What is Transrating?
Transrating changes the bitrate of encoded audio or video, generally by re-encoding it with a different quality or rate-control target. The codec, resolution, and sampling rate may remain unchanged.
How Transrating works
A transrating job targets a new data rate while attempting to preserve the source coding family and major presentation properties. Implementations may perform a full decode and encode or use codec-specific shortcuts, but both redistribute or discard coded information to meet the new rate objective. It differs from simple repackaging because the compressed media changes. In a rendition workflow, it creates bandwidth or storage variants without necessarily changing frame dimensions.
Key facts
- Matching the source codec and resolution does not make transrating lossless: another lossy encode can soften detail, amplify blocking, and spend bits reproducing artifacts already present.
- Rate-control mode determines whether the output pursues a fixed average, a constrained peak, or a quality target, so the requested bitrate alone does not fully describe delivery behavior.
- Shortcut modes that preserve the source frame structure inherit its keyframe spacing, so producing segment-aligned keyframes requires a full re-encode, which can change local bitrate demand.
When Transrating matters
Create lower-bitrate variants when delivery bandwidth or storage limits outweigh the cost of another encode. Aggressive reduction can introduce artifacts even when the original resolution and codec are retained.