On Wed, 14 Apr 2004 11:50:18 -0000, Jim Jarvis wrote:
>How do you distinguish between a 180 degree phase shift,
>and polarity reversal? Unless you're timing logic edges,
>or counting cycles, they look the same to me.
Only if you use a sine wave as a source, and look only at a single
frequency. Use a broadband signal (or try to get broadband performance
from an antenna system) and you will see significant differences (an AM
broadcast signal driving a directional array). You get a VERY
different result with a square wave (or pink noise, or white noise, or
a modulated signal). With that square wave, for example, a network that
produces 180 degrees of phase shift at the fundamental will have very
different phase shift at each harmonic.
An easy example. Feed broadband noise into the two loudspeakers of your
home stereo system (tune between stations, or use a pink noise source).
Invert the polarity of one loudspeaker and listen with one ear facing
the loudspeakers, precisely on centerline between them. Try to set them
up and position yourself so that you are hearing ONLY the direct sound
from the loudspeakers, without reflections from floor, ceiling, and
walls. You will hear lots of cancellation of bass, and as you move side
to side so that you are closer to one than the other, you will hear the
character of the noise change drastically.
What's happening in the above example? On centeline, ALL frequencies
are very close to 180 degrees out of phase (you have reversed the
polarity), and those frequencies which are most precisely 180 out and
equal in level will cancel to the greatest extent. As you move off
center, the lower frequencies (the bass) will still be quite close to
180 out and equal, but the highs will experience significant phase
differences due to the arrival time differences (you are closer to one
loudspeaker than the other), and these arrival time differerences will
cause phase shift that is proportional to frequency. A time difference
of 0.5 ms is 180 degrees around 800 Hz, 360 degrees around 1600 Hz, 540
degrees around 2400 Hz, and so on. (the frequencies are approximate,
depending on the speed of sound, which varies with temperature). The
resulting frequency response is commonly described in the audio world
as "comb filtering."
While we're at this example, restore the polarity to "normal" and
listen again. You won't hear the bass cancelling, but you will hear the
comb filtering.
Next, listen FACING the loudspeakers (in the previous examples you
should have had only one ear facing them), still IN polarity. When you
face them, you will hear far less comb filtering at the higher
frequencies. Why? Because deep cancellation only occurs when the
amplitudes are very nearly equal. Human hearing becomes increasingly
directional above 1 kHz (the head gets in the way), so the right ear
hears less of the left loudspeaker and vice versa. When you turn
sideways you take this directivity out of the equation.
Jim Brown K9YC
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