[ARC5] Lopsided modulation

[robinson at tuberadio.com]

Hi Gang,

Just to help visualise asymmetry,
attached are 2 waveforms (time domain, male speech).

The first is "There's usually a valve."
shows a symmetrical waveform,
until the last word "valve" which is asymmetrical.

The second is part of the word "valve" (expanded at 1.1 seconds)
showing asymmetry.

The convention used is,
positive airpressure = positive voltage
I continue this with in transmitters with
positive voltage = positive modulation

I make no claim that this affects sidebands
in the frequency domain.

Regards
Ray

>     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
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Theres_usually_a_valve.jpg
Type: image/jpeg
Size: 58379 bytes
Desc: not available
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20180228/8f6c4f46/attachment-0002.jpg>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: valve.jpg
Type: image/jpeg
Size: 44193 bytes
Desc: not available
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20180228/8f6c4f46/attachment-0003.jpg>