide about 15 dB of noise reduction (A-weighted). It is constructed by combining the effect of two Dolby B systems together with an expansion to lower frequencies. This is primarily accomplished by splitting the side-chain companding action between two dynamic range processors. Each dynamic range processor is operating within a separate input level range. In record mode, a high level (-30 to -10 dB) dynamic range compressor is followed by a low-level (-50 to -30 dB) dynamic range compressor.

Dolby C noise reduction is effective above approximately 150 Hz and achieves about 18 to 20 dB of improvement in noise reduction / signal-to-noise ratio in practice. Although Dolby C noise reduction employs more treble boost than Dolby B noise reduction, Dolby C includes a special recording characteristic to avoid saturating the cassette tape being recorded to at extremely high audio frequencies - i.e. audio signals above 10,000 Hz. Therefore, Dolby C noise reduction tends to have more headroom than Dolby B noise reduction – making Dolby C freer from tape saturation when recording high frequency audio signals at high recording levels. Tape saturation not only causes harmonic distortion to rise to unacceptable levels but it also reduces the recorded treble level – a phenomenon called foldback. With anti-saturation and spectral skewing techniques, Dolby C noise reduction was intentionally made to be a much improved version of the Dolby B noise reduction it was first set out to supersede.

Dolby C first appeared on high-end cassette tape decks in the early 1980s. The first commercially available cassette tape deck equipped with a Dolby C noise reduction system was the NAD 6150C, which came into the market in 1981. And unbeknown to most audio enthusiasts at the time, the Dolby C noise reduction system was also used on professional video equipment for audio tracks of the Betacam (that used Betamax videotapes) and the Umatic SP videocassette formats.

In actual use, prerecorded cassette tape that used the Dolby C noise reduction system during recording tend to sound much worse when played on equipment that doesn’t have a Dolby C noise reduction system “decoder”. Or maybe its just at the time in the early 1980s, most prerecorded cassette albums recorded with Dolby C are intended for audiophile audiences – i.e. Classical and acoustic Jazz. If Dolby C was used on heavy metal rock albums of the time, consumers would have viewed the increased high-frequency harshness as added clarity akin to a very loud stadium rock concert circa 1980s.

Another criticism of Dolby C is that – especially with piano and other transient rich relatively quiet acoustic recordings – the cassette tape’s background hiss can be heard to vary with the main recorded music signal. A phenomenon now called breathing and / or pumping. Nevertheless, by the time the consumers’ complaints about Dolby C noise reduction became widely heard, the powers-that-be of the “cassette tape cartel” were already formulating a way to make a better version of Dolby C at the very tail end of the 1980s.