There is an art to pitch shifting that goes back to the earliest audio recordings. At first pitch shifting was accomplished by slowing down or speeding up the play speed of the recording. This presented some problems to early audio engineers because as the pitch was changed the speed of the phrase would be out of synchronization of the original recording. Initially this was an effect that would deepen and slow down a human voice in order to create a bass or demonic effect or could be used to speed up and raise the pitch in order to create a “chipmunk” effect. Early cartoons and movies would use this effect to create new voices for characters while using the same voice actor for the initial unaffected recording.
Now there are electronic and software based versions of this effect that can change the pitch of an audio recording without changing the speed of the original recording. This is especially useful for music and vocal tracks that might need fine tuning. This is also how the Auto-Tune effect was developed.
Take for example a voice recording that is ten seconds long. If this audio recording were to be pitch shifted by one whole step down then the recording would be pitched down to the desired key but the length of the recording would now be fifteen seconds long. Inversely, if the speed if the recording were to be increased the pitch would be raised one whole step and the length of the recording would be shortened to five seconds. Here lies the problem with transposing the pitch of a vocal in a song. If the song is set at a certain beats per minute the pitch shifted vocal track would not match the time signature or tempo of the song. Time stretching will leave the pitch at the original frequency while causing the sample to become longer. The combination of these effects used together can create pitch tuning while retaining the speed of the original recording.
Time compression and expansion are useful parameters that audio engineers will use to create a new sample of audio from the original content in the desired pitch while retaining the original length of the sample thus retaining the time signature and tempo of the other audio material that the altered sample will later be mixed with. Pitch shifting is different than frequency shifting in that frequency shifting can create an undesirable flanging effect that can be harsh and metallic. Frequency shifting is not recommended for vocal or guitar tracks that need to retain their natural tone color while being tuned to the desired pitch and tempo of the original audio material.
Time compression and/or expansion may be used to retain the original pitch of the audio program while making the length of the sample longer or shorter as desired by the audio mixing engineer. This can be useful for correcting small mistakes in tempo by the artists and create a sense of unity between performers creating a very tight sound and sense of timing overall.