Constant Bit Rate encoding maintaining fixed data rate throughout the video.
Technical Details
CBR encoding works with fixed time windows (usually 1 second) in which exactly the specified amount of data is used. With complex image content featuring many details or fast movements, quality can decrease as fewer bits are available. Simple scenes with static images, on the other hand, waste storage space. Standard CBR values: DVD 9.8 Mbps, Blu-ray 25-40 Mbps, Digital Cinema Packages (DCP) 250 Mbps. The MPEG-2 codec uses CBR by default, while H.264 and H.265 support both modes.
History & Development
CBR emerged in 1993 with the first MPEG-1 implementations, as early hardware decoders could only process constant data rates. The 1995 DVD specification mandated CBR encoding to ensure compatibility with simple decoders. From 2003 onwards, more powerful processors also enabled Variable Bitrate (VBR), but CBR remained dominant for broadcast applications. Streaming services have primarily used CBR for live broadcasts since 2010.
Practical Application in Film
Television broadcasters use CBR for live transmissions, as the constant data rate prevents buffering issues. "Avatar" (2009) used CBR encoding for its 3D cinema copies at 125 Mbps per eye. Netflix encodes live streams with CBR 6-8 Mbps, while on-demand content uses VBR. In action films with complex explosion scenes, CBR often leads to visible compression artifacts because there are not enough bits available for the high image complexity.
Comparison & Alternatives
Variable Bitrate (VBR) adjusts the data rate to the image complexity and achieves better quality at the same file size. Constrained VBR (CVBR) combines both approaches with a maximum bitrate ceiling. CBR is suitable for real-time applications and hardware with limited buffer capacity. VBR is used for archiving and on-demand streaming, where quality is more important than constant data rates. Modern codecs like AV1 prefer VBR modes for optimal efficiency.