Describes gradual speed changes during media playback, either in the source recording or the transfer machine, or an incorrect playback speed chosen by the transfer engineer. The pitch of the recording will be wrong in both cases (not to be confused with Wow and Flutter).
Can it be fixed?
Incorrect speed is a fairly easy problem to correct. However, If the incorrect speed is the result of a malfunctioning playback deck or turntable during digitization, have the equipment serviced.
If the item was transferred at the wrong speed, re-transfer it at the correct speed. IASA-TC04 recommends transferring at the item’s native speed rather than correcting in the digital domain.1
Concerns with Open Reel Tape
You may find that a tae was recorded at a speed that your playback equipment is incapable of playing (1 ⅞ ips or 15/16 ips). Hardware limitations may require that you transfer at the closest higher speed, and adjust it post-production using pitch change features in your audio editing software. Changing the pitch also changes the speed, but changing the speed does not affect the pitch. A full octave pitch change in either direction is the equivalent of doubling or halving the speed of the recording. Open reel machines have different replay equalization curves for each speed however, so one tradeoff in this approach is that the equalization will be incorrect.
Concerns with Disc playback
There are many concerns when evaluating correct playback speed of discs. The first one is the difficulty of identifying the pitch the musicians (if a musical sound recording) were performing at. While this is beyond the scope of this entry it is important to be aware musical pitch was not standardized (mostly) until the mid twentieth century and even to this day many ensembles still perform tuned either higher or lower than A=440 Hz.
Playback speed was not standardized until late in the 78 rpm era presenting many challenges. While acoustic era transfers can be freely digitally re-speeded without concern for equalization, electric era recordings if transferred with EQ when re-speeded post transfer will have incorrect turnover and roll off.
If an electric era disc is outside the adjustable range of a turntable it could be digitized flat, re-speeded, then run through a DAC to a disc pre-amplifier with appropriate setting and digitally captured. This of course introduces many steps into the signal chain and suggests a more flexible turntable may be be a better solution.
It has been observed during the acoustic era that disc recording speed was often unstable with the pitch of the recording changing dramatically from the beginning of the side to end during constant angular speed playback.
What do I do if the tape was recorded at two or more different speeds?
1) Preview the tape and determine the correct speed at the start of the tape.
2) If the speed changes, mark the time that this occurs, but do not stop the transfer.
3) After the transfer, go back to the section where the speed changed, and capture only that section at its native speed.
Making multiple preservation master files is beneficial because the speed, pitch, and EQ will be native at those speeds. A seamless access copy can be created by joining together these separately captured parts.
Listen to a tape played back at an incorrect speed on SoundCloud
Listen to the first part of an audio cassette recording on SoundCloud where the speed is correct
30 minutes into the recording from above, the speed has changed
1. Kevin Bradley (ed), Guidelines on the production and preservation of digital audio objects, IASA TC-04 (2nd ed., 2009), 57-59. ↩