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Bring on the music CDs!
Did you ever wonder how the CD player works? Let me explain it to you.
The components of the compact disk player consist of:
- Low Power Laser
- Optical Sensor
Compact disc players use optical technology instead of magnetic tape for sound reproduction. The DISC is plastic, 4.7 inches in diameter and can contain up to 74 minutes of two channel stereo, or up to 37 minutes of 4 channel sound. The disc player uses a LOW POWER LASER to read an audio signal encoded as a series of microscopic pits in the reflective metallic layer covered in clear plastic.
During the play mode, the low powered laser beam, mounted on a movable assembly, reads the microscopic pits on the surface of the disc. The laser is focused through the transparent rear surface of the disc. There, it is reflected onto an OPTICAL SENSOR. The signal is then converted to analog format for output to the SPEAKERS.
Compact discs have several advantages over conventional tape or records. The frequency response is more uniform and accurate. Background noise is completely eliminated. Dynamic range (the difference between soft and loud sounds) is very wide. Also an absence of wear since nothing touches the surface of the disc while it is being played. During playback the laser is focused precisely on the microscopic pits, making small scratches and dust out of focus. The speed of the disc is variable to accommodate the difference in circumference between the 1st or inner track to the last or outer track. Speed past the laser remains constant at about 4 feet per second.
Noise reduction or error correction is accomplished by a high speed MICROPROCESSOR by sampling the sound 44,000 times a second per channel and holding the audio wave form momentarily and comparing it to one of 65,535 possible stored values. It then selects the code that most closely represents the sampled code. Each sample is a binary code of 16 ones and zeros. A total of 1.4 million code bits are sampled for each second of music. At the time of recording, an extra bit is added to the end of each digital code, indicating whether the code is even or odd.
In playback, this count is repeated to detect if any bits have changed. By this crosschecking, it is possible to see if any bits are wrong and correct them before playback. The sampled bits, along with error correction codes, tracking codes, and cueing data are all recorded on a master tape, then used to make compression molded plastic discs.