Factors in Writing to CD

copyright 1994 Adaptec Software Products Group
Used with permission



Why You Should Read This

Writing data to recordable compact disc is a complex process which demands a great deal from both hardware and software. Most of this complexity is hidden from the user by the power of the program, but you should be aware of these factors which influence the success of the operation.

The Data

The amount of data you are writing is less important than whether it contains a few large or many small files; in the latter case the system may have trouble locating and opening the files quickly enough to send them smoothly to the CD recorder.

The System

By system we mean your computer and all the software running on it. ANY interruption is fatal to CD recording, so ensure that your CONFIG.SYS and AUTOEXEC.BAT do not load any TSR utilities which may periodically or otherwise interrupt operations. Screen savers, alarms or reminders, and incoming faxes will kill disc writing. You should also turn off network sharing so that no one tries to access the files you're trying to write - this would also fatally interrupt writing.

The Hard Disk


To write a virtual image on the fly to compact disc, the hard disk from which you are writing MUST have a transfer rate fast enough to keep the memory buffer full in the CD recorder. This usually means an average hard disk access time of 19 milliseconds or better.


Having to search all over a very fragmented hard disk for the data to be written (whether on the fly or from a real ISO image) causes operations to slow down, possibly fatally. Defragment your hard disk!

Thermal Calibration

Most hard disks automatically periodically perform a thermal calibration to ensure proper functioning. Calibration interrupts hard disk operations for as much as one and a half seconds. Some hard disks force a calibration at fixed intervals, even if the disk is in use, causing interruptions fatal to real-time operations such as CD writing. This problem arises particularly when the amount of data to be written is large.

Some hard disks calibrate intelligently, postponing it when the disk is being used. Fujitsu and many Micropolis hard disks work this way.

The CD Recorder

Recording Speed

Many new CD recorders (and some old ones) are capable of writing at two (or even four) times the standard writing/playback speed of 150 kilobytes (75 sectors) per second. Ideally, it should be possible to choose the speed (and wherever the unit allows it, our drivers do give you the choice), because, although writing faster is a timesaver in some situations, in others it causes problems. Let's see why: CD recording occurs in real-time. Data is transferred from the computer's hard disk to the buffer memory of the CD recorder, and thence to the recording laser, which inscribes pits into the surface of the CD at a steady speed which cannot be altered or interrupted. The speed at which data must be transferred depends on the mode in which the data is written and on the speed selected:

Format/Mode                        Required transfer rate at:        
(data bits/sector)               1x            2x            4x
CD-ROM Mode 1 (2048)   	     150 kb/sec    300 kb/sec    600 kb/sec
CD-ROM Mode 2 (2352)         172 kb/sec    344 kb/sec    688 kb/sec
Audio (2352)                 172 kb/sec    344 kb/sec    688 kb/sec


When you write a real ISO image from hard disk to CD, speed is rarely a problem because the image is already one gigantic file in which the files and structures are already in order and divided into CD-ROM sectors, so it is only necessary to stream off the data to the recorder.

When you write a virtual image, things get trickier. Now, in order to write to CD, the program must consult its database to find out where each file should go in the image and where it actually is stored on hard disk. It must then open the file, divide it into CD-ROM sectors, all the while sending the data in a smooth, continuous stream to the recorder. Locating and opening the file is often the more time- consuming part of the process, which is why on-the-fly writing is more difficult when you have many small files.

The Recorder's Buffer

The CD recorder's buffer memory helps to ensure that there is always data ready to be written, by storing extra data as it arrives from the computer. The size of the buffer is therefore critical to trouble-free writing: a slow-down or small interruption in the transfer of data from the computer will not interrupt writing so long as the buffer is not completely emptied. Obviously, the larger the buffer, the more safety margin you have in case of interruptions. If your CD recorder has a very small buffer and your hard disk is slow, you may find it difficult or impossible to write virtual images on- the-fly to CD. Your options are (a) make a real ISO image on hard disk and record from that, (b) buy a faster hard disk, or (c), if possible, get your recorder upgraded.

When to Write at Single Speed

Given all of the above, if you want to write a virtual image on-the- fly to CD and you have a slow hard disk, it is generally safest to write at 1x speed. If you have a CD recorder which forces you to write at 2x, in many situations it will be difficult to write a virtual image straight to CD. Your only choices are (a) make a real ISO image on hard disk and record from that, or (b) if possible, get your CD recorder upgraded.

When to Write at Higher Speeds

In most situations where your hardware configuration is adequate (fast, defragmented hard disk, few small files, good-sized recorder buffer), you can successfully write virtual images straight to CD. However, it is always advisable to test first.

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