Offset Technology
'Sample Offset' is another new feature of EAC, it will help to always get the same WAVs compared to a different reader and to prevent generation losses. Nearly all drives can not position the head correctly. That means if the program tells the drive to read block 10000 it will probably read data somewhere in block 9998 instead. But this is not visible to the reading program, it won't know if it is really the data it wanted. Usually the head will be set always to a fixed offset before or after the correct read position. So it is possible to detect this offset once and use it for all CDs coming afterwards. Because it is a hard task to determine the exact absolute offset for each drive, I determined it once on my Plextor 14/32. To find out the offset of any other drive the offset is calculated relative to that absolute offset. To implement an offset detection for other drives I took some bytes from some common CDs I and my friends own. These are the reference each CD-ROM drive has to compare with. Of course I have implemented only a limited selection of CDs that can be used to detect this offset, a list follows below. Sometimes there are different versions (releases) of the same CD, but only the same press
the one I used will work. That is why the table below shows a code, that is written on the inner ring of the music CD, that should identify the release of a CD. The red codes are from CDs which do not work with this test. In time I will add more CDs to test this offset. Furthermore, drives that have jitter are unable to position their heads correctly. So you should activate the secure or fast extraction method and moreover if your drive does caching, activate no-caching emulation. The 'Searching Track Start' algorithm tries to find the correct start position even if jitter occurs. But this is not always possible, mainly if the drive jitters too much. But it can be shown that nearly 80% of the reads will get the same results. A drive's characteristic offset can be found automatically from the CD from on the list of reference CDs. Because of the mentioned jitter error the value given back is also not 100% sure. You should start the test several times and remember to activate emulate no-caching if necessary. Then you should get one value that occurs more often than other values. YOU SHOULD USE THIS TEST ON TWO DIFFERENT CDS AT LEAST! BOTH TESTS SHOULD GIVE BACK THE SAME VALUE! My Plextor 14/32 has an offset value of +679 samples, that means that 679 samples usually are missing at the end of a wav file.
As different modells of common CD-R writer usually do not add the same offset on writing, it seems that also big CD manufactures also do not always press the same offset on their CDs. So I determined the most common offset of pressed CDs and integrated it into the offset detection routines.
Please help us measure more reference CDs. If you have a Plextor 14/32 32x CD-ROM drive, we know the offset. So all you have to do is run some popular disks from your own collection through it with a utility you could download here and send me the output.
http://www.exactaudiocopy.de/FYI,
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