As Radio Looks at Enhanced Audio Formats, Consider Options And History of Surround Sound
Before considering the options for adding surround sound to digital radio, it’s a good idea to gather some background on the technology. Where it’s going is easier to predict if you know where it’s been.
This is particularly useful given that radio has tried to move beyond stereo before, with little success. Will results of the current movement prove any different?
It started with quad
Many readers will remember the quadraphonic movement of the early 1970s, which started as simply an “ambience recovery” technique. In this arrangement, a listening system sends standard stereo left and right signals to two speakers normally, with “inverted pair” wiring (i.e., left-channel hot lead with right channel ground, and vice-versa, via appropriate buffering) sent to a second set of speakers. The four speakers generally are set up in a square, with the inverted pair considered “rear” or “back” speakers, although it doesn’t much matter.
The most popular commercial implementation of this system was Dynaco’s Dynaquad box, a simple passive network fed by the powered outputs of a stereo amplifier, with four pairs of output terminals labeled LF, RF, LB and RB for connection to four speakers (the term “back” was preferred in lieu of “rear” to avoid confusing the resultant R with “right” in any acronyms). The device also included an L-pad to attenuate the relative level of the “back” speakers, as they were often placed closer to the listening location than the “front” speakers. The two speaker sets might also have been of different types in those days, so the pad helped adjust for sensitivity differences.
The effect was a pleasant addition of spaciousness, with occasional interesting “panning” effects as particular sounds appeared to move around in the soundfield on certain recordings. Of course, these were random and unintentional effects that occurred simply as a result of stereo mixing techniques producing surprise artifacts when monitored with this type of four-speaker arrangement; such effects later became known in the audio industry as “magic surround.”
Several of these ambience recovery systems appeared in the marketplace – along with numerous home-brew systems from hobbyists building the inexpensive circuits from plans printed in numerous hi-fi enthusiast publications of the day – including one from Electro-Voice. That company’s profile in both the consumer and professional audio industries prompted it to begin marketing a professional “encoder” product, generally considered the first commercial product intended to purposefully create quad effects in an ambience-recovery matrixed listening system.
The record industry began to take note, and realized that there might be a value in creating quadraphonic content. Discrete four-channel tape formats (open-reel and 8-track) initially were offered, but these required new four-channel playback hardware, and products released in quad format were not properly playable on two-channel systems. The industry therefore sought a way to make the predominant LP format of the day quad-capable, ideally in a fully backward-compatible way. The goal ultimately was to produce a single LP release format that equally satisfied mono, stereo and quad listening.
The first standard quad encoding formats extended the ideas of the ambience-recovery approach, using variants of what became known as the Scheiber matrix (after Peter Scheiber, its inventor).
Scheiber’s design described a method of encoding four discrete audio channels into two and then recovering them – thus the genesis of the generic “4-2-4” nomenclature – albeit with significant crosstalk in the recovered output (only about 3 dB of separation between adjacent channel pairs). CBS and Sansui each improved on the crosstalk performance of the original Scheiber matrix by adding fixed phase shifting, in different ways; and these were adopted by the recording industry as the SQ and QS formats, respectively.
The studio quad mixes (or remixes of previously released stereo records) were mastered to four discrete channels, typically on 1/2-inch, 4-track tape. These masters were then transferred to master discs through the SQ or QS encoders. It soon became common studio practice to mix the final four-track masters while monitoring through a matrix encode-decode loop, so the compromises of the matrix could be accommodated and perhaps even “pre-corrected” via specialized techniques.
For example, there were certain panning techniques that did not fare well through a given matrix when compared to how they sounded in a discrete path. These weaknesses became the lore of the land for each format, and mixers began to adjust their quad mixdowns accordingly.
Meanwhile, JVC had developed a system called CD-4, which was intended to avoid the compromises of matrix encoding, while still producing a backward-compatible LP release format. CD-4 added supersonic FM carriers onto the stereo 45-45 groove. These carriers contained front-minus-back difference information for each channel (i.e., an LF-LB signal in its left channel and RF-RB signal in its right channel). Taking a page from the FM-stereo multiplex system, CD-4’s normal baseband stereo signals included front+back signals, so a stereo-only listener heard a full left/right downmix of all four channels. When decoded through a CD-4 system, the subcarriers were demodulated to baseband and mixed via sum-and-differencing with the regular stereo signals to produce four signals, essentially full representations of the original discrete quad program.
The system’s supersonic subcarriers occupied bandwidth from 20 kHz to 50 kHz, requiring great care in the mastering pressing and playback of the discs. (Remember that LPs used the RIAA equalization curve, which greatly increased the high-frequency content relative to low frequencies as cut on the disc, so this additional HF content taxed many systems’ headroom.)
At the user end, playing a CD-4 LP with a standard spherical or elliptical stylus wouldn’t track the supersonic carrier waveforms, and generally wore them down after only a few plays. A special, super-elliptical stylus called the Shibata (again, after the inventor) was developed to resolve the high frequencies for CD-4 decoding, and preserve them after repeated plays of an LP.
Then as now, how the four discrete tracks of the quad mix would fold down into stereo and then into mono was a critical concern. In CD-4, as stated, the rear channels each folded neatly into their respective front channel for stereo, and the resulting stereo summed to mono as well as it would have in a plain stereo context. The matrixed systems didn’t really have to downmix to stereo because they already were in a two-channel form on the LP. (In the matrixed surround world, stereo channels that include encoded surround signals are referred to as LT and RT, where T stands for “total.”) But because matrixed systems encoded signals with purposeful phase differences across LT and RT, there was some cancellation in mono, This generally affected only back-channel audio, so its removal from monaural signals was not seriously problematic in most cases.
Next time we’ll continue the story of surround with the introduction of second-gen surround systems, and how quad moved from the audio world to the sound for picture environment.