Compact Audio Disc track

The Format of file HALF-VALUE LAYER is a format Microsoft, in conformity with the specification RIFF.

The files HALF-VALUE LAYER (Compact Audio Disc) are generated by Windows when this operating system reaches a Audio CD. Each title of Audio CD is then seen like a file of 44 bytes, and extension " .cda". The files are named " Track01.cda" , " Track02.cda" , etc

The files HALF-VALUE LAYER do not exist as such on Audio CD. They are a kind of short cut created by Windows, pointing towards the zone of CD containing the sound samples.

Organization of a file HALF-VALUE LAYER

The size of a file HALF-VALUE LAYER being fixed, like its organization, it there always and single chunk, did not name one " CDDA" (meaning Compact Disc for DIGITAL Audio).

The identifier created by Windows is used by the reader of CD of Windows 95 and Windows 98 (cdplayer.exe). This reader cannot connect himself to FreeDB or CDDB. So that it can post the name of the artist and the title of the pieces, it is necessary to enter manually this information the file cdplayer.ini (in the repertory of installation of Windows), in a section named according to this identifier. This identifier does not have any relationship with the DiscId used by FreeDB or CDDB, it is a creation purely Microsoft, for the above mentioned use.

The position and the length of the beaches use frames like unit. There are 75 frames by second. It is the smallest storage block which can be read since a CD audio, corresponding to a sector of CD.

All information which requires several bytes is coded with order- thebyte Intel (small boutist).

Example of analysis of a file HALF-VALUE LAYER, in Perl

The demonstration program provided is a script Perl. It is necessary to launch script with a parameter: the way towards the repertory or the disc containing the files HALF-VALUE LAYER.

#! /bin/perl - W use strict; my $path_cda; # way towards the HALF-VALUE LAYERS (parameter of script) my $data_cda; # contained of a file HALF-VALUE LAYER my @liste_cda; # the list of the files HALF-VALUE LAYER of the repertory my $fic_cda; # the name of the current file HALF-VALUE LAYER my $mins; my $secs; my $frames; # to test that an argument was provided $path_cda = shift; die (" argument: the name of the répertoire") yew (! defined ($path_cda)); # to open, read, then to close again the file HALF-VALUE LAYER to opendir (DIR_CDA, $path_cda) gold die (" impossible to open the repertory $path_cda"); @liste_cda = grep/.*cda$/i, to readdir (DIR_CDA); to closedir (DIR_CDA); print " beach beginning lasted beginning lasted \ n" ; print " frames frames min: dryness: frm min: dryness: frm \ n" ; foreach $fic_cda (@liste_cda) { # to charge the contents with the file HALF-VALUE LAYER in a variable open (HALF-VALUE LAYER, " <: raw" , $path_cda.$fic_cda) gold die (" impossible to open the file $fic_cda"); $data_cda = ; closed (HALF-VALUE LAYER); # to check the size of the file, and the header die " cut invalide" yew (length ($data_cda)! = 44); die " heading invalide" yew (substr ($data_cda, 0,0x0f) " RIFF \ $ \ x00 \ x00 \ x00CDDAfmt"); # to post the number of the plage printf sprintf (" %02d " , ord (substr ($data_cda, 0x16, 1))); # to post information on the beach: position in frames $frames = unpack (" V" , substr ($data_cda, 0x1c, 4)); printf sprintf (" %06d " , $frames); # to post information on the beach: duration in frames $frames = unpack (" V" , substr ($data_cda, 0x20, 4)); printf sprintf (" %06d " , $frames); # to post information on the beach: position min: dryness: frames ($frames, $secs, $mins) = unpack (" CCC" , substr ($data_cda, 0x24, 3)); printf sprintf (" %02d: %02d: %02d = %06d frm " , $mins, $secs, $frames, ((($mins * 60) + $secs) * 75) + $frames); # to post information on the beach: min duration: dryness: frames ($frames, $secs, $mins) = unpack (" CCC" , substr ($data_cda, 0x28, 3)); printf sprintf (" %02d: %02d: %02d = %06d frm" , $mins, $secs, $frames, ((($mins * 60) + $secs) * 75) + $frames); print " \ n" ; }

In the columns posting the position and the duration with the format minutes: seconds: frames, script adds the number of frames equivalent to this duration. The position in the two systems should be identical. In practice one however observes a difference of 150 frames, corresponding to the 2 seconds of pre-gap (see the Red Book ).

Results of script

Script above was launched on the album " Nap Great Reward " group Depeche Mode. The results are recopied below, a column " pochette" duration; containing the duration indicated on the small pocket was added.

beach beginning lasted beginning lasted lasted small pocket frames frames min: dryness: frm min: dryness: frm 01 000032 017015 00:02: 32 = 000182 frm 03:46: 65 = 017015 frm 3:46 02 017047 022780 03:49: 22 = 017197 frm 05:03: 55 = 022780 frm 5:03 03 039827 017398 08:53: 02 = 039977 frm 03:51: 73 = 017398 frm 3:51 04 057225 021387 12:45: 00 = 057375 frm 04:45: 12 = 021387 frm 4:45 05 078612 014570 17:30: 12 = 078762 frm 03:14: 20 = 014570 frm 3:14 06 093182 020045 20:44: 32 = 093332 frm 04:27: 20 = 020045 frm 4:27 07 113227 018970 25:11: 52 = 113377 frm 04:12: 70 = 018970 frm 4:12 08 132197 021068 29:24: 47 = 132347 frm 04:40: 68 = 021068 frm 4:40 09 153265 028952 34:05: 40 = 153415 frm 06:26: 02 = 028952 frm 6:26

One finds the difference in 2 seconds between the two fields " position".
The duration posted by a turntable CD or the software of reading can be:

duration indicated in the file HALF-VALUE LAYER, the number of frames having been ignored
duration indicated in the file HALF-VALUE LAYER, with rounded at the second nearest.

Random links: