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?