FFmpeg VBR Settings
There are various FFmpeg encoders that support variable bit rate / constant quality encoding (learn more about rate control modes here). This gives you a much better overall quality when file size or average bit rate are not constrained (e.g. in a streaming scenario). Variable bit rate is usually achieved by setting -q:v (or -q:a for audio) instead of -b:v (or -b:a), which just sets a target bit rate.
The problem is that every encoder uses a different range of values to set the quality—and they’re hard to memorize. This is an attempt to summarize the most important ones.
Notes for reading this table:
- Qmin stands for the setting to be used for achieving lowest quality and Qmax for highest. These are not just lowest and highest values.
- Qdef is the default value chosen if no other is specified. This means that (most?) encoders will use one or the other VBR mode by default, e.g. libx264. I wasn’t able to research whether this applies to all encoders.
- Some encoders use private options instead of the regular
-q. Read the second column Param for the correct option to use. - Rows highlighted with green refer to encoders that allow you to use VBR. Rows in yellow aren’t really VBR or I simply couldn’t find out whether they support it. Rows in red mean: No VBR support.
| Encoder | Param | Qmin | Qmax | Qdef | Recommended | Notes |
|---|---|---|---|---|---|---|
| libx264 | -crf |
51 | 0 | 23 | 18–28 | Values of ±6 result in double/half avg. bitrate. 0 is lossless. Specifying -profile:v lets you adjust coding efficiency. See H.264 Encoding Guide.
|
| libx265 | -crf |
51 | 0 | 28 | 24–34 | Values of ±6 result in double/half avg. bitrate. 0 is lossless. Specifying -profile:v lets you adjust coding efficiency. See H.265 Encoding Guide and x265 docs.
|
| libvpx | -qmin-qmax-crf-b:v
|
63 | 0 | 10 | -qmin: 0–4-qmax: 50–63-crf: 30 |
-b:v sets target bitrate, or maximum bitrate when -crf is set (enables CQ mode). See also VP9 Encoding Guide. Setting -maxrate and -bufsize is also possible. |
| libxvid | -q:v |
31 | 1 | n/a | 3–5 | 2 is visually lossless. Doubling the value results in half the bitrate. Don't use 1, as it wastes space. No VBR by default—it uses -b:v 200K unless specified otherwise. |
| libtheora | -q:v |
0 | 10 | n/a | 7 | No VBR by default—it uses -b:v 200K unless specified otherwise. |
| mpeg1, mpeg2, mpeg4, flv, h263, h263+, msmpeg+ | -q:v |
31 | 1 | ? | 3–5 | 2 is visually lossless. Doubling the value results in half the bitrate.-q:v works for mpeg4, but haven't tested others. |
| prores | -profile:v |
0 | 3 | 2 | Depends | Not VBR. Corresponds to the profiles Proxy, LT, Std, HQ. ProRes might support -q:v?Target bitrates are in the ProRes Whitepaper. |
| Encoder | Param | Qmin | Qmax | Qdef | Recommended | Notes |
|---|---|---|---|---|---|---|
| libfdk_aac | -vbr |
1 | 5 | ? | 4 (~128kbps) | Currently the highest quality encoder. |
| libopus | -b:a |
6–8K (mono) | – | 96K (for stereo) | – | -vbr on is default, -b:a just sets the target, see FFmpeg documentation. |
| libvorbis | -q:a |
0 | 10 | 3 | 4 (~128kbps) | Make sure not to use vorbis, which is the (bad) internal encoder. |
| libmp3lame | -q:a |
9 | 0 | 4 | 2 (~190kbps) | Corresponds to lame -V. See FFmpeg Wiki. |
| aac | -q:a |
0.1 | 2 | ? | 1.3 (~128kbps) | Is "experimental and [likely gives] worse results than CBR" according to FFmpeg Wiki. Ranges from 18 to 190kbps. |