Go wrapper for Opus

This package provides Go bindings for the xiph.org C libraries libopus and libopusfile.

The C libraries and docs are hosted at https://opus-codec.org/. This package just handles the wrapping in Go, and is unaffiliated with xiph.org.

Details

This wrapper provides a Go translation layer for three elements from the xiph.org opus libs:

• encoders
• decoders
• files & streams

Import

import "gopkg.in/hraban/opus.v2"

Encoding

To encode raw audio to the Opus format, create an encoder first:

const sampleRate = 48000
const channels = 1 // mono; 2 for stereo

enc, err := opus.NewEncoder(sampleRate, channels, opus.AppVoIP)
if err != nil {
    ...
}

Then pass it some raw PCM data to encode.

Make sure that the raw PCM data you want to encode has a legal Opus frame size. This means it must be exactly 2.5, 5, 10, 20, 40 or 60 ms long. The number of bytes this corresponds to depends on the sample rate (see the libopus documentation).

var pcm []int16 = ... // obtain your raw PCM data somewhere
const bufferSize = 1000 // choose any buffer size you like. 1k is plenty.

// Check the frame size. You don't need to do this if you trust your input.
frameSize := len(pcm) // must be interleaved if stereo
frameSizeMs := float32(frameSize) / channels * 1000 / sampleRate
switch frameSizeMs {
case 2.5, 5, 10, 20, 40, 60:
    // Good.
default:
    return fmt.Errorf("Illegal frame size: %d bytes (%f ms)", frameSize, frameSizeMs)
}

data := make([]byte, bufferSize)
n, err := enc.Encode(pcm, data)
if err != nil {
    ...
}
data = data[:n] // only the first N bytes are opus data. Just like io.Reader.

Note that you must choose a target buffer size, and this buffer size will affect the encoding process:

Size of the allocated memory for the output payload. This may be used to impose an upper limit on the instant bitrate, but should not be used as the only bitrate control. Use OPUS_SET_BITRATE to control the bitrate.

-- https://opus-codec.org/docs/opus_api-1.1.3/group__opus__encoder.html

Decoding

To decode opus data to raw PCM format, first create a decoder:

dec, err := opus.NewDecoder(sampleRate, channels)
if err != nil {
    ...
}

Now pass it the opus bytes, and a buffer to store the PCM sound in:

var frameSizeMs float32 = ...  // if you don't know, go with 60 ms.
frameSize := channels * frameSizeMs * sampleRate / 1000
pcm := make([]byte, int(frameSize))
n, err := dec.Decode(data, pcm)
if err != nil {
    ...
}

// To get all samples (interleaved if multiple channels):
pcm = pcm[:n*channels] // only necessary if you didn't know the right frame size

// or access sample per sample, directly:
for i := 0; i < n; i++ {
    ch1 := pcm[i*channels+0]
    // For stereo output: copy ch1 into ch2 in mono mode, or deinterleave stereo
    ch2 := pcm[(i*channels)+(channels-1)]
}

Note regarding Forward Error Correction (FEC):

When a packet is considered "lost", DecodeFEC and DecodeFECFloat32 methods can be called on the next packet in order to try and recover some of the lost data. The PCM needs to be exactly the duration of audio that is missing. LastPacketDuration() can be used on the decoder to get the length of the last packet. Note also that in order to use this feature the encoder needs to be configured with SetInBandFEC(true) and SetPacketLossPerc(x) options.

Note regarding Packet Loss Concealment (PLC):

When a packet is considered "lost", DecodePLC and DecodePLCFloat32 methods can be called in order to obtain something better sounding than just silence. The PCM needs to be exactly the duration of audio that is missing. LastPacketDuration() can be used on the decoder to get the length of the last packet. This option does not require any additional encoder options. Unlike FEC, PLC does not introduce additional latency. It is calculated from the previous packet, not from the next one. Note that DecodeFEC and DecodeFECFloat32 automatically fall back to PLC when no FEC data is available in the provided packet.

Streams (and files)

To decode a .opus file (or .ogg with Opus data), or to decode a "Opus stream" (which is a Ogg stream with Opus data), use the Stream interface. It wraps an io.Reader providing the raw stream bytes and returns the decoded Opus data.

A crude example for reading from a .opus file:

f, err := os.Open(fname)
if err != nil {
    ...
}
s, err := opus.NewStream(f)
if err != nil {
    ...
}
defer s.Close()
buf := make([]byte, 16384)
for {
    n, err = s.Read(buf)
    if err == io.EOF {
        break
    } else if err != nil {
        ...
    }
    pcm := buf[:n*channels]

    // send pcm to audio device here, or write to a .wav file

}

See https://godoc.org/gopkg.in/hraban/opus.v2#Stream for further info.

API Docs

Go wrapper API reference: https://godoc.org/gopkg.in/hraban/opus.v2

Full libopus C API reference: https://www.opus-codec.org/docs/opus_api-1.1.3/

For more examples, see the _test.go files.

Build & installation

This package requires libopus and libopusfile development packages to be installed on your system. These are available on Debian based systems from aptitude as libopus-dev and libopusfile-dev, and on Mac OS X from homebrew.

They are linked into the app using pkg-config.

Debian, Ubuntu, ...:

sudo apt-get install pkg-config libopus-dev libopusfile-dev

Mac:

brew install pkg-config opus opusfile

Using in Docker

If your Dockerized app has this library as a dependency (directly or indirectly), it will need to install the aforementioned packages, too.

This means you can't use the standard golang:*-onbuild images, because those will try to build the app from source before allowing you to install extra dependencies. Instead, try this as a Dockerfile:

# Choose any golang image, just make sure it doesn't have -onbuild
FROM golang:1

RUN apt-get update && apt-get -y install libopus-dev libopusfile-dev

# Everything below is copied manually from the official -onbuild image,
# with the ONBUILD keywords removed.

RUN mkdir -p /go/src/app
WORKDIR /go/src/app

CMD ["go-wrapper", "run"]
COPY . /go/src/app
RUN go-wrapper download
RUN go-wrapper install

For more information, see https://hub.docker.com/_/golang/.

Linking libopus and libopusfile

The opus and opusfile libraries will be linked into your application dynamically. This means everyone who uses the resulting binary will need those libraries available on their system. E.g. if you use this wrapper to write a music app in Go, everyone using that music app will need libopus and libopusfile on their system. On Debian systems the packages are called libopus0 and libopusfile0.

The "cleanest" way to do this is to publish your software through a package manager and specify libopus and libopusfile as dependencies of your program. If that is not an option, you can compile the dynamic libraries yourself and ship them with your software as seperate (.dll or .so) files.

On Linux, for example, you would need the libopus.so.0 and libopusfile.so.0 files in the same directory as the binary. Set your ELF binary's rpath to $ORIGIN (this is not a shell variable but elf magic):

patchelf --set-origin '$ORIGIN' your-app-binary

Now you can run the binary and it will automatically pick up shared library files from its own directory.

Wrap it all in a .zip, and ship.

I know there is a similar trick for Mac (involving prefixing the shared library names with ./, which is, arguably, better). And Windows... probably just picks up .dll files from the same dir by default? I don't know. But there are ways.

License

The licensing terms for the Go bindings are found in the LICENSE file. The authors and copyright holders are listed in the AUTHORS file.

The copyright notice uses range notation to indicate all years in between are subject to copyright, as well. This statement is necessary, apparently. For all those nefarious actors ready to abuse a copyright notice with incorrect notation, but thwarted by a mention in the README. Pfew!