Encode Opus in C# · oneaudio cookbook

Encode WAV, FLAC or MP3 to Ogg Opus in C# with oneaudio, a managed Opus encoder: choose the bitrate and mode, add tags, and encode speech with the VoIP application.

using OneAudio;
using OneAudio.Opus;
 
// One call with the defaults: VBR at the encoder's automatic bitrate.
Audio.Transcode("music.wav", "music-auto.opus");
Console.WriteLine($"music-auto.opus: {new FileInfo("music-auto.opus").Length:N0} bytes");
 
// Choose the bitrate and add tags.
var settings = new OpusFileWriterSettings
{
    Encoder = new OpusEncoderSettings
    {
        BitrateBps = 64_000,
        BitrateMode = OpusBitrateMode.ConstrainedVbr,
    },
    Comments = ["TITLE=Four chords", "ARTIST=Test signal"],
};
Audio.Transcode("music.flac", "music-64k.opus", new TranscodeOptions { Opus = settings });
Console.WriteLine($"music-64k.opus: {new FileInfo("music-64k.opus").Length:N0} bytes");
 
// Speech: the VoIP application, mono, 16 kbit/s. The writer resamples the
// 16 kHz input to 48 kHz.
AudioClip voice = Audio.Load("voice.wav");
var voiceSettings = new OpusFileWriterSettings
{
    Channels = 1,
    InputSampleRate = voice.Format.SampleRate,
    Encoder = new OpusEncoderSettings
    {
        Application = OpusApplication.Voip,
        BitrateBps = 16_000,
    },
};
using (var writer = new OpusFileWriter(File.Create("voice-16k.opus"), voiceSettings,
                                       leaveOpen: false))
{
    writer.Write(voice.Samples.Span);       // call as often as you like
    writer.Finish();
    Console.WriteLine($"voice-16k.opus: {writer.FramesWritten} frames, pre-skip {writer.PreSkip}, " +
                      $"{new FileInfo("voice-16k.opus").Length:N0} bytes");
}
 
// Read them back: the decoded length is the audio written, at 48 kHz.
foreach (string f in new[] { "music-64k.opus", "voice-16k.opus" })
{
    using IAudioDecoder d = Audio.Open(f);
    Console.WriteLine($"{f}: {d.Format.SampleRate} Hz, {d.Format.Channels} ch, " +
                      $"{d.LengthFrames / 48000.0:F3} s");
}

Text

music-auto.opus: 404,461 bytes
music-64k.opus: 260,511 bytes
voice-16k.opus: 289201 frames, pre-skip 312, 11,805 bytes
music-64k.opus: 48000 Hz, 2 ch, 32.000 s
voice-16k.opus: 48000 Hz, 1 ch, 6.025 s

Settings

OpusEncoderSettings.BitrateBps takes 500 to 300,000 bits per second per channel, or OpusEncoderSettings.AutoBitrate (the default) or MaxBitrate. BitrateMode is Vbr (the default), ConstrainedVbr or Cbr.
Application is Audio for music and general sound, Voip for speech, or RestrictedLowDelay. Complexity runs from 0 to 10, and the default is 10.
The encoder picks SILK, hybrid or CELT for each packet, the way the reference encoder does. Bandwidth and MaxBandwidth pin the bandwidth. InBandFec with a PacketLossPercent above 0 adds forward error correction, and Dtx makes silence cost one byte per packet.
Input that isn’t at 48 kHz is resampled by the file writer, which is why voice-16k.opus has about three times as many frames as the 16 kHz source. The header records the original rate.
OpusFileWriterSettings takes 48 kHz stereo input unless you set InputSampleRate and Channels. Audio.Transcode sets both from the source. FrameMilliseconds sets the packet length: 2.5, 5, 10, 20 (the default), 40 or 60 ms.

Good to know

With the same settings and 16-bit input, the packets are byte-identical to the RFC 6716 reference encoder on all 269 configurations the project tested. The same input gives the same bytes on a platform.
More than two channels are written as channel mapping family 1, up to eight channels, in WAV speaker order.
OpusEncoder alone encodes raw packets of 2.5 to 60 ms at 48 kHz, for a network stream. See Decode Opus for a round trip through it.
Encoding allocates nothing after the encoder is built.

Opus files and raw packets, compared with Concentus and OpusSharp: Opus in C#.