Magnetic recording tape

Magnetic tape is used with both analogue and digital multitrack and 2-track recorders. Although formats and standards may differ (chemical formulation, length, width, thickness, cassette or open reel housing etc) the essential mechanical and electo-magnetic functions remain the same. The tape must record, store, and playback the source waveform as faithfully as possible.

The structure of magnetic tape

Magentic tape is constructed using a base film. The magnetic coating is applied to one side and a non-magnetic coat to the other. Professional recording tapes employ gamma ferric oxide (Fe2O3) as the main magnetic material together with additives to secure the coat to the base (binder), to assist with its dispersion over the base (wetting agent), to prevent the build up of static electricity (anti-static agents), to keep the head clean (abrasives), and to stop the tape sticking to the heads (lubricants).

The mechanical properties

These are its length, width, and thickness. The maximum tape thickness and standard widths are defined in the IEC (International Electrotechnical Commission) Standard.

The electroacoustic parameters

The parameters of interest to the audio engineer are ...

1. Sensitivity. This is the ratio between the input and output of the recorder for a stated input frequencey.
2. Distortion. The harmonic distortion imparted to the recorded signal by the tape.
3. Bias noise. Noise produced by the tape which as a result of the introduction of bias (a high frequency signal introduced which biases the tape towards a more linear part of its operational range) without any audio input.
4. Direct current noise. Direct current noise indicates modulation noise created by an input signal.
5. Print-trough. The layer to layer transfer of signals during storage. Tapes should be stored “tails out” to avoid this condition.

Recording charecteristics

These are a series of standards defined by various recording organisations (ie IEC, NAB and AES). If a tape is played with the wrong characteristic an incorrect frequency response will result. The recording characteristic is sometimes known as the equalisation curve or EQ (ie NAB eq, AES eq). Some machines are switchable. NAB is standard for 15ips in the US and UK.