Monday, February 4, 2013

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. 

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