MFDigital CD DVD Information Library
Friday, June 11, 2004
Compact Disc Interactive
Introduction to CD-i
Compact Disc-interactive (CD-i) was developed by Philips and Sony during the mid to late 1980s as a multimedia system for the home, education and training. The CD-i specification was originally launched in 1986, then updated but players did not appear until around 1990. A few years later Philips pulled out of the consumer market, after several hundred thousand had been sold world-wide, but now CD-i is finding success in training and other professional applications, particularly in the USA.

CD-i was developed as a more friendly version of a home computer, with excellent multimedia capabilities. Full screen motion video was not included in the original specification but was added later. As it uses MPEG-1 CD-i players make excellent Video CD players. CD-i players are much simpler than today's PCs and are still easier to use and much quicker to boot up from cold. With their TV video output they are more at home in the living room than PCs.

The CD-i specification (the Green Book) is still the largest and most comprehensive of all the CD specifications. This is because it defines not just the disc format and on-disc data coding, but also the player hardware and software operating system.

CD-i Disc Formats
Unlike CD-ROM, which was conceived as a general purpose data storage medium, CD-i discs were designed at the outset for multimedia, ie the presentation of audio, video, graphics and text data together.

Therefore CD-i discs comprise, like CD-ROM XA, mode 2 form 1 and 2 sectors. Each sector contains data of only one type: audio, video (still or motion) or other data. The multimedia data on a CD-i disc is therefore interleaved sector by sector and can be readily de-interleaved in the player.

Each sector contains a subheader used to describe the contents of that sector. The submode byte in the subheader defines the data type (audio, video or other) and other information to facilitate de-interleaving the data. The audio and video data must conform to one of a number of formats defined in the CD-i specification..

CD-i data can comprise one or more 'streams' of audio data, one for each language, for example, together with motion video, still images and/or graphics data. The subheader in each sector can also contain trigger bits which create events which the software can respond to.

CD-i Audio
Audio data comprises three quality levels of ADPCM (from voice to hifi) plus CD-DA. ADPCM (Adaptive Delta Pulse Code Modulation) compresses the PCM data by allocating 4 or 8 bits per sample and using a range parameter (per group of samples) to, in effect, define the volume for that group. The three audio levels have the following parameters (all levels can be stereo or mono):

Level | Sample rate | Bits/sample | % sectors used | Quality
A | 37.8 kb/s | 8 | 50% | Hifi
B | 18.9 kb/s | 8 | 25% | FM radio
C | 18.9 kb/s | 4 | 12.5% | Voice

The sectors used column assumes stereo. Level C mono can be used for up to 16 voice channels, eg in different languages. The sector structure facilitates switching between languages on the fly, as the sectors are interleaved on the disc.

Still Images
CD-i provides two image planes which can be combined using mixing, transparency and other effects. Image resolutions are normally 384 x 240 (NTSC) and 384 x 280. Double resolution images are 768 pixels wide.

Motion Video
Motion video is available as an option using MPEG-1, by adding a plug-in video decoder. CD-i players were the first to make use of MPEG-1 video and can be used to play Video CDs.

CD-i offers a range of image sizes within the MPEG-1 constrained parameter system, which defines the maximum number of macroblocks (each 16 x 16 pixels) as 396. This allows image sizes of up to 352 x 288 which is very close to the CD-i PAL screen size of 384 x 280. All but the earliest video decoders expand the video image from 352 to 384 pixels to maintain the aspect ratio of the original video image.

The MPEG decoder provides a third plane underneath the two still image planes offering a wide range of visual effects combining still images, animated cartoons and full screen motion video.

CD-i Players
CD-i players are based on the Motorola 68000 processor with 1MB memory (increased to 1.5MB if the MPEG decoder is used), two-plane video decoder (plus optional MPEG) with visual effects, audio processor, single speed CD-ROM drive, non-volatile memory and user interface (normally a remote handset with pointer device).

CD-i players are available from Philips, although these have now been withdrawn from consumer outlets. The CD-i player is now finding success in professional, training and educational applications and, now that stocks of consumer players have dried up, Philips is apparently having to increase production to fulfill the demand particularly in the USA.

The MPEG-1 video decoder was not included in the base case CD-i player, but is an add-in module necessary for many applications. It also makes the CD-i player and excellent Video CD version 2.0 player.

CDRTOS CDRTOS is the real-time operating system designed for CD-i. It allows multi-tasking and facilitates event-driven programming. In many ways CD-RTOS is superior to Windows 3.1 in its multimedia facilities, which only Windows 95 can provide in a comparable way.

CDRTOS was developed by Microware and is based on OS9, an industrial real-time multi-tasking operating system. It comprises a kernel with managers for CD file access, user interface, non-volatile RAM (NVRAM) and audio and video decoding plus a video driver.

The hardware features of CD-i , including the 2-plane video decoder, MPEG decoder, user interface and NVRAM are fully supported by CDRTOS.