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diff --git a/libparanoia/README.paranoia b/libparanoia/README.paranoia new file mode 100644 index 0000000..4864c32 --- /dev/null +++ b/libparanoia/README.paranoia @@ -0,0 +1,83 @@ +# @(#)README.paranoia 1.1 97/04/04 J. Schilling from Monty (xiphmont@mit.edu) +README.paranoia + +Paranoia II: +(c) Monty +xiphmont@mit.edu + +Hi there. + +All CDROM drives are not created equal. You're probably using this +patch because yours is a little less equal than others-- or maybe you +just keep your CD collection in a box of full of gravel. Jewel cases +are for wimps; you know what I'm talking about. + +This patch adds extra-robust interframe syncronization, code to detect +scratches (and hold sync across the scratch) and finally routines to +filter out scratches as best possible. These are all handled +automatically by fairly modular code. + +1) extra interframe syncronization ------------------------------------- + +Some CD drives can read audio data from an exact starting point to an +exact ending point flawlessly; these are rare. A larger number of +drives read from only an approximately correct starting point, but do +manage to get all the data in-tact from wherever they manage to begin +the read. Stock cdda2wav is coded with this in mind. + +More drives, especially IDE-like and recent ATAPI drives, suffer from +framing misalignments within atomic multi-sector read operations. +Cdda2wav is also set, by default, to read 75 sectors at a time from +ATAPI drives, which Linux's IDE driver breaks into multiple 8 sector +reads to conserve kernel memory. Both of these things will break +cdda2wav; the symptoms include cracks or pops within the read sample. + +The "Paranoia" patch will verify the alignment of *every* byte read by +cdda2wav. The goal is data integrity, not performance: a minimum 50% +speed hit will result from the patch, likely more if your CDROM +suffers from the framing bugs the patch corrects. + +2) scratch detection and tolerance ------------------------------------ + +Because overlap syncronization requires matching exact, perfect sample +sections, scratched CDs typically cause normal cdda2wav to bail when +overlap syncronization is turned on. The second part of the +"Paranoia" patch ignores scratches in the read bitstream and +syncronizes using the data that can still be "trusted". This step +also collects first-pass scratch detection information used by the +third section of the patch. + +Expect performance to drop through the floor on a badly scratched CD; +the code can no longer use a simple case of exact matching for speedy +correlation. Maintaining solid sync across a scratch is processor +bound; later I'll add speedier algorithms for correlation than the +brute force method currently used. Scratch detection imposes little +additional overhead on a non-scratched CD. + +3) scratch repair ----------------------------------------------------- + +Scratches are an irrevocable loss of data. Still, it's usually +possible to reconstruct a sample closer to the original data than the +raw hissing, fluttering and crackling that severe scratches in the CD +surface produce. + +Scratch filtering is the 'hard part' of this patch. It needs to do +two things; first, find the 'pops' that escaped detection in section 2 +of the patch and secondly fill in the gaps. Both are acheived using +delta (slew) averaging and forward and backward linear predictive +interpolation (with IIR filters) to fill in known gaps as well as look +for 'problem values' in stretches of sample known to be scratched. + +THE SCRATCH DETECTION IS NOT PERFECT. Nor is the repair perfect, +although it is easier than detecting scratches reliably. Both are +cases of coming arbitrarily close to perfection; Paranoia is designed +with light to moderate damage in mind, but will still recover a +listenable sample from heavy damage. + +Paranoia still has quite a bit of room for improvement in the scratch +repair code... If cdda2wav + Paranoia is not doing an acceptable job +on CDs that you just gotta have, drop me a line; I didn't want to sink +months into a project I wasn't certain anyone would ever use :-) + +Monty + |