How does the MusicBrainz calculate CD Index Disc Ids?

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center; margin: 1em 0"> <form action="https://web.archive.org/web/20030813231654/https://www.paypal.com/cgi-bin/webscr" method="post"> <input type="hidden" name="cmd" value="_xclick"> <input type="hidden" name="business" value="donations@musicbrainz.org"> <input type="hidden" name="item_name" value="Donation to MusicBrainz"> <input type="image" src="/web/20030813231654im_/http://musicbrainz.org/images/paypal.gif" style="border: 0" name="submit" alt="Donate to MusicBrainz!"> </form> </div> <div class="SideBarMoreLink"> <span class="sidemenusmall"> <a href="/web/20030813231654/http://musicbrainz.org/sponsors.html">more >></a> </span> </div> </div> </div> </td> <td bgcolor="FFFFFF" align="left" valign="top"> <table cellpadding="15" cellspacing="0" border="0" width="100%"> <tr> <td align="left" valign="top" class="body" width="100%"> <a id="maincontent"></a> <h1>How does the MusicBrainz calculate CD Index Disc Ids?</h1> <p> Let us first have some words about how data is organized on a CD, going top down. </p> <img src="/web/20030813231654im_/http://musicbrainz.org/images/cdhand.gif" align="left" border="0" alt="a compact disc"> <p> A digital audio compact disc (CD-DA) can hold up to 99 audio tracks. </p> <p> Sampling is done at a rate of 44.1 kHz using 16 bits resolution per channel, thus there are 44100 x 2 bytes x 2 channels (Stereo) = 176400 bytes of PCM data stored per second. </p> <p> This audio data is contained in logical blocks of 2352 bytes each on the CD, holding 2352 / 176400 = 1 / 75 seconds of sound. </p> <p> A logical block plus 882 bytes of error correction and control data forms a raw block of 3234 bytes that is spread among 98 frames of 33 bytes each that are all together written on one spiral track among the CD. </p> <p> At present we can stop at the level of logical blocks for our task. The deeper stuff will be needed in case the CD-Text format becomes popular, which stores extra data (like artist and track information) in the above mentioned control data parts. </p> <p> Now let us have a look at a real world CD-DA. Using a tool like <a href="https://web.archive.org/web/20030813231654/ftp://ftp.freebsd.org/pub/FreeBSD/FreeBSD-current/src/usr.sbin/cdcontrol/cdcontrol.c"> <tt>cdcontrol</tt></a> (FreeBSD) on <a href="/web/20030813231654/http://musicbrainz.org/showalbum.html?discid=MUtMmKN402WPj3_VFsgUelxpc8U-&tracks=15&toc=1+15+325882+150+18791+34817+56500+77156+106244+125879+149935+169035+186060+205979+230292+246809+265764+289629">this CD</a> will yield: </p> <center> <table width="95%" cellspacing="0" cellpadding="10" border="0" bgcolor="#efefff" summary="a closer look at a digital audio CD"> <tr> <td> <pre> marc@oranje$ cdcontrol -f /dev/cd0c info Starting track = 1, ending track = 15, TOC size = 4 bytes track start duration block length type ------------------------------------------------- 1 0:02.00 4:10.41 0 18641 audio 2 4:10.41 3:35.51 18641 16026 audio 3 7:44.17 4:51.08 34667 21683 audio 4 12:33.25 4:37.31 56350 20656 audio 5 17:08.56 6:29.63 77006 29088 audio 6 23:36.44 4:23.60 106094 19635 audio 7 27:58.29 5:22.56 125729 24056 audio 8 33:19.10 4:16.50 149785 19100 audio 9 37:33.60 3:49.00 168885 17025 audio 10 41:20.60 4:27.44 185910 19919 audio 11 45:46.29 5:26.13 205829 24313 audio 12 51:10.42 3:42.17 230142 16517 audio 13 54:50.59 4:14.55 246659 18955 audio 14 59:03.39 5:20.15 265614 23865 audio 15 64:21.54 8:05.28 289479 36253 data 170 72:25.07 - 325732 - - </pre> </td> </tr> </table> </center> <p> This is an example of a CD with an extra track of data (what you know as CD-ROM), marketed in this case as CD-Extra, featuring a video and some pictures. (More precise: this is disc with two sessions, audio and data) </p> <p> You should note the special track 170 (<tt>0xaa</tt>), that marks the beginning of the leadout track, consisting of the remaining blocks on the CD. </p> <p> The CD Index algorithm simply takes the following pieces of data and runs them through the <a href="https://web.archive.org/web/20030813231654/http://www.w3.org/TR/1998/REC-DSig-label/SHA1-1_0">SHA-1</a> hash function: </p> <ul> <li> First track (normally one): 1 byte <li> Last track: 1 byte <li> Leadout track: 4 bytes <li> 99 frame offsets: 4 bytes for each track. If there are less than 99 frame offsets (almost certainly), the value 0 will be used instead. </ul> <p> Before the data is fed through the SHA-1 hash, it is converted to ASCII using </p> <center> <table width="95%" cellspacing="0" cellpadding="10" border="0" bgcolor="#efefff" summary="1 byte hex output"> <tr> <td> <tt>printf("%02X", value);</tt> </td> </tr> </table> </center> <p> for byte values and </p> <center> <table width="95%" cellspacing="0" cellpadding="10" border="0" bgcolor="#efefff" summary="4 byte hex output"> <tr> <td> <tt>printf("%08X", value);</tt> </td> </tr> </table> </center> <p> for 32 bit integer values. </p> <p> Code is a better definition than English, so here is the code that calculates the disc Id: </p> <center> <table width="95%" cellspacing="0" cellpadding="10" border="0" bgcolor="#efefff" summary="CD Index Id algorithm"> <tr> <td> <pre> sprintf(temp, "%02X", pCDInfo->FirstTrack); sha_update(&sha, (unsigned char*) temp, strlen(temp)); sprintf(temp, "%02X", pCDInfo->LastTrack); sha_update(&sha, (unsigned char*) temp, strlen(temp)); for (i = 0; i<100; i++) { sprintf(temp, "%08X", pCDInfo->FrameOffset[i]); sha_update(&sha, (unsigned char*) temp, strlen(temp)); } sha_final(digest, &sha); </pre> </td> </tr> </table> </center> <p> Note that the leadout track is stored in <tt>pCDInfo->FrameOffset[0]</tt>. </p> <p> The resulting 20 byte SHA-1 signature is converted to a base 64 encoded character ASCII string that is the disc Id. </p> <p> One uses base 64 encoding to map arbitrary bytes onto a string of printable ASCII characters. It does this by redistributing the 24 bits of three 8-bit codes each into four 6-bit codes from a table of 64 very common ASCII characters. </p> <p> Thus we end up with an Id string of 28 characters, in the above example with "MUtMmKN402WPj3_VFsgUelxpc8U-". </p> <p> Note: This base 64 string is not the same one specified in RFC822. The RFC822 spec uses <tt>+</tt>, <tt>/</tt>, and <tt>=</tt> characters, all of which are special HTTP/URL characters. To avoid the problems with dealing with that, I (Rob) used <tt>.</tt>, <tt>_</tt>, and <tt>-</tt>. For details on this, please refer to <tt>base64.c</tt> in the source code. </p> <p> This scheme has the advantage of being very simple (simple to understand, simple to implement) and it is not ambiguous. </p> <p> However, two CDs pressings may not have the same Ids. To handle this case, the CD Index system will let a user check to see if the CD already exists in the system under a different Id. If so, the system creates a new association for the different pressing of the same CD. </p> <p> If you'd like to know more about this, please <a href="download.html">download</a> the client source code and check it out. The code is clean and self documenting. If you are interested in creating other CD Index clients and need the SHA-1 source code, you can either dig through the CD Index source, or check the <a href="https://web.archive.org/web/20030813231654/http://www.w3.org/tr/1998/rec-dsig-label/sha1-1_0">W3C page</a> on SHA-1. </p> <p> Note: The use of the MD5 algorithm has been discontinued in favour of public domain SHA-1. </p> <p> <font size="+1">Links</font> </p> <ul> <li> The How Stuff Works site has a very readable <a href="https://web.archive.org/web/20030813231654/http://howstuffworks.com/cd.htm">introduction</a> on CD technology intended for the technically curious. Written by Marshall Brain. <li> A superb coverage of CDs and formats on a scientific level is given by Prof Kelin J Kuhn from University of Washington. Get the basics lecture <a href="https://web.archive.org/web/20030813231654/http://www.ee.washington.edu/conselec/CE/kuhn/cdaudio/95x6.htm">here</a> and the format/encoding stuff lecture <a href="https://web.archive.org/web/20030813231654/http://www.ee.washington.edu/conselec/CE/kuhn/cdaudio2/95x7.htm">here</a>. <li> There is actually a very fine <a href="https://web.archive.org/web/20030813231654/http://www.fadden.com/cdrfaq">FAQ about toasting</a> available that has lots of interesting facts. Written by Andy McFadden. <li> Find out more about how CD-DA and CD-ROM formats have been enhanced through <a href="https://web.archive.org/web/20030813231654/http://www.discmfg.com/PDF/enhanced.pdf">this</a> PDF document provided by Cinram. <li> Another introduction from Kelin Kuhn can be found <a href="https://web.archive.org/web/20030813231654/http://www.ee.washington.edu/class/ConsElec/cd.html">here</a>, again at University of Washington Electrical Engineering.  </ul> <div style="margin-top: 1em"> </div> </td> </tr> </table> </td> </tr> </table> <div id="FooterBar"> <table width="100%" cellpadding="2" cellspacing="0" border="0"> <tr> <td width="10%">   </td> <td width="15%"> <a href="https://web.archive.org/web/20030813231654/http://www.creativecommons.org/">Creative Commons</a> </td> <td width="15%"> <a href="https://web.archive.org/web/20030813231654/http://www.eff.org/"><abbr title="Electronic Frontier Foundation">EFF</abbr></a> </td> <td width="15%"> <a href="https://web.archive.org/web/20030813231654/http://www.gnu.org/copyleft/gpl.html"><abbr title="General Public License">GPL</abbr></a> </td> <td width="15%"> <a href="https://web.archive.org/web/20030813231654/http://www.gnu.org/copyleft/lesser.html"><abbr title="Lesser General Public License">LGPL</abbr></a> </td> <td width="15%"> <a href="https://web.archive.org/web/20030813231654/http://www.freshmeat.net/">Freshmeat</a> </td> <td width="10%">   </td> </tr> </table> </div> <div id="CreditsBar"> <table width="100%" cellpadding="2" cellspacing="0" border="0"> <tr> <td width="30%" style="text-align: left"> <a href="/web/20030813231654/http://musicbrainz.org/support/contact.html">contact details</a> </td> <td width="40%" style="text-align: center"> original design|<a href="https://web.archive.org/web/20030813231654/http://vacubomb.com/">vacubomb.com</a> </td> <td width="30%" style="text-align: right"> server version: 2003-07-25 </td> </tr> </table> </div> </body> </html>

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How does the MusicBrainz calculate CD Index Disc Ids?