This article is based on MestreLion's great research posted at EAC forum

The goal of this little tutorial is:

To save beginners a lot of time
To explain what a gap is what how the different gap menu options work in EAC
To explain what a CUE Sheet is
Why there is so many options for creating them, and how they relate to Gap settings

Here's what this tutorial should help you achieve:

Extract a CD into several WAV files, so I could rip, encode and tag them with a single click (using Shift + F5)

Burning an exact copy of the original CD without having to read the CD all over again. So, the already extracted wavs should be used fo this job

READING GAPS

Lets imagine an hypotetical CD with this layout:

 Track 1
 - Index 01: 1 minute
 
 Track 2
 - Index 00: 2 secs
 - Index 01: 2 minutes
  
 Track 3
 - Index 00: 3 secs
 - Index 01: 3 minutes

 Track 4
 - Index 00: 4 secs
 - Index 01: 4 minutes

If you don't "Action -> Detect Gaps (F4)" before extraction (for 'extraction' I mean "Action -> Copy Selected Tracks -> Uncompressed"), the gaps will be automatically appended to the end of the previous track in the wav files. So you wav files will be like this:

 Track1.wav : 1:02 (1st song + gap of the 2nd track)
 Track2.wav : 2:03 (2nd song + gap of the 3nd track)
 Track3.wav : 3:04 (3rd song + gap of the 4nd track)
 Track4.wav : 4:00 (4th song)

But, if you do detect the gaps before extracting the wavs, you can have more control over them. Doing so, you have now 3 ways to handle gaps (the last 3 options in the Action menu):

Leave Out Gaps
Append Gaps to Previous Track
Append Gaps to Next Track

Leave Out Gaps will simply ignore Gap data (00 indexes). Only indexes 01 and over will be recorded to the wavs. Wavs will look like this:

 Track1.wav : 1:00 (1st song)
 Track2.wav : 2:00 (2nd song)
 Track3.wav : 3:00 (3rd song)
 Track4.wav : 4:00 (4th song)

The big problem with this option is that the gap data from the CD is lost in this process. If our goal is an exact copy this option is not a good idea.

But this option might be useful under some circunstances. The gap data lost may be only "useless" silence between tracks, and you might want to store only actual music content. Or perhaps you want to manually change the silent between tracks when writing the CD, so having "trimmed" wavs can help controlling the silent spacing when writing the CD-R.

But gaps are not only silence. This may be true for most albums, but not all. Rembember that gaps can be silence, but also can be anything from audience clapping to actual music.

You can test if the gaps only contain digital silence using *"Action - Test Gaps On Silence"*.

Nevertheless, we want our copy to be as exact as possible. So if the gaps are in the original, they must be in the copy just the way they are.

Append Gaps to Previous Track: this is the default action, works the same way as if no gap detection is made, and was explained above.

Append Gaps to Next Track: similar to the default, but the gap is written in the beginning of the track, like this:

 Track1.wav : 1:00 (1st song)
 Track2.wav : 2:02 (pregap of the 2nd track + 2nd song)
 Track3.wav : 3:03 (pregap of the 3nd track + 3nd song)
 Track4.wav : 4:04 (pregap of the 4nd track + 4nd song)

This might look the most "natural" way to store the gaps, but actually its very odd: Usually the gaps are silences, so it's a bit weird for a track to start with a silence, instead of ending with it.

Also, when you use the seek button in any standalone player, for, say, Track 3, it will begin playing the Track 3 - index 01 (aka 3rd song), not it's 00 index!

So the only way for you to listen to the 3 secs pregap is to listen all the way from Track 2. The player will play Track 2 - index 01, then the pregap (will display negative -3s).

So, if the only way to listen this gap is by playing track 2 until the end, it makes sense to say that the 3s pregap actually belongs to the 2nd track, not the 3rd! (even if in the actual CD layout the 3s gap is called "track 3 - index 00").

So, bottom line for reading gaps: For things to go perfect, you dont even have to detect gaps! If you don't, EAC will automatically read them and store them in the end of the previous track. If you do, make sure that the default option is selected, or select something else being very sure of what you're doing.

WRITING GAPS

Wavs recorded, you're ready to burn the CD-R. If you just want the CD copy to sound the same as the original, just go to Tools -> Write CD-R -> Layout -> Append Files As New Tracks (Index 1), select all the wavs, and be happy.

As the gaps are recorded within the wav files, the resulting cd will have the same "song, gap, song, gap" pattern. Also, gaps will behave like they ought to: if you seek to Track 3, it will jump directly to the 3rd song. To hear the gap before track 3, just listen track 2 until the end.

But, if you look carefully, something is different: In the CD layout, there will be no Index 0 in any of the tracks. After all, all data on the gaps were appended to the songs (01 indexes).

Should you care? Probably not. As far as I learned here, no software or hardware make any useful use of indexes. Most (if not all) PC Players just ignore this information. The only perceptible difference is the time count used in standalone players.

In the original CD, the 1st track will play from 00:00 to 01:00, then a negative -00:02 countdown will be displayed while the gap (track 02, index 00) is being played, and finally track 2 will start at 00:00. In the CD-R copy, track 1 will be 01:02 long. Display will go from 00:00 to 01:00 and continue to 01:01, 01:02 as the 1st song and the subsequent gap is played, and then track 2 will start at 00:00.

If you close your eyes and just listen, all will be the same. If you use seek and search buttons, all will be the same. Only the fancy -2s display countdown will now be displayed as part of the 1st track. Thats it.

But do you care for this fancy display effect? Do you want to keep the index marking for the sake of fidelity, even if this is completely useless? Then the job can easily be done using CUE Sheets.

CUE Sheets are a text file that defines a CD Layout. It tells the burner program what tracks and indexes the CD will have, its length, and how the source wavs will be "sliced" in these tracks and indexes. So, before burning the blank CD, you must first Create a CUE Sheet based on the album you're reading (and also base on how you extracted the wav files from it).

EAC Gives 4 options for creating this CUE Sheet, the only difference, as far as I can see, being the way they handle the gaps found in the original CD. The options, accessed through the "Action -> Create CUE Sheet", are:

Current Gap Settings
Single WAV File
Multiple Files With Gaps (Noncompliant)
Multiple Files With Left Out Gaps
Multiple Files With Corrected Gaps

"Single WAV File" generated a CUE Sheet where a single .wav is sliced in all the tracks in indexes. So, as the name suggests, it must be used when you extracted the full album to a single .wav file (presumably using the Action -> Create Image option). It creates both 01 and 00 indexes, thus preserving original CD layout and content.

"Multiple Files With Left Out Gaps" creates a CD Layout identical to the original, but, as the source wavs presumably lack the gap content (hence the name), its data is entirely recorded in the 01 indexes, and the 00 indexes are filled with silence.

So, although it perfectly preserves the CD layout, the copy is only identical if the content of the gaps in the original CD is only silence too! In other words, all the data in the gaps is lost and replaced by silence of the same length.

"Multiple Files With Gaps (Noncompliant)" slices the wavs assuming that the gaps were appended to the end of each (previous) track. So, using our imaginary album, the first 2 minutes of Track2.wav would be recorded in Track 02 - Index 01, and the last 3 seconds would be Track 03 - Index 00.

No need to say that this is the option that should be used when the wavs are extracted the default way. The scary "Noncompliant" term means that many other burners, like CDRWin, cant correctly recognize CUE Sheets where wavs are sliced this way as valid CUE Sheets. So that's something you should care about if you only use EAC for extraction (thus for creating the CUE Sheets) and another external programs for burning.

If you use EAC for extraction AND burning, there's nothing to fear, as EAC can properly read its own generated CUE Sheet quite well.

"Multiple Files With Corrected Gaps" slices the wavs assuming that the gaps were appended to the beginning of the next track, i.e., this option should be used to correctly recreate the original CD layout and content when you used the "Append Gaps to Next Track" option when extracting the wavs.

Last but not least, "Current Gap Settings" creates a CUE Sheet based on the currently selected gap settings.

So, create the CUE Sheet that best represent the gap format you used when extracted the wavs. Now it's very easy to burn a CD-R. Just go to Tools -> Write CD-R -> File -> Load CUE Sheet. Now see how the wavs are properly sliced? (if not, you did something wrong.) Time to burn! Go to Cd-R -> Write CD and be happy!