Analogue Recording

Analogue signals are continuously variable.
Analogue recording is the process of converting an electrical waveform into an analogous pattern of modulation on a physical medium.
In tape recording this modulation is in the pattern of magnetism (strength and direction of field).

Magnetic Tape

Plastic (polyester) with a coating (metal oxide or metal alloy particles) capable of retaining magnetic flux.
Most common coating is gamma-ferric oxide; other types available are chromium dioxide and metal particles.
Reel-to-reel machines use tape widths of ¼″, ½″, 1″ and 2″.

Magnetic Recording

When an electrical current flows through a coil of wire a magnetic field is created.
The audio signal is AC; this current results in an alternating magnetic field whose direction changes according to the amplitude and phase of the audio signal.
Magnetic flux flows from one pole to the other; a recording head is basically a circular electromagnet with a small gap in it. The tape passes across the gap as the flux flows across it, which leaves the tape magnetised.
On replay, the magnetised tape passes through the head gap and induces a current in the coil, therefore providing an electrical output. This can be achieved by the same head (a record/play head) or a seperate playback head to allow for off-tape monitoring.

Most pro machines have three heads in this order; erase - record - play.
The magnetic characteristics of tape is not linear. By adding a high-frequency signal (a sine wave of between 100 and 200kHz) the tape is biased towards a more linear part of it's operating range. This is known as (surprise!) bias. The bias signal's frequency is too high to be retained by the tape. Recordings without bias retain little magnetisation and suffer from a high level of distortion.
The bias level needs to be set to the coercivity of the tape. High-coercivity tape needs a higher bias so that the signal is recorded more deeply into the oxide layer, and visa versa.

Print-Through

When storing tape for long periods of time a problem called print through can occur. What this means is that as the tape is left in storage, the signal recorded on one layer of the tape (which remember is just a magnetic pattern) will induce it's magnetism in the adjacent layer in much the same way that a needle left sticking to a magnet will become magnetized itself. This induced signal will manifest itself as either pre-echo or post-echo depending on which way the tape was wound off the machine. Pre-echo means a faint version of a sound can be heard a second or so before the sound was actually meant to start, whereas post-echo is a faint repeat of the last seconds of a sound, usually only heard at the end of a piece.
Storing tapes tail out (i.e. played right to the end and left on the take up reel) helps to avoid the much more unnatural sounding pre-echo, with the added benefit that if a tape has just been played right through it will be much more neatly spooled (with the benefit of reducing tape edge damage in storage) than if it has been fast wound in either direction. Post-echo is also much more easily masked by natural reverberation after a sound, so is more desirable.
However unless you will be storing a tape for long periods there is no real need to worry too much about whether to store it tail out or not. It only really affects how you load the tape back into the machine, i.e. whether you thread the tape back through from the take up wheel or forward from the supply wheel. To avoid having to unthread a tape that has been loaded incorrectly it is a good idea to record on the tape box whether it has been stored tail out or not.

Track Formats

Mono, Two-Track and Stereo

Full Track: Mono format which uses the entire width of the tape
Quarter Track: Used in domestic reel-to-reel and cassette systems. Left and right channels are recorded in segments one and three; the tape can then be turned over and two more tracks recorded. The reduced track width causes higher noise and poorer signal-to-noise ratios.
NAB: A two-track format which is designed so that the two tracks can be recorded at seperate times. This requires good separation between the two tracks, so there is a wide 'guard band' between the tracks.
DIN: A stereo format very similar to NAB, except that the guard band is smaller. As it is designed for stereo recording a small amount of crosstalk between tracks is acceptable.

Multitrack

Scale is taken from ¼″ two-track mastering tape. A four-track uses ½″ tape, an 8-track uses 1″ tape, a 16-track uses 2″ tape and a 24-track also uses 2″ tape. Comparable quality is therefore maintained.

Record Levels

If the record level is high, the high flux level causes distortion and saturation.
If the record level is low, the wanted audio signal will be masked by the tape's noise floor.
There is an optimal 'window' where the flux level produces a clean, clear signal. The tape machine needs to be well set up and the engineer needs to know where this 'window' is in relation to the meters.

This site provides some great info about working with analogue tape.

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