A cassette is a simple object doing a demanding job: holding a long ribbon of magnetically coated plastic, keeping it clean, and presenting it to a machine that will pull several centimetres of it out of the shell and wrap it around a spinning drum without creasing it.
The shell
The shell is two moulded halves holding a supply spool and a take-up spool, with a hinged lid across the front edge. A spring holds that lid shut and holds the spools against rotation until the machine releases both. Inside, the tape runs from one spool past guide rollers, behind the lid, and onto the other. Dust that gets past the lid ends up between the tape and the head, which is why a cassette left out of its sleeve gathers picture faults faster than one that was put away.
Threading and helical scan
When the cassette is loaded, arms reach inside the open lid, draw a loop of tape out of the shell and wrap it most of the way around a drum tilted a few degrees from the tape's direction of travel. The drum spins while the tape creeps past it, so each pass of a head writes a long diagonal stripe across the tape rather than a short line along it. That geometry – helical scan – is what makes it possible to record a television picture on tape moving at a few centimetres per second instead of a few metres.
Speeds and running time
- Tape width
- Half an inch, wound between two spools inside the shell.
- Standard speed
- The fastest and best-looking of the domestic speeds, used for recordings meant to be kept.
- Extended speeds
- Slower transports that multiply running time and visibly coarsen the picture, used for long broadcasts and overnight recording.
- Linear audio track
- A narrow track along the tape edge, recorded by a fixed head at the same slow speed as the picture.
- Hi-fi audio
- Sound recorded by the spinning drum in the same diagonal stripes as the picture, a marked improvement over the linear track.
- Control track
- A timing track along the opposite edge that tells the machine where each diagonal stripe begins.
How a recording fails
Most visible faults come from the contact between tape and head. A speck of dirt or a patch where the magnetic coating has shed lifts the tape away from the head for a fraction of a stripe and produces a drop-out: a white or coloured dash across the picture. A stretched or creased edge disturbs the control track and the picture tears at the top of the frame. None of these are electrical faults, which is why handling and storage matter more than the age of a recording.