More on CDs

1. More on CDs

2. CD-DA Review • Recall that the CD-DA standard as set out in the “Red Book,” defines a sector or block of 3234 bytes. • Of those 3234 bytes, • 2352 are actual audio data • 98 are control (analog of servo information, synchronization and location) • 784 are EDC/ECC

3. CD-DA Percentages • Each CD-DA block or sector is • 72.7% actual audio data • 24.2% EDC/ECC • 3% control data • For every 8 bits of data, there are 3 more bits of control/error data. • If a CD-DA error cannot be corrected, one can interpolate.

4. CD-ROM • The red-book CD-DA standards were adapted by the “Yellow Book” standards to handle non-audio data, i.e. regular data files, programs, etc. • These standards are for CD-ROMs, compact disc – read only memory –information written by the manufacturer and not changed by the user.

5. By any other name • The original CD-ROM standard did not completely specify the logical structure (i.e. they did not nail down the analog of the hard disk’s file-system). • This was leading toward proprietary solutions and incompatibility. • The leading manufacturer’s met at the High Sierra Hotel and Casino in Lake Tahoe, NV, in 1985, to standardize CD-ROMs more fully. • The outcome is known as the High Sierra Format. • A slight modification was adopted by ISO as their standard (#9660).

6. CD-ROM Mode 1 • CD-ROM Mode 1 starts with the basic CD-DA sector division 3234 = 98 (control) + 784 (error) + 2352 (data) and devotes some of the data portion to additional error code and control yielding 3234 = 98 (control) + 784 (error) + 304 (more error/control) + 2048 (data).

7. CD-ROM Mode 1 Percentages • Each CD-ROM block or sector is • 63.3% actual data • 33.6% EDC/ECC • 3% control data • More than one-third error detection and error correction code

8. Less Data/Fewer Errors/More Control • The CD-ROM standards impose more control because one must be able to locate data with more precision. • The CD-ROM standard imposes more error detection and error correction because by the nature of the data it stores it does not have available the interpolation approach to dealing with errors that CD-DA does.

9. CD-DA Review • SPEED: 44,100 Hz sampling frequency, 2 bytes to represent the levels and another factor of two for recording in stereo • 176400 bytes/second. • 75 blocks/second using 2353 bytes/block • CAPACITY: 74 minutes of music • 333000 blocks on a CD • 783549000 bytes or 765184 KB or 747 MB.

10. Mode 1 Speed and Capacity • SPEED: • 75 blocks/second • 153600 byes/second (using 2048 bytes/block) = 150 KB/s • CAPACITY: • 333000 blocks • 681984000 bytes or 666000 KB or 650 MB.

11. Speeds • The 150 KB/s is the base data transfer rate for CD-ROM. • Higher speeds are reported as multiplicative factors of this base: 2, 3, etc. • Recall that as disc speeds got higher, they switched from CLV (constant data rate) to CAV (variable data rate). • The latter data rates may be reported with the term “Max” to indicate that the reported data rate is the maximum of the range (i.e. When one is on the outer edge of the CD). • Such drives must still support CLV at lower speeds so that they can play audio CDs.

13. The Other Speed Factor • The data transfer rate is the rate for reading consecutive data. Another speed factor is average access time which is the typical time required to locate a “random” address. • As with hard disks, this involves the radial positioning of the head (seek time) and the angular spinning of the disk (latency).

14. CD-ROM Mode 2 • The CD-Rom standards define a second mode that contains less error correction code. Mode 2 has 3234 = 98 (control) + 784 (error) + 16 (more error/control) + 2336 (data). (which is 72.2% actual data)

15. Going Backward? • Mode 2 almost returns to the the CD-DA standard and was meant for data such as compressed audio or video which didn’t require as much data integrity as regular files (again because interpolation becomes available). • The original Mode 2 standard was not used.

16. Extended Architecture • While the original Mode 2 standard was not used, a variation on it allowed regular data and audio or video data to be stored together. • The Extended Architecture standard CD-ROM XA allows the two types of data to be “interleaved” on the same track.

17. Extended Architecture • The Extended Architecture has two modes • Mode 2, Form 1, which is like Mode 1 and is used for regular data files • Mode 2, Form 2, which is like Mode 2 and is used for compressed audio/video files. • The two modes allowed for the mixing (interleaving of data). • The use of a CD-ROM XA disc requires a special drive. The drive understood the two formats being mixed. It also might be able to uncompress the data “on the fly.”

18. CD-Interactive • An extension of the CD-ROM-XA idea was the CD-i (i for interactive). In CD-I the disk stores data in different formats, but the data is related. One could mix programs and text and audio and video together to create a multimedia experience for the user that was controlled by the user. • The CD-I format was led by Philips and Sony (the same two that gave us the CD-DA standards) and are contained in the Green Book.

19. Bridge CD • Another set of standards that spanned the CD-ROM-XA and CD-I technologies was developed in the White Book. • CDs of this format are sometimes called bridge CDs since they work with both technologies. • The category includes Bridge the Kodak Photo CD format and the Video CD format. • Is being replaced by DVDs.

20. Video CD • Using MPEG compression one can put a 74-minute movie (video plus audio) in the same space used by CD-DA for just audio. • So-called video CDs or VCDs were of limited quality, required a special player and have been surpassed by DVDs.

21. Photo CD • The Photo CD is an example of CD-I technology developed by Kodak specially for photographic data. • Since under normal circumstances one is not going to make a master Photo CD and create from it many copies. The Photo CD standard are found in the Orange Book, which sets out the standards for CD-R, recordable CDs. • But they can be written following the CD-I standards.

22. CD-R • As typical files grew larger and larger, the limitations of the floppy disk as a software/data delivery mechanism became evident. • The CD-ROM had hundreds of times the capacity of a floppy, but was limited in that it was “read only.” The standards essentially required a manufacturing process to write data. • So the CD-R standards were introduced. They are laid out in the orange book mainly by Phillips in 1990.

23. CD-R • CD-R (compact disc, recordable) are also sometimes known as CD-WORM (write once read many) or just CD-WO (write once). • While the Yellow Book, Green Book and White Book outline variations in the logical overlay of the physical specifications laid out in the Red Book, the Orange book must outline new physical requirements that allow a user to record a CD. • Basically the same logical overlay as CD-ROM will be maintained for compatibility.

24. Changing Medium • Recall that a CD-ROM is stamped from a master. The stamping provides the pits. When reading, he pits and distinguished from the lands because the lands yield specular reflection (clean, organized) while the pits yield diffuse reflection (scattered). • Think of light reflecting off a mirror versus light reflecting a rippling body of water (pool, lake, etc.) • So all that is needed is to produce this change in the way light is reflected.

25. Before Recording (Burning) • A CD-R starts off with a plastic substrate. The substrate is not flat, rather it has a wobbly spiral groove. • Think of it as lined paper, the lines guide us as to where we will write the information. • On top of the plastic is a photosensitive dye. • On top of that is the reflective layer (gold, silver or aluminum alloy). • On top of that is a protective layer of plastic. • On top of that is the label.

26. Before Burning From http://entertainment.howstuffworks.com/cd-burner2.htm

27. Photosensitive Dye • The chemical between the plastic and the metal is photosensitive, meaning that it changes its properties when exposed to light. • The process is known as burning. • In this case the light is laser light, the changes are permanent, and the important aspect here is that the changed property affects the way light is reflected.

28. After Burning

29. All at once? • The recording is permanent but it does not necessarily have to be done all at once. • If the recording is allowed to be done in more than one sitting, then the recording is said to be multi-session as opposed to single-session. • Another term is Track-At-Once (TAO) versus Disc-At-Once (DAO).

30. Table of Contents • A CD-ROM has a table of contents (TOC) at the beginning. It serves a similar purpose to the file allocation table and root directory found on a hard disk, it allows the files to be found. • A.k.a. index of the disc. • Newer so-called multi-session CDs allow data to be written in various sessions, that is for data to be appended (not overwritten) at a later time. • Such CDs have a table of contents for each session. • The new TOC contains the old info plus the new. • Such CDs cannot be read by ordinary drives unless they are “finalized.”.

31. Single Session vs. Multiple Session • With single-session CDs, the TOC is easily located by the drive. Multi-session CDs drives need to be able to find the latest TOC. • Many older CD drives do not have this capability. • Reading a CD-RW requires a multi-session capability, so it is becoming standard.

32. CD-R Drive • The burning of a CD requires a special laser, different from the one in an ordinary CD-ROM drive. • CD-Rs can typically handle many of the various logical formats (CD-DA, CD-ROM, CD-I, etc.) • A CD-R drive can also read. Typically it reads at a higher speed than it writes. • Writing a CD requires a steady flow of data and can be demanding. • A simple CD-ROM drive is often IDE/ATAPI but CD-R drives are more likely to be SCSI, which tend to perform better and allow other things to occur simultaneously.

33. Keep that data flowing • To keep the flow of data steady, a CD-R drive may use a buffer. • Another approach is to make am image of the disc to be burned on the hard drive (collect all the files from their various locations, add the error code, etc.) and then have a have a nice, steady, fast continuous read of consecutive data.

34. Compatibility • With differences in physical makeup (CD-ROM vs. CD-R vs. CD-RW) as well as differences in how writing is done (single-session vs. multi-session) and differences in logical overlay (CD_DA, CD-ROM, CD-I, etc.), compatibility is an issue with CD drives and CDs. • Another difference is whether a drive can extract CD-DA data or just play it. • Compatibility issues are being resolved, newer CD players tend to accept a wider variety of formats, but it remains an issue to be aware of.

35. Compatibility Table

36. Usefulness of CD-R • CD-Rs had floppies beat on capacity by a (multiplicative) factor of several hundred, but floppies could be written and re-written. • For files that are in the editing/updating process, the ability to rewrite is crucial. • CD-Rs have come down in price, but they’re too expensive to be throwing away regularly – let alone the impact that would have on the environment.

37. CD-RW • CD-Rs are “write once” because the photo-sensitive dye is permanently changed when it is written/burned. • In CD-RW this material is replaced with a phase-change layer. • A phase is a state that a material can be in. For example, H2O can be in one of three phases: ice (solid), water (liquid), stream (gas). • Materials may have several different solid phases. • A phase is stable (the material can remain in a given phase indefinitely), but a phase change is reversible (heating for example may return the material to its original state).

38. Dr Jekyll and Mr Hyde • The two phases in question are a matter of how “organized” the material is. • In one phase, the material is a very orderly crystal that reflects light specularly. • In the other phase, the material is disorderly and reflects light diffusely. • Heating the material a little can give it a little bit of movement and flexibility so it can organize. Heating it up a lot throws it into disorder, but it cools quickly (quenches) and remains in the disorderly state.

39. The price one pays • The differences in these phases is more subtle than the differences in CD-ROMs and CD-Rs. • Thus a drive must have a more sensitive photo-detector in order to read a CD-RW. • Many older (“legacy”) drive do not have such a sensitive detector and cannot read CD-RWs.

40. Digital Versatile/Video Disk (DVD) • The size of software and data files continues to grow, so the CD with its 650 MB capacity is becoming too limited. • A newer, higher capacity alternative is the DVD which stands for either digital video disk or digital versatile disk. • Although some will now say DVD doesn’t stand for anything.

41. DVDs • Physically DVDs are very similar to CDs. • The differences are more on the logical side. • As with CDs there are various formats and one has to be careful about compatibility.

42. References • PC Hardware in a Nutshell, Thompson and Thompson • http://www.pcguide.com • http://entertainment.howstuffworks.com/cd-burner2.htm