VBR stands for Variable Bit Rate. The bit rate of a coded audio file is the number of bits (binary digits - 0s or 1s - bits of digital information) that are required to store 1 second of audio. An mp3 at 128kbps requires 128,000 (ish!) bits to store 1 second of audio.
The point of psychoacoustic coding is that you only store the audible part, but just how much information is needed to represent the audible part varies from 1 moment to the next. Silence doesn't contain very much information at all - rather than storing thousands of zeros, you could just store a code meaning "so many seconds of silence".
A Variable BitRate Coder will vary the bit rate, depending on how much information is needed to store the audible part of a signal from moment to moment. If a decoder expects the same number of bits every second, this can really confuse it!
MPEG 1/2/2.5 layer 3 files (.mp3 files) are encoded with a specified bit rate, usually 128 kbps (thousand bits per second) or, sometimes 160 or 192 kbps.
Greater bit rates generally mean better sound quality and a closer representation of the original sound. With variable bit rate MP3s, the encoder automatically detects which bit rate is most apropriate for the sound being encoded, live.
This means that when there is little sound to be heard, the encoder encodes the MP3 with a low bit rate (as low as 16 kbps or less), but when there is very complex sound to be heard, the encoder uses a much higher bit rate (up to 320 kbps) to attain better sound reproduction.
What are the major advantages of VBR encoded MP3 files?
- VBR encoded MP3s are generally smaller than standard MP3s of the same sound quality and generrally sound better, especially in the high frequencies.
Also, it's great for spoken word audio as there are often pauses and silence between sentences.
What are the major disadvantages of VBR encoded MP3 files?
- Some older MP3 players cannot play them.
- Must be re-encoded in order to be streamed. Although variable speed compressed files are difficult to stream continuously over most Internet connections, those that download the entire file before viewing are unaffected.
LAME MP3 Encoder
: is a variable bitrate MP3 encoder, generally considered the best of the MP3 encoders.
: is another freeware MP3 encoder.