mirror of
https://github.com/aaru-dps/libaaruformat.git
synced 2025-12-16 19:24:40 +00:00
20 lines
1.3 KiB
Plaintext
20 lines
1.3 KiB
Plaintext
[appendix]
|
||
== Claunia Subchannel Transform
|
||
|
||
The subchannel structure in CompactDisc media—and compatible formats—consists of eight interleaved components: `P`, `Q`, `R`, `S`, `T`, `U`, `V`, `W`.
|
||
|
||
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.
|
||
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%).
|
||
|
||
To address this, the **Claunia Subchannel Transform** is applied:
|
||
|
||
- All bits are **de-interleaved** so that each subchannel (`P` through `W`) is formed into distinct byte streams.
|
||
- All `P` bytes from all sectors are written sequentially, followed by all `Q` bytes, then `R`, and so on up to `W`.
|
||
|
||
While this transform temporarily increases memory usage (approximately 32MiB additional), the benefits are substantial:
|
||
|
||
- Compression speed improves up to **10× faster**
|
||
- Compression gains reach approximately **96%**, particularly on media lacking `R`–`W` subchannel data—as is the case with ~99% of discs.
|
||
|
||
NOTE: For implementation specifics or updates to this method, refer to the authoritative `libaaruformat` source.
|