What is bitrate? What is the difference between CBR and VBR?

The bitrate is a fundamental parameter of video compression. It expresses the overall compression ratio of the stream and therefore determines the required data channel bandwidth. The higher the bitrate, the more picture details can be preserved and the more realistic the resulting video appears. Choosing a bitrate always involves a trade-off between encoding quality and channel bandwidth.

There are two basic approaches to distributing the bits within a stream: constant bitrate (CBR) and variable bitrate (VBR). Engineers can mean two different things by the terms constant or variable bitrate. In the context of broadcasting, it means the constant or variable number of bits transmitted per second. In the context of coding, it means the constant or variable rate at which the buffer is being filled or emptied.

Visual scheme of difference between CBR and VBR

CBR

CBR shows a stable bitrate level regardless of the scene and does not cause abrupt load changes. It is not optimal in terms of quality, because the stream does not change depending on dynamics and complexity of video.

  • CBR is ideal for data transmitted over limited-bandwidth multimedia channels because it makes it possible to use full capabilities of such channels.
  • This mode affects quality when encoding complex (highly dynamic) fragments that normally require more space.
  • Static video fragments are encoded using the same number of allocated bits, and available channel bandwidth is wasted.
  • Sometimes during direct broadcasting over a low-bandwidth channel (CBR), bitrate deviations up to ~10% are allowed. Use video stream analysis software to check if the encoder is using correct bitrate.

VBR

VBR allows an increase in bitrates and reduction of compression ratio in complex scenes to achieve better picture quality. In this mode, the bitrate is adjusted depending on the rate at which the picture changes. For example, a static video fragment (sunset) would have much lower bitrate than a fragment that features fast action (car chase).

  • VBR responds only to activity that occurs in the frame and enables considerable disk space savings when working with files.
  • This mode makes it possible to achieve much better picture quality than CBR at smaller encoded file size.
  • VBR is well-suited to Internet broadcasting (ОТТ).
  • As activity in the frame intensifies, the bitrate increases correspondingly so that the system throughput can approach the limit very quickly. This can cause lots of artifacts and distortions in the resulting picture or even complete loss of the picture.
  • The upcoming changes are hard to predict. The bitrate increase takes place with a time lag during recording.

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