Opus Recommended Settings: Difference between revisions
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== Mono or Stereo == | == Mono or Stereo == | ||
Opus tends to start '''downmixing stereo inputs to mono''' from roughly '''30 Kb/s and lower'''. | Opus tends to start '''downmixing stereo inputs to mono''' from roughly '''30 Kb/s and lower'''. | ||
You can check the details in the '''[https://github.com/xiph/opus/blob/master/src/opus_encoder.c# | You can check the details in the '''[https://github.com/xiph/opus/blob/master/src/opus_encoder.c#L148 opus_encoder.c]''' source file. | ||
You can force downmixing at any bitrate by using the following command-line parameters: | You can force downmixing at any bitrate by using the following command-line parameters: | ||
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The following table shows where Opus decides to switch between its different '''[https://tools.ietf.org/html/rfc6716#section-2 audio frequency bandwidths]'''. This could be useful if you are encoding audio that has already been bandpassed, or should go through a bandpass filter (e.g. VoIP speech). | The following table shows where Opus decides to switch between its different '''[https://tools.ietf.org/html/rfc6716#section-2 audio frequency bandwidths]'''. This could be useful if you are encoding audio that has already been bandpassed, or should go through a bandpass filter (e.g. VoIP speech). | ||
The table was copied from the '''[https://github.com/xiph/opus/blob/master/src/opus_encoder.c# | The table was copied from the '''[https://github.com/xiph/opus/blob/master/src/opus_encoder.c#L121 opus_encoder.c]''' source file. | ||
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Revision as of 06:03, 2 August 2016
Depending on what kinds of sounds you want to encode with Opus, you should use different bitrate (quality) settings.
The settings in the table below are meant to start you off with a decent tradeoff between good quality and small filesize (or bitrate usage, if you're streaming).
You should test the suggested bitrate by actually listening to your encoded audio and then tweaking the bitrate:
- down if you think the quality is good, but the filesize (or bitrate) is too big
- up if you think the quality is bad, and you can afford having bigger files (or bitrate)
| Use Case | Channels | Bitrate (Kb/s) | Notes |
|---|---|---|---|
| Ham radio | 1 (mono) | use Codec 2 | Opus only supports bitrates down to 6 Kb/s. Codec 2 handles speech from 0.7 to 3.2 Kb/s. |
| VoIP | 1 | 10-24 | 10 Kb/s will deliver narrowband most of the time, 24 Kb/s should give fullband. More details in the relevant table further down this page. |
| Audiobooks / Podcasts | 1 | 24 | bitrates from here on up tend to deliver fullband audio. |
| 2 (stereo) | 32 | ||
| Music Streaming / Radio | 2 | 64-96 | Opus has better quality than MP3, AAC and Vorbis at these rates. (test results here and here) |
| Music Storage | 2 | 96-128 | Opus at 128 KB/s (VBR) is pretty much transparent |
| 6 (5.1 surround) | 128-256 | for surround sound, Opus uses surround-sound bitrate allocation | |
| 8 (7.1 surround) | 256-450 | ||
| Music Archiving | any | use FLAC | if you are archiving audio, use a lossless audio format to prevent generation loss |
Technical Details
For the more technical Opus users, here are some details to help you fine-tune your decision on which bitrate best fits your needs.
Mono or Stereo
Opus tends to start downmixing stereo inputs to mono from roughly 30 Kb/s and lower. You can check the details in the opus_encoder.c source file.
You can force downmixing at any bitrate by using the following command-line parameters:
--downmix-mono - downmixes input channels to mono
--downmix-stereo - downmixes input channels to stereo (if there are more than 2 e.g. if your input is surround sound)
Bandwidth Transition Thresholds
The following table shows where Opus decides to switch between its different audio frequency bandwidths. This could be useful if you are encoding audio that has already been bandpassed, or should go through a bandpass filter (e.g. VoIP speech).
The table was copied from the opus_encoder.c source file.
| (bitrates in Kb/s) | Mono | Stereo | ||
|---|---|---|---|---|
| Voice | Music | Voice | Music | |
| NB ↔ MB | 11 ± 1 | 12 ± 1 | 11 ± 1 | 12 ± 2 |
| MB ↔ WB | 14 ± 1 | 15 ± 1 | 14 ± 2 | 18 ± 2 |
| WB ↔ SWB | 17 ± 1 | 18 ± 1 | 21 ± 2 | 21 ± 2 |
| SWB ↔ FB | 21 ± 1 | 22 ± 2 | 28 ± 2 | 30 ± 2 |
The HydrogenAudio wiki also has some great information on Opus and its usage.
Framesize Tweaking
Opus uses a 20 ms frame size by default, as it gives a decent mix of low latency and good quality.