Appendix A
Archive Material Selection-Technical Considerations
Although there is no difficulty in stating preferred minimum technical specifications for recording quality regarding the usual parameters of frequency response, signal to noise ratio, wow and flutter and distortion, there is considerable difficulty in measuring them on a suspect piece of material. This is because to make such measurements it is necessary to have test tones on the tape, recorded by the machine on which the recording proper has been made. By definition a doubtful recording will not have this. Therefore a decision concerning technical suitability will usually be made on a subjective basis.
The basic requirement of any recording is that it be intelligible to a person with average hearing ability. There are a number of techniques available to enhance the performance of a “borderline” case to bring it closer to an acceptable performance. By using conventional “mid” tone controls or a graphic or parametric equaliser, it is possible to accentuate the 1 to 3 KHz frequency range to which the ear is most sensitive. Tape hiss can have its annoyance value reduced by careful setting of a gentle treble “roll-off” using a graphic equaliser. This, or a parametric or notch filter can also be used to reduce hum and associated harmonic frequencies provided they are not too wideband (it can be difficult to filter out large amplitude harmonics that extend across the whole audio band). Unwanted low-frequency rumble can be reduced by using a high-pass filter with either a 6dB or 12dB per octave slope with a switchable range of say 50 to 160Hz - most modern mixing desks incorporate this feature.
It should also be borne in mind that improvements in signal processing techniques are almost inevitable in years to come. For this reason it is probably advisable to either copy poor material “flat”, noting details of necessary filtering to be carried out on any subsequent copies, or for the minimum of filtering to be used for acceptable results on a master tape1.
1. Note (added in 2010): modern practice is not to “improve” the original recording when copying for archival purposes but only when making access copies.
