Audio phase

[Steve - K4HX]

The best way to be louder is to run more power and/or have a better antenna. The rest is mostly a matter of taste.

The distortion compensation of the recent SDR transmit chains is not to improve the on channel audio quality. It's to reduce/remove the off-channel RF that could result in interference. Given most receive audio chains have several percent (or more) THD, even if there were on-channel improvement, you couldn't hear it.

[AB2EZ]

Steve

I respectfully disagree, in part, with your post (above)

I agree that pre-distortion is primarily employed to reduce out of channel odd order distortion products that are introduced in a non-linear RF amplifier chain.

However, if the RF amplifier has a significant amount of non-linearity (i.e. output envelope vs input envelope)... as in the case of my Elecraft KXPA 100 when operated at 100W peak output on 40m...
then there will also be significant amounts of in-channel odd order distortion products that correspond to distortion of the demodulated audio.

It is easy to see (on a scope)/hear (with my off air monitor) this distortion when operating 40m AM. The KXPA100 produces about 3dB of compression at full 100 watt output (3rd order IMD only about 20dB down in a two-tone test).

See the attached curves of amplifier output envelope v. input envelope (blue) and Pure Signal (adaptive predistortion) linearity correction (red).


Without pre-distortion applied, the off-air monitor output sounds very distorted.
With pre-distortion applied (around the KXPA100 amplifier), the off air monitor output sounds excellent.

Stu

[W3RSW]

If the pure signal digital / mathematical algorithm has little problem removing distortion from RF then it has even less problem removing distortion from the concomitant modulation envelope controlling the RF at much lower frequencies.

By superposition theorem, the Audio and RF waveforms are now one waveform, choppable in minute discrete intervals for manipulation.

[K1JJ]

My technology tale of woe:    

As a side note about "Pure Signal" and the new RF digital technology...  I find it curious that I spent a ham lifetime experimenting with many, many conventional homebrew RF amplifier configurations looking for the cleanest, perfect high power signal without splatter, etc.  The best I could do was about -30db 3rd order IMD at best. When the amplifier was pushed hard, it got nasty down below -25db 3rd.

After almost giving up over 30 years, I eventually settled on a Class A low level system that drove cascaded class AB systems, giving over -50dB 3rd IMD at high power. In 2012, I had finally arrived! For once my high power linear signal was truly "linear."

Then the signal processing software became mainstream. Now a crap CB splatter-maker amplifier can be made to run as clean or cleaner than my low efficiency, pristine nightmare. It's not fair!  

I thought I had finally achieved something that was unattainable to the average ham and I could have pride in running it on the bands - the elite few!  But time and technology march on. Nobody cares or knows anymore how the signal was generated.

This pattern has repeated itself throughout history and will continue. Just think of the ham rigs we thought were hot-sh@t in the 50's, 60's, 70's, etc.

T   (dragged thru time kicking and fighting)

[AB2EZ]

It is interesting to think about (but perhaps not for too long) the difference between

case 1: y(t) = [a m(t) + b m(t)**2 + c  m(t)**3 ...] x [cos(2pift)]  

and

case 2: y(t) = a [m(t)cos(2pift)] + b [m(t)cos(2pift)]**2 + c [m(t)cos(2pift)]**3 ...


where m(t) is: 1+ an audio waveform whose peak magnitude is less than 1

Case 1 corresponds to the low-pass filtered output of a high-level AM modulated (e.g. a Class C plate modulated transmitter or a Class E drain modulated transmitter) transmitter... where the modulation process is not perfectly linear.

Case 2 corresponds to the output of a (not perfectly linear) RF linear amplifier driven by an ideal AM modulated RF input signal (before using a low pass RF filter to attenuate RF harmonics)

It is even more interesting to think about (but definitely not for too long) how adaptive predistortion would be implemented in Case 1 or Case 2.

Stu

[Opcom]

I'm interpreting this thread as a discussion of "If asymmetry then how much?"

Rather than take my word for it, here's the lesson learned 50+ years ago in the professional broadcast world:

http://www.w3am.com/SymmetraPeakBrochure.pdf

For those who desire a total understanding, here's the patent:

http://www.w3am.com/SymmetraPeakPatent.pdf

100% of AM broadcast processors include an all-pass filter. I like experimenting but not reinventing the wheel...

It's a perfect solution but the units are rare now and the good quality inductors are as rare in the values that work. As an exercise I drew up an electronic version using tubes in a Philbrick GAP/R K2-W op-amp circuit and otherwise based on that 8 op-amp circuit board shown. It would not be exactly compact with sixteen 12AX7s and a regulated +/-300V 50mA supply. I have not built it so anyone who uses the schematic would have to debug it themselves. It's mainly presented an example of what could be done with tubes if someone wished it.

[AB2EZ]

All radio receivers contain "automatic volume control" functionality. In most radios (naturally including traditional AM radios), the AVC circuitry does not respond to (capture and hold) the actual peaks of the i.f. output signal

Depending upon the details of how the actual "peak" detector is designed, it responds to a short term average of the envelope of the received signal.. which lies somewhere between the true peak and the long term average (carrier) level. [Note that, by analogy, most power meters, including high-end power meters, do not respond to and/or display the true the peak envelope power of an AM signal... even though they are specified to be peak reading... because their internal peak detector is not fast enough to capture and hold the actual peaks].

If you reduce the carrier level (the average of the received envelope) of an AM signal... the AVC will compensate by increasing (to some extent) the gain of the i.f. and/or audio amplifiers.... even if the peak RF envelope stays the same.

When someone tunes across the "stations" with an AM radio... without touching the volume control... the stations with a higher ratio of peak envelope power to carrier envelope power will sound louder... due to the action of the AVC circuitry.


That says nothing about the effect of having high positive peaks (v. negative peaks) on whether the received audio sounds good or not.


This effect will hold for SSB as well as AM if the AVC/ALC/AGC circuitry of the receiver is not fast-acting enough to capture and hold the actual peaks of the modulated received signal.

Stu

[AB2EZ]

The above (my last post) indicates that if you want to fight in the loudness wars... with a given PEP limitation... you want adjust your voice processor to produce the largest ratio of positive peaks-to- whatever characteristic of the modulated signal produces the smallest response from a typical AM radio AGC "peak" detector.

Taking advantage of the AGC behavior, in this way, is conceptually equivalent to reaching across to my receiver, and turning up the audio gain control, when it is your turn to transmit.

Stu

[W3RSW]

Analog AVC in those am radios are r/c integrators, that is they use a real interval or passage of time as the needed dimension to operate.
Come to think of it, we all do. 

Information is therefore averaged .  Informatiom integration also allows lower signal levels to be utilized.

[N1BCG]

Further support for thoughtfully clipping the positive peaks.

[AB2EZ]

On another web site: Yahoo Groups... Apache Labs Group,

Paul (W9AD) posted the following excerpt from the Orban 9300 manual (note the double negative at the beginning of the excerpt)



"Have a look at the Orban Optimod-AM 9300 operating manual, referring to page 1-15:

 “There is really nothing lost by not modulating asymmetrically: Listening tests easily demonstrate that modulating symmetrically, if 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.”

 http://www.orban.com/orban/products/manuals/9300_1.0.5_Operating_Manual_rev_02.pdf "

This is an example of a customer driven approach to selling a product:

My interpretation: 

Our processor can be adjusted for 125% positive peaks... as you have requested, when talking to our sales and marketing folks (but its really not good for you).