VBR (variable bitrate) encoding lets the bit rate rise and fall with the complexity of the audio, spending more on dense, transient-rich passages and less on simple or quiet ones. For a given average size this yields better quality than CBR, because bits go where they're needed, which is why Opus and AAC use it by default and why it dominates file-based and on-demand audio. The downside is unpredictability: peak rates can spike, sizes vary, and it is harder to fit into a strictly rate-limited channel — so constrained-VBR and CBR modes exist for streaming and broadcast where steady bandwidth matters more than squeezing out the last bit of efficiency.
The book · Volume 4 of 7
Audio for Video: A Complete Guide to Sound in Video Products: From PCM and Codecs to Loudness, Accessibility, and Real-Time Voice Processing
VBR gives better audio per average bit, but it can surprise teams that budget bandwidth or file size by a fixed number. Volume 4 covers VBR in detail within its codec chapters: Opus and AAC rate control, constrained versus unconstrained modes, bit depth and how bitrate choices affect perceived quality. Written by Nikolay Sapunov, CEO at Fora Soft.
Choosing between VBR and CBR audio for your streams or downloads?
We will look at your content and network limits and give you concrete VBR or CBR settings for Opus or AAC, with expected bitrates and quality per profile. 250+ video products shipped since 2005.
