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.
