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


Chapter Three
Sound Reproduction

Page 4

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


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Broadcast Radio
          Let’s return to a time when the first Beach Boys harmonies drifted with the breeze from portable radios on the beaches of Southern California.  How did a Brain Wilson masterpiece of harmony travel from a distant DJ to our radios?  
           It
began with a broadcaster generating electromagnetic waves via a broadcast antenna. The electromagnetic waves caused electrons at our receiving antennas to budge, dislodge, and regenerate an electromagnetic current.  This modulating current, an analog of the microphone’s original voltage, was then amplified and fed to our raido's speaker.  Surf’s Up, the Beach Boys were alive on our beach.  

AM/FM
           Amplitude Modulation (AM) andav solutions, av installation, best buy auburn, home theater, stereo audio Frequency Modulation (FM) are the  most common methods of radio broadcasting.

AM
          AM broadcasters are assigned a frequency within the limited band
of the radio dial.  The broadcaster’s transmitter
modulates the amplitude of this frequency.  This is analogous to changing the brightness of a distant invisible beacon of light up and down.  Fast changes reproduce the higher frequencies of sound.  Slower changes reproduce the low bass frequencies. Consequently, brighter is louder and the converse is less loud.


FM
          FM broadcasters are also assigned a frequency in the radio band. But FM broadcasters are given room to move plus or minus of their assigned frequency.  
          T
hink of the assigned frequency as a specific color of invisible light.   Then as the name implies, frequency modulation shifts the frequency
plus or minus
of the assigned frequency. This is like varying the shade
of the color.  
          Fast changes are higher frequency sound,
and the slower changes
are the lower frequencies.
 Broader change is louder and the converse is less loud.          
          AM and FM each have broadcast advantages.  AM can travel longer distances than FM.   But AM is sensitive to other amplitude modulated noise such as lightning, your car’s ignition system, and hair dryers.  FM typically avoids this interference, rendering it more acceptable for music.


Accurate Standards

          High fidelity audio refers to the faithful reproduction of the originally recorded sound.
It is an adherence to accurate standards.  High Fidelity standards are generally measured and expressed in terms of some type of distortion.

          For example, a frequency response test measures the uniform amplitude of a range
of sound at the output of a component.  The measured result is compared to the amplitude
of the original input.  A typical test will measure the range from 20HZ to 20KHz.   Any deviation is defined as distortion.

          Harmonic distortion is a measurement that feeds the input of a component from
20Hz to 20KHz in 5Hz steps.  A measuring device searches for any evidence of unintended signal at up to six octaves above each step.  The results are summed and computed as a percentage of the original.
           A signal-to-noise ratio test measures the ratio of the input signal to any unwanted interference that is created by the component.

              This is a typical example of an amplifier’s specifications:

              Total continuous power = 100 watts per channel

              Frequency response plus or minus 3db from 20Hz to 20KHz
              less than 0.05 % total harmonic distortion
              Signal-to-noise ratio = 92db


          This concept of high fidelity standards can be applied to any component in the playback chain. This includes speakers, interconnecting cable, and the acoustics of a listening room. However it is important to recognize this HiFi truth.  All systems that sound good measure good.  But not all systems that measure good sound good.  Do not forget to use the best tool in your sound evaluation box -- your ears.
            This link 
Rane.com offers a comprehensive list of audio specification definitions.  

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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