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.  

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