[AMRadio] Asymmetry


Jay Bromley jayw5jay at cox.net
Fri Oct 4 18:05:59 EDT 2013


In the Orban 9200 manual says the following:  >>>>There is really nothing lost by not modulating asymmetrically: Listening tests easily demonstrate that modulating symmetrically (provided that time dispersion has been applied

to the audio) produces a considerably louder and cleaner sound than does asymmetrical

modulation that retains the natural asymmetry of its program material.

Some of the newer transmitters of the pulse-width modulation type have circuitry for

holding the carrier shift constant with modulation. Since artificial asymmetry can introduce

short-term DC components (corresponding to dynamic upward carrier shift), such carrier

shift cancellation circuitry can become confused, resulting in further distortion.

 

This has me bugged a little bit and looking for comments from the forum to broaden my thinking on this.

After a discussion with a friend on this, I stayed up most the night reading in books and on the internet about this subject.  I have included below the whole section from the 9200 manual below my text on Asymmetry so you can read the whole text in its entirety.  

 

I got to playing around on the air using Asymmetrical modulation up to near 200% down to a little over 100 percent to test this in real life.  Using an REA AMM-HF1, my station does get louder with more positive peaks.  However I know there is a point that most diode detectors can’t handle this.  My first question at what point is a good point not to go beyond on the positive peaks?  Most ham stations are using enveloped non-synced detectors.

 

To me it makes sense that you would be louder with Asymmetrical modulation as anytime you are increasing the current and voltage in the waveform that will have more power ultimately.  Most processor manuals like the Orban will also warn you about some transmitters not handling the increase in power well without problems.  So this must be making a huge difference using processing and or asymmetrical modulation.  I also noticed the average power was up or at least was on my scope.  Reading carefully, if you are not creating Asymmetrical modulation artificially does a male voice still have both odd and even order harmonic and IMD distortion to be concerned about?  In other words if you voice has asymmetry naturally and our transmitter is capable of reproducing what it is fed, is this even a concern like the Orban manual seems to indicate?

 

I know that processors like Orban are made for broadcasting, not the amateur market.  Broadcasting processors need to be equipped to handle both music as well as voice and maybe that is the rub here?  Using a mix of programing that ham station never has to deal with.  Looks like to me to achieve louder/cleaner (I know an oxymoron!)  audio symmetrically, broadcast processors do all sort of neat tricks like phase rotators, all pass filters, etc.  BTW, I am not using a multiband processor like the Organ here.

 

So if a person has the capability to modulate asymmetrical without distortion and doesn’t have a big gun processor like an Orban, is trying to maximize asymmetry worth the effort?  I always thought it was reading from very old handbooks it was worth the effort to maximize and now I am wondering if it really is.

 

See full text below on Asymmetry from the Orban 9200 manual I got off the internet.  Many thanks for any input on this!!   73 de w5jay/jay..

 

 

 

Asymmetry

While any AM modulation system is limited by the physics of carrier pinch-off to an

absolute negative modulation limit of 100%, it is possible to modulate positive peaks as high

as desired.

In the United States, the FCC permits positive peaks of up to 125% modulation. Other

countries have similar restrictions.

However, many transmitters cannot achieve such modulation without substantial distortion,

if they can achieve it at all. The transmitter’s power supply can sometimes be strengthened

OPTIMOD-AM Digital INTRODUCTION 1-11

to correct this. Sometimes, RF drive capability to the final power amplifier must be

increased.

Voice, by its nature, is substantially asymmetrical. So asymmetrical modulation became

popular in an attempt to increase the loudness of voice. Traditionally, this was achieved by

preserving the natural asymmetry of the voice signal. An asymmetry detector reversed the

polarity of the signal to maintain greater positive modulation. The peaks were then clipped

to a level of −100%, +125%.

OPTIMOD-AM takes a different approach: OPTIMOD-AM’s input conditioning filter

contains a time dispersion circuit (phase scrambler) that makes asymmetrical input material,

like voice, substantially symmetrical.

OPTIMOD-AM permits symmetrical or asymmetrical operation of both the safety clipper

and multi-band distortion-canceling clipper. Asymmetrical clipping slightly increases loudness

and brightness, and will produce dense positive peaks up to 125% if this is desired.

However, such asymmetrical processing by its very nature produces both odd and even-order

harmonic and IM distortion. While even-order harmonic distortion may sound pleasingly

bright, IM distortion of any order sounds nasty.

There is really nothing lost by not modulating asymmetrically: Listening tests easily

demonstrate that modulating symmetrically (provided that time dispersion has been applied

to the audio) produces a considerably louder and cleaner sound than does asymmetrical

modulation that retains the natural asymmetry of its program material.

Some of the newer transmitters of the pulse-width modulation type have circuitry for

holding the carrier shift constant with modulation. Since artificial asymmetry can introduce

short-term DC components (corresponding to dynamic upward carrier shift), such carrier

shift cancellation circuitry can become confused, resulting in further distortion.



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