![accurip database accurip database](http://audiotools.sourceforge.net/screenshots/trackverify.png)
If your DAE software is configured to trim silence or to do anything with gaps other than put them at the end of the preceding track (as normally happens when gaps aren't taken into account), then it's unlikely the checksums of your rips will be submitted to or compared against those in the AccurateRip database. This means that any silence or "gap" at the beginning or end of the track, except before the first track, must be included in the rip. Technical details on checksum calculation can be found here: AccurateRip-CRC-CalculationĪ Linux command-line tool to compute AccurateRip checksums can be found here: Įach AccurateRip checksum is based on a complete track rip, from the beginning of the track to the end, as determined by the track's entry in the disc's table of contents. DAE software can use this info to decide whether to try re-reading a track that produced a different checksum than was expected. The AccurateRip database contains over 2 million unique discs. See the secure ripping article for more on this subject. 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). 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. If the data is the slightest bit different, the checksums will usually be very different. Identical data will produce identical checksums.
![accurip database accurip database](https://tom2tec.files.wordpress.com/2016/03/ripping-to-flac.png)
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. However, since all submissions are calibrated to the same reference, and the apparent error so small (~680 µs), the reliability of AccurateRip data is not compromised in any way whatsoever. Wiethoff feels that it's too late to change to a different reference now that the database is populated. The offsets are 30 samples too low, or the correction values are 30 samples too high, however you want to look at it. The reference measurements Andre Wiethoff made and which were adopted by Spoon for the AccurateRip database were challenged in late 2006 to support a claim that the reference is actually off by 30 samples. The DAE software can correct for this by padding the track with digital silence so it's the correct length.ĭetermining the actual read offset of a drive is difficult. If the drive can't overread, then there will be samples missing from the extracted track. 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. 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. 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. There are 2352 bytes, or 588 samples, in each sector of an audio CD, corresponding to 1/75th of a second of sound. One "sample" on an audio CD is 4 bytes, consisting of a 2-byte left-channel value and a 2-byte right-channel value. 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 AccurateRip database allows one to find out the read offset, which is normally constant for given make & model of CD drive. 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. 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). Very few CD drives actually start reading data from audio CDs exactly at the sector requested by DAE software.