How does magnetic tape work? | The Basics

THE GIST

Magnetic tape recording works by converting electrical audio signals into magnetic energy, which imprints a record of the signal onto a moving tape covered in magnetic particles. Playback is achieved by converting the recording on tape back into electrical energy to be amplified.

Tape recording relies on a plastic film coated with tiny magnetic particles on one side (the tape) moving at a consistent speed through a tape machine. This is accomplished by unwinding the tape from one reel, passing it through a series of stabilizing rollers and guides, and spooling it onto a second takeup reel to be stored. If the speed is changed during playback or recording due to improper settings or faulty motors, the pitch will be distorted.

Studer A820 24 Track Recorder

Between the reels, the tape passes over a series of magnetic heads that convert audio signals into magnetic energy and back again. First the tape passes over the erase head, which (if the track is armed) scrambles anything stored on that track.

Next comes the record or sync head, which is essentially a stack of magnets (one per track), each wound with a coil of wire. Between the positive and negative poles of each magnet is a tiny gap where an electromagnetic field is created that fluctuates in response to the changing signal. As the tape passes by, these pulses align the tiny magnetic particles into patterns, leaving a record of the sound.

Finally, the tape moves across a dedicated playback head, which “reads” the magnetic information stored on the tape and converts it back into electrical signals that are sent to the machine’s outputs. The record head can also play back what’s on the other tracks while recording to enable real-time overdubbing.

The Sound of Tape

Otari MTR12-IV 1/2" Four Track Master Recorder

The physics behind magnetic tape give the medium an idiosyncratic sound, which is coveted for its unique saturation properties. The tape can only absorb so much magnetic energy, but instead of distorting like an amplifier when overloaded with signal, it compresses the sound in a soft, flattering way.

This saturation effect, combined with all the analog circuitry on the inputs and outputs, gives tape an unmistakable sonic signature that can be subtly pleasing or intentionally exaggerated for an effect.

Modern Usage

Studer A820 24 Track Recorder

Magnetic recording has been obsolete since digital technology took over the industry, but tape is still used by many engineers who praise its unique tonal properties, the inherent workflow restrictions involved, and the stability of the storage medium.

Somewhat ironically, those first two positives have historically been seen as flaws, with manufacturers designing tape and equipment to produce the purest sound possible.

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