CompuServe Thread

OK, here goes again. Broadly speaking, the XA spec. defines two sector types, Form 1 and Form 2. Form 2 sectors contain no error correction and are used for storing audio and video information in a file format defined by the XA spec. These audio/video files that are stored in Form 2 sectors using the XA audio/video format are commonly called XA streams. The XA spec. designates Form 1 sectors for storing any files that are not XA format audio/video files. Form 1 sectors do contain error correction. Pictures stored on a PhotoCD disc are stored in Form 1 sectors. PhotoCD picture files do not use the XA audio/video format. To read PhotoCD pictures, a CD-ROM drive only has to support Form 1 XA sectors. Most of the new PhotoCD compatible CD-ROM drives do only support Form 1 sectors. They do not support the Form 2 sectors used for storing XA audio/video ("XA streams"). Since there are currently many more drives that support MPC than support Form 2 XA, converting XA titles to MPC means those titles can run on many more CD-ROM drives. In theory, XA is a superiour technology. It is designed to run on many operating systems and should provide faster, smoother multimedia. However, MPC currently has a much larger installed base.

<<What confuses me — and frankly that isn't a difficult feat — is that I thought XA was a Kodak requirement for PCD, and that a PCD ready computer would be able to handle XA on its own.>> Full XA compatibility (including hardware chips for XA decoding) is not a requirement of the CURRENT PhotoCD specification, although "XA readyiness" (Mode Two) IS a requirement. We chose to describe the environment required for SoftwareXA~ as PhotoCD ready MPC computer in order to distinguish those MPCs with Mode Two CD-ROM drives which are necessary for playing back CD-ROM XA data streams. SoftwareXA~ does the splitting of the audio and "other" data, and the conversion from compressed XA audio (ADPCM) to MPC audio (PCM using only the CPU) of the MPC computer system. In this way, SoftwareXA~ gives the MPC owner a taste of the fast response and high performance of CD-ROM XA (but not the high quality audio) and creates a bridge between the MPC multimedia system of today and the high performance CD-ROM XA platform of the near future. Hope this helps!