Variable Bit Rate (VBR)

A variable bit rate (VBR) is an encoding method that is used mainly in communications and computing to achieve improved audio quality in comparison to file size ratio. Depending on the nature of the audio, the bit rate is continuously changed during the encoding process to achieve the VBR.

VBR is most commonly used in the transmission of compressed audio and video data, such as videoconferencing.

Using the variable bit rate (VBR) method has advantages and disadvantages.

VBR advantages are as follows:

• Compared to a constant bit rate (CBR), VBR produces a better quality-to-space ratio for the same file size.
• The available bits are used to more flexibly and accurately encode the audio or video data.
• Higher bits are used in more difficult to encode passages and lower in less demanding passages.

VBR disadvantages are as follows:

• The encoding process requires more time.
• The VBR process is more complex and thus more susceptible to errors.
• Hardware compatibility can be an issue.

VBR encoding types include:

• Quality-based VBR encoding: The focus is on a specific quality level for the media stream and not the bit rate. This encoding also ensures that the generated file has a consistent quality. However, because this encoding cannot provide required size and bandwidth criteria, it may not be suitable for those with restricted memory or bandwidth, like portable media players.
• Unconstrained VBR encoding: This makes use of two encoding passes. Like CBR, unconstrained VBR encoding uses a specific bit rate, which is used as an average bit rate. Unconstrained VBR encoding's main advantage is the ability to provide the highest quality for the compressed stream, while remaining within an assumable bandwidth.
• Constrained VBR Encoding: This allows the specification of a maximum and minimum bit rate. Using the maximum values, the codec then determines and compresses the data.