- Written by Administrator
- Published on 27 November 2007
In the audio industry, nothing has revolutionized the way we listen to recorded music like the compact disc (CD). Commercial availability of compact discs and players is a little more than a decade old, and over this time, CDs have slowly replaced the Long Playing (LP) record to the point that music stores (except for the specialty market) carry only CDs and cassette tapes. LPs represent the last vestige of analog music reproduction, where the musical sound wave is recorded continuously.
With CDs, music is recorded in digital fashion. Instead of a continuous recording of the waveform, the music is sampled 44.1 thousand times per second (44.1 kilo Hertz or 44.1 kHz) for each of the two channels (left and right stereo). The information is stored on a spiral track, beginning near the center and winding its way to the outside edge. LPs are recorded in a spiral groove beginning at the outside edge and winding their way to the center. The difference between a CD and an LP is what is in the spiral. The spiral track of a stereo LP is a series of peaks and valleys engraved at 45 degree angles to the surface, one for the left channel and one for the right. In a CD, the spiral contains a series of small bumps (usually called "pits") and flat areas (called "land") in between and beside the bumps. A small laser beam is aimed onto the spiral as the disc spins, and light is reflected from an area about three times as wide as one of the bumps. When the beam is on a spot where there are no bumps, all the light is reflected together. However, when the beam reaches a bump, light is reflected from the land on either side of the bump, and from the bump itself. Since the bump is higher than the land, its reflected light is slightly out of synchronization (called being "out of phase") with the light being reflected from the land, and the total amount of light reflected is reduced because of partial cancellation. The reflected light is directed onto a sensor which reads the light as a series of "on" (when only reflected from land) or "off" (when reflected from a bump and surrounding land) "bits" of digital data. An on bit is transmitted or stored as a one (1), and an off bit is transmitted or stored as a zero (0).Â The process of encoding different portions of the waveform by digital words of a given number of bits is called "Pulse Code Modulation" or PCM. This means that the waveform (the modulating signal, i.e., the music) is represented by a set of discrete values. In the case of music CDs using 16 bit words!
, there are 216 word possibilities (65,536). A value within a given range is assigned one of the 216 16 bit words. PCM is a "coded modulation", whereas the video and two analog channels on laserdiscs are represented by a noncoded modulation, called "Pulse Width Modulation" or PWM. In this case (PWM), the values can be any number, defined by the width of the pulse (the length of the pit in the track). Because the pit can represent any number, rather than one of a discrete set of numbers, PWM signals are analog. The PCM tracks in CDs are represented by 216 values, and hence, are digital. Furthermore, for current CDs, the bits are encoded onto the disc in what is called "EFM" or "Eight-to-Fourteen Modulation", meaning that an 8 bit packet of data is "Modulated" to 14 bits. An original 8 bit segment such as 0 0 0 0 0 0 1 1 would have the following code in EFM: 0 0 1 1 1 0 0 0 0 0 0 0 0 0, and 1 1 1 1 0 0 1 0 would have this code: 0 0 0 0 0 0 1 1 1 1 0 0 0 1. square wave, put onto an AC waveform so that a 0 is represented by 2 volts, while a 1 is represented by 3 volts. Then the AC The coding is part of the "Overhead" that allows the process to run smoothly. When the disc is played, the bits are read as a waveform is extracted, and the overhead bits removed, so that the "music" bits can be presented to the Digital-to-Analog Converter (DAC) as 1s and 0s after the process of "Demodulation". The DAC converts the 1s and 0s to the analog signal (which is fed to the CD player's output jacks for connection to an amplifier).