Compact Disk (CD) FAQ

How can I clean a dirty CD?

Use a drop of dish detergent and lots of clean water. Do not rub. Never rub or wipe in a circle. If you must stroke the disc do it with a soft cotton cloth in a straight line from the center outwards (radially). Rinse the disc in running clear water, shake off most remaining drops, and lightly pat dry with a soft, clean cloth.

What about CD spray treatments (ArmorAll et al)?

In a nutshell, save your money.

Current wisdom is to avoid any disc coating or spray. Some will definitely damage the disc.

There are many theories on what ArmorAll can do to a disc. One is that it reduces static which will attract the delicate head of the laser detector to the disc. Another theory is that the cleaner will fill voids in the disc with silicone, thereby making it easier to read by reducing diffraction effects.

Scientific studies of the data coming off of the disc have failed to show any difference between a virgin disc and a treated disc. I have not heard of double blind listening comparisons that have proved that there are people who can hear the difference.

One of the strongest proponents of ArmorAll issued a "recall" on his advice. He now warns that ArmorAll can damage the disc. He also advises that you can clean ArmorAll off treated discs with Dawn dish detergent.

Bold text YES YOU CAN this is how

FUCK YOUR MAA

Do CDs deteriorate with time? What is their life span?

A CD consists of a polycarbonate top layer, an aluminum (or gold) metal reflective layer, a polycarbonate bottom layer, and some miscellaneous printing ink. Of these materials, polycarbonate seems to be extremely stable with time provided that it is well cared for. Do not use any liquids on a CD that contain silicone or solvents. Do not leave CDs in sunlight or other bright light. Do not stick labels on CDs. Do not write on CDs. Do not expose CDs to temperatures higher than normal room temperatures. Don't leave a CD under water. Even the top side of a CD is critical and subject to damage.

Some pressings from the early 1980s used ink which damaged the polycarbonate top layer and eventually got into the aluminum. These inks are not in use today. Some earlier discs were made with imperfect sealing around the perimeter of the disc. This was evident because the aluminum in the disc extended all of the way to the disc edge. These discs were known to fail due to moisture getting to the aluminum and causing it to oxidize. Modern CD factories have solved this problem as well.

With those cautions, modern CDs will last for more than 30 years without deterioration. Most of the CDs which were made in 1983 are still around today and still sound good.

How much music can you possibly cram into a CD?

The longest seen so far (reported by Stuart Kahler) is a MiC bootleg of Depeche Mode "Evolution", at 81:09. Next are 'Live Evil' by Black Sabbath at 80:03, 'No Quarter' by Jimmy Page and Robert Plant at 79:38, the collected singles CD release by The Sisters Of Mercy at 79:30, an MCA reissue of Steely Dan: Greatest Hits at 79:17 and a Musical Heritage recording of Bach: Goldberg Variations at 79:02. Modern CDs are pressed using tighter track spacing than the first CDs, because modern equipment is capable of holding tighter tolerance than the original machines.

Why is CD digital data written in 44.1 kHz samples?

The rate of 44.1 kHz was picked to be compatible with existing 50 Hz and 60 Hz video-based digital audio storage, where an integral number of frame buffers could fit in a single horizontal scan. Quote from Watkinson and Rumsey, "Digital Interface Handbook" 2.7.6 Choice of Sampling Rate: "In 60 Hz [525 line, 60 Hz vertical refresh) video there are 35 blanked lines, leaving 490 lines per frame, or 245 lines per field for samples. If three samples were stored per line, the sampling rate becomes 60*245*3=44.1 kHz. In 50 Hz video [625 line, 50 Hz vertical refresh), there are 37 lines of blanking, leaving 588 active lines per frame, or 294 per field, so the sampling rate becomes 50*294*3=44.1 kHz. The sampling rate of 44.1 kHz came to be that of the Compact Disk. Even though CD has no video circuitry, the equipment used to make CD masters is video based and determined the sampling rate."

The length of 74 minutes is determined by the physical nature of the reading system. It's based on the encoding method, the wavelength of the laser used (different wavelengths are incompatible with current CDs) and the necessary support information. During the development of the CD, von Karajan was allegedly asked how long a CD must be, to which he responded it must be long enough to hold HIS performance of Beethoven's 9th symphony, but the parameters had pretty much already been nailed down at that point.

How do the letters ADD on my CD relate to sound quality?

The simple answer to this question is that there is no relation between the three letter code and sound quality. Those three letters refer to the recording and mastering tools used in making the CD.

The first letter refers to the recording process. For example, a disc labeled ADD was ANALOG recorded, where a disc labeled DDD was DIGITALLY recorded. Analog recording means that some form of conventional analog tape recorder was used, whether it be a two-track home-quality recorder or a very expensive wide-tape, high-speed, multitrack recorder. Digital recording could be as simple as a two-track DAT recorder, or can be a much fancier multitrack digital recorder.

The second letter refers to the recorder used in the mixing and editing process. Mixing and editing is the process of combining a multitrack master recording, setting levels, editing out defects, adjusting equalization, and creating a two-track final tape. There are good machines available for this which are analog and good machines which are digital.

The third letter refers to the final master, which for a CD is always digital. I have seen discs that are labelled as AAD, ADD, DAD, and DDD.

Future releases may not have this three letter code on them because they don't tell you anything that is significant. Also, some codes have been used incorrectly on some discs, which makes the information that much more meaningless.