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iTunes 5 Brings Variable Bit Rate (VBR) AAC Encoding
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2005-09-08 15:22:31
Apple, yesterday, released iTunes 5, which brought with it many enhancements over the previous versions. One improvement that many may have missed (we have seen no discussion of it as yet) is the addition of variable bit rate (VBR) encoding for AAC files. While iTunes has long supported MP3 VBR encoding, until this release all AAC encoding in iTunes has been using a fixed bit rate.
Now for a quick explanation of how VBR encoding works: There are many types of sounds within a given audio track. There are periods of soft melodic drifts, thundering orchestral crescendos, and spans of almost total silence. With fixed bit rate encoding, every part of a track is encoded at the same bit rate, which is wasteful of disk space when you consider that, say, the thundering crescendo is a much more detailed sound than a period of almost total silence. Under VBR encoding, the encoder examines the type of audio that is about to be encoded and adjusts the actual bitrate used on that section of sound accordingly; for silence or simplistic waveforms the bitrate is lowered while, for complex areas of sound, a higher bitrate is used.
At this time, it is unclear just how the user-specified bit rate corresponds to the resultant VBR file with iTunes' new AAC VBR encoder. If you examine the preferences for "Custom..." MP3 encoding in iTunes 5 (under the "Advanced" icon in iTunes Preferences), you see that, when VBR is enabled, a "Quality" setting becomes active. An indication that when using VBR, the selected bit rate serves as "a guaranteed minimum bit rate" can also be seen. There is no such indication or "Quality" setting in the AAC "Custom..." settings. Hopefully clarification will surface on this point soon and, when that happens, we will pass on the information directly.
We would heartily encourage anyone who encodes music into AAC or MP3 to utilize VBR encoding as standard practice.
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