last updated: 13 february 99
If you have any suggestions, comments, questions or additions, mail me at erik.deppe@pandora.be
Introduction
Introduction
There are a lot of CD-ROM players which don't perform like they should.
The difference between two drives with the same speed rating can be very big. It is not rare that a 24x drive from one manufacturer can be twice as fast as another 24x or that a 16x drive is outperformed by a 12x drive.
The best way to see how well your drive performs, is to test it with a benchmark like CD Speed. Comparing this result to another drive that has the same speed rating should tell you if the drive matches it's specifications or not.
If it doesn't there might be a problem with the drive or the configuration of your computer.
How to test the CD-ROM drive?
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This section explains how to use CD Speed to achieve the best results. Offcourse you can use another benchmark, but most programs will only give you a number which doesn't tell you much. There are also a lot of CD-ROM benchmarks which only test the first part of the disc which will give you poor results when used on a (P)CAV drive.
1. Use a disc with at least 600 MB data on it.
All high speed drives (> 12x) read data faster on the outer part of the disc. If you use a small disc with such drives, you will get bad results.
2. DOS only: Use the ORIGINAL driver from the manufacturer.
Although most drivers will work with any CD-ROM drive, using a different driver can result in poor performance.
3. Don't use a badly scratched CD for the test.
If the CD surface is so badly scratched that the drive has trouble reading certain parts, it will have to re-read some of the sectors. Needless to say this will give bad results too.
4. Use a silver CD for the test.
Many drives have trouble reading CD-R's (recordables) and will slow down to read them.
The reason for this is that the reflectivity of CD-R's is about 80% compared to silver CD's.
However, if your drive has a feature called AGC (Automatic Gain Control) then reading CD-R's shouldn't give you any troubles.
A CD-RW (rewritable) should NEVER be used, since their reflectivity is only 20% compared to silver discs and even if your drive supports Multiread (which will indicate that the drive can read CD-RW's), then it's very likely that the reading speed will be much slower.
5. Enable DMA.
It is VERY important to enable DMA (Direct Memory Access) when testing a high speed IDE drive. If DMA is not enabled, the CPU usage of the drive will be so high that there's no time left to process the data which will result in very poor performance.
The graphs below shows the performance of the AOpen CD-948E with and without DMA enabled.
DMA disabled
DMA enabled
As you can see, without DMA enabled, the speed is three times lower and the CPU usage five times higher.
What result should I expect from my drive?
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That depends on what technology your drive uses.
There are three different types: CLV,CAV and P-CAV.
First it's important to know that, unlike hard disks, the data density on a CD is the same on every part of the disc. This means that there is more data on the outer part of the disc then on the inside.
CLV (Constant Linear Velocity)
The data transfer rate is kept at a fixed level by changing the rotation speed.
This means you should get the number that's displayed on the box.
CAV (Constant Angular Velocity)
The drive maintains a consistent rotation speed which will result in a variable transfer rate.
If you're buying a new CD-Rom unit, it's very likely that you will only see the maximum speed the drive can achieve.
To know what the real speed is, you have to look in the manual or on the website from the manufacturer. There you will see two speeds: the minimum and the maximum.
The average transfer rate is (min+max)/2.
P-CAV (Partial Constant Angular Velocity)
This is a combination of CLV and CAV.
The transfer rate goes up until the drive reaches it's maximum speed (CAV). From that point the drive will slow down and the transfer rate will be constant (CLV).
Because P-CAV drives reach their maximum speed much sooner than CAV drives, the average transfer rate should be higher.
The drive does not perform like it should.
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First make sure you have followed the steps described in "How to test the drive".
1. I have a (P)-CAV drive and the graph only shows a flat line.
The drive has propably trouble reading the disc you have inserted and will therefore slown down. Use another disc.
2. The graph looks good but at a certain point the speed goes down for one value and then goes up again.
There might be a scratch on the CD. Use another one.
3. At first, the graph looks normal but then the speed will stay constant without ever reaching it's maximum.
This is the thing that can happen if you don't use the original driver from your manufacturer. Go to their website to download the latest version.
This behaviour might also indicate that DMA is not enabled.
Some games crash with my new high speed drive.
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All high speed drives will spin down after a while. This is done because letting it spin at high speed all the time would dramatically reduce the life of the drive.
If the game (or other program) wants to access the drive again, the drive has to spin up.
This will take a few seconds and might be enough to cause trouble with some games.
However there is a utility which will allow you to change the spindown time.
It can be found at:http://www.ncf.carleton.ca/~aa571
Some drives (Plextor) allow you to select the spindown time.
The spinup/down time can be measured with CD Speed 99.
Will my configuration affect the speed of my drive?
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1. Motherboard, CPU, memory.
No, unless you have a very slow system (386 or below) with a high speed drive.
2. Bus speed.
Yes, since the CD-ROM controller is attached to the system bus, higher bus speeds will result in a higher transfer rate. The differences will be small however.
3. CD-ROM controller.
Yes.
IDE
New drives use PIO Mode 4. If your controller doesn't have this mode (normal IDE) the speed will be slower.
SCSI
Using a slow SCSI controller with a high speed drive will certainly slow down your CD-ROM drive.
Also the number of devices attached to the controller can reduce the performance.
Disclaimer: the information in this document is provided "as is".
Copyright 1999 by Erik Deppe
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