The actual structure of a CD track is really quite complicated and this is not an attempt to list and explain all of the intricacies of the elements that a CD track contains. Ken Pohlmann's excellent The Compact Disc Handbook has a completely detailed description of a CD track and I encourage you to try and find this book if you are interested . For my purposes, I'll define CD tracks as consisting of digital data containing two index points which determine where tracks begin and/or end. Although one can have up to 99 indexed locations within a track, the two most often used indexes are Index 00 and Index 01. These two indexes are present on all audio CD tracks.

Index 00 precedes the audio portion of the track and is often referred to as a pregap or pause with a countdown to the lead-out area (Index 01). Index 01 is where the audio portion of the track starts. The general behaviour of CD players has the player detecting indexes via the disc's table of contents in the lead-in area (the innermost band of a disc) and P and Q channel data contained in every frame. A frame is the smallest recognizable section of a disc. All of this data allows CD player manufacturers to perform certain actions such as identifying separations between tracks, resetting a time display to 00:00 or actually playing audio. PC applications also handle CD audio tracks in the same basic way.

Generally, there is a two second gap between songs on a CD. Live CDs and other tracks that segue into one another are examples of where this is not necessarily true. This is mentioned here as the CD standard requires that the very first track (Track 1) have an Index 00 starting a minimum of two seconds before Index 01 (the start of audio on Track 1). Tracks 2 through 99 don't have this requirement and usually have varying Index 00 and Index 01 locations although the two second pregap is widely used. 99 tracks is the maximum number of tracks allowed on a CD.

Manipulation of the space and data between Index 00 and Index 01 comprises the most interesting of CD oddities contained on these pages. Other oddities covered on these pages include multiple tracks within one track and other intentional deviations from the CD standard which may or may not cause certain amounts of havoc on different stand-alone and PC application players.

Manipulating Index 00 usually involves putting audio (data) into the space between Index 00 and Index 01. This results in both semi- and fully hidden tracks within and between normal audio tracks. This works in two distinct ways depending on the Track number in which the data is placed. Accessing the hidden track(s) also depends on this:

Index 00 data on Track 01 (fully hidden)
If the space between Index 00 and Index 01 of Track 01 is used for hiding a track, the "hidden" track can ONLY be accessed by playing the CD at Track 01 and then holding the reverse button down, shuttling back beyond Track 1's Index 01 (at 00:00). Once beyond this point, most stand-alone CD player's time displays will go into increasing negative time numbers (-00:01...-00:02, and so on) until Index 00 is reached. The track between Index 00 and Index 01 will never play unless it is accessed in this manner. It is also possible to overshoot Index 00, when holding the reverse button down, causing most stand-alone players to simply stop. Here are some examples of what your ordinary stand-alone CD player's time display would look like when accessing this type of track:

[PLAY] Starting play at Track 1 shows positive time, like:	TRACK	INDEX	MIN		SEC
	1	1	00	:	01	~	...00:02, 00:03
[REW] Index 01 of Track 1 is at 0:00, like:	TRACK	INDEX	MIN		SEC
	1	1	00	:	00
[REW] Space between Index 01 and Index 00 counts up negative time, like:	TRACK	INDEX	MIN		SEC
	1	0	-00	:	01	~	...-00:02, -00:03
[PLAY] Index 00 of Track 1 in negative time, like:	TRACK	INDEX	MIN		SEC
	1	0	-02	:	12	~	...-02:11, -02:10

Index 00 data on Tracks 2 through 99 (semi hidden)
If the space between Index 00 and Index 01 of any track other than Track 1 is used, then the "hidden" track can be accessed only if the two tracks containing the audio data between Index 00 and Index 01 of the second track are played back-to-back.

For example, if Track 6 of a CD has Index 00 with audio data starting some amount of time before Index 01 then it can be accessed by playing Track 5 (the previous track) and Track 6 back-to-back. As with the Track 1 case, you can see the effects of this on the time display of the stand-alone CD player:

[PLAY] Track 5 will show positive time, like:	TRACK	INDEX	MIN		SEC
	5	1	04	:	32	~	...00:02, 00:03
[PLAY] Index 00 of Track 6 will show negative time, like:	TRACK	INDEX	MIN		SEC
	6	0	-02	:	12	~	...00:02, 00:03
[PLAY] Index 01 of Track 6 will show positive time, like:	TRACK	INDEX	MIN		SEC
	6	1	00	:	14	~	...00:02, 00:03


Note: If one uses "Shuffle mode" on the stand-alone CD player this type of "hidden" track will never play because "shuffle" forces the player to treat each track as a separate entity and start each track at Index 01.

PC Application behaviour
On CD players and rippers for PCs, such as WinAmp and CDex (two popular PC CD programs), these types type of tracks are represented in different ways.

on Track 1
In WinAmp, if Track 1 was listed on the CD liner notes as 4:32 and the space between Index 00 and Index 01 of Track 1 was 1:28, then Track 1 would show a time of 6:00. In CDex, the only noticeable difference on Track 1 is the "Start Time". With the previous example, the start time of Track 1 would be "01:28:00". Discs without this would show a Track 1 start time of "00:00:00".
on Tracks 2 through 99
On Tracks 2 through 99, this is often represented as extra time on the previous track. In WinAmp, if Track 5 was listed on the CD liner notes as 4:32 and Track 6 was listed as 2:47 and the space between Index 00 and Index 01 of Track 6 was 0:30, then Track 5 would show a time of 5:02. Currently, the time display in CDex gets completely confused by this type of track. Once the CDex gets to Index 01 of the track, however, the time display starts to work normally.

Note: Please don't send me any listings about specially die-cut CDs, business -card CDs or other such items UNLESS they have hidden tracks or somehow deviate from the audio CD standard.