What is Variable Bitrate?

Video encoders compress visual data into bits. The rate control method determines how many bits each frame receives.

In Constant Bitrate (CBR), the encoder targets a fixed data rate, such as 5 Mbps. It forces every frame to fit this target. Simple scenes, like a static talking head, use fewer bits than available, creating buffer space. Complex scenes, like fast sports action, may exceed the target, causing quality loss. CBR ensures the output stream stays within a strict bandwidth ceiling.

In Variable Bitrate (VBR), the encoder adjusts the data rate per frame or scene. It allocates more bits to complex visuals and fewer to simple ones. The average bitrate matches the target, but instantaneous rates fluctuate. This preserves detail in high-motion areas without wasting data on static backgrounds. VBR produces smaller files for the same perceived quality.

For live streaming, contribution feeds often use CBR to prevent buffer underruns on unstable uplinks. For VOD, VBR is standard because viewers expect consistent quality regardless of scene complexity. Adaptive bitrate streaming (ABR) uses multiple VBR renditions to serve the best quality for each viewer's connection.

Choosing between VBR and CBR directly impacts bandwidth costs and viewer experience. Bandwidth is your largest variable cost. VBR reduces the total data transferred for on-demand content. If you serve a one-hour movie with VBR, you may save significant data compared to CBR, especially in scenes with low motion. This lowers egress fees and storage costs.

For live events, network stability is critical. CBR provides a predictable load on your encoder and CDN. If your uplink fluctuates, a CBR stream is less likely to drop frames or buffer because the output rate is constant. VBR live streams can spike in bitrate during high-action moments, potentially overwhelming a constrained uplink.

Viewer perception also differs. VBR maintains visual consistency across scene changes. CBR can show visible artifacts during complex scenes because the encoder cannot exceed its fixed rate. For premium content, VBR is the safer choice for quality. For live sports or news, CBR is often preferred for reliability. Most operators use CBR for live ingest and VBR for the final VOD output.

• Using CBR for VOD: This wastes bandwidth and can degrade quality in complex scenes. Use VBR for on-demand content to optimize file size and visual fidelity.
• Using VBR for unstable live uplinks: Bitrate spikes can cause buffering or dropped frames. Stick to CBR for live contribution feeds on unpredictable connections.
• Ignoring average bitrate targets: VBR still has an average target. If you set it too high, you lose the cost benefits. If too low, quality suffers.
• Confusing VBR with ABR: VBR is an encoding method. ABR is a delivery method that switches between multiple VBR files. You need both for optimal streaming.