Multimedia Pre-mastering


Introduction to CD-ROM

Since compact discs store audio in a digital format they are suitable for storing other digital information. In 1984, Phillips and Sony released the Compact Disc Read Only Memory (CD-ROM) specification, known as the Yellow Book. This defines the necessary additions to the Red Book for the storage of computer data. 

The physical parameters of CD-ROMs are identical to those defined in the Red Book. The use of the data it contains is different. While audio CDs can be played at only one speed, CD-ROM drives exist with a range of speed options from normal (1x) to four times normal speed (4x) and, now, 52x. As the speed increases the access time also decreases. 

CD-ROM discs differ from CD audio discs in two important ways. 

The data on a CD-ROM disc are divided into sectors which contain both user data and other data for control and (additional) error protection. The data on a CD-ROM are contained in files. All CD-ROMs therefore need a file system to enable the computer to access the required file easily and quickly. 

The user normally will not need to be bothered with the sector structure but will be aware of the file structure of the files on a CD-ROM. 

In order to use CD-ROMs you need to buy a CD-ROM drive (sometimes called a CD reader) and connect it to your computer. A CD-ROM has several advantages over other forms of data storage, and a few disadvantages. It can hold 650 megabytes (MB) of data, the equivalent of nearly 500 high density floppy disks. The data on a CD-ROM can be accessed much more quickly than data on a tape, but CD-ROMs are considerably slower than hard discs. Like audio CDs you cannot write to a CD-ROM (but CD recordable discs do exist), so they are only used for pre-recorded data. In addition CD-E (Compact Disc Erasable) is now being introduced. Which will enable you to re-record data on the CD.  

CD-ROM Specification

The CD-ROM specification is defined in the Yellow Book which is based on the CD audio format but for computer data. The salient parameters are shown below: 

Parameter Value Comments
Data capacity 650 MB 70 minutes
Raw data bit rate 1.41Mb/s all bytes
User data rate 150 kB/s at normal speed
Block (sector) size 2352 bytes  
User data per sector 2048 bytes full error correction  
Sector rate 75 sect/s at normal speed
Sector Modes 1 or 2  
Sector Forms 1 or 2 only with Mode 2

You must use "Disk at Once Mode" Do not attempt to send us masters using "Track Incremental". Hewlitt Packard and Sony burners are notorious for defaulting to this mode so be aware! Check with your operational manuals for details. 

Storage capacity of our CDROMs:

650Mbytes of user data in CD-ROM mode 1 (2048 bytes/record) OR up to 78 minutes of digital audio @44.1Khz OR a combination of the above (enhanced CD).

CD-ROM Sectors & Modes

Data stored on a CD-ROM disc is divided into sectors which are equivalent to the audio frames for a CD audio disc. At normal playback speed 75 sectors are read every second. For double speed CD-ROM drives this increases to 150 sectors per second and so on. Seek times, while the disc rotates to the required starting position, will also reduce as speeds increase. 

Because CDs were designed primarily for audio, their use for computer data requires the addition of header data and error correction codes which are included in every sector. There are two different types of sectors defined in the CD-ROM specification, mode 1 and mode 2 (the latter being used for CD-ROM XA discs). 

Mode 1 Sectors

Mode 1 sectors are intended for the storage of computer data. They comprise of: 

Header which consists of Minutes, Seconds, Sectors (frames) and Mode (= 1). 

2048 bytes of user data. Error detection and correction codes to give the CD- ROM data extra protection. While CD audio players are able to conceal uncorrectable errors which may result from a scratched disc, CD-ROM data (especially application code) needs to be error free. 

Mode 1 sectors are the simplest type and are used for most CD-ROM based formats which follow the Yellow Book.

CD-ROM Based Formats

The first CD-ROM discs contained Mode 1 sectors designed for general computer storage on CD. Although not specifically designed for multimedia there are a number of important multimedia formats which use mode 1 CD-ROM as the basic method for storing data on CD.

Format File system Comments
PC CD-ROM ISO 9660 Most common format
MPC 1 & 2 ISO 9660 Specifies PC configuration
Mac CD-ROM HFS Based on Mac operating system
3DO Proprietary Data on disc is encrypted
CD32 ISO 9660 Not 100% ISO 9660
Mixed Mode ISO 9660 Audio and Data combination

How do I make sure my CDROM is going to work?

It is vital that you test your CDROM one off master on as many different machines as possible. If you follow the formats listed in this section then you should have success. While we at OmniDisc wish you the greatest satisfaction with your order, we can not be held responsible for discs that fail due to improper pre-mastering. 

What about errors and glitches?

OmniDisc digitally transfers your master and does not normally test for errors. If an error or glitch is found on your finished CDs we will reference your original master. If it is the fault of our glass master and not your master we will rerun your order free of charge to you. CDs will not be replaced unless returned COD to us. - No CDs may be returned without first obtaining an RMA number. 

What about track "0" and special mastering needs?

There is a feature that can be obtained on a CD-R master that utilizes track "0". This can be used as a special data area or hidden area from standard audio CD players. While this clever usage of the CDROM is possible it does carry a premium price for glass mastering. Please ask customer service about these charges and any other nonstandard formats that you may require.