FOCUS ON: CD ROM STANDARDSAND APPLICATIONS

Since the introduction of the audio compact disc (CD) in 1982,this digital information carrier has been the nucleus of manydevelopments. Today, the CD is not only used for music, butalso for computer data and video. This article discusses thevarious CD standards that have evolved over the years. Theseare world standards which have been defined in the Red Book,Yellow Book, Green Book, Orange Book and White Book.

By our editorial staff

THE Compact Disc was born inEiridhoven, The Netherlands, at the

Philips Research Laboratories. There,the concept of optical registration of digi-tal data was developed. In 1980, whenPhilips N.V. and Sony agreed to promotethe CD as a carrier of digital audio, itwas difficult to fore see tbe overwhelmingeffect the silver disc would have on themusic industry. In five years' time, some30 million CD players were operational ,and 450 million CDs sold. The gramo-phone, Edison's invention, and the mostimportant sound medium for BO-addyears, was dead and buried at 'record'speed.

Since then , there's Da stopping theCD. The shining silver disc is used for anincreasing number of applications. TheWbite Book, for instance, published in1994 by Philips and JVC, gives details ofthe video CD, a CD variant which lookspoised to open the attack on the pre-

recorded video tape. In the course of thisyear, both tbe computer industry andmanufacturers of consumer electronicswill introduce various systems whichsupport digital video based on the videoCD. This puts the video CD in a positionto become accepted just as rapidly as theCD-DA (audio CD).

Today there are so many CD stan-dards that it is difficult to see the woodfor the trees. These standards are, there-fore, briefly discussed below.

Red Book, CD-DAThe standard for digital audio, CD·DA,may be found in the Red Book, whichcontains a11technical specifications con-cerning registration and error correctionpertaining to the CD. The Red Book hasbecome the basis for later CD standardsand developments, which were onee be-yond the specs laid down in the Red Book

because of high technical demands on theCD.

The information on a CD consists of aspiral-shaped string ofholes pressed intoa transparent polycarbonate layer. Next,a reflecting aluminium layer is appliedwhich is covered with a transparent pro-tection layer. The space between twoholes is called an islaud. A laser bearnscans the spiral from centre of the disc.Because the recording device in the laserdrive unit is capable of detecting whetherthe light is reflected by a hole 01' an is-land, it is possible to recover the informa-tion pressed on the CD. Variations in theintensity ofthe reflected light are used tofocus the laser via a complex servo sys-tem.

The spiral on the CD is divided intothree sections: the lead-in, the program(data) and the lead-out. To make the bestpossible use of the space on the CD, thespiral is scanned at a constant speed.The CD's rotation speed is therefore de-pendent on the position of the laser. Inthe program section, the rctation speedcan vary between 197 and 539 revolu-tions per minute. Because a mechanicalspeed control alone does not enable aconstant data flow, the player has abuffer memory which has to be half filledall the time. If less than 50% ofthe avail-able space is used, the drive speed is in-creased. If more than 50% is used, thespeed is lowered acccrdingly. The dataread by the laser is clocked out of thebuffer with the aid of a quarta-controlledoscillator. The buffer/clock system guar-antees a perfectly stable datastream intothe DAC which follows the CD drive unit.

All data on an audio CD representssound signals whicb have been sampledat a frequency of 44.1 KHz. Each soundchannel is linearly digitized at a resolu-tion of 16 bits. The result is a datastreamwith a speed of 1.4112 Mbit per second.

Yellow Book, CD-ROMThe next standard to appear on the mar-ket was defined in the Yellow Book,again produced jointly by Philips andSony. Once the enormaus storage capac-ity of CDs was fully recognized, the ideaof a CD·ROM was born. The CD-ROM ismarketed as the re placement for mag-netic media such as floppy disks andhard disks. It has a number of advan-tages and disadvantages. The advan-tages include low cost, large storagecapacity, non-volatile character, and reli-ability. On the down side, a CD-ROM is aread-only medium.

Ta maintain compatibility with the

ELEKTOR ELECTRONICS MARCH 1995

CD·DA, the CD·ROM also contains onelang spiral. This is in stark contrast withthe long-establiehed system of paralleltracks on magnetic media. Because theCD-ROM 1Salso read at constant speed,it has a fairly large randorn read accesstime. Any time a file has to be read, themotor speed has to be adjusted until theright reading speed is achieved, and thattakes time. Furthermore, the data trans-fer speed offered by the CD·ROM is rela-tively low at about 150 KByte per second.Fortunately, double, tripIe and quadru-pIe speed CD-ROM drives have errived,and we may see further improvements inthe near future.

The information in the Yellow Bookdoes not go much further than a deserip-tion of the idea to store computer infor-mation in the program section. Further >

it indicates the types of error correctionmethods which can be added to the sys-tem. The Yellow Book contains neitherdescriptions of tbe structure of files anddirectories as used with computers, nOTinformation on how audio-visual (multi-media) information is to be stored.Consequently, the ISO·9660 and CD·ROM-XA standards were introducedlater.

Tbe ISO·9660 standard was initiallydeveloped by the High Sierra Group, andtaken over by the ISO in 1988. It de-scribes the file structure used with IBM-compatible PCs, and has become thebasis for later standards. The UNIX plat-form uses the RRIP (Rock RidgeInterchange Protocol), while AppleMacintosh users have the HFS(Hierarchical File System) for their CD·ROMs.

Green Book, CD-iIn 1986, less than a year after the intro-duction ofthe CD·ROM, the CD Inter-ac-tive (CD-i) was announced as amulti-medie system for the consumermarket. The complete system descriptionmay be found in the Green Book. Multi-media systems which process audio-visualinformation are an interesting applicationarea of the CD·ROM. As already men-tioned in the description of the CD-DA,audio information is recorded at a speedof 1.4112 MBit/s. Because video informa-tion is far more complex than audio infor-mation, the amount of da ta required fordigital video will be even greater. Bothwith audio and video, there is an interac-tion between the necessary datastreamand the quality of the encoded signal. TheCD-i specification describes lots of possi-bilities for the recording of multi-medieinformation such as sound, still-video, an-imations and video. Moreover, the infor-mation is stored in an interleavedpattern, which means that, for instance,audio and video are interwoven in blocks.That allows the CD·i player to read sev-eral types ofinformation simultaneously.

The essential difference between CD-i

ELEKTOR ELECTRONICS MARCH 1995

FOCUS ON CD·ROM STANDARDS AND APPLICATIONS

, :-:tl

Fig. 1. The CD-DA (audio CD) was the first CD for a big audience. This Philips CD player wasa market hit during the early nineteen-eighties.

and CD·ROM is that the specifications ofthe former include a complete descrip-tion of the hardware wh.ich is necessaryto be able to use the CD· i softwate. TheGreen Book also provides infcrmation onthe structure of the data pressed on aCD·i disc. A marked difference with theYellow Bock, which has nothing on datastorage formats.

Cbapter 9 of the Green Book describeshow MPEG 1 full-screen video encodedaccording to ISO 11172 may be added toCD-i information. This system is called

'digital video on CD-i' and has beenadded to the CD·i system as an option.

CD-ROM XACD·ROM XA (aXtended Architecture)denotes an extension of the CD-ROMstandard drawn up to make the mediumbetter suited to multi-medie applica-tions. Broadly speaking, the XA standardenables a number of CD-i compatibleaudio and video formats to be used witbCD-ROM also. Furthermore, it intro-

Fig. 2. The CD-ROM drive is on the way to becoming just another standard peripheral in com-puter land.

GENERALINTEREST

duces the possibility of different start di-rectories, which enables you to run a CDunder different operating systems.Finally, the CD-ROM-XA standard, likethe CD-i, is structurally compatible withthe 180-9660 standard. That makes theCD-ROM-XA suitable for many differentcomputer systems. This type of disc is,however, unsuitable for CD-i players be-cause they often lack the appropriateprograrns. Despite the fact that the XAstandard has been with us for some time,little software has become available sorar which utilizes this standard.

Sector structureAccording to the definitions in the YellowBook, sectors on a CD have a length of2,352 bytes, and are subdivided into anumber of fields to make them accessibleto computers. The start of a sector is in-dicated by a synchronization pattern of12 bytes. This is followed by a 4-bytehe ader which contains its absolute ad-dress in minutes, seconds and sectors, aswell as a mode byte. That leaves2,336 bytes available for data storage ineach sector.

All sectors in one track use the samemode. Sectors exist in three differentmodes. In Mode-O sectors, all remaining2,336 bytes are empty, i.e., zero. Mode-lsectors contain 2,048 bytes of user data,protected by an EDC (error-deteetingcode) and an ECC (error-correcting code).This error protection is additional to theprotection already used for CDs.Although the standard error correctionfor CDs is app1ication independent, it isnot powerful enough for computer appli-cations. This is because small data errorsare unacceptable in computer systems,although they may go by unnotieed inaudio systems.

In Mode-2, which is the format usedfor normal CD-ROMs that do not requireextra error correetion (for instance, digi-tized audio/video), the remaining2,336 bytes are available to hold data.The structure of Mode-2 sectors is fur-ther detailed in the Green Book to ensureeompatibility with the 180-9660 stan-dard. The extension with Mode-2 is usedwith CD-i and CD-ROM-XA. It involvesthe storage of data in sectors by assign-ing a sub-header to each sector. This sub-header contains a file and channelnumber coding infortnation for details onthe type of data contained in the sector,as weIl as a sub-mode byte. The file num-ber is used to distinguish between sec-tors of different, interleaved, files. Thechannel number is used to support thedifferent channels a file rnay be COID-

posed of. The sub-mode byte contains theend-of-file (EOF) and end-of-reeord(EOR) bits, an interrupt trigger bit to en-able synchronization, and a real-time bitwhich indicates that the file is used inreal time mode.

Finally, three bits follow whieh indi-

Table 1.

standard bookRed BoakYellow BookGreen Baok

Standard

CD-DACD-ROMCD-iCD-ROM XACD-ROM MO/CD-WOCD-BridgePhoto CDVideo CD

Orange Book

White Book

system holderPhilips, So nyPhilips, 8anyPhilips, SonvPhilips,8onyPhilips, SonyPhilips, SonyKodak, PhilipsJVC, Philips

introduction19821985198719891990199119921994

Fig.3. A CD~iplayer is compatible with many different CDformats.

cate the type of data (video, audio, com-puter, etc.) in the seetor. This extensionhas two opt.icns: mode-2/form-l andmode-2/form-2, wbere the form bit indi-cates whether or not the additional errorcorrection used in Mode-l sectors is em-ployed. Mode-2/form-1 sectors do havethis extra error correction (EDC andECC), while mode-2/form-2 sectors havean EDC block only. The absence of extraerror correction is justified if the CD ccn-tains audio or video information. Withthese file types, the absence of the real-time bits (even occasionally) is muchmore important than small data errors.Because the extra error correction is notused, 280 bytes are available for datastorage. With a real-time file stored in amode-z/form-I sector, the error detectionand correction operations have to be per-formed in real time also, which makeshigh demands on the relevant hardware.

The eight spaee bytes in a Mode-1 see-tor are usually zero, and used for theheader in Mode-2. Consequently, the 10-

cations for the data and the EDC fieldsare moved by eight bytes. A mode-2/form-2 sector is elosed off by a 4-byte EDCfield only, which may contain parity bitswhich serve as quality indicators in theCD production process. These bytes mayalso be made zero. If they are used, it isrecommended to employ tbe same EDCas with mode-2/form-l sectors.

Orange Book, CD-MO andCD-WOThe Compaet Disc Magneto-Optieal (CD·MO) and Compact Dise Write Onee (CD·WO) are specified in the Orange Book.This extension has given the CD an evenwider application area, because it speci-fies how CDs may be produced in sm allquantities.

Tbe CD-MO enables tbe informationon a CD to be re-written several times.By contrast, the CD-WO system allowsdata to be written onee only. The CD-MOhas a magneto-optical layer in which the

ELEKTOR ELECTRONICS MARCH 1995

information 1S stored in a completely dif-ferent manner than with a normal CD.The read section of the laser recognizes aCD-MO disc from a change in the polar-ization direction of the laser light. A spe-cial drive unit is required for the readingand writing ofthis type of CD.

A CD-WO, a.k.a. CD Recordable (CD-R) has three status levels: 'it is eitherblank, partly written on, or completelywritten on. Like the CD-MO, the CD-WOcontains a special pre-recorded trackwith information required for trackingand timing. The CD-WO contains a lightabsorbing layer of which the reflectioncharacteristics are modified with a spe-cial laser during recording. A CD-WO en-ables CD's to be 'burnt' which arecompatible with the Red, Yellow andGreen Book. These discs can be played onany conventional CD player or CD-ROMdrive.

The CD-WO standard also supportsthe use of multiple partitions on a singleCD (tmulti-sesaion'). Each of these parti-tions has its own lead-in, program andlead-out sections, and is compatible withthe standards in the Red, Green orYeI10w Book. Such a multi-session CDcan only be read by a special multi-ses-sicn drive. An ordinary CD-ROM drivecan only read the data in the first ses-sion.

CD-BridgeThe CD-Bridge standard was developedto bridge the CD-i and CD-ROM stan-dards. It 1S a very open standard withplenty of room for the implementation ofthe various system specifications. A CD-Bridge disc is a CD-ROM-XA disc whichalso contains a CD-i program. Conse-quently, this type of disc may be used ina computer as well as in a CD-ROM-XAdrive. The popular photo-CD and Video-CD are examples of CD-Bridge discs.

Photo-CDThe Photo-Cf) is a CD·Bridge disc ofwhich the standard was designed byPhilips and Kodak. This type of CD is in-tended for the storage of photographswhich have been digitized at a very highsolution, and so allows you to view pho-tographs on your TV or computer screen.The relevant file may be processed fur-ther on the PC, or printed on paper.

A Photo-CD may be an ordinary CD,produced with conventional means, or aCD·WO. The latter format allows photosto be added to the CD during further ses-sions. Obviously, pre-recorded Photo-CDs have one session only, and can beread by all types of CD-ROM.

The standard version of the Photo-CDis based on 35-rnm photographic nega-tives which are digitized at aresolutionof 3782 x 2048 pixels. The file recordedon the CD consists of five sub-files:Base/Iß, Base/4, Base, 4xBase and

ELEKTOR ELECTRONICS MARCH 1995

FOCUS ON CD·ROM STANDARDS AND APPLICATIONS

Table 2.Compatibility between disk format and player.

Disk playerlsystemCD·ROM·XA CD·i Photo CD Video CD

x x x xxx

xx xx x xx x x

CD·DA CD·ROMCD-DACD-ROMCD-ROM XACD-iCD-BridgePhoto CDVideo CD

x xx

x = compatible- = not compatible

16xBase. Thanks to data compressiontechniques, an ordinary CD can hold1II0re than 100 digitized Sö-mm pho-tographs. These days it is also possible to

put larger film formats onto a Photo-CD.This is particularly interesting for pro-fessioual users of photographic material.

Mode 0

Cdiyn,.Hea.. ; I4 bytes o

c;u"."Q12bytes E :: iii ~

o 12 1~6---------------------------:::!2352

Mode 1Sync. Header

4 bytes

12bytes il~I~I) User dataEDC Space Eee

4 bytell 8 bytes 276bytes

2076 235212 16 20" 20"o

Mode 2Sync. Header

4 bytes

12bytes ~I:I~I~ User data

o 16 235212

Mode 2 Form 1Sync. Header Subheader.--

12bytes ~I:I~I~8bytQ

User dataEDC ecc

4 bytes 278 eytee

2076 235216 24 20"12

Mode 2 Form 2Sync. Header Subheader..,....

12bytes !lil~l§8bytes

User dataEoe..,....

2348 2352

950018·1124o 12 16

Fig. 4. The data structure on a CD-ROM is strongly dependent on the application. This draw-ing shows the sector structure of the CD-ROM, CD-ROM-XA and Cd-i formats.

GENERAL INTEREST

Fig.5. The pnoto-cn format was designed Kodak and Philips. It allows over 100 as-mm neg-atives to be stored on a single CD without lass of quality.

White Book, Video-CD

The last CD-Bridge variant discussed inthis article is the Video-CD. This wasdeveloped for the storage of fuH-motion,full-screen, video, as already described inChapter 9 of the Green Book (CD-i). Thepictures on the CD are encoded to theMPEG-l standard. Anormal 12-cm di-ameter CD has storage capacity for74 minutes of digital video. The technicalspecifications of the Video-CD are de-scribed in the White Book. Initially, thisversion was also known as the Karaoke-CD standard.

Ta further the acceptance of thismedium, the standard explicitly de-scribes the possibility of adding full-mo-tion video decoder to a conventionalaudio CD player. AB a matter of course,that is only possible ifthe player is modi-fied accordingly.

Other formatsCD-Video, not to be confused withVideo-CD, is a CD format which has beenin use for some time to record five to sixmiuutes of analogue video on a CD. Thisformat is described as an extension inthe Red Book. In addition to the video in-formation, the CD has spare capacity forabout 20 minutes of digital audio. After afew false starte, this fcrmat soon wentinto ob1ivion. Today, it is hardly pro-dueed any more.

The CD Background Music (CD·BGM) format was developed for back-ground musie ('Muzak') systems, and isbased on teehnologies derived from the

CD-i standard. By virtue of the ADPCMencoding used, a single CD ean hold up to20 hours of background musie at reaeon-able quality.

CD+MIDI and CD+G were also de-veloped on the basis of the audio CD.These formats enable graphics and MIDIdata to be put on a CD, together with dig-ital audio.

The last format to be rnentioned iscalled CD-i Ready. With these audioCDs, an amount of CD-i software isstored in the background. When the CD

is played on an audio CD player, thisextra functionality is not noticed beeausethe dise behaves just like any othermusie CD. The CD-i software comes tolife onee the disc is inserted in a CD-Iplayer. Additional functions which arethen available include song texts, pho-tograpbs, discographies, etc. To makesure that the CD-i information does notinterfere with the music reproduction,the CD-i player first reads its own infor-mation, and stores it in a buffer. Next,the associated audio track is played.

FinaleA plethora of CD standards is currentlyin use, and the overall situation is com-plex enough to look pretty bewildering atfirst glance. As a general tip, formulateyour requirements before you go out andpurehase a CD player or a CD-ROMdrive. Use the information in Table 2 tocheck for yourself which standards areactually required for your particular ap-plications, and then select a player whichsupports at least those standards.

(950018)

Source:Compact Disc Standards, anIntroductory Overview, by Jan Korst andVerus Pronk, Phi1ips ResearchLaboratory, Eindhoven, The Nether-lands.

Fig. 6. The Video-CD is expected to take a large market share away from the pre-recordedvideo tape. Computer users, too, can use this CD format to load and display digitized photoswith the aid of an MPEG insertion card.

ELEKTOR ELECTRONICS MARCH 1995