Games of Daze

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CD-ROM Technical Summary From Plastic Pits to "Fantasia" Andy Poggio March 1988 Abstract This summary describes how information is encoded on Compact Disc (CD) beginning with the physical pits and going up through higher levels of data encoding to the structured multimedia information that is possible with programs like HyperCard. This discussion is much broader than any single standards document, e.g. the CD-Audio Red Book, while omitting much of the detail needed only by drive manufacturers. Salient Characteristics 1. High information density -- With the density achievable using optical encoding, the CD can contain some 540 megabytes of data on a disc less than five inches in diameter. 2. Low unit cost -- Because CDs are manufactured by a well-developed process similar to that used to stamp out LP records, unit cost in large quantities is less than two dollars. 3. Read only medium -- CD-ROM is read only; it cannot be written on or erased. It is an electronic publishing, distribution, and access medium; it cannot replace magnetic disks. 4. Modest random access performance -- Due to optical read head mass and data encoding methods, random access ("seek time") performance of CD is better than floppies but not as good as magnetic hard disks. 5. Robust, removable medium -- The CD itself is comprised mostly of, and completely coated by, durable plastic. This fact and the data encoding method allow the CD to be resistant to scratches and other handling damage. Media lifetime is expected to be long, well beyond that of magnetic media such as tape. In addition, the optical servo scanning mechanism allows CDs to be removed from their drives. 6. Multimedia storage -- Because all CD data is stored digitally, it is inherently multimedia in that it can store text, images, graphics, sound, and any other information expressed in digital form. Its only limit in this area is the rate at which data can be read from the disc, currently about 150 KBytes/second. This is sufficient for all but uncompressed, full motion color video. CD Data Hierarchy Storing data on a CD may be thought of as occurring through a data encoding hierarchy with each level built upon the previous one. At the lowest level, data is physically stored as pits on the disc. It is actually encoded by several low-level mechanisms to provide high storage density and reliable data recovery. At the next level, it organized into tracks which may be digital audio or CD-ROM. The High Sierra specification then defines a file system built on CD-ROM tracks. Finally, applications like HyperCard specify a content format for files. The Physical Medium The Compact Disc itself is a thin plastic disk some 12 cm. in diameter. Information is encoded in a plastic-encased spiral track contained on the top of the disk. The spiral track is read optically by a noncontact head which scans approximately radially as the disk spins just above it. The spiral is scanned at a constant linear velocity thus assuring a constant data rate. This requires the disc to rotate at a decreasing rate as the spiral is scanned from its beginning near the center of the disc to its end near the disc circumference. The spiral track contains shallow depressions, called pits, in a reflective layer. Binary information is encoded by the lengths of these pits and the lengths of the areas between them, called land. During reading, a low power laser beam from the optical head is focused on the spiral layer and is reflected back into the head. Due to the optical characteristics of the plastic disc and the wavelength of light used, the quantity of reflected light varies depending on whether the beam is on land or on a pit. The modulated, reflected light is converted to a radio frequency, raw data signal by a photodetector in the optical head. Low-level Data Encoding To ensure accurate recovery, the disc data must be encoded to optimize the analog-to-digital conversion process that the radio frequency signal must undergo. Goals of the low level data encoding include: 1. High information density. This requires encoding that makes the best possible use of the high, but limited, resolution of the laser beam and read head optics. 2. Minimum intersymbol interference. This requires making the minimum run length, i.e. the minimum number of consecutive zero bits or one bits, as large as possible. 3. Self-clocking. To avoid a separate timing track, the data should be encoded so as to allow the clock signal to be regenerated from the data signal. This requires limiting the maximum run length of the data so that data transitions will regenerate the clock. 4. Low digital sum value (the number of one bits minus the number of zero bits). This minimizes the low frequency and DC content of the data signal which permits optimal servo system operation. A straightforward encoding would be to simply to encode zero bits as land and one bits as pits. However, this does not meet goal (1) as well as the encoding scheme actually used. The current CD scheme encodes one bits as transitions from pit to land or land to pit and zero bits as constant pit or constant land. To meet goals (2) to (4), it is not possible to encode arbitrary binary data. For example, the integer 0 expressed as thirty-two bits of zero would have too long a run length to satisfy goal (3). To accommodate these goals, each eight-bit byte of actual data is encoded as fourteen bits of channel data. There are many more combinations of fourteen bits (16,384) than there are of eight bits (256). To encode the eight-bit combinations, 256 combinations of fourteen bits are chosen that meet the goals. This encoding is referred to as Eight-to-Fourteen Modulation (EFM) coding. If fourteen channel bits were concatenated with another set of fourteen channel bits, once again the above goals may not be met. To avoid this possibility, three merging bits are included between each set of fourteen channel bits. These merging bits carry no information but are chosen to limit run length, keep data signal DC content low, etc. Thus, an eight bit byte of actual data is encoded into a total of seventeen channel bits: fourteen EFM bits and three merging bits. To achieve a reliable self-clocking system, periodic synchronization is necessary. Thus, data is broken up into individual frames each beginning with a synchronization pattern. Each frame also contains twenty-four data bytes, eight error correction bytes, a control and display byte (carrying the subcoding channels), and merging bits separating them all. Each frame is arranged as follows: Sync Pattern 24 + 3 channel bits Control and Display byte 14 + 3 Data bytes 12 * (14 + 3) Error Correction bytes 4 * (14 + 3) Data bytes 12 * (14 + 3) Error Correction bytes 4 * (14 + 3) TOTAL 588 channel bits Thus, 192 actual data bits (24 bytes) are encoded as 588 channel bits. Editorial: A CD physically has a single spiral track about 3 miles long. CDs spin at about 500 RPM when reading near the center down to about 250 RPM when reading near the circumference. Disc with a 'c' or disk with a 'k'? A usage has emerged for these terms: disk is used for eraseable disks (e.g. magnetic disks) while disc is used for read-only (e.g. CD-ROM discs). One would presumably call a frisbee a disc. First Level Error Correction Data errors can arise from production defects in the disk itself, defects arising from subsequent damage to the disk, or jarring during reading. A significant characteristic of these errors is that they often occur in long bursts. This could be due, for example, to a relatively wide mark on the disc that is opaque to the laser beam used to read the disc. A system with two logical components called the Cross Interleave Reed-Solomon Coding (CIRC) is employed for error correction. The cross interleave component breaks up the long error bursts into many short errors; the Reed-Solomon component provides the error correction. As each frame is read from the disc, it is first decoded from fourteen channel bits (the three merging bits are ignored) into eight-bit data bytes. Then, the bytes from each frame (twenty-four data bytes and eight error correction bytes) are passed to the first Reed-Solomon decoder which uses four of the error correction bytes and is able to correct one byte in error out of the 32. If there are no uncorrectable errors, the data is simply passed along. If there are errors, the data is marked as being in error at this stage of decoding. The twenty-four data bytes and four remaining error correction bytes are then passed through unequal delays before going through another Reed-Solomon decoder. These unequal delays result in an interleaving of the data that spreads long error bursts among many different passes through the second decoder. The delays are such that error bursts up to 450 bytes long can be completely corrected. The second Reed-Solomon decoder uses the last four error correction bytes to correct any remaining errors in the twenty-four data bytes. At this point, the data goes through a de-interleaving process to restore the correct byte order. Subcoding Channels and Blocks The eight-bit control and display byte in each frame carries the subcoding channels. A subcoding block consists of 98 subcoding bytes, and thus 98 of the 588-bit frames. A block then can contain 2352 bytes of data. Seventy-five blocks are read each second. With this information, it is now straightforward to calculate that the CD data rate is in fact correct for CD digital audio (CD-DA): Required CD digital audio data rate: 44.1 K samples per second * 16 bits per sample * 2 channels = 1,411,200 bits/sec. CD data rate: 8 bits per byte * 24 bytes per frame * 98 frames per subcoding block * 75 subcoding blocks per second = 1,411,200 bits/sec. The eight subcoding channels are labeled P through W and are encoded one bit for each channel in a control and display byte. Channel P is used as a simple music track separator. Channel Q is used for control purposes and encodes information like track number, track type, and location (minute, second, and frame number). During the lead-in track of the disc, channel Q encodes a table of contents for the disk giving track number and starting location. Standards have been proposed that would use the remaining channels for line graphics and ASCII character strings, but these are seldom used. Track Types Tracks can have two types as specified in the control bit field of subchannel Q. The first type is CD digital audio (CD-DA) tracks. The two-channel audio is sampled at 44.1 Khz with sixteen bit linear sampling encoded as twos complement numbers. The sixteen bit samples are separated into two eight-bit bytes; the bytes from each channel alternate on the disc. Variations for audio tracks include pre-emphasis and four track recording. The other type of track specified by the subchannel Q control bit field is the data track. These must conform to the CD-ROM standard described below. In general, a disc can have a mix of CD digital audio tracks and a CD-ROM track, but the CD-ROM track must come first. Editorial: This first level error correction (the only type used for CD Audio data) is extremely powerful. The CD specification allows for discs to have up to 220 raw errors per second. Every one of these errors is (almost always) perfectly corrected by the CIRC scheme for a net error rate of zero. For example, our tests using Apple's CD-ROM drive (which also plays audio) show that raw error rates are around 50-100 per second these days. Of course, these are perfectly corrected, meaning that the original data is perfectly recovered. We have tested flawed discs with raw rates up to 300 per second. Net errors on all of these discs? Zero! I would expect a typical audio CD player to perform similarly. Thus I expect this raw error rate to have no audible consequences. So why did I say "almost always" corrected above? Because a sufficiently bad flaw may produce uncorrectable errors. These very unusual errors are "concealed" by the player rather than corrected. Note that this concealment is likely to be less noticeable than even a single scratch on an LP. Such a flaw might be a really opaque finger smudge; CDs do merit careful handling. On the two (and only two) occasions I have found these, I simply sprayed on a little Windex glass cleaner and wiped it off using radial strokes. This restored the CDs to zero net errors. One can argue about the quality of the process of conversion of analog music to and from digital representation, but in the digital domain CDs are really very, very good. CD-ROM Data Tracks Each CD-ROM data track is divided into individually addressable blocks of 2352 data bytes, i.e. one subcoding block or 98 frames. A header in each block contains the block address and the mode of the block. The block address is identical to the encoding of minute, second, and frame number in subcode channel Q. The modes defined in the CD-ROM specification are: Mode 0 -- all data bytes are zero. Mode 1 -- (CD-ROM Data): Sync Field - 12 bytes Header Field - 4 User Data Field - 2048 Error Detection Code - 4 Reserved - 8 Error Correction - 276 Mode 2 -- (CD Audio or Other Data): Sync Field - 12 bytes Header Field - 4 User Data Field - 2048 Auxiliary Data Field - 288 Thus, mode 1 defines separately addressable, physical 2K byte data blocks making CD-ROM look at this level very similar to other digital mass storage devices. Second Level Error Correction An uncorrected error in audio data typically results in a brief, often inaudible click during listening at worst. An uncorrected error in other kinds of data, for example program code, may render a CD unusable. For this reason, CD-ROM defines a second level of error detection and error correction (EDC/ECC) for mode 1 data. The information for the EDC/ECC occupies most of the auxiliary data field. The error detection code is a cyclic redundancy check (CRC) on the sync, header, and user data. It occupies the first four bytes of the auxiliary data field and provides a very high probability that uncorrected errors will be detected. The error correction code is essentially the same as the first level error correction in that interleaving and Reed-Solomon coding are used. It occupies the final 276 bytes of the auxiliary data field. Editorial: This extra level of error correction for CD-ROM blocks is one of the many reasons that CD-ROM drives are much more expensive than consumer audio players. To perform this error correction quickly requires substantial extra computing power (sometimes a dedicated microprocessor) in the drive. This is also one reason that consumer players like the Magnavoxes which claim to be CD-ROM compatible (with their digital output jack on the back) are useless for that purpose. They have no way of dealing with the CD-ROM error correction. They also have no way for a computer to tell them where to seek. Another reason that CD-ROM drives are more expensive is that they are built to be a computer peripheral rather than a consumer device, i.e. like a combination race car/truck rather than a family sedan. One story, probably apocryphal but not far from the truth, has it that a major Japanese manufacturer tested some consumer audio players to simulate computer use: they made them seek (move the optical head) from the inside of the CD to the outside and back again. These are called maximum seeks. The story says they managed to do this for about 24 hours before they broke down. A CD-ROM drive needs to be several orders of magnitude more robust. Fast and strong don't come cheap. The High Sierra File System Standard Built on top of the addressable 2K blocks that the CD-ROM specification defines, the next higher level of data encoding is a file system that permits logical organization of the data on the CD. This can be a native file system like the Macintosh Hierarchical File System (HFS). Another alternative is the High Sierra (also known as the ISO 9660) file standard, recently approved by the National Information Standards Organization (NISO) and the International Standards Organization (ISO), which defines a file system carefully tuned to CD characteristics. In particular: 1. CDs have modest seek time and high capacity. As a result, the High Sierra standard makes tradeoffs that reduce the number of seeks needed to read a file at the expense of space efficiency. 2. CDs are read-only. Thus, concerns like space allocation, file deletion, and the like are not addressed in the specification. For High Sierra file systems, each individual CD is a volume. Several CDs may be grouped together in a volume set and there is a mechanism for subsequent volumes in a set to update preceding ones. Volumes can contain standard file structures, coded character set file structures for character encoding other than ASCII, or boot records. Boot records can contain either data or program code that may be needed by systems or applications. High Sierra Directories and Files The file system is a hierarchical one in which directories may contain files or other directories. Each volume has a root directory which serves as an ancestor to all other directories or files in the volume. This dictates an overall tree structure for the volume. A typical disadvantage in hierarchical systems is that to read a file (which must be a leaf of the hierarchy tree) given its full path name, it is necessary to begin at the root directory and search through each of its ancestral directories until the entry for the file is found. For example, given the path name Wine Regions:America:California:Mendocino three directories (the first three components of the path name) would need to be searched. Typically, a separate seek would be required for each directory. This would result in relatively poor performance. To avoid this, High Sierra specifies that each volume contain a path table in addition to its directories and files. The path table describes the directory hierarchy in a compact form that may be cached in computer memory for optimum performance. The path table contains entries for the volume's directories in a breadth-first order; directories with a common parent are listed in lexicographic order. Each entry contains only the location of the directory it describes, its name, and the location in the path table of its parent. This mechanism allows any directory to be accessed with only a single CD seek. Directories contain more detailed information than the path table. Each directory entry contains: Directory or file location. File length. Date and time of creation. Name of the file. Flags: Whether the entry is for a file or a directory. Whether or not it is an associated file. Whether or not it has records. Whether or not it has read protection. Whether or not it has subsequent extents. Interleave structure of the file. Interleaving may be used, for example, to meet realtime requirements for multiple files whose contents must be presented simultaneously. This would happen if a file containing graphic images were interleaved with a file containing compressed sound that describes the images. Files themselves are recorded in contiguous (or interleaved) blocks on the disc. The read-only nature of CD permits this contiguous recording in a straightforward manner. A file may also be recorded in a series of noncontiguous extents with a directory entry for each extent. The specification does not favor any particular computer architecture. In particular all significant, multibyte numbers are recorded twice, once with the most significant byte first and once with the least significant byte first. Multimedia Information Using the file system are applications that create and portray multimedia information. While it is true that a CD can store anything that a magnetic disk can store (and usually much more of it), CDs will be used more for storing information than for storing programs. It is the very large storage capacity of CDs coupled with their low cost that opens up the possibilities for interactive, multimedia information to be used in a multitude of ways. Programs like HyperCard, with it's ease of authoring and broad extensibility, are very useful for this purpose. Hypercard stacks, with related information such as color images and sound, can be easily and inexpensively stored on CDs despite their possibly very large size. Editorial: The High Sierra file system gets its name from the location of the first meeting on it: the High Sierra Hotel at Lake Tahoe. It is much more commonly referred to as ISO 9660, though the two specifications are slightly different. It has gotten very easy and inexpensive to make a CD-ROM disc (or audio CD). For example, you can now take a Macintosh hard disk and send it with $1500 to one of several CD pressers. They will send you back your hard disk and 100 CDs with exactly the same content as what's on your disk. This is the easy way to make CDs with capacity up to the size of your hard disk (Apple's go up to 160 megabytes). True, this is not a full CD but CDs don't need to be full. If you have just 10 megabytes and need 100 copies, CDs may be the best way to go. If you are buying a CD-ROM drive, there are several factors you might consider in making your choice. Two factors NOT to consider are capacity and data rate. The capacity of all CD-ROM drives is determined solely by the CD they are reading. Though you will see a range of numbers in manufacturers' specs (e.g. 540, 550, 600, and 650 Mbytes), any drive can read any disc and so they are all fundamentally the same. All CD-ROM drives read data at a net 150 Kbytes/sec for CD-ROM data. Other data rates you may see may include error correction data (not included in the net rate) or may be a mode 2 data rate (faster than mode 1). All drives will be the same in all of these specs. End of article.