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CBR vs. VBR
Most of the information you'll read in this book and elsewhere assumes that the bitstream is being encoded at a constant bitrate (CBR). In other words, if you specify a 128 kbps encoding, then that's what you're going to get, start to finish. The drawback to CBR is that most music isn't structured with anything approaching a constant rate. Passages with many instruments or voices are succeeded by passages with few, simplicity follows complexity, and so on. The response to this situation has been the development of variable bitrate (VBR) encoders and decoders, which vary the bitrate in accordance with the dynamics of the signal flowing through each frame. VBR technology was first implemented by Xing, which is now owned by Real Networks, but is now supported by dozens, if not hundreds, of third-party products.
Rather than specifying a bitrate before encoding begins, the user specifies a threshold, or tolerance, when encoding with VBR. All notions of bits per second go right out the window, of course; instead, one selects VBR quality on a variable scale. Confusingly, this scale is represented differently in different encoders. While MusicMatch Jukebox gives you a scale of 1 to 100, the LAME command-line encoder lets you specify a quality of 0 to 9, where the scale represents a distortion ratio. Therefore, you can't just assume that higher numbers mean higher quality-see the documentation for your encoder before proceeding, or run the tests yourself. In any case, the scales are essentially arbitrary; think of them as though you were using a slider to control the overall quality versus file size ratio as you might with a JPEG editor.
While VBR files may achieve smaller file sizes than those encoded in CBR at a roughly equivalent fidelity, they present a number of drawbacks of their own. First, these files may not be playable in older-generation decoders, which had no notion of VBR concepts (although the ISO standard specifies that a player must handle VBR files if it's to be considered ISO-compliant). Second, VBR files may present timing difficulties for decoders. You may expect your MP3 player to display inaccurate timing readouts-or no timing information at all-when playing back VBR files. However, VBR techniques conveniently take some of the guess work out of trying to find an optimal bitrate for any given track-whereas you might have to encode a file several times with CBR to find the perfect balance, you can just set your encoder to use a relatively high quality level and let the computer figure out an optimal bitrate for each frame automatically.
NOTE
In general, the header data in most CBR files is same for each frame, while header data is necessarily different for each frame of a VBR file. However, VBR files don't incur more processing power, as all MP3 players read the header data for each frame regardless of whether they're playing a CBR or VBR file.
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Topic: media editor keeps closing (Read 2168 times)