DETERMINING THE OFFSETS PART I

If you want to make a 100% duplicate of an audio CD you must use offset correction. If you don't know what exactly offsets are and what influence they have on the extraction, make sure you read The Truth About Offsets. Actually everyone should read it, because there's a lot of misinformation about offsets confusing people. On this page you can read that there are 2 ways to correct the offset issue:

• Using the combined read/write offset correction
• Using the separate read offset correction and write offset correction

The second method is preferred as this is the only one that gives no missing samples (again, read The Truth About Offsets for in depth information about this issue). Another advantage of the separate read offset correction and write offset correction is that the tracks extracted to the harddisk are also offset corrected, while when using the combined read/write offset correction only the burnt target disc is offset corrected - the tracks on the harddisk still have an offset!

So when do you have to use the combined read/write offset correction then? Currently Exact Audio Copy is the only software available that can use the separate write offset correction. Thus if your burner is not supported by EAC for writing you have to use alternate writing software like CDRWin, Feurio! or Nero, and are not able to use the separate read offset correction and write offset correction to make perfect duplicates of CDs. In this case you'll have to use the combined read/write offset correction. Some users may not like the EAC write feature and prefer their favorite recording program to record the tracks to CD. They have to use the combined read/write offset correction too. Thus the conclusion is that when you do not record to CD with EAC you must use the combined read/write offset correction. In all other cases it's recommended to use the separate read offset correction and write offset correction.

The drawback of using the combined read/write offset correction is when you use EAC to extract tracks to your harddisk that those tracks will not be offset corrected. In other words, when you extract tracks to your harddisk, not meant for CD recording but MP3 encoding for example you'll want to use the separate read offset correction and write offset correction too make sure that the tracks on your harddisk are also offset corrected. Thus this means that you sometimes will have to switch between using the combined read/write offset correction and the read offset correction. Users also writing CDs with EAC don't have this problem since they use the separate read offset correction and write offset correction all the time.

On the other hand some users will not be able to determine the separate read offset correction and write offset correction for their drives, even when writing with Exact Audio Copy. In this case they also have to use the combined read/write offset correction. Not yet confused? Well then, let's start!

Determining the read offset

About reference CDs:

Determining the real read offset of a drive is very difficult, but Andre Wiethoff (EAC) managed to determine the offset for the Plextor UltraPlex 32TS. Once a single drive's offset was known Andre used this drive to create the reference CD list included in Exact Audio Copy. If you own a CD from this list, it is possible to determine the read offset in just a few mouse clicks. EAC will compare a small range of audio data extracted from your CD with the reference data included in EAC. From this EAC calculates the read offset of your drive.

Of course this only works if you own CDs on the EAC reference CD list. Each new version of EAC contains more new reference CDs so the chances become bigger that you or someone you know owns such reference CDs. View the current list of included reference CDs (scroll down a little on that page).

Excellent you'd think, but too bad a problem raises. It seems that different pressings of CDs sometimes also have an offset to each other! Thus you may perform the read offset detection and get a wrong result while you used the same CD as included on the EAC reference CD list. Therefore you should also check the CD's ID code which can be found on the inner ring of the CD. See the two examples below:

When you lookup those CDs in the current list of included reference CDs you'll see that the ID codes match. Sometimes CDs with a different ID code still have the same offset as the reference CD, sometimes the offset between the CD you have and the original reference CD is that much that it is no longer recognized as a reference CD, but what's worse... sometimes CDs with the same ID code have a different offset. Thus the ID code is just for reference, it does not guarantee correct read offset detection.

Therefore read offset detections performed with only 1 reference CD are not reliable. You should at least use 2 different reference CDs and get at least 2 times the same read offset.

For example, you may have 5 CDs included on the reference list, but only 2 out of those give the same read offset. If all CDs give a different read offset you'll have to look for more reference CDs as you still can't be sure about the detected read offset. If 3 or more CDs give the same read offset you don't have to worry, you have found the correct read offset!

Although it should be obvious I'll mention it anyway: The read offset detection must be performed on original pressed CDs! Copies are not suitable and may give incorrect results.

Thus as you can see, determining the read offset comes down to finding the correct reference CDs.

Performing the offset detection:

Now you know what you should look for, it's time to explain how to perform the read offset detection. Insert a reference CD in your drive. Select the device you want to use for extraction in the top left drop down box. This is in most cases your CD-ROM. If you want to use multiple devices you'll have to repeat all steps for each device you want to use.

From the EAC menu select Drive Options... or press F10. Click OK to pass the warning message that may appear.

The Options screen for your drive should now appear. You can verify that you have selected the correct drive by the displayed name in the title bar of the dialog window.

On the Offset / Speed tab select Use read sample offset correction. Make sure a CD on the list of included reference CDs is inserted in the drive and click the Detect read sample offset correction... button.

A new dialog box Analyzing will popup showing the status of the read offset detection process. First thing that's detected is whether or not the drive is capable of overreading into the Lead-In and/or Lead-Out. This is an important feature if you don't want missing samples when using offset correction. Read The Truth About Offsets to learn more about overreading into the Lead-In and/or Lead-Out.

If the CD is found in EAC included reference list Found CD will display its title. Meanwhile Exact Audio Copy determines the offset of the data extracted with your CD to the reference offset data. This process may take a few minutes but is usually done in a few seconds.

As soon as EAC was able to calculate the read offset of your drive the Sample Offset is displayed. Note that the returned value is the read offset correction, thus in fact the opposite of the real read offset! Thus if EAC returns a positive read offset correction that means that your drive actually has a negative read offset, and vice versa!

Some drives do not have a constant read offset, therefore you should perform the test at least a few times for each CD. If you get a different read offset on each read offset detection (with the same reference CD) your drive sadly enough has no constant read offset. Continuing the read offset detection does not make much sense as you will never be able to get 100% offset corrected duplicates of CDs. If EAC returns the same read offset on each detection you can be sure you have a drive with a constant read offset. Some cases are known where the first read offset detection gives a wrong value and the following read offset detections all return the same correct values. I'm not sure why the first detected value is incorrect, but these drives seem to have a constant read offset anyway.

If your drive has a negative read offset or a positive read offset correction it also needs overreading into the Lead-Out in order to have no missing samples on extraction. If the drive has a positive read offset or a negative read offset correction it needs overreading into the Lead-In. Anyway, don't worry too much if your drive cannot overread as only very few samples are missing at the beginning or end of a CD as can be read in The Truth About Offsets.

Remember to get reliable read offset values you should at least use 2 different reference CDs and get at least 2 times the same read offset! If you only use 1 CD you can never be sure that the detected read offset is correct. You can eject and load a new CD without going back to the Exact Audio Copy main screen. Just close the Analyzing dialog box by clicking the OK button to return to the Options screen for your drive. Eject the current CD and load another one. Wait a few seconds for the drive to initialize the CD, then click Detect read sample offset correction... again to perform a new read offset detection.

Once you have 2 CDs which return the very same read offset information you know that both CDs are equal to the reference CDs which were used to create the EAC reference CD list. From then on it's enough to use one of those two CDs to detect the read offset since the results they return are proven reliable.

The above screenshots show the detected read offset values for the Plextor UltraPlex 32TSi. Both CDs used for the read offset detection returned the same read offset so it's safe to assume that the detected Sample Offset (read offset correction) is correct.

The Plextor UltraPlex 32TSi is thus capable of overreading into the Lead-In and overreading into the Lead-Out, and has a read offset correction of +679 samples (the real read offset is thus -679 samples!).

Click the Apply button to parse your settings to EAC. Clicking the OK button will not pass the settings to EAC!

On the left you can see the settings passed to Exact Audio Copy for the Plextor drive.

Click OK to save the settings and to return to EAC's main screen.

Help other users:

Not everyone is able to determine the read offset correction or overreading for his drive. Therefore an online User Reported Drive Features Database was set up. If you were able to determine your settings you are strongly encouraged to contribute to this database by sending in your settings. See the Help Other Users section for detailed instructions. Even if your drive is already listed in the database you still may send in the information. The more people report their settings for a certain drive the more accurate it becomes.

No reference CDs available or found:

Problems raise when no CD you own is on the reference list. Also when having CDs of a different pressing than the original reference CDs Exact Audio Copy may detect wrong offsets or not be able to determine the read offset at all.

Does this mean that you won't be able to determine the read offset? There's still a chance you can find the read offset with some external help: the online User Reported Drive Features Database. If your drive is listed in this database you are lucky, but keep in mind that the information in this database is sent in by other users and absolutely not verified by myself. Also, different firmware or hardware revisions may have a different read offset.

It's also possible to determine the read offset of any drive if you own a CD-writer of which the write offset is known. Just keep on reading to find out how. The next chapter discusses the write offset determination, but it also applies if you already know the write offset and want to find the read offset.

Continue reading Determining The Offsets Part II.