VBR (variable bitrate)
VBR (variable bitrate) is an encoding mode where the data rate rises for busy, detailed moments and drops for calm, simple ones. VBR delivers better overall quality than constant bitrate at the same average file size. Encoders usually cap VBR with a maximum bitrate so streams stay deliverable.
Why it matters
VBR spends bits where viewers would notice and saves them where they would not, which improves quality at the same average delivery cost. Uncapped VBR can spike above what viewers' connections handle, causing buffering, so most streaming services use capped or constrained VBR. VBR peaks do not need their own budget line on FastPix, because on-demand streaming is billed per minute watched at each resolution, not by the data each scene uses.
How it works
- You set an average target bitrate and usually a maximum bitrate.
- The encoder measures how complex each scene is.
- It raises the rate for detailed, fast scenes and lowers it for simple, still ones.
- A buffer limit keeps short spikes within the maximum so players can download them in time.
VBR vs CBR: which gives better quality?
VBR gives better quality at the same average bitrate because it shifts bits to the scenes that need them. CBR spreads bits evenly regardless of content. That is why on-demand services and YouTube's upload guidance favor VBR, while live ingest often uses CBR for steadiness.
What is capped VBR?
Capped VBR is variable bitrate with a maximum rate set. The encoder varies the bitrate with content but never exceeds the cap over a short buffer window. For example, a stream might average 5 Mbps with a cap of 7.5 Mbps. This keeps quality high without spikes that stall slower connections.
Where it's used
OTT services encode films with capped VBR so action scenes stay sharp while the average bitrate, and the monthly delivery bill, stays the same as constant bitrate.
Where it usually goes wrong, and how FastPix handles it
VBR peaks can exceed what some viewers' networks deliver, so a stream that looks fine on average still buffers for part of the audience during busy scenes.
Every view in FastPix Video Data carries a time-weighted average bitrate next to buffer count and buffer ratio, with breakdowns by ISP, device and CDN. That shows whether VBR peaks are causing stalls on the networks your audience actually uses.
By the numbers
- YouTube's recommended upload encoding settings call for variable bitrate, with no bitrate limit required. (YouTube Help: Recommended upload encoding settings)
