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20 lines
1.3 KiB
Plaintext
20 lines
1.3 KiB
Plaintext
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[appendix]
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== Claunia Subchannel Transform
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The subchannel structure in CompactDisc media—and compatible formats—consists of eight interleaved components: `P`, `Q`, `R`, `S`, `T`, `U`, `V`, `W`.
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In their raw form, each byte read from the disc contains a single bit from each of these elements, resulting in a highly interleaved data stream.
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This structure, while efficient for playback, poses challenges for compression algorithms such as LZMA, which struggle with apparent randomness and achieve poor compression ratios (typically less than 2%).
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To address this, the **Claunia Subchannel Transform** is applied:
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- All bits are **de-interleaved** so that each subchannel (`P` through `W`) is formed into distinct byte streams.
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- All `P` bytes from all sectors are written sequentially, followed by all `Q` bytes, then `R`, and so on up to `W`.
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While this transform temporarily increases memory usage (approximately 32MiB additional), the benefits are substantial:
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- Compression speed improves up to **10× faster**
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- Compression gains reach approximately **96%**, particularly on media lacking `R`–`W` subchannel data—as is the case with ~99% of discs.
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NOTE: For implementation specifics or updates to this method, refer to the authoritative `libaaruformat` source.
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