Why Don’t Podcasts Use VBR MP3s?
AVFoundation, the low-level audio/video framework in iOS and macOS, does not accurately seek within VBR MP3s, making VBR impractical to use for long files such as podcasts. Jumping to a timestamp in an hour-long VBR podcast can result in an error of over a minute, without the listener even knowing because the displayed timecode shows the expected time.
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Three simple solutions to accurate VBR stream-seeking have existed for almost twenty years to embed seek-offset tables at the start of VBR MP3s for precise seeking[…] But AVFoundation supports none of them.
Update (2016-08-21): See also: Accidental Tech Podcast.
