This is a consumer electronics format that uses the optical disc in combination with a computer to provide a home entertainment system that delivers music, graphics, text, animation, and video in the living room. Unlike a CD-ROM drive, a CD-I player is a standalone system that requires no external computer. It plugs directly into a TV and stereo system and comes with a remote control to allow the user to interact with software programs sold on discs. It looks and feels much like a CD player except that you get images as well as music out of it and you can actively control what happens. In fact, it is a CD-DA player and all of your standard music CDs will play on a CD-I player; there is just no video in that case.

For a CD-I disk, there may be as few as 1 or as many as 99 data tracks. The sector size in the data tracks of a CD-I disk is approximately 2 kbytes. Sectors are randomly accessible, and, in the case of CD-I, sectors can be multiplexed in up to 16 channels for audio and 32 channels for all other data types. For audio these channels are equivalent to having 16 parallel audio data channels instantly accessible during the playing of a disk.

If you want information about Philips CD-I products, you can call these numbers:

      US: Consumer hotline:    800-845-7301
          For nearest store:   800-223-7772
          Developers hotline:  800-234-5484

      UK: Philips CD-I hotline: 0800-885-885

"Discovering CD-I" available for $45 from Microware Systems Corporation

Other books by Philips IMS and published by Addison Wesley:

  "Introducing CD-I"             ISBN 0-201-62748-5
  "The CD-I Production Handbook" ISBN 0-201-62750-7
  "The CD-I Design Handbook"     ISBN 0-201-62749-3

A CD-ROM has several advantages over other forms of data storage, and a few disadvantages. A CD-ROM can hold about 650 megabytes of data, the equivalent of thousands of floppy discs. CD-ROMs are not damaged by magnetic fields or the xrays in airport scanners. The data on a CD-ROM can be accessed much faster than a tape, but CD-ROMs are 10 to 20 times slower than hard discs.

You cannot write to a CD-ROM. You buy a disc with the data already recorded on it. There are thousands of titles available.

CD-XA

Announced in August 1988 by Microsoft, Philips, and Sony, the CD-ROM XA (for Extended Architecture) format incorporates audio from the CD-I format. It is consistent with ISO 9660, (the volume and the structure of CD-ROM), is an application extension of the Yellow Book, and draws on the Green Book.

CD-XA defines another way of formatting sectors on a CD-ROM, including headers in the sectors that describe the type (audio, video, data) and some additional info (markers, resolution in case of a video or audio sector, file numbers, etc).

The data written on a CD-XA can still be in ISO9660 file system format and therefore be readable by MSCDEX and Unix CD-ROM file system translators. A CD-I player can also read CD-XA discs even if its own `Green Book' file system only resembles ISO9660 and isn't fully compatible. However, when a disc is inserted in a CD-I player, the player tries to load an executable application from the CD-XA, normally some 68000 application in the /CDI directory. Its name is stored in the disc's primary volume descriptor. CD-XA bridge discs, like Kodak's Photo CDs, do have such an application, ordinary CD-XA discs don't.

A CD-DA drive is a CD-ROM drive but with some of the compressed audio capabilities found in a CD-I player (called ADPCM). This allows interleaving of audio and other data so that an XA drive can play audio and display pictures (or other things) simultaneously. There is special hardware in an XA drive controller to handle the audio playback. This format came from a desire to inject some of the features of CD-I back into the professional market.

Cell Compression (from Sun Microsystem Inc.)

There are two flavors of Cell: the original called Cell or CellA, and a newer flavor called CellB. CellA is designed for use many times video, where one does not mind that the encoder runs at less than real time. For example, CD-ROM playback, distance learning, video help for applications. CellB is designed for use once video where the encoder must run at real time (interactive) rates. For example, video mail and video conferencing.

Both flavors of cell use the same basic technique of representing each 4x4 pixel block with a 16-bit bitmask and two 8-bit vector quantized codebook indices. This produces a compression of 12-1 (or 8-1) since each 16 pixel block is represented by 32 bits (16-bit mask, and two 8-bit codebook indices). In both flavors, further compression is accomplished by checking current blocks against the spatially equivalent block in the previous frame. If the new block is "close enough" to the old block, the block is coded as a skip code. Consecutive skip codes are run-length encoded for further compression. Modifying the definition of close enough allows one to trade off quality and compression rates. Both version of Cell typically compress video images down to about .75 to .5 bits/pixel.

Both flavors have many similar steps in the early part of compression. For each 4x4 block, the compressor calculates the average luma of the 16 pixels. It then partions the pixels into two groups, those whose luma is above the average and those whose luma is below the average. The compressor sets the 16-bit bitmask based on which pixels are in each partition. The compressor then calculates a color to represent each partition.

In Cell, the compressor calculates an average color of each partion, it then does a vector quantization against the Cell codebook (which is just a color-map). The encoded block is the 16-bit mask and the two 8-bit colormap indices. The compressor maintains statistics about how much error each codebook entry is responsible for and how many times each codebook entry is used. It uses these numbers to adaptively refine the codebook on each frame. Changed codebooks are sent in the bitstream.

In CellB, the compressor calculates the average luma for each partition and the average chroma for the entire block. This gives two colors [Y_lo, Cb_ave, Cr_ave] and [Y_hi, Cb_ave, Cr_ave]. The pair [Y_lo, Y_hi] is vector quantized against the Y/Y codebook and the pair [Cb_ave, Cr_ave] is vector quantized against the Cr/Cb codebook. Here the encoded block is the 16-bit mask and the two 8-bit VQ indices. Both of CellB's codebooks are fixed. This allows both the compressor and decompressor to run at high-speed by using table lookups. Both codebooks are designed with the human visual system in mind. They are not just uniform partition of the Y/Y or Cr/Cb space. Each codebook has fewer than 256 entries.

Cell (or CellA) is supported in XIL 1.0 from SMI. It is part of Solaris 2.2. CellB is supported in XIL 1.1 from SMI. It will be part of Solaris 2.3 when that becomes available. Complete bitstream definitions for both flavors of cell are in the XIL 1.1 programmer's guide. There is some discussion of the CellA bitstream in the XIL 1.0 programmer's guide.

CellB was used for the SMI Scott McNealy holiday broadcast, where he talk to the company in real-time over Sun Wide Area Network. This broadcast reach from Tokyo Japan to Munich Germany with over 3000 known viewers.

CELP (Code-Excited Linear Prediction)

The uncompressed bit rates for transmitting CIF at 29.97 frames/sec is 36.45 Mbit/sec.

Coded order