Activity 6
Send Them a Recording
Background Information
Long-playing (LP) records and compact discs (CD) provide a nice example of the difference between analog and digital information storage. In the LP, the needle of the phonograph picks up vibrations from the shape of the record groove. On stereo records, each side of the groove contains one channel of the sound track.
When the volume increases, the amplitude of the waves in the groove is larger. When the frequency increases, the number of wiggles in the groove per centimeter (along the groove) increases. The walls of the groove go back and forth just like the compressions and rarefactions of air in the sound wave. The needle bounces along the record surface, slowly wearing away the grooves with each playback. Specks of dust add extra vibrations. The turntable speeds up and slows down, causing a corresponding fluctuation in the sound.
The CD is completely different. The sound wave is represented by a stream of numbers. These numbers encode the value of the audio signal, which has been sampled about 40,000 times per second. The numbers, written as ones and zeroes, are etched onto the surface of the disc as tiny pits. A laser reflects off the surface to read the sequence of pits. Contrast this binary process with the playing of an LP. Since no needle touches the CD, there is virtually no wear on the disc as it is played. Background noise from the recording and reproduction process is eliminated. The digital information is stored in a computer memory temporarily and then read out at a precise rate, so imperfections in the rotation of the disk cannot affect the sound. Each disc can store 74 minutes of sound, more than a long-playing record. And on top of all this, new disc technologies are coming which will substantially increase the amount of music stored per disc.
To investigate the change in information density from the LP to the CD, students measure the number of grooves per centimeter of each with interference. The laser beam reflects from the surface of the recording rather than passing through the grating, as in the experiment in Activity 5. The drawing shows light rays reflecting from the grooves.
The geometry is shown on page 324 of the Student Book. The distance x and L are measured in cm and d is in cm-1. The wavelength of the laser light is 0.67 X 10-4 cm.
d (x / L) = 0.67 3 10-4
Solving for 1/d, the number of grooves per centimeter, gives
Send Them a Recording
Background Information
Long-playing (LP) records and compact discs (CD) provide a nice example of the difference between analog and digital information storage. In the LP, the needle of the phonograph picks up vibrations from the shape of the record groove. On stereo records, each side of the groove contains one channel of the sound track.
When the volume increases, the amplitude of the waves in the groove is larger. When the frequency increases, the number of wiggles in the groove per centimeter (along the groove) increases. The walls of the groove go back and forth just like the compressions and rarefactions of air in the sound wave. The needle bounces along the record surface, slowly wearing away the grooves with each playback. Specks of dust add extra vibrations. The turntable speeds up and slows down, causing a corresponding fluctuation in the sound.
The CD is completely different. The sound wave is represented by a stream of numbers. These numbers encode the value of the audio signal, which has been sampled about 40,000 times per second. The numbers, written as ones and zeroes, are etched onto the surface of the disc as tiny pits. A laser reflects off the surface to read the sequence of pits. Contrast this binary process with the playing of an LP. Since no needle touches the CD, there is virtually no wear on the disc as it is played. Background noise from the recording and reproduction process is eliminated. The digital information is stored in a computer memory temporarily and then read out at a precise rate, so imperfections in the rotation of the disk cannot affect the sound. Each disc can store 74 minutes of sound, more than a long-playing record. And on top of all this, new disc technologies are coming which will substantially increase the amount of music stored per disc.
To investigate the change in information density from the LP to the CD, students measure the number of grooves per centimeter of each with interference. The laser beam reflects from the surface of the recording rather than passing through the grating, as in the experiment in Activity 5. The drawing shows light rays reflecting from the grooves.
The geometry is shown on page 324 of the Student Book. The distance x and L are measured in cm and d is in cm-1. The wavelength of the laser light is 0.67 X 10-4 cm.
d (x / L) = 0.67 3 10-4
Solving for 1/d, the number of grooves per centimeter, gives