dbx: The Forgotten Cassette Tape Noise Reduction System?

Though it was touted during the cassette tape’s heyday as the noise reduction system that can make tapes as silent as digital recordings, has dbx now been relegated to the technological dustbin of history?

By: Ringo Bones

As a budding hi-fi enthusiast during the latter half of the 1980s and a semi-pro studio musician during the late 1980s and the early 1990s, it seems – from my own perspective – that the dbx noise reduction system is more popular in the pro-audio field than in home hi-fi. And even though in this day and age of barely-legal and barely-audiophile sounding digital music downloads, has dbx been largely forgotten?

Even though the dbx name had only been familiar to hard-core audiophiles who want to extract the last ounce of performance from cassette during the 1980s, dbx – the company – is much more than a mere “cassette tape noise reduction company”. The company, I think, has two main business addresses – their consumer gear division can be contacted at dbx, Incorporated, 71 Chapel Street, Newton, MA 02195. While their pro audio division can be contacted at (well, at least according to the business card they gave me back in 1990) at dbx Professional Products, A division of AKG Acoustics, Inc. 1525 Alvarado Street, San Leandro, California 94577, USA.

Product wise, dbx is a family of noise reduction systems developed by the company of the same name that was used not only for cassette tape noise reduction for the home, but also for professional applications in the “modern” recording studio and for noise reduction of sound reinforcement systems used for  live stadium concert applications. The original dbx Type I and Type II systems were based on so-called “linear decibel companding” – that is the audio signal to be recorded onto a recording medium has its dynamic range compressed and then its dynamic range is expanded upon playback to not only make it sound as close to the natural recorded event as possible, but also to transcend the dynamic range limitations of most existing analog recording mediums at the time. The dbx companding-expanding system was invented by David E. Blackmer of dbx, Inc. in 1971.

In practice, the dbx noise reduction system encodes the audio signal to be recorded (or transmitted) with 12-dB preemphasis (boosting the high frequency part of the audio spectrum) prior to compression. The purpose of which is to further boost the high frequency signal content above the hiss level. An additional preemphasis network is placed in the control voltage (detector) signal path. The extra boost further reduces the compressor gain, keeping recorded high-frequency program content below the saturation point of the tape. This ensures that high-level input signals will not saturate the tape.

There are actually two types of dbx noise-reduction systems, Type I is intended for high speed – 15 inches per second tape speed or greater – tape recording and playback operation and for use with any other wide-band audio transmission system. For slower tape speeds and other restricted bandwidth media – like cassette tapes and vinyl LPs – the dbx Type II noise-reduction system is used. The main difference between Type I and Type II dbx noise-reduction systems can be found in the level detection circuits.

At around 1981, if one wants to record and plays back his or her prerecorded dbx tapes or wants to dbx encode his or her DIY cassette tape recordings, he or she can use an external stand-alone dbx noise reduction processor like the dbx Recording Technology Series Model 224 which was advertised in Stereo Review magazine’s January 1981 edition with a MSRP of about 300 US dollars – which is already a lot of money in 1981, and I have the good fortune to use this myself near the end of the 1980s. At the time, the dbx Recording Technology Series Model 224 was touted as the then “cheapest” way to achieve recordings having a signal-to-noise ratio of 90dB – akin to then existing digital recordings – for home use / making DIY recordings. In cassette tape use, the dbx noise-reduction / dbx NR system operates throughout the audio range and does not require matching record and playback levels. Thus, there is no tracking problem that can affect treble response.

However, it has been noted that playback response at the frequency extremes, particularly at the treble end, tends to be not as good with dbx as with Dolby B and C NR – i.e. Dolby B and C NR tend to sound more natural on acoustic Classical and Jazz music compared to dbx. The dbx system achieves about a 30dB reduction in noise while Dolby B NR only achieves about 10dB while Dolby C NR 20dB. So why did dbx, despite its excellent specs, got “crowded out” in the market place by lesser-performing Dolby noise-reduction systems?

The general lack of acceptance of the dbx noise-reduction system in the consumer electronic marketplace during the go-go 1980s is that all dbx encoded recordings sound unacceptable – i.e. has an over-compressed dynamic range or no variation between loud and soft musical passages – when played on playback equipment without dbx encoding despite the very excellent improvement in sound quality. The first generation of dbx stand alone noise-reduction processors intended to be connected to your existing playback equipment – like your cassette tape deck and turntable for vinyl LP playback – were kind of prohibitively expensive for most hi-fi enthusiasts back in 1981. And despite allowing you to play your cassette tapes and vinyl LPs with the same lack of noise and hiss as that of the late 1970s era professional digital recording system – the JVC DAS-90 system - mainly used in digitally recording and mastering mainly Classical music performances around 1980 – dbx died barely a whimper around 1982. Even dbx encoded vinyl LPs released during 1973 to 1982 were capable of having a signal-to-noise ratio of 120 dB, by the way, Redbook spec 16-bit 44.1-KHz sampled CDs were only capable of around 98dB signal-to-noise ratio - and the JVC DAS-90 digital recording system probably has a lower signal-to-noise ratio than this - even with hiss free full digital recordings.

In my own experience, the dbx noise-reduction system seems more suited to recording and listening pop/rock music without the residual hiss from the cassette tape medium. Even though the cut-price dbx decoding of my own Technics RS-T55RP cassette tape deck, a cassette tape recording of Night Ranger’s Sister Christian was played back with a dynamic range punch reminiscent that of a live stadium rock concert, though as the strongly stuck drums died down, the softer piano parts of this particular Night Ranger song tend to pump and wheeze the cassette tape’s hiss – the oft criticized aspect of dbx artifacts known as breathing / noise modulation. And it seems that even pro audio gear made by dbx primarily aimed for live concert stage noise reduction – i.e. the dbx 563X Hiss Reducer – which around 1990 was primarily used to reduce the hiss of guitar effects stomp boxes using the rather noisy LM 741 op amp IC as the active gain element – also works as an excellent external cassette tape noise reduction / hiss reduction box too. 

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 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.