Most of us are familiar with audio CD's and the tracks of music which they contain. When mastering a CD, those tracks may either be laid down separately, with the laser turning off between each pair; or in a continuous flow. If the laser writes Track at Once (TAO), a gap of two seconds is introduced (set by the Redbook standard) to ensure proper synchronization. If the tracks are written continuously in Disc at Once (DAO), no gap is required. There are schemes for approximating DAO by adjusting the gap in spite of the standard; the results may or may not be satisfactory depending on the player used and there is always a small gap with that approach.
DAO also has a role in data recording: masters for mass production are required to be in that format. Jerry Hartke, President of Media Sciences, Inc. http://www.mscience.com/ has provided the following on why pressing plants require DAO masters. Note that going through the 8-mm intermediate adds to the cost of mastering and that it will also (slightly) change the size of the disc being pressed.
Mastering facilities must have perfect source data or their laser beam recorder will abort and ruin an expensive glass master. Track-at-once recording leaves "link-blocks" at the end of lead-in and at the beginning of lead-out. These are read as defects, or errors, by the LBR. Disk-at-once recording has no link blocks. Many mastering houses have learned from a rather gruesome history, and transfer information from a CD-R to 8-mm tape using methods that remove problems such as link blocks. Mastering is then conducted from the perfect image on the 8-mm tape, not from the "bad" CD-R.
Not all CD-Recorders are capable of DAO and not all software supports even those that can do it. The best information on DAO recording is provided by Jeff Arnold at http://www.goldenhawk.com/ If you believe that you will want DAO either for mastering pressed discs or for making CD-DA's without gaps, check there before you pick your recorder.
Session at once is a relatively new capability used on some CD Extra discs. With SAO, an audio session is written without intertrack gaps, just as though it were DAO. The difference is that only the session is closed, so one or more additional sessions can be written. Since an audio player can only see the first session of a disc, it makes no sense to write audio after SAO (unless you want to play the disc only on your computer).
Not all software supports SAO; Easy CD Creator 4.0 and above does, but earlier versions did not. Similarly, not all writers support this mode. Many which handle DAO, such as the Ricoh 62xx, are not compatible with SAO. Of course, that may be changed with firmware and new drives are almost certain to support SAO.
E-mail me at cdrecording@mrichter.com
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