|Audio Tape Digitisation Workflow
Although speed correction is also possible in the digital domain, it is better to avoid such later digital corrections. It is recommended to choose the correct tape speed already at the transfer process, for reasons of accurate levelling, bandwidth limitation, and sampling rate limitation on the A/D conversion, as well as selecting and using the corresponding equalisation.
Music or language content of the recording may help to determine the correct playback speed (e.g., piano is usually tuned to a standard middle A at 440 Hz). Checking background electrical hum (50 Hz in Europe, 60 Hz in North America) with test equipment may also help to determine the correct speed. Note that the speed can vary throughout the recording, especially with battery powered field recordings. In addition, consumer tape recorders are more likely to exhibit inaccurate speed characteristics because of faults, poor alignment, or unstable power supply.
Wikipedia: Reel-to-reel tape recorder