Glossary

What is CBR or Constant Bitrate in Video Compression?

Constant Bit Rate (CBR) rate control is used to ensure that compressed videos (or video streams) occupy a certain number of bits per second when they are played back.

A fundamental part of any video compression system is in the control of the bit rateᅳhow the available data gets distributed across the video frames. A simple and predictable way of doing this is to allocate the same number of bits per elapsed unit of time (e.g., seconds) for each video and this is how CBR works.

In other words, no matter how intricate the scenes being encoded are, or how simple and straightforward, each segment of video is given a fixed amount of data in the CBR rate control mechanism.

Advantages of CBR Rate Control

By assigning a constant set number of bits to every part of the video, the CBR file size can be figured out before the actual encoding process is even completed. This is great when you need to have precise control over the final product, like when you’re fitting a video into a video game console’s memory or you’re trying to meet broadcast specs.

The beauty of CBR is that it is well integrated into most video editing software and encoding tools. CBR is especially well-suited for real-time streaming, where you’re trying to render an experience on the viewer’s end that is as close to perfect as you can come. Applications such as video conferencing also benefit when using CBR.

Drawbacks of using CBR Rate Control

The CBR’s fixed bitrate allocation can result in wasted data. This is because the allocation is extremely simple. It breaks up the video into different scenes, determines the length of each scene, and then divvies up all the bits for that video across all its scenes.

If a video has more bits allocated for it than it can use, then that, by definition, is wasted space. Some videos are just a lot simpler than others, and for those videos, the CBR forces the allocation of more bits than are necessary.

Quality problems often occur when a video’s content changes significantly from one part to another. A “fast-action” scene, for example, with a lot of detail moving across the screen, requires more data to represent it accurately than does a simple “talking head” sequence in an office.

Where should we use CBR Rate Control?

Consider using CBR in the following instances –

Streaming over limited bandwidth can rely on CBR to get the job done. Allocations of bits per second are guaranteed to go off without a hitch, ensuring the viewer a solid, steady experience every time.

Limited encoding resources: when the encoding resources are limited and cannot run algorithms suited for variable bitrate encoding, or 2-pass encoding, then a single-pass CBR encode can provide a lot of benefits.

CBR is also useful for Internet-based delivery, where variations in bandwidth can cause all kinds of issues for video that is a bit too close to the bleeding edge.

Finally, CBR may not always achieve the best compression, its output is highly predictable, and it guarantees that video will play back at the same rate it was compressed at, which is not the case for most of the other methods.

If you are interested in rate control mechanisms, then you should consider reading this article on VBR rate control.

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