We like to think of the computer reading and writing information perfectly. In fact, we seldom even think about that at all because perfect operation is so common. Unfortunately, perfection is not achievable by man. In Islam, that reality is recognized by requiring that a flaw be woven into each 'Persian' rug. In computers, real hardware provides all the errors we need.
According to the specification, CD-R errors start out as being not more than one in a million. Sounds good, right? 99.9999% accuracy should satisfy even the most critical person - but it won't do for computers. There may be 8*650*1024*1024 bits on a CD-ROM. If so, 5,000 of them may be faulty and reading that disc will be a major problem. So almost 15% of the space on the disc is 'wasted' in coding which helps automatic error correction - ECC or Error Correction Code. With ECC, the specified error level is cut by a factor of 1,000 so that there would be only five bits wrong on a disc. Even those may be transient so that when an uncorrectable error is found the sector is reread in the hope that good information can be retrieved. In addition, most discs will exceed the specification significantly. Even so, if rereading is necessary the data flow is reduced substantially. The read mechanism must stop its rapid progress, back up and start the sector over.
That saving ECC layer is not applied to audio. If it were, the longest playing time on a CD-DA would be about 64 minutes instead of 74. Similarly, on a VCD there's no ECC on the video files. The idea is that the extra playing time costs only slight errors on playback and that you won't notice the ones that sneak through any error correction applied on the analogue side. For video, the fault is usually a momentary spot in the picture and no correction is used. For audio, the player has some limited ECC of its own, but if it is used there can be a loss of brilliance in the sound.
Another key factor in the quality of information read from a CD-ROM or CD-R is the quality of reading. Some errors derive from the way the track is formed on the disc and are related to the term 'jitter'. Even more significant is that, in general. the faster the data fly past the pickup, the greater the chance that a bit will be misinterpreted. With a pressed disc and ECC, that is not significant. Listening to a CD-DA or watching a VCD occurs at 1x regardless of the drive, so again speed errors don't matter. However, when you extract digital data through DAE or in raw form from a VCD, the story becomes less happy. All other terms being equal, the faster you run DAE, the greater the number of ticks and pops - bit errors - you will encounter.
There is also a quality issue here - and in this, I am providing personal opinion, not objective data. With the increasing pressure from the marketplace for faster and faster drives, other things are being sacrificed. The general public neither knows nor cares about DAE, so the ability to get a good signal without ECC is not a selling point on CD-ROM readers. Therefore the manufacturer will work to find a way to claim 24x for a poorly made drive which might work well if designed for, say, 12x and provide competent DAE at 1x or 2x. In practice, the 24x will be slowed by the need to reread even data and may deliver very little more in effective speed than a 12x, while being unable to provide satisfactory DAE even at 1x. Even a pressed CD-DA may sound dull on that drive, though typical computer speakers are unlikely to show that since they, too, seem aimed at bass boom instead of clean high-end response. And as has been reported elsewhere, running a disc at high speed increases vibration and mechanical noise.
Finally, a CD-R is not a pressed disc and it does not perform as well. In particular, a pressed disc has great contrast between its strongest reflection and its maximum scattering of the reader's light. A CD-R or CD-RW has a much lower maximum signal and a much higher minimum signal for the reading diode. As a result, the chances of reading errors from a CD-R or CD-RW are higher than for a pressed disc and a copy of a CD-DA copy (especially audio) is likely to sound noticeably worse than the original. Of course, the better the hardware, firmware and software are for the job, the better the results are likely to be. But there is no free lunch, there is no guarantee of perfect writing or reading, and if you do not take that into account as you choose your reader and test your CD-Rs, you may regret it.
E-mail me at cdrecording@mrichter.com
Return to Mike's home page