AccurateRip is a online ripping accuracy database, using submissions from people all around the world, a bit-perfect verification of a track rip can be assured with a positive match. AccurateRip also allows the offsets of drives to be determined.
The database is maintained by Illustrate Ltd., the company run by "Spoon", primary developer of dBpoweramp. All of the data is submitted by the Windows-based ripping programs dBpoweramp and Exact Audio Copy via a DLL licensed from Illustrate. Other rippers and tools, such as CUETools and Mac OS-based rippers, can receive info from the database in order to look up drive offsets or check rip quality, but they cannot submit their own rip results to it.
Drive read offsets
Very few CD drives actually start reading data from audio CDs exactly at the sector requested by DAE software. There are drives that are off by over 1 sector (1/75th of a second), but most are off by much less (1/250 to 1/350 second). Most modern CD drives have "Accurate Stream" technology, so there's no "jitter", meaning in this case that the variance is consistent from read to read, and will tend to be the same for all drives of a certain make & model.
The AccurateRip database allows one to find out the read offset, which is normally constant for 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. One "sample" on an audio CD is 4 bytes, consisting of a 2-byte left-channel value and a 2-byte right-channel value. There are 2352 bytes, or 588 samples, in each sector of an audio CD, corresponding to 1/75th of a second of sound. Therefore, an AccurateRip offset of +134 means the drive consistently delivers data from 536 bytes behind (earlier than) where it was asked to read from, so the DAE software needs to look that far ahead (hence the positive offset) in order to get the right data.
When offsets are taken into account, the DAE software might have to ask the drive to "overread" into the lead-in or lead-out portions of the disc, where there's no audio data. Some drives can't be asked to do it, some drives will try to do it and fail, and some will just return null samples (a stream of "0" bytes, a.k.a. digital silence). If the drive can't overread, then there will be samples missing from the extracted track. The DAE software can correct for this by padding the track with digital silence so it's the correct length.
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 over 2 million unique discs. DAE software can use this info to decide whether to try re-reading a track that produced a different checksum than was expected.
Technical details on CRC calculation can be found: AccurateRip-CRC-Calculation
The checksum algorithm ignores the first 2940 samples (5 frames, a little over 0.013s of audio) at the beginning of the first track, and ignores an equal amount at the end of the last track. The largest drive offset in the database, as of October 2009, is only 1776 samples, so when an offset-corrected drive "overreads" beyond the boundaries of the audio data, it normally won't affect the checksum. If the overread were to exceed 2940 samples, though, the algorithm assumes the data is padded with digital silence (nothing but zeroes). If your drive isn't capable of overreading and your DAE software doesn't zero-pad the rip to simulate an overread, then it's unlikely the checksums of your rips will be submitted to or compared against those in the AccurateRip database.
AccurateRip v2 was introduced to lesson the impact of pressings. Pressings are where a batch of discs are created by the pressing plant, then at a later date a 2nd batch of CDs are created with a in-built audio offset, apart from this offset discs across pressings are identical. An AccurateRip v2 compatible program is able to verify a different pressing disc not currently in the database using any of the other pressing values held in the database.
Spin Offs from AccurateRip data
It is possible to run statistic analysis on the 100's of millions of submissions to AccurateRip to determine how accurate a given drive is (with the idea that with enough spread of drive submissions, the data collected with be averaged out of damaged CD). The last such calculation can be found: CD-DVD-Drive-Accuracy-List-2011
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.
Software that uses AccurateRip
AccurateRip was introduced as an add-on for dBpowerAMP Music Converter (dMC) in January 2003.
Notes and references
- As per a Jan. 2010 post by Spoon, "Only two programs submit to AR, EAC and dBpoweramp, these two submit through the standard AccurateRip access dll, which ensures that offsets are correct, we are not talking submissions to freedb where one record does not effect the submission of future records if keyed with a wrong offset. You might not like it, but it is the only way to ensure that the quality of the database is not impaired."