Variable Bitrate MP3 Encoding

To those with trained ears, much of what passes for Internet Audio is defined by its limitations. Some sanguine engineers, realizing the value of balancing speed with pristine sound, have incorporated an oft-overlooked but eminently useable MP3 encoding technology called variable bitrate (VBR). VBR is used for many other encoding systems and codecs, especially video. Part of the MPEG standard, VBR was first implemented for MP3-encoded files by Xing Technologies (the developer of AudioCatalyst - now owned by Real Networks) and has been available long enough that all recent MP3 players support it. Our context here is for MP3 files.

Simply put, VBR technology allows better sounding MP3 files in the same space taken up by your garden-variety constant bitrate (CBR) encoded song. To those who value small file size more than sound quality: think of it as providing the same sound quality in a smaller file or, conversely, better sound quality in a similar file size.

Let's take a closer look at how VBR works. Standard MP3 files are encoded at a single bitrate through the entire file (hence the CBR moniker). In contrast, VBR encoding looks at the audio file and chooses what bitrate to encode based on how much audio information is present at any given moment. A song that begins quietly or with a single musical instrument will, during that section, be encoded at a lower bitrate then the middle of the song when all the instruments are playing together and the volume and frequency range is high. As a result, most songs will be encoded at several different bitrates corresponding to fluctuations in dynamic range. The key conceptual difference between CBR and VBR is that where in CBR encoding you specify your compression by space, with VBR you specify it by quality. With CBR, sonic quality is consistently reduced to maintain the bitrate you specify. With VBR, the bitrate is changed to meet the quality level desired. CBR is inefficient in that 10 seconds of silence encoded at 128kbps requires the same file size as 10 seconds of full-on opera. VBR encoded files of the same audio would result in a very small file for quiet sections and larger sized files for loud sections (depending on the quality you chose). Like CBR, VBR encoding parameters are set at the time the song is encoded.

The MP3 standard is designed so that information about the encoded bitrate level is included throughout the file. This makes it easy for VBR-enabled MP3 players to seamlessly decode and play VBR files. In fact, users are likely to have a more consistent listening experience with frequent bitrate changes in a VBR file than with CBR. In a CBR file, if you suddenly reach a more dynamic piece of a song, you're stuck with the same bitrate. This can result in audio artifacts (most noticed when encoding to lower bitrates - 64kbps and less).

Original text by Jon Luini, Allen Whitman


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