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5. How does an 8-track work, anyway (when it works...)?




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This article is from the 8-track Tapes FAQ, by Malcolm Riviera malco@interpath.com with numerous contributions by others.

5. How does an 8-track work, anyway (when it works...)?

An 8-track cartridge contains a length of 1/4 inch tape. The ends of
the tape are connected by a metal foil splice, thus forming a loop.
The tape itself is divided along its length into 8 channels, or tracks
(hence the name). The playback head plays 2 of these tracks at a time
- 4 programs in stereo. Inside the cartridge, the tape is wound
around a central hub, or spool. Tape pulls out from the center of the
spool. It moves to the top of the cartridge, where it connects with
the playback head in the player through an opening at the top of the
cartridge. A pressure pad in the cartridge presses the tape up against
the playback head. The capstan (part of the player) is spun by the
player's motor. As the capstan spins, it rolls the tape against the
pinch roller in the cartridge. The capstan and the pinch roller move
the tape along its path at 3 and 3/4 inches per second. The tape
finally loops back to the central hub, where it rewraps around the
outside of the spool. When the entire length of tape has gone through
this loop, the metal foil splice in the tape passes by a solenoid
sensing coil which is positioned right next to the playback head in
the player. This moves the playback head along the width of the tape,
and it starts to play a new program (remember, the tape contains 8
tracks, only 2 of which are supposed to be played at once).

From the previous description, it is probably pretty obvious why 8-track
is so terribly prone to malfunctions. If you don't have a cartridge
handy, get out a ruler. Dividing 1/4 inch into 8 separate tracks makes
for very small tracks. Now think about the fact that the playback head
has to pick up only 2 of those tracks at a time. When you further
consider that the playback head itself moves all the time, virtually
assuring that it will eventually become misaligned, it becomes painfully
clear why 8-track so often produces crosstalk or "sound bleeding" from
one program into another. The relatively complex path that the tape has
to travel is another problem. This, combined with the fairly large
number of moving parts in the cartridge, encourages tangling and tape
backups. Since the capstan's movement regulates tape speed and
movement, the somewhat tenuous grip that the capstan/pinch roller
combination has on the tape sometimes leads to tape slowdowns, even if
the motor is moving at a correct and steady speed (which it often
isn't). Furthermore, the tape splice, the most vulnerable part of the
loop, is put under constant pressure. Four times during the playing of
each tape, the splice is pulled past the playback head and through the
capstan/pinch roller wringer. This constant wear on the splice
encourages it to split, which it often does. Lastly, the age of most
8-track cartridges means that some of the parts are likely to be
decayed. Foam pressure pads and rubber pinch rollers are the most
commonly decayed parts of an 8 track, but the adhesive used on the metal
splice also tends to break down.

Abigail Lavine (abbot@pobox.com)

 

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