5.4.12 Corrections for Errors Caused by Misaligned Recording Equipment
5.4.12.1 Misalignment of recording equipment leads to recording imperfections, which can take manifold form.While many of them are not, or hardly correctable, some of these faults can objectively be detected and compensated for. It is imperative to take compensation measures in the replay process of the original documents incurred, as no such correction will be possible once the signal has been transferred to another carrier.
5.4.12.2 Azimuth and Tape Path Alignment: Inaccurate alignment of the record head of the original recording machine means that at replay, the signal retrieved will exhibit a reduced high frequency response, and, in the case of two or more track replay, an altered phase relationship between the two channels. Adjustment of the angle of the replay head such that the relationship of the head is in the same plane as the magnetised field on the tape is termed the azimuth adjustment and this simple adjustment can markedly improve the quality and intelligibility of the retrieved signal. There is no difficulty in training staff in this task, and good binaural hearing is all the measuring technology required. An accurate phase meter or oscilloscope will aid in the adjustment of mono and properly recorded tapes, they may, however, be misleading on tapes recorded on cheap, domestic equipment. In such cases aural judgement of the high frequencies should be relied on. Additionally or alternatively, a software programme providing a real time-spectrogram function can be used. Azimuth adjustment should be a routine part of all magnetic tape transfers.
5.4.12.3 Digital systems may correct the phase relationship of the signal (often described as azimuth correction), however such procedures cannot retrieve the high frequency information that is lost. Azimuth adjustments must be made on the original tape before transfer commences.
5.4.12.4 The vertical alignment of the heads on the original recording machine may present an obstacle to the appropriate reproduction of the signal. This is particularly the case with recordings made on amateur or consumer-grade equipment. In order to obtain a visual representation of the alignment of the tracks on the tape of a recording the following procedure should be followed: Recorded portions of tapes should be protected by a very thin transparent sheet of Mylar or similar transparent material. A powder or suspension of ferromagnetic material, particle size less than 3 µm, is sprayed over the transparent sheet. The magnetic properties of the recorded portion of the tape then make the tracks visible. A carefully marked series of measurement lines on the sheet will aid in detecting misalignment. These tape path adjustments are less frequently required than azimuth adjustment, but if they must be undertaken the replay equipment should be recalibrated by a qualified technician. Every care should be taken to ensure no iron particles remain in contact with the tape as these may damage the replay heads.
5.4.12.5 Cassette Enclosures: The enclosures in which low cost cassette tapes are housed may cause the tape to jam or replay with increased wow and flutter. In such cases it is often beneficial to replace the tape in a high quality screwed enclosure being sure to include the rollers, pressure pad and lubricating sheets.
5.4.12.6 Wow, Flutter and Periodic Tape Speed Variations: There is little that can be done to effectively improve periodic variations in the recorded signal. It is therefore imperative that the replay equipment is thoroughly and carefully checked, aligned and maintained to ensure that no speed related artefacts are introduced.With the availability of high resolution A/D converters and components, it seems possible to retrieve the high frequency (HF) bias signal from analogue magnetic tapes during transfer, which may enable the correction of wow and flutter. There are, however, many significant barriers to realising this, including a lack of available hardware to extract signals of such high frequencies and the inherent unreliability of the bias signal itself. As the procedure is generally time-consuming and complex, and substantial improvements concerning this matter are not to be expected, implementation is unlikely, and even then, only feasible for a limited group of tapes produced under specific circumstances.
