This is a very early proof of concept of the Sendspin protocol. The protocol will likely change. This does work today (10/26), but may not work tomorrow.

SendspinKit

A Swift client library for the Sendspin Protocol - enabling synchronized multi-room audio playback on Apple platforms.

Features

• 🎵 Player Role: Synchronized audio playback with microsecond precision
• 🎛️ Controller Role: Control playback across device groups
• 📝 Metadata Role: Display track information and progress
• 🔍 Auto-discovery: mDNS/Bonjour server discovery
• 🎵 Multi-codec: PCM, Opus, and FLAC support for flexible streaming
• ⏱️ Clock Sync: NTP-style time synchronization

Requirements

• iOS 17.0+ / macOS 14.0+ / tvOS 17.0+ / watchOS 10.0+
• Swift 6.0+

Installation

Swift Package Manager

dependencies: [
    .package(url: "https://github.com/YOUR_ORG/SendspinKit.git", from: "0.1.0")
]

Quick Start

import SendspinKit

// Create client with player role
let client = SendspinClient(
    clientId: "my-device",
    name: "Living Room Speaker",
    roles: [.player],
    playerConfig: PlayerConfiguration(
        bufferCapacity: 1_048_576, // 1MB
        supportedFormats: [
            AudioFormatSpec(codec: .pcm, channels: 2, sampleRate: 48000, bitDepth: 16),
            AudioFormatSpec(codec: .opus, channels: 2, sampleRate: 48000, bitDepth: 16),
            AudioFormatSpec(codec: .flac, channels: 2, sampleRate: 48000, bitDepth: 16),
        ]
    )
)

// Discover servers
let discovery = SendspinDiscovery()
await discovery.startDiscovery()

for await server in discovery.discoveredServers {
    if let url = await discovery.resolveServer(server) {
        try await client.connect(to: url)
        break
    }
}

// Client automatically handles:
// - WebSocket connection
// - Clock synchronization
// - Audio stream reception
// - Synchronized playback

Codec Support

SendspinKit supports multiple audio codecs for high-quality streaming:

• PCM - Uncompressed audio up to 192kHz 32-bit (zero-copy passthrough)
• Opus - Low-latency lossy compression (8-48kHz, optimized for real-time)
• FLAC - Lossless compression with hi-res support (up to 192kHz 24-bit)

All codecs output normalized int32 PCM for consistent pipeline processing. See docs/CODEC_SUPPORT.md for detailed codec documentation, performance characteristics, and implementation guide.

Audio Synchronization

SendspinKit uses timestamp-based audio scheduling to ensure precise synchronization:

• AudioScheduler: Maintains priority queue of audio chunks sorted by playback time
• Clock Sync: Compensates for clock drift using Kalman filter approach
• Playback Window: ±50ms tolerance for network jitter
• Late Chunk Handling: Automatically drops chunks >50ms late to maintain sync
• AsyncStream Pipeline: Non-blocking chunk output for smooth playback

The scheduler converts server timestamps to local playback times and ensures chunks play at their intended moment, not when they arrive from the network.

Testing

• Swift Bring-Up Guide: See docs/SWIFT_BRINGUP.md for codec negotiation, scheduler architecture, clock sync details, and the 5-minute PCM stream test procedure.
• Manual Testing: See docs/TESTING.md for manual testing procedures and validation checklist.

TODO

• Verify hi-res audio (192kHz/24-bit) end-to-end with real hardware
• Test compatibility with other Sendspin Protocol server implementations
• Update implementation as Sendspin Protocol spec solidifies
• Comprehensive audit and bug fixes

See GitHub Issues for detailed tracking and discussion.

License

Apache 2.0