Excerpted from Four Channel Sound, by Leonard Feldman, c. 1973 Howard Sams & Co., Inc.
The most straightforward way of achieving four-channel [quadraphonic] sound is by means of magnetic tape. In four-channel recorders and players, the tape head is equipped with four equally spaced pickup “gaps” (each corresponding to its own pickup coil), as illustrated to the right.
Tapes recorded for four-channel utilize all four tracks, all of which are played simultaneously and in the same direction. The conventional arrangement of tracks with respect to channel placement is shown in Fig. 2-5. Tracks “1” and “3” take care of the front two channels (as in the basic stereo format), while tracks” 2″ and “4” now serve to reproduce the rear-left and rear-right program material. The only sacrifice in this four-channel arrangement is playing time, which has been cut in half. Signal-to-noise ratio is the same as before, since individual track widths have not been altered. Cross talk that may exist between channels is actually less bothersome than in the stereo arrangement, since there are now four programs which are related musically; any slight cross talk which may exist merely represents a small reduction in channel separation and not the unwanted introduction of unrelated program material.
Electronically, a four-channel playback machine (deck) must contain four separate preamplifiers, each feeding a separate output jack. The four outputs are then fed to four amplifiers and speakers arranged for quadraphonic playback (left).
The Q-8 Tape Cartridge Format
Continuous-loop tape cartridges were developed to overcome two serious objections that many people had to open-reel tape use. Many people found it bothersome to have to thread a reel of tape onto a playback machine. In addition, if they did not wish to hear both sides of a given tape reel, it was necessary to rewind the tape onto its original reel when the tape playing came to an end. This usually meant rethreading the tape a second time, from take-up reel back to original reel–since most listeners are not at the machine to “catch” the tape before it leaves the original reel entirely. Often, open-reel tapes have a way of “spilling” tape, resulting in tangles which have been known to frustrate even the most enthusiastic tape user.
Tape cartridges offer solutions to these problems. Inside are several hundred feet of tape arranged in a continuous loop. A small opening at the front of the cartridge exposes a small length of tape that will be in contact with the tape head in the playback mechanism. In addition, a rubber roller is included in each cartridge and is mounted in position to press against the motor drive capstan of the playback machine. The tape itself is 1/4 inch wide (just as in open-reel machines) but contains eight recorded tracks instead of four. Stereophonic eight-track cartridges have four separate programs recorded on them, each of which uses two of the available eight tracks. The tape always travels in the same direction, and the program change is accomplished by a movement (vertically) of the tape head. Its two pickup gaps move, successively, from one pair of program channels down to the next. The tape head itself is usually mounted on a movable bracket, and some form of stepping cam arrangement (actuated by a solenoid or stepping relay) moves the bracket from its uppermost position through all four program positions. Tape speed is standardized for cartridges at 3 3/4 inches per second. If we assume that most high-fidelity, open-reel, prerecorded tapes operate at a speed of 7 1/2 inches per second, it is easy to calculate that a given length of tape in the cartridge format will provide four times the playing time of the same length of tape in open-reel format. (It is played at half the speed and contains twice as many programs.)
Since eight tracks already existed in cartridge tape systems, the easiest way to accommodate the new four-channel sound was to simply reduce the number of programs on a cartridge from four stereo-channels to two quadraphonic channels. The new layout of tracks is illustrated to the left.. Of course, the playback machine must now be equipped with a tape head containing four pickup coils and four gaps. In addition, four preamplifiers (instead of two) must be incorporated in the electronic portion of the machine and, if it is a completely self-contained unit (as in the case of automobile players), four power amplifiers and four speakers must be included as well. An example of such a new four-channel player intended for car use is shown below. Notice that the unit bears the designation “Stereo 4-Channel.”
That is because it has been arranged to play both stereo and quadraphonic cartridges compatibly. When a stereo eight-track cart- ridge is played, only two of the tape-head pickup coils and preamplifiers are used, and four full programs are played sequentially by progressive movement of the tape head assembly. When a quadraphonic cartridge is inserted in the slot, all four preamplifiers and tape-head pickup coils are active, and tape-head motion is restricted to only two positions. How does the machine tell the difference? Close examination of the structure of a quadraphonic cartridge discloses an extra indentation [four channel “sensing” notch] in the plastic case . When this type of cartridge is inserted in the slot, it clears a spring-return switch built into the side wall of the cartridge chamber. On the other hand, when a stereo cartridge is inserted, its unindented side wall actuates this switch, which is used to restore full four-program stereo operation.
RCA must be credited with introducing the Q-8 cartridge format for four-channel sound. Convinced that four-channel sound was an improvement over stereo, RCA was quick to introduce complete home units employing the cartridge format. Since the success or failure of such a new musical medium is largely dependent upon the availability of musical program material, RCA was in the fortunate position of being able to supplement its new “hardware” approach with an impressive list of prerecorded four- channel Q-8 cartridges. The amount of “software” (the term used to describe records, tapes, etc., as distinct from hardware–a term used to describe the equipment upon which software is played) available in Q-8 continues to increase, and four-channel tape players are finding their way into homes as well as cars. At the moment , they represent the simplest, least expensive way into discrete four-channel sound reproduction.