[Libcdio-devel] Re: proposed documentation for pregap interface

[Robert William Fuller]

After considerable feedback, I revised the proposal.  Let me know what 
you think!

6.3 CD-DA discs and gaps

Gaps are possibly one of the least understood topics in audio discs.  In 
the case of CD-DA discs, standards require a silent 2 second gap before 
the first audio track and after the last audio track (in each session.) 
 These are respectively referred to as lead-in and lead-out gaps.  No 
other gaps are required.  It is important not to confuse the required 
lead-in and lead-out gaps with the optional track pre-gaps.  Track 
pre-gaps are the gaps that may occur between audio tracks.  Typically, 
track pre-gaps are filled with silence so that the listener knows that 
one song has ended, and the next will soon begin.  However, track 
pre-gaps do not have to contain silence.  One exception is an audio disc 
of a live performance.  Because the performer may seamlessly move from 
one piece of the performance to the next, it would be unnatural for the 
disc to contain silence between the two pieces.  Instead, the track 
number updates with no interruption in the performance.  This allows the 
listener to either hear the entire performance without unnatural 
interruptions, or to conveniently skip to certain pieces of the 
performance.  Finally, some CD-DA discs--whose behavior will be 
described below--lack track pre-gaps altogether although they must still 
include the lead-in and lead-out gaps.

In order to understand the track pre-gaps that occur between audio 
tracks, it is necessary to understand how CD players display the track 
number and time.  Embedded in each block of audio data is non-audio 
information known as the Q sub-channel.  The Q sub-channel data tells 
the CD player what track number and time it should display while it is 
playing the block of audio data in which the Q sub-channel data is 
embedded.  Near the end of some tracks, the Q sub-channel may instruct 
the CD player to update the track number to the next track, and display 
a count down to the next track, often starting at -2 seconds and 
proceeding to zero.  This is known as an audio track pre-gap.  It may 
either contain silence, or as previously discussed--in the case of live 
performances--it may contain audio.  Almost as often as not, there is no 
pre-gap whatsoever.  Regardless, an audio track pre-gap is purely 
determined by the contents of the Q sub-channel, which is embedded in 
each audio sector.  This has some interesting implications for the track 
forward button.

When the track forward button is pressed on a CD player, the CD player 
advances to the next track, skipping that track's pre-gap.  This is 
because the CD player uses the starting address of the track from the 
disc's table of contents (TOC) to determine where to start playing a 
track when either the track forward or track backward buttons are 
pressed.  So to hear a pre-gap for track 4, the listener must either 
listen to track 3 first, or use the track forward or reverse buttons to 
go to track 4, then use the seek backward button to back up into track 
4's pre-gap, which is really part of track 3, at least according to the 
TOC.  Track 1 pre-gaps are especially interesting because some 
commercial discs have audio hidden before the beginning of the first 
track!  The only way to hear this hidden audio with a standard player is 
to use the seek backward button as soon as track 1 begins playing!

Audio track pre-gaps may be specified in a couple of different ways in 
the popular cue file format.  The first way of specifying a pre-gap is 
to use the "PREGAP" command.  This will place a pre-gap containing 
silence before a track.  The second way of specifying a pre-gap is to 
give a track an "INDEX 00" as well as the more normal "INDEX 01". 
"INDEX 01" will be used to specify the start of the track in the disc's 
TOC, while "INDEX 00" will be used to specify the start of the track's 
pre-gap as recorded in the Q sub-channel.  "INDEX 00" is ordinarily used 
for specifying track pre-gaps that contain audio rather than silence. 
Thus, the cue file format may be used to specify track pre-gaps with 
silence or audio, depending on whether the "PREGAP" or "INDEX 00" 
commands are specified.  If neither type of pre-gap is specified for a 
track, no pre-gap is created for that track, which merely means the 
absence of pre-gap information in the Q sub-channel, and the lack of a 
short count down to the next track.

Various CD-DA ripping programs take various approaches to track 
pre-gaps.  Some ripping programs ignore track pre-gaps altogether, 
relying solely on the disc's TOC to determine where tracks begin and 
end.  If a disc is ripped with such a program, then re-burned later, the 
resulting disc will lack track pre-gaps, and thereby lack the playback 
behavior of counting down to the next track.  Other ripping programs 
detect track pre-gaps and record them in the popular cue file format 
among others.  Such ripping programs sometimes allow the user to 
determine whether track pre-gaps will be appended to the prior track or 
pre-pended to the track to which they "belong".  Note that if a ripping 
program is ignorant of track pre-gaps, the track pre-gaps will be 
appended to the prior track, because that is where the disc's TOC puts 
them.  Thus, there are many different ways an application may chose to 
deal with track pre-gaps.  Consequently, libcdio does not dictate the 
policy a ripping program should use in dealing with track pre-gaps. 
Hence, libcdio provides the cdio_get_track_pregap_[lba|lsn] interfaces 
to allow the application to deal with track pre-gaps as it sees fit.

Note that the cdio_get_track_pregap_[lba|lsn] interfaces currently only 
provide information for bin/cue and NRG images.  (CDRDAO support will be 
added either when needed by the author or requested by someone offering 
the author a free beer.)  Getting the track pre-gaps from a CD drive is 
a more complicated problem because not all CD drives support reading the 
Q sub-channel DIRECTLY at HIGH speed, and there is no interface to 
determine whether or not a drive supports this optional feature, aside 
from trying to read the Q sub-channel, and possibly incurring IO errors. 
 However, all drives DO support reading the Q sub-channel INDIRECTLY 
while playing an audio disc by asking the drive for the current 
position.  Unfortunately, this occurs at normal playback speed, and 
requires a certain settling time after the disc starts playing.  Thus, 
using this SLOW interface requires a more sophisticated algorithm, such 
as binary search or some heuristic, like backing up progressively from 
the end of the prior track to look for the next track's pre-gap.  Note 
that CD drives seek slowly, so it is better to simply use a drive that 
can read the Q sub-channel directly at HIGH speed, and avoid complicated 
software solutions.  (Not to mention that if the user has an older 
system with an analog audio cable hooked up between their soundboard and 
their drive, and a ripping program uses the SLOW interface, the user 
will hear bits of the audio on the disc!)  Consequently, because there 
is no good universal solution to the problem of reading the Q 
sub-channel from a drive, libcdio currently leaves this problem up to 
the application, a problem which is readily approachable through either 
libcdio's MMC interface or libcdio's cdda interface.  For an example of 
one such application, see https://gna.org/projects/cued/.