Kopus

A Kotlin Multiplatform library providing OPUS audio codec functionality for Android and iOS platforms.

Features

• Cross-platform: Supports Android and iOS through Kotlin Multiplatform
• High-quality audio: Based on the libopus 1.5.2 codec
• Flexible encoding: Support for multiple sample rates, channels, and application modes
• Multiple data formats: Encode/decode both 16-bit PCM (Short) and floating-point (Float) audio

Installation

Add the dependency to your build.gradle.kts:

dependencies {
    implementation("io.voxkit:kopus:$kopus_version")
}

Quick Start

Basic Encoding and Decoding

import io.voxkit.kopus.*

// Create encoder
val encoder = Opus.encoder(
    sampleRate = SampleRate.RATE_48K,
    channels = Channels.STEREO,
    application = OpusApplication.AUDIO
)

// Create decoder  
val decoder = Opus.decoder(
    sampleRate = SampleRate.RATE_48K,
    channels = Channels.STEREO
)

// Encode PCM data
val pcmInput: ShortArray = // your audio samples
val frameSize = 960 // 20ms at 48kHz
val encodedBuffer = ByteArray(OpusEncoder.DEFAULT_OUTPUT_BUFFER_SIZE)
val encodedLength = encoder.encode(pcmInput, frameSize, encodedBuffer)

// Decode back to PCM
val decodedBuffer = ShortArray(frameSize * 2) // 2 channels
val encodedData = encodedBuffer.copyOf(encodedLength)
val decodedSamples = decoder.decode(encodedData, frameSize, decodedBuffer)

// Don't forget to close resources
encoder.close()
decoder.close()

Configuration Options

Sample Rates

• SampleRate.RATE_8K (8 kHz)
• SampleRate.RATE_12K (12 kHz)
• SampleRate.RATE_16K (16 kHz)
• SampleRate.RATE_24K (24 kHz)
• SampleRate.RATE_48K (48 kHz)

Channels

• Channels.MONO - Single channel
• Channels.STEREO - Two channels

Application Modes

• OpusApplication.VOIP - Optimize for voice calls
• OpusApplication.AUDIO - Optimize for music/general audio
• OpusApplication.LOW_DELAY - Optimize for low-latency applications

Advanced Features

Packet Loss Concealment (PLC)

// Handle missing packets by passing null data
val missingSamples = decoder.decode(
    data = null, // indicates packet loss
    frameSize = expectedFrameSize,
    pcm = outputBuffer
)

Forward Error Correction (FEC)

// Decode with FEC to recover from errors
val recoveredSamples = decoder.decode(
    data = corruptedPacket,
    frameSize = expectedFrameSize, 
    pcm = outputBuffer,
    decodeFec = true
)

Platform Support

Platform	Status	Implementation
Android	✅ Supported	JNI/NDK with C++ bridge
iOS	✅ Supported	Kotlin/Native cinterop

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

Links

• OPUS Codec
• libopus Documentation
• Kotlin Multiplatform