Peggy and David "sound out" the technologies used to produce CDs and LPs.
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 similarÑor analogousÑto how it was originally created. All sounds begin with vibrations. In music, for example, the instruments vibrateÑthe 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 shouldnÕt 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 discÕs mirror-like surface, or it fails to reflect when it strikes the nonreflective pits. The pattern of reflections creates a sequence of ones and zerosÑa 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.
Additional sources of information
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
Questions
The most expensive violin in the world would sound terrible if it didnÕt have a sound box to increase the amplitude of the wave of the violin stringÕs vibration. You can make a simple one-string violin by punching a hole in the bottom of a large paper or plastic cup, tying a knot in one end of a string, and threading it through the hole. If you pull the string tight and pluck it, the sound will be greatly amplified.
If you have a tape deck that has an LED (light-emitting diode) readout to show the volume of the music, you have a good model for how a sampler works. Play a tape and watch the lights. During loud sections they will jump up, and during soft sections they will just barely move. If you could count the number of lights on each time and turn it into a binary code, you would have a digital record of the song volume.
The original Edison phonograph had no electronics. The device produced sound acoustically, by a vibrating needle attached to a large cone. To duplicate EdisonÕs device, roll up a piece of paper into the shape of a cone and stick a sewing needle through the body near the narrow end. Get an LP that you never plan to use again. (You will ruin it with this experiment.) Stick a pencil through the center hole and spin it like a top or lay the record on a turntable or lazy Susan. If you place the needle in the groove, you should hear the music come out the cone.
Using a good magnifying glass or a dissecting microscope, compare the surface of a CD to that of an LP. What differences do you see? What similarities?
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Newton's Apple is a production of KTCA Twin Cities Public Television.
Made possible by a grant from 3M.
Educational materials developed with the National Science Teachers
Association.