5.4.13 Removal of Storage Related Signal Artefacts
5.4.13.1 It is preferable in most cases to minimise the storage related signal artefacts before undertaking digitisation. In linear analogue magnetic recording, for example, print-through is a well-known and disturbing phenomenon. The reduction of this unwanted signal can only be undertaken on the original tape.
5.4.13.2 Print-Through: Print-through is the unintentional transfer of magnetic fields from one layer of analogue tape to another layer on the tape reel. It reveals itself as the pre and post echoes to the main signal. The intensity of print through signal is a function of the wavelength, tape coating thickness, but primarily the spread of the coercivity5 of the particles in the magnetic layer. Almost all print through occurs soon after the tape is recorded and wound onto the pack. The increase in print-through after this reduces over time. Further significant increases in print-through occur only as a consequence of changes in temperature. When the tape is stored with the oxide facing in to the hub, the most common standard, the print on the layer outside of the intended signal is stronger then the print signal on the layer towards the hub of the spool. Consequently it has been frequently recommended that tapes be stored “tail out”, in which case the post echoes are louder than the pre echoes and less obvious. German broadcast standards specified that tapes be wound with the oxide out, in which case the reverse applies, and tapes should be stored “head out”.
5.4.13.3 Printed signals are reduced by the act of rewinding the tape prior to playing, by a process termed “magnetostrictive action”. Systematic tests have shown, however, that it is wise to rewind a tape at least three times to sufficiently diminish print through (Ref Schuller 1980). If the printed signal is very high and it does not respond adequately to rewinding, some tape machines allow the application of a low level bias6 signal to the tape during playback. This selectively erases lower coercivity particles and hence reduces print-through, though it may also have an effect of the signal, especially if over-applied, and should only be used as a last resort and then very carefully.
5.4.13.4 Though print-through can be reduced on the original tape the same level of restoration is not achievable afterwards. Once copied to another format the printed signal becomes a permanent part of the wanted signal.
5.4.13.5 Vinegar Syndrome and Brittle Acetate Tape: Acetate tape becomes brittle with age which may make it difficult to play a tape without breaking. The brittleness occurs as a result of a process of chemical degradation which occurs when the molecular bonds of the acetate compound break down to release acetic acid giving off the characteristic smell of vinegar. Broken acetate tape can be spliced without any signal loss or deterioration, because, as a result of its brittleness, no elongation of the tape occurs. Brittle tapes, however, are also subject to a variety of deformations which hinder the necessary tape-to-head contact for optimal signal retrieval. Though a process of re-plastification would be advantageous,such processes do not exist as yet. Archivists are warned against the chemical processes sometimes suggested as these may not only jeopardise the further survival of the tape,but also contaminate replay equipment and, indirectly, other tapes replayed on such machines. Instead, it is recommended that such tapes be replayed using a recent machine that permits to lower tape tension. This will enable an acceptable compromise between care of the fragile tape and the application of enough tape tension to permit the best possible tape-to-head contact.
5.4.13.6 Physical Tape Memory: Poorly stored and spooled polyester and PVC tapes may also suffer from deformation of the tape. The tape will often retain a memory of that deformation and so make poor tape to head contact, which reduces the signal quality. Repeated respooling and resting may reduce some of this effect.
5. Coercivity; A measure of the intensity of the magnetic field needed to reduce the magnetization of a ferromagnetic material to zero after it has reached saturation
6. Bias; A high frequency signal mixed with the audio during recording to help reduce tape based noise. Devised by Weber in 1940
