In the Recording Studios


Storing sounds as magnetic patterns

As a child at school you may have made patterns with iron filings on a thin sheet of paper by moving a magnet underneath. Tape recorders work in a similar way - they set up a pattern of magnetised metal particles on a tape, a pattern that corresponds to the sounds of the instruments and voices being recorded.

The tape consists of a thin layer of powder-like metal particles (commonly iron and chromium oxides) in a plastic coating on polyester ribbon. The recording head which creates the patterns is a tiny electromagnet.

To record a tape of anything from a solo singer or speaker to a full orchestra, the sound is directed at a microphone where its pressure waves are converted to weak, variable electrical signals. The signals are fed to an amplifier to boost their strength, and then to a recording head in the tape recorder.
Here, they flow through the coils of the electromagnet and produce a magnetic field. When the tape is carried past the recording head, each particle is magnetised - it becomes a tiny magnet and is attracted more or less towards the electromagnet according to the strength of the signals. So the particles become, as it were, a code for the original sound.

The electrical signals used to create the pattern may be fed from a microphone, or from a radio, compact disc, record player or another tape recorder. Tapes are now normally recorded in stereo. With digital audio tape (DAT), the electrical signals from the microphone are converted into numerical values represented by binary numbers. This system is more precise, and produces a more faithful recording.

Most modern tape recorders use the same head for recording and playback. For recording, the tape must be wiped clean of all previous patterns before it reaches the head. This is done by an erasing head, an electromagnet fed with high-frequency current that upsets the existing alignment of the particles on the tape. The erasing head is switched on automatically when the recording button is pressed.

To play back a recording, all that is needed to reproduce the coded sound is to reverse the process. When the tape is played back, its magnetic pattern sets up an electric current in the electromagnetic head. The current is fed to an amplifier and then to loudspeakers that produce pressure waves, which vibrate your eardrums in the same way as the original sounds.

Musical Technologies