From Hydrogenaudio Knowledgebase
Revision as of 07:14, 6 January 2009 by Mjb (Talk | contribs)

(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to: navigation, search

AccurateRip is a database that accepts and supplies two things: 1. estimates of the accuracy of the digital audio extraction (DAE) capabilities of CD-ROM/CD-RW drives (specifically, their read offsets), and 2. checksums for audio tracks extracted with those drives.

Drive offsets

Very few CD drives actually start reading right at the sector requested by DAE software. They can be off by as much as 4 seconds. Most modern CD drives have "Accurate Stream" technology, which makes the amount of this "jitter" usually be well under 1 second and be the same for all drives of a certain make & model.

The AccurateRip database allows one to find out the read offset, which is a number, for a given make & model of CD drive. This number can then be used by DAE software to ensure that each track is ripped from its exact start to its exact finish.

The offset is given in samples. There are 1176 two-byte samples (588 pairs of 16-bit values for the left and right channels) in each sector of an audio CD, corresponding to 1/75th of a second of sound. An AccurateRip offset of +134, for example, means the drive consistently delivers data from 268 bytes behind where it was asked to read from, so the DAE software needs to look that far ahead in order to get the right data.

Ripped track checksums

Once all the samples for a track have been extracted and put into a file such as a WAV, a checksum can be generated to summarize the sample data. Identical data will produce identical checksums. If the data is the slightest bit different, the checksums will usually be very different. The checksums derived from the same tracks from the same pressings of the same CDs, so long as drive offsets have been accounted for, can be compared in order to determine whether the extraction was error-free. That is, if you rip a track and find that your checksum matches what everyone else got, then you can be confident there are no missing or incorrect samples (or that you've all got exactly the same damage, which is nearly impossible). See the secure ripping article for more on this subject.

The AccurateRip database contains checksum data for the tracks on thousands of CDs. DAE software can use this info to decide whether to try re-reading a track that produced a different checksum than was expected.

Submitting offsets and checksums

The database is designed to be anonymously accessed by DAE software, via HTTP. Submitting data should only be done through such software.