Theoretical question about how CD's work

Short answer - the laser “reads” data on the CD metal layer as a series of reflective and non-reflective areas (lands and pits). The lengths of pits and lands are variable and represent digital strings of data according to a pre-determined scheme. The actual determination of what the laser reads is determined after the laser reading operation is complete. I.e., it has to go through an interpreter.

The photodetector detects only the reflected signal (land) but doesn’t detect the signal when it is on a pit due to light wave cancellation produced by clever design of the geometries involved and the wavelength 780 nm of the CD laser. Unfortunately, the photodetector also detects scattered laser light. Since the photodetector is not too bright it thinks it’s real signal. The vibration of the CD itself produces errors during the laser reading process as the laser servo system is unable to keep up with wobbly and floppy disc.

The short answer: Yes!

A longer answer:

First, the monkey would indeed be busy for a very long time, as a typical CD contains something like six or seven billion 1’s and 0’s (referred to as "bits," which is short for "binary digits"). The bits being physically represented by transitions or lack of transitions between the pits and lands Geoff referred to.

Some of those bits don’t represent data, but rather are provided for control purposes, i.e., to make it possible for the laser mechanism and associated circuitry to identify, track, and read the data. Numerous bits are also provided in the form of "error correcting codes," which allow most and in many cases all erroneous reads that may occur for various hardware-related reasons to be mathematically corrected to bit perfect accuracy by the subsequent processing circuitry in the player.

The rest of my answer, though, pertains just to the bits which represent musical data. We are all familiar with decimal numbers, where each digit in the number can range from 0 to 9. Computers and most digital circuitry uses binary numbers instead, where each digit in the number is represented by either a 0 or a 1. Any decimal number has a binary equivalent; decimal and binary are simply different ways of expressing the same quantities. (It’s actually a little more complicated than that, as numbers expressed by 0’s and 1’s can be in various formats such as "2’s complement," or "offset binary," or "straight binary," but you needn’t get into those distinctions for the purposes of your question).

The musical data on a CD conforming to the standard "Redbook" format contains for each of the two channels 44,100 samples per second that are proportional to the amplitude of the music signal at the instant the sample was taken. While "amplitude" can be thought of as volume, keep in mind that for an audio signal it can be either positive or negative.

Each of those 44,100 data samples that are present for each channel during each second of the recording consists of 16 bits, i.e., a group of 16 digits each of which can be either a 1 or a 0, representing a number that if expressed in decimal form would be integers (i.e., whole numbers) ranging from -32,768 to +32,767. With each such number, as I said, being proportional to the amplitude of the music signal in the corresponding channel at a given instant of time.

So, yes, given enough time and given appropriate and accurate playback hardware and software, an accurate transcription of those numbers by the monkey would allow the music to be reproduced consistently with the data on the CD.

Regards,

-- Al

Hey sealrock -


Imagine a record. On it, you can see the sound waves of the recording. The vinyl track gets deeper, or taller depending on the signal.


Now imagine you could measure the depth of the grove in precise increments and you did this every 1/1000 of an inch of grove length. Write that depth down. That is your digital signal.


Best,

E

The encoded data - the pits and lands do not actually represent digital data, not really. The laser reading process is, frankly, strictly an analog process. The series of pits and lands, their various lengths and the transitions from pits to lands and lands to pits are converted to meaningful digital data downstream. So, since the lengths of pits and lands is variable precise timing is critical to recover the data as it appears on the disc. It is not a difficult task to demonstrate that (1) Reed Solomon is only effective for certain errors, but definitely not all errors. For example it’s extremely ineffective dealing with circular scratches. It’s very good for predictable errors like scratches that are radial. Reed Solomon codes and the laser servo function are both rather ineffective for correcting errors due to vibration, including the fluttering and wobbling of the disc during play as well as seismic vibration, etc., or scattered light detected by the photodetector. Reed Solomon did the best they could under the circumstances, I guess. But that was 40 years ago, for crying out loud. 😢

A lot of errors get past the goalie. For for many people the whole thing works good enough. 🙄

All these issues with CDs and especially the player, have always been there. And there are very important reasons why CDs often sound thin, compressed, congealed, honky, metallic, brittle, bass shy, thuddy, synthetic, like paper mache.

And furthermore...

http://www.laesieworks.com/digicom/Storage_CD.html