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What is a CD Physically | Hitting the colored "Books" Single vs. Multi Session | Stamped Multi Session vs. Mixed Mode Active Audio's Track Zero Approach Single Session vs. Multi Session
Lets start off by asking what is a session? A "session" is the encoding of data on to an optical disc. As we saw earlier, this can happen by "burning" pits into organic layer known as cyanine (as done with CD-R writing) or by "stamping" them in the injection molding process as done with CD and CD-ROM replication. In either case, a "single session" format references a disc in which subsequent data cannot be added to the first recording or "session". There is a "lead-out" code that closes the single session on a "burned" CD-R and prohibits the addition of any new content. A "single session device", such as a drive or writer, refers to a device that can only read or write a single session formatted disc. A single session drive can read a "multi-session" disc but only the first session that was burned.
Multi session discs are recorded in accordance to the Orange Book Part II standard. Herein, discs can be burned and partially closed. As such, subsequent data writes can be added in either the CD ROM or CD ROM XA format. When Kodak teamed with Philips in the early 90's to create the multi-session Photo CD format they choose the CD ROM XA format only for what the improved multi media capabilities the could provide them in the future. Because the Photo CD was the first reason manufacturers had to create multi session CD ROM drives and drivers, they quickly assumed if a disc was multi-session it had to be CD ROM XA. So now when o ne of these drives sees a disc that is not XA, it is coded to assume that its also not a multi session disc and treats it like a single session disc, attempting to read only the first session burned on it. This is the primary reason why drives and drivers today need to be updated to support the multi-session CDE format. Most CD-ROM drive manufacturers have realized by now they have a problem and are coming out with new software drivers which support multi session reading in both the CD-ROM and CD-ROM XA format.
Stamped Multi Session vs. Mixed ModeIn any event, the CD Extra adheres to the Blue Book specification is "stamped multi session" format. It is a two session CD. The first session can hold up to 98 audio tracks and this session is then closed as a CD-DA. The other session contains the data track and should be closed as a CD-ROM. On presently available consumer CD audio players the tracks contained in the first session are played but when the laser reads the lead-out at the sessions end it resets to zero. A multi session compatible CD ROM drive will default to the last session and read the data as expected. With the aid of various tool kits such a s Macro Media's Enhanced CD tool kit this data can include information for playing audio from the audio tracks with track-relative synchronization for lyric-cueing, synced animation etc.
On the other side of the format coin is the predecessor to the "stamped multi session format" and that is the "Mixed Mode". With the Mixed Mode format the Yellow Book data is placed in track 1, followed by the Red Book audio. The immediate problem here is that data in track 1 needs to be manually skipped when playing back on audio CD players. This means no "shuffling" or random play is allowed. (Not to mention ever time a title is to be released in both audio only and Mixed Mode formats the album artwork and linear notes would have to be separately designed and printed to support the absolute track numbering differences between the two formats sequences. This is costly and confusing).
The Track Zero ApproachAs we've seen, the track 1 issue is remedied with the stamped multi session but is also solved by ActiveAudios mix mode TrackZero system. Here the ROM data content is placed not in track 1, but track zero. It precedes the Red Book and with normal operation would never be heard by an audio CD player. Though this format is not the RIAA's choice it purports to offer improvement over the endorsed stamped multi-session in a couple of areas. One having the data first even single session CD ROM drives can read it. This will be an issue of diminishing consequence as more and more drives are fitted properly for multi session play. Secondly by having the data first, it resides on the inner tracks where the track diameter is least. This reduces what is known as latency.
Latency occurs when the drive heads are seeking a particular piece of data and the head happens to be just past where that data is sectored on the disc. As a result, the head needs to wait for nearly an entire revolution to access the sector of the data it seeks. Since CD's use a "constant linear velocity" approach, a greater distance needs to be traveled to complete a single revolution with outer tracks than with inner tracks. Simply because the diameter of the i nner track is less, the latency or wait time is less. Don' t forget the stamped multi session method places ROM data (Yellow Book) on the outer most sectors of the disc while "mixed mode" ROM content occupies the discs inner most sectors. Active Audio reports disc access delays of data placed on outer sectors form 1.5 to 2.4 times greater than with disc data placed on inner sectors.
ActiveAudio furthers its claim of format superiority by stating that anomalies which result during CD pressing tend to generate errors at the outer edge of the disc. They claim discs which have ROM data at the outer edge of the disc can suffer from this problem.
It can be debated at infinite why the RIAA and the "Big Six" distributors have choose the Sony/Phillips approach despite any technical advantages of the track Zero approach. It may have to do with the "community" being what it is. And/or maybe because use of the stamped multi session format does not necessitate any licensing agreement as it appears the ActiveAudio's format does. This is only speculation on part of AIIG. Any input here would be appreciated and posted.