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Disc-At-Once

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Disc-At-Once(DAO) is a general term for a group of writing methods that write an entire CD or DVD including lead-in and lead-out (beginning and end area) without switching off or readjusting the laser in between. Of the writing methods mentioned in the MMC standard, SAO and RAW fall into the group of DAO writing methods.

Advantages are that pauses or gaps (“pregap”, Index-0) between tracks can be set to values that are not allowed in the standard. However, not all drives allow the length of the index-0 to be set to arbitrary values in SAO mode. Since in RAW mode the drive does not evaluate what is being burned, an arbitrary index-0 length is possible on all drives that support RAW mode.

RAW

In raw mode, work is carried out without the specification of a CD or DVD format. In RAW mode, for example, a CD is written in portions as a memory image to a file or written from this to the data carrier from a memory image file without evaluating or changing the format of the data.

RAW is the only write mode that makes it possible to write R-W subchannels, CD text and ISRC information, catalog numbers, CD+MIDI, gaps, indexes, and manipulated TOCs. There are three different RAW modes for this purpose:

  • RAW 16 writes P-Q (16 bytes) subchannel. 2 bytes of CRC data in the Q channel are generated by the recorder.
  • RAW 96P writes P-W (96 bytes) subchannel, but 2 bytes of CRC data are generated in the Q channel by the recorder. This mode is rarely used in practice and is supported by few recorders.
  • RAW 96R writes P-W (96 bytes) subchannel. This is the best mode and allows to write all subchannel information.

This additional data of the subchannels is relevant for advanced functions like CD-Text, but also for copy protection mechanisms. Paradoxically, in these modes it is possible for the recorder to write data that it cannot read; formats such as CD-Text or CD+G can also be duplicated, even if the recorder does not actually support these formats. To do this, however, the data must have been read in correctly beforehand, e.g. using another drive.

In RAW mode, a multi-session CD can theoretically be written in one go. However, since there is no known recording program supporting RAW mode that can write a Program Memory Area (PMA), a write prefix, in RAW mode, such CDs cannot be continued and are even detected as defective and unreadable by Philips recorders.

SAO

In Session-At-Once mode (SAO), the data is also written to the medium without switching off the laser, but a so-called B0 pointer can be set and the drive writes in this case (but with a write interruption) additionally a PMA before the lead-in. This way more sessions can be added. This is used for example with CD-Extra.

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