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
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 [1973], they represent the
simplest, least expensive way into discrete four-channel sound reproduction.