[ARC5] Lopsided modulation

[Richard Knoppow]

    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.

On 2/28/2018 10:17 AM, Tim wrote:
> Yep!
> I'm waiting for Prof. Lee to hit us with the all-defining 
> modulation math.  (leaving it as an exercise for THIS student 
> would not be productive!)
> 
> Also. I can imagine asymmetrical sideband power on either side of 
> an AM radio carrier but I cannot imagine what an "asymmetrical 
> voice" could be. ;o)
> 
> Tim
> N6CC

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