Tuesday, June 4, 2013

Dolby S Noise Reduction System: Savior Of The Cassette Tape?


In spite of its “heroic” audio engineering specs, why hasn’t the Dolby S noise reduction system been able to save the cassette tape from eventual commercial extinction?

By: Ringo Bones 

Introduced back in 1989 with the hopes to extend the cassette tape’s “commercial lifetime” well into the 21st Century despite of the onslaught of the digital media competition, Dolby S was introduced to the hi-fi masses primarily to make commercially produced mass-market prerecorded cassette tapes sound as good – if not virtually indistinguishable from the 16-Bit 44.edbook spec CDs. Sadly, according to the major consumer electronic entertainment firms, tape-based recording and playback systems – like the cassette tape – we are told, is just isn’t acceptable anymore during the early to mid 1990s because it has an old-fashioned image. Not to mention the proliferation of CDs (both brand new and reissue albums – even hard-to-find rarities) only a dollar or two more expensive than their prerecorded music cassette tape album versions sold in music stores and the proliferation of “affordable” (as in between 350 to 500 US dollars each) CD players from Marantz, NAD and Rotel which now sound as smooth as their entry-level vinyl LP playback system counterparts. Despite its inability to save the “commercially doomed” and aging cassette tape format, what makes Dolby S so special in comparison to its other Dolby cassette tape noise reduction system predecessors? 

Dolby B and Dolby C are cassette tape hiss reduction processes which, in essence, boost the signal to be recorded in order to keep it much louder than the tape hiss. On playback, the boost is reversed, drastically reducing the volume of the hiss in the process. A complication arises because the boosted signal must not be allowed to peak so high that the tape cannot accommodate the signal, and it is solutions to this problem that made every Dolby noise reduction system developed so far such an outstanding success. The Dolby B circuit reduces noise above approximately 500 Hz and achieves about 8 to 10 dB of noise reduction while Dolby C is effective above approximately 150 Hz and achieves about 18 to 20 dB of improvement in the signal-to-noise ratio. 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 at extremely high audio frequencies approaching 20 KHz. But his doesn’t mean that it’s perfect, or that Dolby B or Dolby C tackles all of the problems associated with recording unto cassette tape. This is where Dolby S steps in.
Dolby S is the simplified for consumer use / domestic use version of the Dolby SR – a professional signal processing scheme, not just a mere noise reduction strategy. It consists of a number of circuits operating together to reduce tape hiss right across the audio frequency band of 20-Hz to 20-KHz – not just the high frequencies where Dolby B and Dolby C operate – with the minimum possible audible intrusion. Several different measures are taken to ensure that noise reduction is supplied in such a way that the background hiss – including the microphone and first-stage preamplifier hiss, not just tape hiss – can’t be heard to vary, which is the problem with Dolby C in particular, especially with piano and transient-rich music. 

Techniques employed including varying the bandwidth of the signal compressors as the music changes using three separate stages of signal processing to cover the entire 20-Hz to 20-KHz audio frequency band, and using weighed networks and filters that modify the action of the processors where the system is likely to be audible. If you like Dolby S analyzes the nature of the signal to be recorded and responds to it intelligently. In use, Dolby S achieves an improvement of 23 to 25 dB of signal-to-noise ratio.  

A secondary property or benefit if you will, of commercially produced Dolby S encoded cassette tape recordings is that in the absence of a Dolby S decoder equipped cassette tape playback deck, as in older cassette tape decks, they will operate satisfactorily with a Dolby B processor at a cost of some mild compression, which can actually be beneficial. This is especially noticeable in-car cassette tape playback systems where the ambient noise levels can swamp the quieter musical passages. Given such “heroic” abilities, why didn’t the Dolby S noise reduction system manage to save the cassette tape from being crowded out by the 16-Bit CD and even the strongly resurgent vinyl LP competition during the mid to late 1990s? 

Being fortunate enough to have a hands-on review of the two most popular “almost affordable” Dolby S equipped cassette tapes being sold back in 1994 – the Sony TC-K611S which sold for 460 US dollars at the time and the Kenwood KX-7050S which sold for 560 US dollars. Thanks to our very friendly local hi-fi dealer who at the time used the two Dolby S equipped cassette tape decks in a more or less in an almost permanent demo basis, given that the two were the only high-ticket items practically flying out of the shelves back then. Sadly, Dolby S doesn’t make up for a lack of talent elsewhere. I mean the rather audible – from a musician / music lover’s ears’ perspective – wow and flutter of the 100 US dollar cheaper Sony TC-K611S cassette tape deck made a rather unacceptably “wow and fluttery” rendition of the prerecorded cassette of Tori Amos’ Little Earthquakes album, while the rendition of the Kenwood KX-7050S was less musically convincing than a similarly priced Nakamichi cassette deck which our friendly hi-fi shop owner owns and often uses as a reference for reviewing cassette tape decks – and no, the Nakamichi doesn’t have Dolby S. Compared to the similarly priced Nakamichi, the Kenwood had a less than musically convincing performance – especially in the mid-band. Although both the Dolby S equipped Sony and Kenwood cassette tape decks delivered a hiss-free performance. 

The resulting hands-on review only serves to reinforce my preconception of consumer electronic products I since harbored since my childhood during the 1970s that mechanical sophistication can be replaced by electronics, bringing enormous cost savings in the process. These days – or back in the post Operation Desert Storm days of 1992, electronics come cheap, but mechanical parts don’t. Cassette tape decks are a complex amalgam of mechanics and electronics. The Japanese being undisputed masters of the former managed to manufacture the best and relatively affordable transport mechanisms and record / playback heads during the cassette tape’s heyday in the 1980s. For example, Nakamichi makes great recording and playback heads for cassette tape decks, but you can bet they are a far more costly way of obtaining an increased dynamic range than the Dolby S modules in the cassette tape deck’s signal path. Even though it failed to prevent the commercial extinction of the cassette tape, at least Dolby S may have extended its commercial lifetime of the cassette tape up to until 2008. 

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