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).
"Discrete Four-Channel Sound
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.
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