[Boatanchors] Voice asymmetry and processing

Tue Oct 6 06:09:40 EDT 2015

Don,

Thanks for adding information on the Symmetra-peak operation.  We had them in the racks at Greenville VOA, but I think they were not in use.

On the HF side, we've used the Orban Optimod-HF 9105A series, but I think most or all are gone.  Currently, we are using the Orban 9200 on both the HF and MW transmitters.  On our FM transmitters, Orban 2200 or 2300 units.

73
Sheldon

-----Original Message-----
From: boatanchors-bounces at mailman.qth.net [mailto:boatanchors-bounces at mailman.qth.net] On Behalf Of Donald Chester
Sent: Tuesday, October 06, 2015 5:08 AM
To: boatanchors at mailman.qth.net
Subject: Re: [Boatanchors] Boatanchors] Voice asymmetry (was 813 grid to filament short)

> On voice "asymmetry" what is typically observed is due primarily to 
> distortion introduced in the speech amplifiers most hams use, not a 
> natural feature of human vocalization.

>Dennis D.  W7QHO  Glendale, CA

On the contrary. Random distortion in typical Hammy Hambone speech amplifiers is more likely to reduce or eliminate, rather than to introduce, asymmetry to the waveform.

Most human voices, particularly male, have substantial natural asymmetry, which in some cases may be as high as 2:1. This asymmetry will be preserved and appear at the modulated signal if and only if the total distortion and phase shift remain minimal throughout the system, from the microphone all the way to the final modulated stage. For maximum sideband power with asymmetrical waveforms, the phase polarity must be adjusted so that the half-cycle with the greater amplitude is in the positive direction. If no "processing" (which is, by definition, a form of distortion) is introduced to the signal, positive peak headroom must exceed 100% in order to attain 100% modulation in the negative direction without flat-topping on the positive.  If the transmitter is capable of modulating only up to 100% positive, attempting to modulate 100% negative with an asymmetrical audio signal will result in exactly the same flat-topping, distortion and splatter on positive peaks, as results from over-modulation on negative peaks.

As an example, take a voice with a 2:1 natural asymmetry. When that voice is used to modulate to its maximum capability, a transmitter with no positive peak headroom beyond 100%, the negative peaks cannot exceed 50 percent without positive peak flat-topping.  Effectively, the total maximum percentage of modulation cannot exceed 75% without  splatter and distortion.  Fortunately, most voices have a natural asymmetry more like 1.5:1 or less.  The Kahn Symmetra-peak is described as an "all-pass" filter, designed to rotate the phase linearly across the audio frequency range in such a way as to reduce or eliminate whatever asymmetry that may show up in voice or music program material.  Its purpose was to allow a higher effective percentage of modulation with transmitters that lacked positive peak headroom.  It was NOT designed to introduce or enhance asymmetry to the waveform.

With a ribbon (velocity) microphone,  the polarity of the voice waveform may be reversed simply by rotating the microphone 180 degrees in front of the speaker. Some broadcast equipment, such as the Urei BL-40 modulimiter, have a built-in phase sensor that automatically reverses the polarity so that the half-cycle with the  greater amplitude is always phased in the same direction.

Natural asymmetry is not to be confused with so-called "ultramodulation, negative peak clipping and negative cycle loading, all of which deliberately distort the signal by attenuating or clipping the negative half of the audio cycle. With natural asymmetry, the energy content of the positive and negative peaks remain identical; peaks with the greater amplitude are proportionally of shorter duration than the peaks of lesser amplitude, so that the area under the curve of a graphical representation is exactly the same on both sides of the baseline. Therefore, a transformer or coupling capacitor can pass the asymmetrical waveform intact, since no rectification has occurred, unlike the case with the above forms of audio processing; transformers and coupling capacitors cannot pass rectified DC. With systems that deliberately attenuate the negative peaks, such as ultramodulation, rectification occurs causing the carrier to shift upwards during modulation. The resulting signal is equivalent to controlled-carrier combined with even harmonic distortion.

Don k4kyv

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