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Compact Discs

 

Overview

A hot debate rages today around the question of which has better sound, compact discs (CDs) or long-playing records (LPs). Both do a good job of reproducing sound. They differ, however, in the way they encode sound and retrieve it for transmission back to our ears. LPs use analog technology. They capture and reproduce sound in a way similar?or analogous?to how it was originally created. All sounds begin with vibrations. In music, for example, the instruments vibrate?the strings of a guitar, the reed of a clarinet, the head of a drum. These vibrations create disturbances in the air, which we hear as sound. To make an LP, air vibrations are changed into electrical signals by a microphone. These then activate a tool that carves the pattern of the vibrations into the LP as tiny notches in the side of a groove. Etched in the same pattern in which they moved through the air, they are analogous to the original sound wave. To play back an LP, the phonograph stylus sits in the groove of the rotating LP, bouncing off the notches as it moves along. This causes the stylus to vibrate, and those vibrations are then electronically amplified and sent out through the speaker, making a sound wave in the air. Because the stylus constantly rubs in the record groove, LPs can wear out, and if they get scratched, they are all but ruined. In addition, when the stylus hits a piece of dust in the groove, it reads it like a notch, so we hear a noise that shouldn?t be there. CDs avoid this problem by using digital technology, which eliminates the contact and the vibrations. To make a CD, incoming sound waves are captured electronically, as with an LP. But instead of keeping them intact, special electronics take them apart and turn them into a code, using technology called signal sampling. As the electric signal comes in, its amplitude is read or sampled at regular intervals, and assigned numerical values between 0 and 65,000. The greater the number of samples per second, the greater the resolution and the more accurate the sound. Most CDs use around 44,000 samples per second, creating extremely accurate reproductions. After sampling, circuitry encodes the sound and stores it on the CD as a series of reflective points and nonreflective pits. As the laser beam scans the surface of the CD, it either reflects off the disc?s mirror-like surface, or it fails to reflect when it strikes the nonreflective pits. The pattern of reflections creates a sequence of ones and zeros?a binary code. The computer translates this binary code as a series of instructions. Using the code, the computer reassembles the samples, feeds the signal to the amplifier and speaker, and recreates the original sound wave recorded.

Activity

CD technology works because an extremely large number of samples (44,000 per second) is used to encode individual sound waves. In this activity, you will discover how increasing the number of samples changes the resolution of an image. Materials
  • 3 pieces of 8 1/2" ? 11" unlined tracing paper
  • metric ruler
  • pencil
  1. Using the ruler, construct a 7 cm X 7 cm (2.8" X 2.8") square in the middle of the first sheet of tracing paper. Mark off points 10 mm (.4") apart from each other around the edge of the square and connect the dots across the square so that you have a grid with 10 mm (.4") boxes.
  2. Lay the grid over the picture below or a picture of your choice. With the pencil, color in each box that contains a line of the drawing. Leave the other boxes empty. Make sure you color the entire box. Once you have finished coloring the boxes, label the drawing “grid A” and put it off to the side.
  3. On a fresh piece of tracing paper, construct another 7 cm X 7 cm (2.8" X 2.8") grid, only this time make the individual boxes 5 mm (.2") on each side. You should have twice as many boxes in each direction as the first grid. Using the same procedure as in step 2, color in all the boxes that a line from the drawing enters. Label this "grid B."
  4. On the last piece of tracing paper, construct a third 7 cm X 7 cm (2.8" X 2.8") grid. This time make the individual boxes 2 mm (.08") on a side. (You may have to sharpen your pencil for this one!) Place it over the picture and fill in the boxes as before. Label this "grid C."
  5. Compare the three drawings to the original. Drawing A is the standard. Drawing B has double the sampling points, and drawing C has five times the sampling points. Questions

Resources

  • Berger, M. (1987) Lights, lenses and lasers. New York: G.P. Putnam?s Sons.
  • Birchall, S. (1985, Jan) The CD: It?s the pits that make the music. Digital Audio, pp. 26?30.
  • Hewitt, P. (1992) Conceptual physics (2d ed.). New York: Addison-Wesley.
  • Klein, L. (1989, Nov) The sound of CD. Radio-Electronics, pp. 74?75.
  • Macaulay, D. (1988) The way things work. Boston: Houghton Mifflin.