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.

Dolby B: Noise Reduction Alphabet For The Masses?

Famed for lifting a lowly dictation medium – the Philips’ compact cassette – to true high fidelity status, does the Dolby B noise reduction system deserve to be called the noise reduction alphabet for the masses?

By: Ringo Bones

The Dolby B type noise reduction system makes use of single chain signal processing. The filter is a fixed-gain variable bandwidth device. The resulting output of the Dolby B-type encoder is a boosted high frequency output response which gradually flattens out as the high-frequency output level rises to avoid tape saturation. The Dolby B circuit reduces noise above approximately 500 Hz and achieves about 8 to 10 dB of noise reduction. Despite becoming the ubiquitous noise reduction system of choice during the heyday of the compact cassette, the Dolby B type noise reduction system would not have existed at all without the help of Henry Kloss.

During his early work in radio astronomy, Ray Dolby got inspiration on how to make an effective noise reduction system for analog tape based recording form his own work in trying to extract very weak cosmic signals from background radiation noise, which later became the working principle behind Dolby A, a noise reduction system that then became in standard use in professional recording studios during the start of the Golden Age of Stereo near the end of the 1950s. Despite its effectiveness, Dolby A was deemed too complicated for domestic use that domestic hi-fi enthusiasts during the 1950s up to early 1960s had been doing their do-it-yourself audio recordings on their consumer-grade open reel tapes without the help of any form of noise reduction whatsoever.

In 1967, after hearing about Ray Dolby’s famed noise reduction system – the Dolby A type noise reduction system – which then became more or less the standard noise reduction system used in every professional recording studios, Henry Kloss tried to urge Ray Dolby to develop a much simplified version of Dolby A for domestic use. Dolby later on complied and developed a simplified consumer version of the Dolby A-type noise reduction system which we know today as Dolby B, which Kloss originally saw as a boon to home / domestic open reel tape users. Dolby B then became a runaway success in the domestic open-reel tape recording front that one of the first open-reel tape machines intended for home use that adopted the Dolby B type noise reduction system – the now famed Revox A77 open-reel tape deck – got a very favorable review on the August 1972 issue of Stereo review magazine that reviewer Julian Hirsch said “he cannot imagine how the sound quality of this machine could be improved in any way” when he tested the Revox with the Dolby B noise reduction engaged. By the way, Dolby B was also used on 8-Track tape systems back then. 

Somewhat later, Henry Kloss linked the Dolby B noise reduction system with a previously unsuccessful Du Pont product – the chromium-dioxide tape. And thanks to the magical consumer product midwifery that Kloss excels, these inventions helped make the Philips compact cassette tape – introduced primarily for office dictation purposes – become a viable musical storage medium that went on to surpass the vinyl LP in sales during the very end of the 1970s.

  For any type of Dolby noise reduction system to work properly, the level in playback must be matched to the level in recording. Without such “tracking”, treble frequencies can sound muffled or suppressed. Fortunately, the Dolby B type noise reduction system seems to function more or less adequately if there’s no gross mismatch in playback and recording levels. And what makes Dolby B very popular to consumers despite the next generation of Dolby noise reduction systems slated to replace it is that Dolby B also works adequately in cassette tape playback equipment that are not equipped with any Dolby noise reduction system whatsoever, thus gaining the approval of the masses who during the 1980s more than a half of them probably don’t own a cassette tape playback equipment equipped with any form of Dolby noise reduction system. 

Dolby A: Breakthrough Legacy of the Dolby Noise Reduction Alphabet?

It might be superseded by newer “alphabets” in the Dolby Noise Reduction series, but does Dolby A truly qualify as a “breakthrough legacy” in noise reduction of tape-based recording?

By: Ringo Bones

There is a little-known fact that Ray Dolby of analog audio tape noise-reduction fame did his early work in radio astronomy. In fact his inspiration for his famed series of “alphabets” of noise reduction schemes which later on eventually lifted the lowly Philips compact cassette tape into the realm of high fidelity and earned Ray Dolby enormous wealth – came from his work in trying to extract very weak cosmic signals from background radiation noise. But the question now is, did the first of the series of the “alphabet” of Dolby’s noise reduction systems – namely Dolby A – truly qualify as a breakthrough legacy when it comes to noise reduction in the high-fidelity tape-based recording and playback?

After American open-reel tape recording pioneer 3M studied Jack Mullin’s World War II – era “war booty” -i.e. Helmut Krüger’s open-reel tape recording and playback system used by the Reichs-Rundfunk-Gesellschaft to broadcast both live Orchestral music recordings and Adolf Hitler’s speeches, it was only capable of a dynamic range of 60 decibels and was later deemed “inappropriate” for true high-fidelity sound during the very dawn of the Golden Age of Stereo. Ushering in a series of innovative methods to improve the dynamic range of tape-based recording and playback by increasing its dynamic range / signal-to-noise ratio to make it much closer to sound like a live musical performance.

The Dolby type-A noise reduction system first used during the start of the post World War II studio recorded music trend that compresses certain portions of the audio signal prior to recording. It does not operate on high-level signals. Dolby A uses a side chain operation where the signal is split into four bands, each of which is processed separately. In the pro-audio / studio recording world, Dolby A almost gained a universal acceptance when it comes to recording and mastering in the recording studio.

Some “purist” analog recordings made as relatively recently as during the late 1960s to the 1970s – either of Classical music or studio recorded pop/rock music persuasion – that does not use Dolby A or any other form of noise reduction seem to have developed a cult-like following of achieving a more life-like sound in comparison to their counterparts that used Dolby or other form of noise reduction. And despite these master-tapes possessing slight residual tape hiss due to the fact that these tapes were recording music while run at their highest possible speed, they do seem to cast doubt as to whether we need noise reduction at all when recording in the studio when all that noise reduction does is ruin the naturalness of the recorded sound. And some pop/rock recordings made during the late 1960s and 1970s that don’t use any form of Dolby noise reduction even sound natural despite of the overdubs.

 Surprisingly, some hi-fi enthusiasts were even surprised by the naturalness that resulted when recording onto cassette tapes without any form of Dolby noise reduction, though the cassette tape deck used in this recordings were modified to run at 3 ¾ inches per second – twice that of standard cassette to enable it to record with signal peaks at +10dB on the deck’s VU meters. So there are times that better more natural sounding recordings can be achieved by not using Dolby A or any other form of noise reduction for that matter. Well, at least Dolby A eventually paved way for its “younger brother” that’s intended for home / domestic / consumer applications called Dolby B.