Playback speed correction
The playback speed of a digitized recording may be incorrect. The original recorder may have been running at a wrong speed. Sometimes the tape has been copied onto another tape recorder. The speed change has taken place at some stage of the recording's history.
If the recording plays too fast, the pitch is too high and the duration too short. When the speed is too low, the pitch is too low and the duration too is long. Especially music should play in the right key, but also human speech sounds weird if it is even 2% too high pitched.
It is quite easy to correct a speed error. There are different ways to find out the original, or correct speed. In magnetic tape recordings there is very often some "leftovers" of mains hum, which should be 50 Hz in Europe and 60 Hz in the USA. The hum frequency from the tape gives a cue how to adjust the playback speed.
You can also look up the music sheet and read it, and adjust the speed to match the key. Another way is to play a tuned instrument and compare the pitch with the recording. There are also analyzers that show the note value of a sound.
Playback speeds up and the pitch gets higher
Sometimes the batteries of a Cassette recorder have weakened during a recording. When you now listen to the Cassette, at some point the speed starts to accelerate and the pitch rises to mickeymouse. Close to the end of the recording there is more noise and distortion.
The beginning of the tape may play fine. The recorder battery voltage may have dropped for example somewhere after 30 minutes of the recording. This short demo shows how the pitch and speed error can be corrected, so that it is possible to listen to the recording.
Wow and flutter
Analog recordings very often have wow and flutter, which makes them sound like "out of tune". Wow and flutter are not easy to remove, but the task is not totally impossible either.