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 Brian Wilson masterpiece of harmony travel from a
distant DJ to our radios.
It began with a broadcaster generating electromagnetic waves,
continuous modulating sparks, via their broadcast
antenna. The electromagnetic waves caused electrons in
our receiving radio antennas to budge, dislodge, and
regenerate electromagnetic current. The modulating
current, an analog of the microphones original voltage, was
amplified and fed to our radio speakers. Surf's Up!
Amplitude Modulation (AM) and Frequency Modulation (FM)
The two most common methods of radio broadcasting.
AM broadcasters are assigned a frequency within a limited band
of the radio dial. The broadcaster's transmitter
modulates the amplitude of its assigned frequency.
This is analogous to changing the brightness of a distant
invisible beacon of light up and down. Fast change
reproduce the higher frequencies of sound. Slower change
reproduce the lower frequencies. In addition, brighter
is louder and the converse is less loud.
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. Think 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, much as varying the
shade of the color. Fast changes are the higher
frequencies, and slower changes are the lower
frequencies. Broader change is louder while the converse
is less loud.
AM vs FM
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, and hair dryers. FM typically avoids this
interference rendering it more acceptable for music.
High fidelity audio refers to the faithful reproduction of the
originally recorded sound.
It adheres to standards of accurate measurement -- frequency
response, distortion, noise.
A frequency response test measures the uniform amplitude of
the audible range of sound at the output of an audio
component. A typical test measures the range from 20Hz t
20KHz. The measured result compares the amplitude to the
original input. Any deviation is 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 up to
six octaves above each step. The results are summed,
compared, and computed as a percentage of the original.
Signal to Noise Ratio
A signal to noise ratio is calculated by measuring a unit's
output noise with no signal present. The measured noise
is used to calculate a ratio between it and a fixed output
Typical example of an
amplifier's measured specification:
-- Total continuous power = 100 watts per
-- Frequency response = 20Hz to 20KHz
-- Total harmonic distortion (THD) = less
-- Signal to noise ratio (S/N) = 92db
Frequency response, harmonic distortion, and signal to noise
ratios are base measurements.
Select his link Rane
list of audio measurement.
This concept of high fidelity standards applies to any
component in the playback chain. This includes electronics,
speakers, interconnecting cable, and the acoustics of the
listening room. However, recognize this HiFi
truth. All systems that sound good measure good.
But not all systems that measure good sound good.
Therefore, do not forget the most sensitive measurement tool
in your audio toolbox -- your ears.