DETERMINING THE OFFSETS PART II

Determining the write offset and the combined read/write offset

Using a writer supported by EAC

Even if your writer is not supported by EAC or you do not intend to use the write feature it's still interesting to read this part as it explains how to calculate the read offset or write offset when you know the combined read/write offset correction and either the write offset and read offset respectively and gives other relevant information.

In the Detecting The Write Features part of this tutorial you can find out whether or not your CD-recorder is supported by Exact Audio Copy for writing. Don't worry if your burner is not supported by EAC. Instructions on how to determine the write offset of a non-supported writer follow later on this page.

Select the device you want to use for recording in the top left drop down box. Most people only have a single writer so that's an easy choice. In our example we'll use the Plextor PlexWriter PX-R820TSi.

Insert a blank (empty) CD-R disc in the drive. Note that this time there will be data written to the disc making the disc useless for further usage. On the other hand we're talking about just 1 CD-R disc that will be wasted... With the current low prices of CD-R media you won't be ruined from that :)

You can of course also use an erased CD-RW disc for this test. Then you can erase that disc again after you are finished with the testing.

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

The Options screen for your CD-recorder 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.

Exact Audio Copy features the creation of a so-called offset test CD. This is a CD that is recognized by EAC as a valid reference CD to perform read offset detection with. This offset test CD is then used to perform a read offset detection. Since we have determined the read offset earlier the write offset is the difference between the detected read offset with this offset test CD and the previously detected read offset. The exact calculation follows later. Oh, and don't let the word calculation scare you, it's no more than a plain simple sum!

The offset test CD can be created by clicking the Create Offset Test CD button on the Writer tab of the Options dialog for your CD-recorder. Make sure an empty recordable disc (CD-R) or erased rewritable disc (CD-RW) is loaded in the writer and verify that write samples offset is set to 0! Contrary to its intended use, we will write the offset test CD not offset corrected.

After clicking the button a warning message appears informing you that EAC will actually write data to the disc. We already knew that, so ignore this message and click the Yes button to continue.

Exact Audio Copy will then create the offset test CD. This writing process takes a few minutes and a progress bar is indicating the status.

When this is finished it is time to perform a new read offset detection with this offset test CD. Therefore eject the CD and insert it in the drive of which you detected the read offset earlier on this page. If this is the very same drive you just used to write the CD with it is still recommended to open and close the drive's tray. Some CD-writers do not refresh the CD contents after writing and thus falsely assume that the just written CD is still empty. Those burners have to be forced to do a refresh by opening and closing the tray.

Go to the Options dialog for your drive by choosing Drive Options... from the EAC menu or press F10. Click OK to pass the warning message that may appear. If the drive you know the read offset of is different from the drive you wrote the offset test CD with, do not forget to change EAC to that drive. You can change the drive in the top left drop down box as seen before. In our example here we use the Plextor PlexWriter PX-R820Ti for writing and a Plextor UltraPlex PX-32TSi for reading. Earlier the read offset of the UltraPlex 32x was determined so we will do the new read offset detection with this drive.

On the Offset / Speed tab click the Detect read sample offset correction... button. A 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, but for some reasons this information is not always reliable when tested with an EAC offset test CD. Thus only trust the information you got with a pressed CD.

After the read offset detection finished EAC returns the detected sample offset. Do not apply this value! You already have the correct read offset. We only need this detected sample offset to calculate the write offset. The sample offset Exact Audio Copy returns is also called the combined read/write offset correction and it represents the total offset of your writer and reader combination. If you use another program for writing than EAC you should use this combined read/write offset correction for reading audio CDs. More about that later. To calculate the write offset:

write offset = combined read/write offset correction - read offset correction = 649 - 679 = -30

Thus the write offset of the PlexWriter 8/20 is -30 samples. You should enter this value in the write samples offset field on the Writer tab of the Options dialog for your CD-writer. Click the OK button to save the settings. You are now able to make 100% offset corrected duplicates of audio CDs!

If you not already did, read The Truth About Offsets to find out why it is sometimes impossible to have a 100% perfect duplicate of a CD, even when using offset correction. The problem is the overwriting capability of the CD-recorder. If the drive is not able to overwrite into the Lead-In (actually, it's the pregap of the CD - for a positive write offset) or overwrite into the Lead-Out (for a negative write offset) there will always be missing samples at the beginning or end of the copied CD respectively.

Again, like said in The Truth About Offsets this is such a small time it may be ignored.

If you already knew the write offset of your recorder and wanted to determine the read offset correction you should use following calculation:

read offset correction = combined read/write offset correction - write offset = 649 - (-30) = 679

It may be useful to know that EAC is somewhat inconsistent when it comes to naming things. The sample offset you have entered for reading is actually the read offset correction, thus the opposite of the real read offset, while the write offset you have to enter in EAC to compensate the offset is the real write offset of the recorder. Confusing...

read offset correction = - read offset

Some people might prefer a program for writing audio CDs other than Exact Audio Copy. Since EAC is the only program that uses the separate read offset and write offset, other programs record with write offset 0. That means that you'll have to use the combined read/write offset correction on reading in order to be able to make perfect offset corrected duplicates.

In the screenshot on the left you can see how the Plextor 32x CD-ROM drive which is used for extraction is set to use the combined read/write offset correction. Again, using the separate read offset and write offset is the preferred method of copying, thus if your recorder is supported by EAC you better use EAC for burning too. Leave the combined read/write offset correction only for use with recording programs that do not allow to specify the write offset.

Help other users:

Not everyone is able to determine the write offset 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.

Using a writer not supported by EAC

Basically, what we'll do is identical to calculating the write offset by determining the combined read/write offset correction of a recorder supported by EAC. It just gets a little more complicated. A track is burned to CD, then extracted again with the known read offset. This file is compared to the original and the difference between both is the write offset.

Since EAC is not used for writing this time I'll use Goldenhawk's CDRWin to show everything. You may however use your own favorite writing program. All we need it for is to write an audio track to CD. Even without any help, most users should be able to perform that simple task. If you want to follow my instructions to the letter you can obtain a demo version of CDRWin at Goldenhawk's website. CDRWin is very simple to use (for basic tasks), does not require any configuration to make it work and supports almost every writer available.

First thing is getting a track. Since Exact Audio Copy is an audio extractor and you obviously intend to use it for extraction (hey, why else are you reading this :) we'll use it to obtain the track.

It's important to extract the track with read offset correction disabled, in other words a read offset of 0 samples!

If you already determined the read offset do not forget to set the read sample offset correction value back to the correct value after you performed the write offset detection!

Which track should you extract? I really doesn't matter. Just take an audio CD you have nearby and extract a track to your harddisk. See Extracting Tracks for instructions if you haven't figured out yet how to extract tracks with Exact Audio Copy.

For the ease of use I renamed this track to Reference Track.wav.

Then burn this track to CD with your favorite recording program.

Since some users may be totally new to audio recording (there's nothing special about it though) I'll explain how to write this reference track to CD with CDRWin. Launch CDRWin and the CDRWin main screen (see image on the left) should appear.

Insert a blank (empty) CD-R disc in the drive. Note that there will be data written to the disc making the disc useless for further usage. On the other hand we're talking about just 1 CD-R disc that will be wasted... With the current low prices of CD-R media you won't be ruined from that :)

You can of course also use an erased CD-RW disc for this test. Then you can erase that disc again after you are finished with the testing.

Click the top left Record Disc icon on the CDRWin main screen.

The Record Disc dialog window should open. If you have multiple recorders, make sure you select the correct one in the CDROM Recorder drop down box on the top. Then click the Load Tracks... button on the right. This opens the Load Tracks dialog box. Use the Add... button to browse to the file Reference Track.wav we want to burn to CD and add it to the project. When that's done click OK to return to the Record Disc dialog window. If everything went right the Recording Information field should display the current project information.

Click Start Recording to start the writing process. When the recording is finished eject the CD and insert it in the drive you use for reading. If this is the very same drive you just used to write the CD with it is still recommended to open and close the drive's tray. Some CD-writers do not refresh the CD contents after writing and thus falsely assume that the just written CD is still empty. Those burners have to be forced to do a refresh by opening and closing the tray.

Then back in Exact Audio Copy, extract the freshly burnt track to the harddisk. Again with read offset set to 0. This is very important! If you use a read offset different from 0 you will not detect the correct combined read/write offset correction. For the ease of use I renamed this track to Extracted Track - write offset 0 - read offset 0.wav.

Now you need a tool to compare the reference track with this new extracted track. Luckily EAC comes with a WAV comparison tool that can be used for these purposes. From the Tools menu select Compare WAVs... or press ALT+C. EAC will ask for 2 WAV files to load. Make sure you select Reference Track.wav as first track (in the Select First WAV open file dialog box) and Extracted Track - write offset 0 - read offset 0.wav as second (in the Select Second WAV open file dialog box). The order hasn't any effect on the quality of this test, but the way EAC displays the comparison information makes that you want the reference track as first track. If you don't pay attention to the file order you may get an offset with the opposite sign.

After EAC processed both files and determined all differences the information is displayed. The information is shown in 2 columns as can be seen on the screenshot on the left. On the left Reference Track.wav and on the right Extracted Track - write offset 0 - read offset 0.wav.

If the reference track is missing samples or the extracted track has extraneous (repeated) samples the offset is a positive number. If the extracted track is missing samples or the reference track has extraneous (repeated) samples the offset is a negative number.

In our example the extracted track has 649 repeated samples. The offset determined with this method is the combined read/write offset correction. See Using a writer supported by EAC to find out where to enter this value to make EAC use combined read/write offset correction.

To calculate the write offset we use this new detected combined read/write offset correction and the earlier detected read offset correction:

write offset = combined read/write offset correction - read offset correction = 649 - 679 = -30

Keep in mind that there's not really much use of knowing the write offset when you do not use Exact Audio Copy for recording. You must use the combined read/write offset correction on extraction and record with write offset 0.

If you already knew the write offset of your recorder and wanted to determine the read offset correction you should use following calculation:

read offset correction = combined read/write offset correction - write offset = 649 - (-30) = 679

Help other users:

Not everyone is able to determine the write offset 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.