Specifications

CD-i FAQ 2000 Edition Revised February 21, 2001
Latest version and more CD-i info: http://www.icdia.org
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To enable the simultaneous retreival of both audio and video information, data is interleaved on
the CD-i disc. This means that each sector contains a piece of either audio or video information,
and that those sectors can be put in consequence next to each other.
Since a CD-i disc is read at a constant continuing speed, the designer needs to be aware of the
load of a certain choice of quality for audio or video in the disc's datastream. When a lower audio
quality is used (refer to 3.x for more information about audio coding), fewer sectors will be
occupied than with a higer quality. For example, when level A stereo sound is used, only half of
the remaining sectors can be used for other information like video.
It's also possible to read for example only the sectors belonging to one audio channel at a time,
and then move back to the beginning of the disc and read the sectors of another audio channel.
Since the lowest audio quality only uses one in every sixteen sectors, and a CD-i disc lasts for 74
minutes, the total time for audio can be (16 x 74 min = ) over 19 hours!
Because of this realtime reading of sectors, every CD-i player reads data at the same speed,
sometimes refered to as normal speed or single speed. It would be unnecesary to make a CD-i
player with a higher speed CD-drive, since data is to be read in realtime accoring to the
specifications (thus single speed) and audio, video and animation would be out of sync when
being read at a higher speed. The designers of the CD-i system put more attention to the
development of encoding techniques that enable for high quality audio and video within the single
data speed and hence resulting in a longer playing time, instead of utilising a high speed drive
and by such reducing the playing time.
3.6 What's CD-i's sector format? What about mode 1 and 2, form 1 and 2?
For CD-ROM, the Yellow Book defines the mode 1 sector format. This format allows for 2048
bytes of user data in every sector, with an accompanying 280 bytes of error correction information
in each sector. When data is read at 75 sectors per second (the normal CD speed), this results at
a datarate of of 150 KB per second.
For CD-i it was not always necesary to have error correction in each sector. For example, audio
and video need a much lower degree of correction than data or programs. Instead, the 280 bytes
used for error correction in mode 1 could be added to the 2048 of user bytes, resulting in 2324
bytes of user data per sector. This larger sector size then results in an improved datarate of about
170 KB per second. The new sector format was called mode 2. Within mode 2, 2 forms were
defined: form 1 does incorporate the original error correction and is used for data and programs,
and form 2 which lacks the error correction and is to be used for audio, video and other media
information.
Mode 2 added an additional subheader to the header of mode 1, which holds information about
the type of data that is contained in a sector (audio, video, data, etc.), the way it is coded (for
example which audio level is used), and an indication of the used sector form. This subheader is
interpreted by the CD-i system for each sector, which is then processed by the appropriate
decoders. Both forms of mode 2 sectors can be interleaved, so that program data and audio and
video can be read instantaneously from the disc.
Because of the mixage of both form 1 an form 2 sectors, it is impossible to tell the capacity of a
CD-i disc. When all sectors are form 1, the disc wil hold 648 MB. When all sectors are form 2, the
capacity is 744 MB. CD-i's disc capacity will hence be between 648 and 744 MB.
Altough a CD-i disc consitsts of only mode 2 sectors, a CD-i system must be able to read mode 1
sectors on CD-ROM discs, and of course the audio sectors that are defined for CD-Audio in the
Red Book.