Crystal is a scalable lossless audio codec designed for real-time wireless audio over Bluetooth. It delivers bit-perfect lossless audio when the channel permits, and degrades gracefully to perceptually transparent lossy audio under interference — never to silence, glitches, or dropouts.
| Property | Value |
|---|---|
| Architecture | Two-layer: lossy base + lossless enhancement residual |
| Lossless mode | Bit-perfect at 0–2% packet loss |
| Lossy mode | Perceptually transparent at up to 20% packet loss |
| Latency | ≤ 20 ms algorithmic (encode + decode) |
| Decode complexity | ≤ 40 MIPS on ARM Cortex-M4 |
| Compression | 1.5:1 – 2.5:1 (lossless); base layer ~320 kbps |
| Patent status | All techniques public domain or prior art |
| License | GPLv3+ / CC BY-SA 4.0 |
| Topic | File |
|---|---|
| Design goals and patent avoidance | spec/01-introduction.md |
| Two-layer architecture diagram | spec/02-architecture.md |
| MDCT, psychoacoustic model, Huffman coding | spec/03-base-layer.md |
| Lossless residual, LPC, Rice coding | spec/04-enhancement-layer.md |
| Reed-Solomon FEC, interleaving, adaptive bitrate | spec/05-wireless-resilience.md |
| Bitstream format (superframe, transport packet) | spec/06-stream-format.md |
| Encoder profiles (wireless / low-latency / archival) | spec/07-encoder-profiles.md |
| Decoder state machine | spec/08-decoder.md |
| Street-condition performance vs aptX Lossless | spec/09-scenarios.md |
| Conformance requirements | spec/14-conformance.md |
| Theorem | Property | Status |
|---|---|---|
| 1 | Lossless reconstruction (both layers present) | Proven |
| 2 | Mid-Side stereo invertibility | Proven |
| 3 | LPC residual bounded bit width | Proven |
| 4 | Rice coding bijectivity | Proven |
| 5 | RS erasure recovery capacity | Proven (classical) |
| 6 | Convolutional interleaving burst protection | Proven |
| 7 | UEP probability bounds (exact binomial) | Proven |
| 8 | Crystal > aptX Lossless under independent loss | Proven (all p ≤ 0.25) |
| 9 | Crystal > aptX Lossless under burst loss | Proven (Gilbert-Elliott) |
| 10 | Adaptive FEC allocation optimality | Proven (convex optimization) |
| 11 | Concealment continuity and energy decay | Proven |
Full proofs: formal/
The Crystal specification is released under the Creative Commons Attribution-ShareAlike 4.0 International License (CC BY-SA 4.0).
The reference implementation is GNU GPLv3+. All contributors waive patent claims under GPLv3 Section 11. No patents. No royalties. No CLA required.