Saturday, October 26, 2013

Dolby HX-Pro: For Rockers Only?


After it became a practical and commercially viable signal processing scheme in domestic cassette tape recording when self adjusting cassette decks dramatically dropped in price, is the Dolby HX-Pro reserved for rock and heavy metal music album recording use only?

By: Ringo Bones 

Even though Dolby HX-Pro was developed in 1982 by high-end (then) West German hi-fi manufacturer Bang & Olufsen in conjunction with Dolby Laboratories as a viable add on for high end cassette tape decks destined for domestic use due to high end recording bias self adjusting cassette tape decks hat were introduced at the very end of the 1970s started dropping in price due to widespread availability. After all, the process behind Dolby HX-pro only works when cassette tape decks has a built in circuit that can dynamically, in real time, change the recording bias level being applied on the cassette tape being recorded on. And one of the first cassette decks to use it was Bang & Olufsen's B&O Beocord 9000 cassette deck. But despite only becoming a common add-on on domestic cassette tape decks during the very end of the 1980s and only used on commercially available prerecorded cassette tape albums in the early 1990s – and overwhelmingly only on hard rock and heavy metal music albums on cassette – is the Dolby HX-Pro deserve the reputation as something “for rockers only”? But first, here’s a primer on how the Dolby HX-Pro process works. 

Bias – as in AC pre-magnetization bias, usually at 50,000 to 120,000 Hz – is an essential ingredient in the analog tape recording process since the days of Helmut Kr├╝ger experimented with two-channel stereophonic recording onto open reel tapes. However, most cassette tape decks – especially those destined for domestic use – use only a fixed level of bias. High level high frequency musical information – as in strongly struck ride and crash cymbals in drum kits (especially hard rock and heavy metal drum kits) – tends to provide a certain amount of bias to the tape being recorded on its own, so whenever you are recording music which contains high frequencies at high recorded levels, the tape being recorded on will end up being slightly over biased – that is, getting more bias current than necessary. 

The results of excessive tape bias are dull-sounding high frequencies and a certain amount of midrange compression. In such circumstances, it would be better to reduce the level of fixed bias, so that the total bias remains constant, irrespective of the tendency of the high frequency music signals to bias the tape it is being recorded into. The effect of reducing the fixed level of recording bias when high frequency signals intended to be recorded are present is that the total amount of energy being recorded onto the tape is reduced. This means that there is more room for the music signal. This results in increased headroom for recording high frequency music signals. Dolby’s HX – or Headroom eXtension – circuit monitors the level of high frequency energy going into the cassette tape deck’s record head and dynamically adjusts the “fixed” recording bias level to that the total recording bias reaching into the magnetic particles of the tape used in the recording session is always the same. 

Back around 1991 and 1992, most prerecorded cassette albums that use Dolby HX-Pro in the recording process where overwhelmingly of the hard rock and heavy metal music genre – as in Megadeth’s Countdown to Extinction, Skid Row’s Slave To The Grind and Metallica’s eponymous black album just to name a few - while a prerecorded commercial cassette copy of The Three Tenors that was recorded with Dolby HX-Pro are as rare as hen's teeth. So when I managed to acquire a Dolby HX-Pro equipped cassette tape deck a few years later, I tried experimenting to use its “Headroom eXtension” capabilities while recording our local Classical musicians playing some Bach cello suites using their very own cellos – using a 12AX7 vacuum tube equipped microphone preamplifier connected to the Dolby HX-Pro equipped cassette tape I had at the time, a Pioneer CT-W606DR. 

Well, the results are quite interesting – cello music, or most live acoustic music in fact that’s not too loud as in averaging under100 decibels sound pressure level tends to sound better when recorded at a bit higher bias that is being “reduced” by the deck’s Dolby HX-Pro processing. I mean Dolby HX-Pro tends to make live cellos sound a tad steely and bright once recorded onto cassette. Is this due to increased total harmonic distortion on the recorded cello music signals on cassette tape via the Dolby HX-Pro being switched in? While hard rock and heavy metal tracks being dubbed from LPs and CDs for my own use tends to give better results with the Dolby HX-Pro process working overtime while recorded at the maximum level before overload distortion or cassette tape saturation becomes audibly obvious to audiophile trained ears. Maybe not just for rockers only, but the Dolby HX-Pro process really shines when you want to record music with really loud cymbals and percussion onto cassette tape at louder than average levels.  

Doubling The Running Speed: Secret To A Better Cassette Tape Sound Quality?



It may be a “long-shot” but does doubling the cassette tape’s running speed from 1 7/8 inches per second to 3 ¾ inches per second really improve its sound quality? 

By: Ringo Bones 

When it was introduced primarily as an office dictation recording medium back in 1963 by Philips, the compact cassette tape’s rather low 1 7/8 inches per second tape speed was the main engineering challenge of coaxing something approaching high fidelity sound from such an unlikely tape medium. Not to mention the narrow tape width of just 3.8 mm which contrasted with the higher fidelity sounding domestic music recording and playback mediums of the period – i.e. the quarter track open reel tape and the 8-Track tape cartridge’s 6.3 mm tape width. But can increasing the compact cassette tape’s speed from 1 7/8 i.p.s. to 3 ¾ i.p.s. really improve its sound quality? 

Back in the early 1980s, a consumer electronics company by the name of Teac made such the prospect of experimenting to improve the sound quality of the compact cassette tape medium by doubling its running speed much easier for domestic hi-fi enthusiasts by introducing the C-3X cassette tape deck that has the option – via a simple flick of the switch – to be run at 3 ¾ inches per second. But in order for the better sounding DIY recording – on cassette – to be playable other than the lone Teac C-3X cassette tape deck that you probably own (or more likely just afford to own) in your hi-fi rig, your other cassette tape playing equipment that you own or have access to – i.e. your boom box portable, Walkman, car stereo’s cassette tape deck, etc. should also be capable of running at 3 ¾ inches per second, which posed a problem for compatibility to anyone jumping into the “cassette running at 3 ¾ i.p.s. bandwagon” back then. 

As I was fortunate enough to “toy” with the Teac C-3X cassette tape deck in our hi-fi repair shop given that it was very widely available in second hand hi-fi shops across Vietnam during the mid 1990s, running cassette tapes at double its normal speed - the results can be quite spectacular. Using good quality cassette tapes like TDK SA, Sony Metal XR and related tapes – the results can be outstanding and hiss free even without switching in / using any form of Dolby noise reduction. When you can tape at recording levels right up to +10 dB (and even a bit more on the cassette tapes I’ve just short listed since those never saturated on the Teac C-3X cassette tape deck) hiss is no longer an issue. By the way, the best loved rock and pop recordings known for their pristine sound quality from the late 1960s up to the 1970s were recorded and mastered without any form of Dolby noise reduction whatsoever – though its on 2-inch thick open reel tapes being run at 30 inches per second. 

Although the midrange purity on the 3 ¾ i.p.s. cassette tape recordings – both LP test dubs for my own use and live recordings of local Classical and rock musicians via a 12AX7 vacuum tube equipped microphone preamp into the Teac C-3X tape deck – is about half a notch below what’s possible with a well maintained Elcaset deck or a hi-fi 8-Track record playback deck like the Pioneer RH-65 or a quarter track open reel tape running at 3 ¾ i.p.s. Though a metal particle cassette tape running at 3 ¾ i.p.s. on the Teac C-3X is miles ahead in sound quality when compared to a current i-Pod – even when that said i-Pod is playing hi-rez digital music downloads!

Sadly, despite their stellar sound quality – dual-speed cassette tape decks where a short-term innovation of the early 1980s. They contravened Philips standard which stipulates one speed for the compact cassette – i.e. 1 7/8 inches per second – for guaranteed compatibility, so were discouraged and hence since discontinued. Teac finished their line of dual-speed cassette tape decks similar to the C-3X cassette tape deck back in 1984. Were Teac the only ones making dual-speed cassette tape decks back then? 

Tuesday, September 17, 2013

Remembering Ray Dolby – Father of Tape Noise Reduction


Though his iconic invention made him a very wealthy man, could the world of hi-fi and music be very different today if Ray Dolby hasn’t bequeathed his invention to the public for almost nothi

By: Ringo Bones   

Sadly, Ray Dolby passed away Friday, September 13, 2013 aged 80. Forever remembered after the iconic noise reduction system that bears his name, the music and hi-fi world would probably not exist as we know it today without his inventive solution in tackling the perennial problem of magnetic recording tape hiss.
Ever since the US Armed Forces introduced working samples of World War II era German analog open-reel tape recordings and their associated tape recording and playback equipment near the end of the 1940s in various trade expos, many American audio engineers suddenly got an epiphany that tape hiss or noise is not going to be an easy problem to solve. This is so because tape noise results mainly from the lack of homogeneity of the magnetic coating. Even with existing technology – then and now – the ferrous particles can never be distributed absolutely uniformly throughout the coating and the resulting aggregations of these particles create discrete magnetic fields which – during replay – manifests itself as noise e.m.f. or tape hiss at the playback head and thus amplified along with the desired audio signal. 

Luckily, a then young electronics engineer named Ray Dolby managed to formulate – i.e. engineered - a very cost effective solution that became a noise reduction system that is named after him. Thanks to Ray Dolby, the lowly cassette tape that was primarily created by Philips as an office dictation medium was raised to high fidelity status and became a very cost effective analog tape based music recording medium since the latter half of the 1970s and even displacing vinyl LP in popularity as a domestic hi-fi playback medium during the 1980s. In a sense Ray Dolby’s iconic noise reduction system virtually raised every cheap magnetic tape recording based media into audiophile status.   

In 1967, Henry Kloss heard about Ray Dolby’s noise reduction system – i.e. the Dolby A which was intended for professional studio recording noise reduction applications during the Rock N’ Roll era. It was Kloss who pushed for a consumer version of the Dolby A noise reduction system which is now known as the Dolby B, which Kloss originally saw as a boon to home/domestic open-reel tape users – and made the Philips compact cassette – which Philips originally intended as an office dictation medium – into a runaway commercial hi-fi success that virtually out-competed the better sounding vinyl LP near the end of the 1970s.
Thanks to his financial success, Ray Dolby also paved the way for various inventions and inventors to make domestic high fidelity a much more affordable hobby. Using the same Peter Scheiber patents of the old quadraphonic sound systems that expired with barely a whimper back in 1975, Dolby Laboratories managed to create Dolby Pro Logic – a surround sound system that made possible those relatively affordable surround sound capable home cinemas during the early 1980s that also made low-cost mediums like the VHS or Betamax video cassette tapes – and even cassette tapes – capable of life-like surround sound reproduction when used with a designated Dolby Pro Logic decoding box. 

Unbeknown to most of us, Ray Dolby first cut his teeth in the field of radio astronomy. In fact, his Dolby Noise Reduction System came from his earlier work in trying to extract very weak cosmic radio signals from background radiation noise mainly caused by our very own radio telecommunications traffic. 

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. 

Tuesday, March 5, 2013

Surround Sound From Cassette Tapes?


Even though they were relatively widely available during the late 1980s and early 1990s, why didn’t the surround sound encoded cassette tapes became more popular than they should?

By: Ringo Bones 

As someone who grew up during the heyday of “physical music media”, music lovers back then gauged the popularity of a newfangled music recording gimmick by how soon it is applied to the hard rock and heavy metal music genre. And though it is not as popular as it should have, finding cassette tapes that are Dolby Pro-Logic surround sound capable of hard rock and heavy metal live concerts during the late 1980s and early 1990s is enough for one old enough to ask why are these types of cassette tapes never became standard issue? But first, let’s discuss a brief history of surround sound. 

 When the concept of surround sound – as in quadraphonic sound – was let loose to consumers during the early 1970s, the best surround sound systems during quadraphonic’s heyday were discrete quadraphonic on four-track open reel tapes. This was by far the best delivery system for the quadraphonic sound system but these tapes were just too expensive for the average audiophiles of the day. Even though quadraphonic sound systems – i.e. 1970s era surround sound – expired with barely a whimper in 1975, it was a concept that was introduced rather too late for a more consumer friendly priced quadraphonic sound system, a matrix decoder type surround sound with logic steering that became the basis of the Dolby Pro-Logic surround sound system that eventually reintroduced the concept of surround sound for the home around the middle of the 1980s. 

Based on the same Peter Scheiber patents as the old quad systems which Dolby Laboratories added their own proprietary improvements, the Dolby Pro-Logic system managed to rekindle the consumers’ interest in of surround sound in the home during the mid 1980s because its surround sound encoding system can be seamlessly introduced – more or less – into two-track stereophonic audio already widely used in cassette tapes and the VHS and Betamax videocassettes of the time while still allowing the very same Dolby pro-logic encoded cassette tapes and videocassettes to be played on ordinary two-channel stereo systems with no loss of sound quality. The great thing about the Dolby Pro-Logic surround sound system is that it can impart relatively accurate surround sound steering data even on bandwidth restricted recording and playback media like the Philips compact cassette. 

All the analog based encode / decode surround sound systems - like Dolby Pro Logic – use a sum-and-difference matrixing system to shoehorn front and rear and the center channels into the main stereo pair. Front center, of course, was left-signal / right-signal mix, because L+R is what produces a phantom center in stereo. The rear channel was recorded out-of-phase in the front left and right channels, so that the process that recovers the signal will cancel out the other. Strictly speaking, the surround signal lagged the left stereo channel by 90 degrees of phase and led the right channel by 90 degrees, so that it was anti-phase between the stereo channels but only a symmetrical 90 degrees out-of-phase with either stereo channel.      
  
Unfortunately, when using the scheme as is, channel separation between adjacent channels is a lousy 3 dB, so a technique called logic steering was ultimately used to monitor the stereo signals, then “decide” which is the dominant channel at a given instant, and subtract the channel’s signal from the ones adjacent to it. For instance, when the left front signal is dominant, its signal is cancelled from left back and center front. When the surround channel is dominant, its signal is subtracted from the left and right fronts. Even with logic steering, there’s usually some leakage of the front left and right signals into the rear, so a small time delay is put in the surround channel to harness the ears’ tendency to localize sounds in the direction they are first heard from. 

Despite the good results obtained from Dolby Pro Logic encoded cassettes to recreate a believable surround-sound at the home even with a budget Dolby Pro Logic equipped receiver, most analog matrix encoded surround sound were eventually superseded by full-digital surround sound systems like Dolby Digital AC-3 surround sound and DTS starting in the early 1990s. As good as Dolby Pro Logic was – even in cassette form – its surround sound channels just don’t have the 20 KHz or greater bandwidth of full digital surround sound systems like Dolby Digital AC-3 and DTS.