One of the more common questions in CD-R is: What's the best writing speed? The answer is, again, the one that works best for you. This page is intended to provide insight into why that is so.
First, having the laser work 'faster' or 'harder' is not the issue. It is designed to work in a specific fashion and its capabilities for speed and power are within its designed use. The issue on the laser is akin to that of a CPU: if the computer is on, it is spending its cycles doing either useful work or nothing. The computer will not last longer by having it spend more time doing nothing. The laser will not work better by being used more gently - assuming that the drive is not overheating, etc. In what follows, I write of dye, but the same sort of thing is true for the alloy in an erasable (with extra complications due to heat transfer).
Think of the old Memorex "Is it live" commercial in which the glass was shattered by a sound. That 'demonstration' had the note matching the glass' resonant frequency. (At least in principle; it was an ad, after all, not reality.) If the note had been a little higher or a little lower, it would have had to be much louder to shatter the glass, but it could still have been done. The dye in a CD-R is akin to the glass. It is designed to work best with the write laser energy supplied in a particular way. If it is provided faster or slower, it will still do the job, but less efficiently. That lack of efficiency will show up as a dark area being less dark (because the energy was not used efficiently) or a light area being darker (because of scatter from an adjacent dark area). To a significant extent, that is corrected by power calibration, but a given type of disc will still write better at one speed than at any other in a given recorder. If the optimal speed is 4x, it will probably still be very good at 2x and 6x, but may not work as well at 1x and 8x.
An erasable disc works by heating a tiny area of an alloy until it melts, then allowing it to harden either as a crystalline or an amorphous material. Since the alloy conducts heat, the cycle of heating and cooling depends on how much energy is dumped into it how quickly and how the heat is conducted away. In order for the melting to be localized and for the cooling to be controlled, early erasable media could be written only at 2x. Recently, media have been developed which can be written at from 1x-4x and the range may be increased with time. Even so, there is a best writing speed for that heat/cool cycle in any given writer.
The limitations of write-once media are different so that media which work over speed ranges of 8-to-1 or even 12-to-1 are available. Even so, there is a best speed and the farther one operates away from that speed, the higher the error rate. At first thought, it would appear that the dye would react based on the total energy, not the speed with which it is applied, but that's not the way the process operates - at least, not completely. If you are familiar with Silly Putty, you know that you can apply a lot of force to it slowly and it will simply deform - ooze out of the way. Apply the same force quickly, perhaps by dropping it onto a hard surface, and it will bounce. But hit it with a hammer and it will shatter. The resilience of the chemical bonds is similar in some ways to that of the Silly Putty; the changes depend both on how much energy is applied and how quickly.
Unfortunately, home recorders do not let us get a handle on the BLER (bit-level error rate), so we cannot read the uncorrected errors. Therefore the only way we know how well the writer, speed and medium combine is by looking for cases when the errors are so many that they get through the ECC. Since we only see those errors when we read the disc, uncertainties due to the reader enter as well. Please see the pages on media in this primer for some illustrations (of a sort). In summary, some readers are better tuned to a darker recording and some to a lighter one, so different readers will have different error rates from a single disc.
This is *not* an exact science - it's closer to psychology than to rocketry, at least at this stage of its evolution.
E-mail me at cdrecording@mrichter.com
Return to Mike's home page