Why would different releases of the same song sometimes be out-of-sync?

Question

You can have a song that was released on an artist album, and the same song released as a single (I'm talking about CDs in this example). These different versions of the same song can differ in overall length by seconds. It's the same for compilation album versions and instrumental versions. Some even sporadically go out-of-sync throughout the song.

Is it a product of the mastering process? Is it a product of the recording device? Or something else?

Over the years I have noticed this on a few different songs. One example I can think of off the top of my head is Tupac's song 'Hail Mary'. The album version, the single version and the instrumental version all fall out of sync and even seem to sporadically speed up and slow down.

Answer 1

It's almost certainly a product of the mastering process as you say. Even if additional audio processing doesn't happen when a previous song is included on a later release the producers or engineers might decide to add or subtract silence around the songs.

In some cases, when songs are included on a mixtape or something like that, it could even be timeshifted to fit the purposes of the new releases, especially if they're trying to do any beat-matching between songs.

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Barring any kind of conscious mastering decision, it's also possible that since Tupac was recording during the era of analog tape that the tape speed could have been minutely different during mastering each of those releases.

Answer 2

It depends on how the "length" is measured. Exactly where does a song "end" that is faded out at the tail (as most popular music seems to be)? Or classical music where the venue ambience is included at the end? Also it depends on whether they are measuring from the start to the start of the next song (i.e. including the inter-song gap). It doesn't seem particularly reasonable (or important?) to expect that the "length" is always going to be measured or reported uniformly.

Answer 3

Sometimes labels re-record the same song for licencing purposes... it can mean, for example, they don't have to pay the backing musicians royalties - or they re-do something because it turns out a sample wasn't cleared. The main vocal might be patched back in from the original, or even re-recorded. It can arise when a track was initially released with low expectations of sales, but has to be hastily re-looked at once it finds its way onto "Hits 2016". I seem to remember Groove Armada coming unstuck with 'At The River', as an example. That might account for some of the things you're finding?

And yes, mastering engineers will play with things to make them work better on compilations, including speed.

And sometimes there are various different mixes of a track made at the time of creation, and they're simply not kept track of because nobody cares that much which one goes where. It could be as tedious as that.