Question on negative peak limiting

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steve_qix:
Quote from: Steve - WB3HUZ on November 19, 2008, 05:09:50 PM

Consider that the clipping is occuring at negative 95+ percent moduation. What is the power of any harmonics/distortion products at that point?


Ahhhh... the question of the hour  ;)   That depends on how MUCH clipping is happening.  I suppose if the unclipped waveform is symmetrical, and the clipped waveform is 200% positive (and clipped off at 95% negative), that would produce a LOT of harmonics.  I've run a few experiments in this area, and the [unfiltered] bandwidth will expand.  But, that is an EXCESSIVE amount of clipping.

If you just clip off the occasional peak, I have not found any significant bandwidth issues, and in fact, it appears to be quite tollerable as compared to blatent overmodulation (in the negative direction), which can produce splatter like nobody's business (remember Irb?).

Regards,

Steve

W8IXY:
Hello AMers!

I have been involved with AM broadcast and amateur radio AM for almost 50 years now....

In AM broadcast, ALL of the audio processing is done prior to applying the audio to the transmitter.  The transmitter is (hopefully) capable of reproducing a modulated wave form identical to the one supplied to its input.

Any manipulation of negative peaks, equalization, increasing average density, and so on is done and filtered before the audio even gets to the transmitter input stage.  Any non-linearity in the transmitter (including negative peak limiting) is a form of DISTORTION and actually works against you.

The ideal would be to build a transmitter that can handle any waveform you feed it, such as an asymmetrical waveform that is processed to have positive peaks that exceed 200% positive modulation, and almost 100% negative modulation.  Phase shift would need to be very low, and frequency response would need to be ruler flat.  Then all "manipulation" of the audio would be done at low level, and the transmitter would faithfully follow that audio wave form.

I designed all the AM processing parameters for the OMNIA One audio processor here at Telos systems, and have some "very highly processed" audio modulating the 50,000 watts here on WKNR/AM850 in Cleveland.  If you are within range, tune it in.  Your transmitter needs to be as "transparent" as possible, and the processed audio will fly through it and result in UNREAL good sounding audio on the air.

73
Ted  W8IXY

steve_qix:
Quote from: W8IXY on November 20, 2008, 01:26:44 PM

Hello AMers!

  Your transmitter needs to be as "transparent" as possible, and the processed audio will fly through it and result in UNREAL good sounding audio on the air.



Hi Ted, good to hear from you!!!

We have a few transmitters on the air with transparent audio.  These are generally the low level modulated transmitters, or the solid state class E, high level modulated transmitters. 

However, we do have a LOT of transmitters that won't do it - so the compromise is to do a bit of negative peak clipping at high level  ;)   It works rather well if not carried to an extreme.  I've done it with broadcast rigs (like the Collins 21E, RCA BTA1-R, etc.) that, while pretty good, would not handle a clipped, asymmetrical waveform very well.

Good stop-gap measure 'till we got an MW5 (this was back in the early '70s).

Regards,

Steve WA1QIX

w1vtp:
<copied from Steve QIX>  Ahhhh... the question of the hour 

My comments.  The question is if the transmitter is at ~95% negative modulation peak (null) and why wouldn't splatter be a non-issue as there is little power at that point coming out of the transmitter.  Harmonic distortion is distortion no matter if it occurs on negative or positive.  Some folks have touched on the key in this thread -- the diode with keep alive is a great way of reducing the HARD square wave corners in negative clipping.  But I think that excessive positive modulation can produce similar results IF the audio peaks are accompanied with some clipping at the positive peaks too.

It always comes down to good design and proper operation of our AM transmitters.  This thread is an excellent one for me.  I would still like to see some spectrum analysis under controlled testing conditions and compare a good negative modulation protection with an honest to goodness negative over modulation.  Could even throw in some positive clipping spectrum displays.

This is fun -- I think we as AMers are much more conscious of proper operation on both the negative peaks and possible flat topping.  We really do not want yellowy distorted sounding modulation and that usually results in good BW performance.

Cheers, Al VTP (who couldn't persuade my bod to shut down for the evening).

steve_qix:
Of course Al has a great point.  If there is clipping of the positive peaks too, then obviously there will be subsequent harmonics (aka splatter) produced.

I think for the purposes of this particular discussion, we are "assuming" the transmitter is not clipping the positive peaks at all, at the actual level of modulation in use at the time.

I wouldn't apply negative peak limiting (at least not much!) to a transmitter with limited positive peak capabilities.  Heck, my voice alone WITHOUT negative peak limiting has a natural 30% to 40% asymmetry - and I have actually observed more than this at times.

I can speak for myself - the negative peak limiting I use is pretty minimal.  It is there only to deal with the occasion high peaks that would otherwise cause overmodulation in the negative direction.  If carried too far, the point of a good, high fidelity transmitter is diminished because there WILL be distortion.

On the other hand, it sure is nice to be able to really pour on the audio when conditions are rough.  At this point, pure fidelity gives way to copyability  :D  But, never to the point of positive peak clipping, too!

Regards,

Steve

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