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FOR ALL the controversy and confusion surrounding quadraphonic discs these days (wir befinden uns in der Hochphase der Quadro- Eurphorie 1973/1974), it seems odd that the tape forms of program sources have made so little progress in the area of four-channel sound. Superficially, at least, tape presents no problems when it comes to "surround sound."

Multi-track recording (as many as 16 or even 24 channels) has been the rule rather than the exception in professional recording studios for years now. Aside from artistic or aesthetic considerations, it is no great problem for a recording mix-down engineer to create a four-channel master tape from such multi-track masters.

Why, then, has tape lagged behind in consumer acceptance of quadraphonic formats?
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Die Fragen der Tonmeister : Wo sitzt der Hörer wirklich ?

First attempts at four-channel musical recording for home use sought to place this ambience where it belonged - all around the listener.

If four-channel recordings (and reproducing equipment) did nothing more, they would be worthwhile. But quadraphonics, once given to recording studios as a new tool, has been put to more ambitious tests.

Musical arrangements which surround the listener with instruments placed around him have become popular, particularly in the area of pop and rock music. Listeners to this kind of fare have, for a long time, wanted to be totally immersed (eingeschlossen) in the musical experience.

Using stereo equipment, they have tried to "turn on the entire room" by employing inordinately loud listening levels. With four-channel sound, they can achieve the same involvement even at moderate listening levels. Four-channel sound may
well be the medium that saves the hearing of our youth !
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No real problem was posed in translating open-reel tape formats to quadraphonics. For years, stereo open-reel tape machines had four-track capability, as shown in Fig. 1.

One stereo program was recorded and played back on tracks 1 and 3, while a second stereo program was recorded and played in the reverse direction, using tracks 2 and 4.

To get four-channel recordings on open-reel tapes, it was only necessary to devise new tape record and play heads which incorporated four magnetic gap circuits instead of two, as shown in Fig. 2.

Driven by additional record and playback preamplifiers, each gap magnetized one of the available four tracks during record; on playback, each recorded track was picked up by a separate section of the playback head and suitable playback preamplifying circuits.

The only element of the system that is altered is the available length of recording time in a given length of tape. (Obviously, with full tape width used for the four channels of recording, playing time is cut in half for any tape speed.)

This open-reel tape format inherently offers best fidelity and the most
"discrete" (getrennten) four-channel sound of all. Separation in excess of 40 or 50 dB between channels is easy to achieve, compared with the 20 or 25 dB of separation offered by even the most carefully recorded (and played back) CD-4 "discrete" phono discs.

• Anmerkung : SQ und QS haben erheblich weniger (eine geringere) Kanaltrennung.

Why, then, has the open-reel four-channel machine become the preferred "pet" of only a limited number of dedicated audiophiles?

If you thumb through any hi-fi equipment catalogue you will be surprised to learn that open-reel machines, stereo or quadraphonic, carry very high price tags.

Unlike a decade ago, when good stereo decks could be bought for $200 or $300, components of this type are now largely confined to the over $500 category and, in the case of good quadraphonic decks, they approach the "$1000.00 and up" bracket.

The answer lies in the rapid development and improvement of cassette tape decks.
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Cassette machines, which just a few years ago were regarded by the high-fidelity fraternity as an interesting curiosity capable of only low-fi voice recording, have undergone the greatest technological advance of any single component in hi-fi.

Frequency response of better machines has improved steadily so that today it is not unusual to find cassette machines capable of response from 30 Hz to above 15,000 Hz (which is as good as present FM broadcasting, by the way).

Record/play heads for cassette machines have undergone major refinements. Tape formulations have improved remarkably, as having machining tolerances, tape-drive systems, and associated electronics. When you consider the fact that cassettes operate at an incredibly low speed of l 7/8 ips and that the tape itself is scarcely wider than 1/8 inch (compared with 1 1/4-inch widths used in both 8-track cartridge and open-reel tapes), the frequency response achieved is truly phenominal.

So, too, are the wow and flutter figures (which, in topquality machines, are as low as 0.1% or better), and the signal-to-noise ratios which approached 50 dB, even before the advent of Dolby noise reduction.
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With Dolby noise reduction, improvement in signal-to-noise ratio is as great as 10 dB, pushing the effective S/N ratio up from 48 to 50 dB to 58 or 60 dB. This is enough to qualify the cassette machine as a true high-fidelity component.

Track layout of a standard cassette differs from that used in open-reel tapes. Here, related stereo tracks are placed side by side, as diagrammed in Fig. 3.

Thus, one stereo program is heard when the cassette is played in one direction; when it is inverted and played in the other direction,.a second full program is heard.

This format was decided upon by Philips Company of Holland, licensors of the cassette principle to all other manufacturers. This arrangement makes cassettes fully compatible, that is, a stereo cassette played on a mono machine has its two channels picked up simultaneously by a wide playback head gap, while a mono cassette played on a stereo machine induces the same magnetic information in both stereo head gaps (and hence both speakers).
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At first glance, the simplest transition to quadraphonic cassettes might be made by using all four available tracks, recorded in the same direction, much as is being done for open-reel pre-recorded tapes.

The track arrangement would be that shown in Fig. 4. New record and play heads would have to be engineered, but this would be no more difficult than the transition in open-reel machines.

The problem lies with the compatibility of cassettes. Since Philips decrees that all cassettes must be compatible with all machines, the solution using the track layout of Fig. 5 would not fill the bill.
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• Anmerkung : Das Philips CC Kassetten-System war genauso patentrechtlich geschützt wie das Grundig C100 Kasetten-System. Philips vergab jedoch kostenlose Lizenzen, wenn die Bedingungen eingehalten wurden. Max Grundg wollte jedoch Lizententgelte haben. So hatte Philips die Japaner hinterlistig gegenüber seinem "Partner" Grundig "überzeugt".

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A listener playing a quadraphonic cassette on a stereo machine would hear, first, the two front channels (with no contribution from the two rear channels). Upon inverting the cassette, he would not only be treated to the back channels only, but would hear their content played backwards.

Accordingly, Philips has repeatedly suggested that the way to four-channel cassettes must involve a further reduction of track width. Specifically, this is eight very narrow tracks all within the width of a little more than 1/8 inch, as shown in Fig. 5.

Each track would be just a few thousandths of an inch wide and, while manufacturers might be able to keep their record/play heads in reasonable alignment so as to prevent cross-talk and poor frequency response, signal-to-noise ratios would take a "step backwards," negating some of the improvement brought about by the Dolby system.

An even more severe problem would be faced by cassette duplicating firms (who "dub" pre-recorded music onto cassettes at much higher speeds for reasons of economy). Cassette tapes, moving at high speeds, have a tendency to wobble up and down. Therefore, with decreased track widths proposed by Philips, alignment problems in duplicating could become horrendous.

Despite the problems envisaged, at least one company has announced production plans for a quadraphonic cassette tape deck following the Philips 8-track proposals.

But, seeing is believing. It has also been suggested that "matrix" encoding from four channels to two channels might be the best solution for cassettes. Indeed, the two encoded channels of a matrix system could be recorded onto cassettes without any change of parameters, format, frequency response, or signal-to-noise specifications, much as is done with matrix-encoded records.

It would seem a pity to have to go this route with any tape format, however, since tape, of all storage media, seems so right for discrete approaches to multi-track recording.
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The last of the tape formats to be considered in terms of its four-channel applicability is the 8-track cartridge from Lear. First dubbed "Quad-8" cartridges, and later Q-8 by RCA, four-channel sound on cartridges appeared very early in the quadraphonic era.

Stereo 8-track cartridges of the "endless loop" variety had been introduced years before, primarily as a background music program source for automobile use.

The track arrangement of stereo 8-track cartridges is shown in Fig. 7. Four full programs of two tracks each is standard. As each program is completed, a metal foil strip bonded to the tape itself actuates a switch and solenoid which moves the tape playback head downward (or upward) an appropriate distance so that the succeeding program can be played.

Play is continuous, of course, and in most machines the cartridge will repeat playings of its various programs until it is physically extracted from its slot.

In Q-8 cartridges, only two program loops are possible, since each uses four of the eight available tracks, arranged as shown in Fig. 8. New machines designed to play these cartridges have a built-in measure of compatibility, that is, a stereo cartridge inserted in the slot of such a machine will have all its four programs played in sequence, while a Q-8 quadraphonic cartridge, equipped with a special slot in its plastic housing, will cause the head to move only once when switching from the first quadraphonic program to the second.

On the other hand, a quadraphonic cartridge inserted in a stereo cartridge player will first play the front channels of program "A," followed by the back channels of program "A," the front channels of program "B" and, finally, the back channels of program B. One might term this one-way compatibility.

Even though 8-track cartridges operate at higher speeds than cassettes (3 3/4 ips) and one would expect the fidelity and S/N ratio to be better than that obtained in slower-speed cassettes, the reverse is true.

The fact that the head itself is required to move from program to program makes permanent, precision head alignment almost impossible. The endless-loop principle in which tape is drawn from the inside of the wound tape and returned to the outer diameter involves a tricky drive system which does not lend itself to the extremely low wow and flutter figures possible with either open-reel or cassette drive mechanisms.

The closed-loop tape also makes it impossible to have a "fast rewind" feature on these machines (there is no rewind of any kind possible, since it is not possible to squeeze the tape back into the center of the roll), although some late-model machines do feature a somewhat faster-than-play "fast forward" mode.

Signal-to-noise ratios (tape hiss) are generally poorer than those found in better cassette machines, even before Dolby is applied and, not surprisingly, Dolby noise reduction has not been applied to either pre-recorded 8-track cartridges or to the playback machines with which they are used.

A few machines now have record as well as playback facilities, but the vast majority are built for playback of pre-recorded music only. In short, the 8-track format just has not caught on with the true audio buff.

That is not to say that these mechanisms could not be built for improved specifications - they simply have not been up to now.

Some of these shortcomings are masked by road and engine noise, of course. And it's in automobiles where cartridges made their mark. Also, 8-track offers a much greater pre-recorded library than other formats.
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Fig. 1. Arrangement of two stereo programs on open-reel 4-track tapes. The "notches" on the pickups are the head gaps here and in other figures.
Fig. 2. In 4-channel open-reel machines, all four tracks are recorded and played at once, in one direction, reducing the playing time.
Fig. 3. Cassette track utilization for stereo differs from that of open-reel tapes.
Fig. 4. Simplest transition to quadraphonic cassettes would be track arrangement shown, but this was not to be, as shown in Fig. 5.
Fig. 5. Further division of cassette tracks to eight would maintain quad-stereo-mono compatibility and playing time while reducing the track width. Tape width remains Vs inch.
Fig. 6. Cartridge stereo tapes have four programs, each using two of the available eight tracks (LI = left of 1st program, etc.)
Fig. 7. Q-8 cartridges have two f full programs of quadraphonic sound, each occupying four of the available eight tracks.
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