[ARC5] Lopsided modulation

[D C _Mac_ Macdonald]

Finally, some 'splainin' that makes some sense!


73 de Mac, K2GKK/5
(Since 30 Nov '53)
Oklahoma City, OK
USAF Retired 61-81
FAA Retired 94-10
________________________________
From: arc5-bounces at mailman.qth.net <arc5-bounces at mailman.qth.net> on behalf of Richard Knoppow <1oldlens1 at ix.netcom.com>
Sent: Wednesday, February 28, 2018 13:31
To: arc5 at mailman.qth.net
Subject: Re: [ARC5] Lopsided modulation

    For the asymmetrical voice begin by visualizing a sine wave
with a line to indicate the zero volts going along the center.
Now, imaging a sine wave with one side flattened. That is
asymmetrical. If the center line is still in the same place the
voltage on one side is larger than the voltage on the other.
    The human voice is generated by forcing air from the lungs
past the vocal cords. The vocal cords act as a valve. Since the
pressure is in one direction only the modulated air stream is
mostly more positive pressure than the average air pressure.
However, since the voice is also partially generated by a number
of resonant chambers in the throat and head, which are after the
vocal cords, the pressures are modified so that its not all more
positive than the average air pressure. The displacement from
average depends on the individual voice: some voices are very
asymmetrical, some are more nearly symmetrical. I don't think any
are more negative going because the voice starts out as a
positive pressure from the lungs. The vocal cords can vary or
modulate this pressure but can not reverse it.
     If one picks up the voice with a purely pressure sensitive
microphone the asymmetry is reproduced as an asymmetrical
electrical wave form. However, since microphones have some
resonances the symmetry or lack of it may be affected. Some
microphones, namely the figure eight type or cardioid type are
sensitive to either air particle velocity (figure 8) or both
velocity and pressure (cardioid). The figure-8 pattern results
from sensing a pressure difference between two points in the
pressure wave so the microphone, in effect, performs a first
order differential on the wave, and thus looses any constant
pressure. That results in a waveform which may be asymmetrical
but not in the same way the original pressure wave was
asymmetrical. All this is made much clearer with a couple of
drawings than it is in words.
     In any case, if you have an oscilloscope attach a microphone
to it and talk into it. You don't even need a modern scope, one
that responds to audio frequencies is adequate. The scope should
be DC coupled. It will show the asymmetry in the voice.

--
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL
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