'Max Speed' Explained

When looking at CD-ROMs, one term always stands out: "Max Speed." Whether it's 24x Max, or 50x Max like the Asus, the term has become quite standard in describing CD-ROM speeds. To understand the significance of Max speed and Min speed, you have to know (just a little) about the different ways CD-ROMs read data. Back in the day, when 2x, 4x, and 8x were standard, everything was pretty simple.

The laser reads the information off the disc as the disc spins. However, remember that the outer edges of a disc spins faster than the inner edge. Data on a CD is packed uniformly across the disc (hence, the outer edges contain more information than the inner edge). In order to read the information at the outer edges correctly, the CD-ROM must either adjust the speed at which the laser reads, or the speed at which disc spins. A drive which adjusts the read speed of the disc is defined as using "Constant Linear Velocity" or CLV. The speed of rotation of the disc decreases as the laser moves towards the outer edge, so the rate at which data can be read remains constant. A 4x CLV drives reads at 4x all the time, a 40x CLV drive reads at 40x all the time.

The other solution is to keep the disc rotating at a constant rate, using "Constant Angular Rotation" or CAV. CAV drives adjust the read speed of the laser - when they reach the faster outer tracks, the laser simply reads the data faster as it passes by. In this way, the read speed at the outer edge of the drive can approach two times the read speed of the inner edge. At first, many companies using CAV claimed their drives were 24x because they read at this speed near the outer edge. They neglected to mention that near the inner edge, the drive was only 12x. Since CDs are read from the inside edge-out, you can see that this becomes a problem, as only a fully-loaded CD (650MB-worth) is likely to see the benefits of the full 24x.

Is this honest?

There was a pretty big uproar about this misleading advertising, so to put themselves in the clear, manufacturers making CAV drives now label them as "24x Max" or "24x/12x" drives - maximum 24x speed, minimum 12x speed. CLV drives are often simply labeled "24x" or "True 24x," as they maintain their maximum transfer rate regardless of which track is being read.

The Asus 50xMax is a CAV drive with a maximum transfer rate of 7,500 KB/s (50x) at the outer edge of the track. Its minimum speed is 3,300 KBps, or 22x near the innermost tracks. Again, the speed you get out of this drive depends greatly on the specific CD-ROMs you're using, particularly whether or not they are at full capacity.