Is Variable Bit Rate encoding really better?

By Steve Perlman

Variable Bit Rate (VBR) encoding is theoretically much better than Constant Bit Rate (CBR). VBR encodes scenes with high motion at a higher bit rate than scenes with low motion. As MPEG compression uses predictive vectors to store information, VBR causes more vectors to be encoded with high motion, and fewer vectors with low motion. Image quality improves.

The key is the difference between single-pass VBR and multi-pass VBR. Single-pass encoders do not analyze the entire video before encoding. With bit rate calculations that are based on information constantly updated during the encode, VBR calculations may not be accurate for several minutes. So, CBR may actually produce a better encode than single-pass VBR!

Multi-pass encoders provide the best quality VBR. Each pass analyzes the source video and builds a bitrate plan ahead of the actual encode. The final pass performs the encode according to the bitrate plan. Multi-pass VBR calculations are accurate and reliable.

Multi-pass VBR is available in some hardware encoders, requiring deck control to accurately run each pass. But, these encoders are expensive, generally priced for high-end production studios. Software encoders require significantly more time (capture, editing and encoding), but offer multi-pass VBR encoding at a fairly low cost.

Visible Light's suggestion? Use CBR with hardware encoders, and multi-pass VBR if available with a software encoder. Don't forget that decoders differ, so always test compatibility!