The Audio CD-ROM Submission Process
Audio CD-ROM background
Before explaining the Audio CD-ROM submission process for MusicBrainz, some background on Audio CD-ROMs is necessary. Below is a quick, fairly non-technical introduction to Audio CD-ROMs. If you want lots more detail about CD-ROMs and how CD Index Ids are calculated, you should take a look at DiscIDCalculation.
An audio CD-ROM does not contain any metadata, like its title, the artist or the track names. There are some newer variants of audio CD-ROMs with CD-TEXT extensions that do contain metadata, but they are not in wide use. So, you need a system like MusicBrainz to identify a CD-ROM and to provide the metadata for that CD-ROM.
An Audio CD-ROM contains a table of contents (TOC) that indicates where each of the tracks on the CD starts. After the TOC, the audio tracks start, each at a given sector offset, as outlined in the TOC. The CD-ROM is logically broken into sectors, where each sector represents 1/75th of a second of sound. So, 75 sectors on a CD equate to 1 second of sound.
The CD Index ID is calculated from the table of contents, and nothing else. So, if one CD differs from another CD by as little as one single track being longer/shorter by one sector, the CD Index ID will be completely different. This ensures that each type of CD has a unique identifier, which is important for looking up metadata for CDs.
However, there is a downside to this as well. Audio CD-ROMs are usually produced in large batches, that may include thousands if not millions of copies of one CD. Once a record company is done selling that one batch, or a record company in another country wants to print up a batch of the same CD, they will make another batch. There are over 10 different pressings for the album 'The Dark Side of the Moon' by Pink Floyd, for instance.
If the CDs from the new batch are off by one sector (as mentioned above) they will have a different unique CD Index ID. Let's assume that a CD from a new batch has one track that is shorter by one sector (as compared to the previous batch). This yields a different CD Index ID, but to the human ear the CD will be identical, since humans cannot tell that one track is shorter by 1/75th of a second.
Inserting Audio CD-ROMs into MusicBrainz
Now to explain the MusicBrainz CD insertion process. Let's say you use one of the CD Submission tools to submit a CD and you find that MusicBrainz doesn't know about the CD. You then proceed to select the artist and then MusicBrainz shows you a list of CDs and your CD is right there is in the list. What's wrong?
There may be a few things going on:
- MusicBrainz may not have any CD Index IDs for this album.
- MusicBrainz may have a different CD Index ID than you have, thus MusicBrainz already knows about a different pressing of the CD.
In either case, you should select the matching CD and let MusicBrainz create another association for you. This means that MusicBrainz will now know about the new CD Index ID that you just reported. The next time you want to look up this CD, MusicBrainz will gladly deliver the data to you.
However, that's not all. Let's say that the CD you're looking for is not found. In that case, you should select 'CD not found' and then proceed to enter the data for that CD, so it can be included in the MusicBrainz database. And, there may be yet another case, where MusicBrainz knows about your CD, but it has a different number of tracks than your CD.
This happens when two different pressings of a CD have a different number of tracks. Let's say that a band in the UK releases a CD that contains 10 tracks and one bonus track for a total of 11 tracks. Once the CD is released in the US, the US record company may decide for some reason to yank the bonus track and issue the CD under the exact same name as the CD released in the UK. As far as MusicBrainz is concerned, they are two completely different CDs, and MusicBrainz should have a record for both of these CDs. So, if you run into this problem, please proceed with entering the data for your CD, even though MusicBrainz already has one version of it.
If you still have questions, please join the MailingList and post your questions there.