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 Krüger 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.