Constant-Quality Modes: CRF and Rate Factor
Constant-quality rate control aims at a distortion target and lets the bitrate fall where it may. Instead of asking the encoder for a fixed average bitrate, the operator names a quality level and accepts the size that follows. This page sets out how that level is expressed across the three main codecs and how to bracket it in a few runs.
What the rate factor means
The number is a scale on the quantiser. A low value keeps more detail and produces a larger stream; a high value discards more detail and produces a smaller one. The scale is not shared between codecs, so a value that looks polite in one encoder can be aggressive in another. Only the distortion curve is comparable, never the raw factor.
Bracketing the factor in three runs
Start from the baseline range for the codec. Encode a short clip at the low end, the mid and the high end of that range, then score each reconstruction. Plot the three points on a distortion curve. The useful zone is the knee: below it, each step yields little visible quality; above it, each step saves little size for a growing visible loss. One extra run on either side confirms the knee rather than guessing it.
| Observation | Likely cause |
|---|---|
| Curve is flat across a wide range | Source is easy: low grain, low motion. |
| Curve is steep near the knee | Source has fine grain or fast detail. |
| Size moves but score does not | Metric is blind to the artefact present. |
| Repeats scatter widely | Frame threading or a thermal limit. |
Anchors, not absolutes
Treat any quoted factor as an anchor for a source class. A talk recording and an action clip do not share a knee, even at the same resolution and frame rate. Record the source class next to the factor so the pair travels together. The desk publishes its anchors on the 1080p and 4K sheets and explains the scoring on the measurement page.
Where constant quality stops
For a fixed channel or a live feed, an unknown size is a problem, so a constrained run or a two-pass target is used instead. Constant quality is strongest for on-demand and archival work, where the budget is a long-run average rather than a hard ceiling per minute.