5.2.5 Speed
5.2.5.1 Despite being referred to as “78s”, it was very often the case that coarse groove shellac discs were not recorded at precisely 78rpm, and this is especially the case with recordings made prior to the mid-1920s. At different times certain recording companies would set different official speeds, and even these were varied by recording engineers, on occasion during recording sessions. There is insufficient space here to discuss specific settings, though they are covered elsewhere in detail (see for example Copeland 2008, Chapter 5).
5.2.5.2 It is imperative that the disc be replayed for transfer as close to the original recording speed as is possible, in order to recapture the sound event originally recorded as faithfully and objectively as possible. However, subjective decisions often have to be made, and to this end knowledge of the recorded content or context in which the recording was made can be useful. The chosen replay speed should be documented in accompanying metadata. This is particularly important where any doubt remains as to the actual recording speed.
5.2.5.3 Recording speeds of commercial replicated cylinders standardised at 160rpm around 1902, although prior to that Edison, at least, applied several short-lived speed standards (all lower than 160 rpm; see Copeland 2008, Chapter 5). Instantaneous cylinders, while often recorded around 160 rpm or so, have been found with recording speeds ranging from below 50 rpm to over 300 rpm. In the absence of a recorded reference pitch (as provided occasionally by some early recordists) these will need to be set by ear, and documented accordingly.
5.2.5.4 Replaying a disc or cylinder at reduced speed may improve the ability to accurately track damaged carriers. There are many ways that this can be attempted depending on the equipment available, but attention should always be paid to the effect this will have on the sample rate of the digital file when adjusted to compensate for the change, and an appropriate sample rate should be chosen accordingly. Half-speed replay may be the simplest to employ, as it can be coupled with a doubled sample rate to produce corrected-speed transfers with a minimum of distortion caused by sample rate conversion. It should be noted that reduced speed playback is just one of many techniques that may be used to solve tracking problems. It is useful to try other procedures first such as adjusting the anti-skate to counter-balance the direction that the stylus jumps from a skip or using more or less tracking force to keep the stylus in the grooves.
5.2.5.5 Although playback with reduced speed may deliver increased surface noise compared with original speed, it is also the case that the action of filtering equipment, digital or otherwise, may be more effective. Playing at reduced speed means that the high frequency signal is halved in frequency, while the rise time of the unwanted impulse noise caused by surface damage remains the same and can be more easily distinguished from each other. However, some sophisticated predictive filtering equipment may be less effective at non- original speeds. Low speed copies must be flat transfers, without applied equalisation which can be introduced later (see 5.2.6).
