Ed's AV Handbook.com
Home Theater & High Fidelity Stereo Audio


Chapter Three
Sound Reproduction

Page 2

Batting practice for the audio/video pro and a primer for the novice
 


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The Compact Disc and Digital Storage
av solutions, av installation, best buy auburn, home theater, stereo audio The compact disc launced the digital audio revolution in 1982.
CD
fabrication begins with the voltage from a microphone or recorded source - analog tape or computer hard disc.  The amplitude of the voltage is sampled (looked at and measured) 44,100 times per second by an analog to digital converter.  This is referred to as a sampling rate of 44.1KHz.       

          The sampled voltage is compared to and assigned a value from a predetermined table of 65,536 values.  The assigned values are a close approximation of the original voltages.  
Each value is represented by a 16-bit binary word derived from a numeric 'alphabet' of two numbers (0 & 1) called bits.
           The binary word is simply a string of 16 zeros or ones with a total of 65,536 possible permutations.   Each bit location in the string represents a percentage of the total possible voltage.  Each bit in the string has its particular voltage turned on or off per its instructions:
0 = OFF and 1= ON.

All 16 bits in the ON position equals the total possible voltage.
The 1st bit in the string equals half of the total possible voltage. 
The 2nd bit equals half of the 1st.
The 3rd bit equals half of the 2nd. 
The 4th bit equals half of the 3rd bit 
This pattern continues to the 16th bit.

As an example: 
 1111111111111111 = total possible voltage 
0000000000000000 = 0 volts
1000000000000000 = of the total voltage
0100000000000000 = of the total voltage
1100000000000000  = of the total voltage

Handbook Note
A micron or micrometer = onemillionth of a meter


         
          The 16 bit data is etched
as microscopic pits onto the surface of a compact disc.  The pits are impressed onto an injected molded piece of polycarbonate in a 0.5-micron wide spiral track that begins at the center of the disc.  The pits are covered in a reflective aluminum layer which is protected by a top acrylic layer. 
          At
playback the disc spins as a laser focuses on the spiral track of pits and the land between the pits which
change the reflection of the laser light.  An opto-electrical pickup converts the reflected light into an electric current.
         
A digital to analog converter samples the converted voltage 44.1 thousand time per second and translates the 16-bit code.   A sampled change created by a land-to-pit  or pit-to-land equals a digital 1 - an ON command.   No change per sampled look equals a digital 0 - an OFF command.  
          The converter reconstructs the 16-bit words and then reproduces their voltages.  The converted voltage is finally amplified and delivered to a speaker.  


                        Handbook Notes:    
               
        CD
is 16-bit code sampled at 44.1K times per second
                        DVD is 24-bit code at sampled at 96K times per second
                        DVD
-A is 24-bit code sampled at 192K times per second
                        HD DVD is 24-bit code sampled at 192K stereo or 96K eight channel audio
                        Blu-Ray is 24-bit code sampled at 192K up to 6 channels, 92K up to 8 channel

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Ed's AV Handbook.com
Batting Practice for the AV Pro and a Primer for the Novice.
Copyright 2007 Txu1-598-288   Revised 2018