TO REMIND ME WHAT ASYMETRIC MODULATION IS ALL ABOUT THE ANSWER IS... TO BE LOUDER ON THE DIAL THAN THE OTHER STATIONS.
TROUBLE IS, ALL THE STATIONS HAVE IT SO THEY'RE ALL LOUD.
RIGHT WING CONSERVATIVE TALK HOSTS GAIN AN EXTRA DECIBEL BY SHOUTING AT THE MICROPHONE. IT MAKES THEIR STATION SEEM LOUDER.
I read the first page linked and I still don't understand the details. There seemed to be a lot of "I think.." and "someone told me.." postings.
Neil
Exactly Neil.
Actually, for Part 15 broadcasting, the goal of asymetrical modulation (at least for me) is to increase range. I don't particularly care about perceived loudness.
This is the best post on the topic I've seen so far at Virtual Engineer, by Henry, found on page 2.
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WB6QED » Wed Oct 23, 2013 8:13 pm
I believe that the original intent of authorizing positive modulation in excess of 100% was to "allow" asymmetry in the way that the CBS Laboratories 4300 Volumax implements it by using a polarity follower and changing the positive AGC threshold and the final clipper bias rather than to "force" it in the way that it is currently being manifested by most "modern" AM broadcast audio processors that use input all pass filters to scramble the polarity of the audio waveform.
For example, the later 4300 Volumax "allows" asymmetry by changing the bias on the positive side of the AGC detection circuitry and uses a different voltage zener diode for the positive output clipper. This effectively raises the point of positive waveform AGC action and positive clipping when the positive peak switch is set to +125%. The polarity follower keeps the polarity rotated in the proper direction.
The reality of the situation is that most program material is symmetrical and when the polarity switch is set to +125%, the negative side of the AGC detection circuitry will being doing all of the work while the positive peaks will seldom initiate AGC action.
To set up the 4300 Volumax for positive peak operation, you would adjust everything to be the way that you want it with +/-95% operation. When you were satisfied with that, then you could move the positive peak switch up to +125% and let the positive peaks land where they will. I would expect that for the majority of program material, there would be very little difference since the AGC action will be mostly working on the negative side and will back the overall envelope down accordingly. When operated this way, I am virtually certain that one could not hear the difference between +/-100% modulation and +125%/-100% modulation. The human ear responds to average energy not transient energy. Who knows, maybe the ear would actually perceive the symmetrical waveform to be louder since both halves of the audio waveform would be clipped as the transient rolls through?
"Modern" audio processing with input phase scrambling that removes natural asymmetry should be adjusted for symmetrical operation. It is sort of like take your pick. Either phase scramble the input and run it symmetrical or use the 4300 Volumax approach with a polarity follower.
Forced asymmetry creates a DC axis shift with modulation and subaudible components that can upset the power regulator in a PWM system. This necessitates a direct-coupled modulator (flat down to and including DC) after any negative-peak clipping.
I suspect that the loudness benefit of input phase scrambling using symmetry is significantly greater than that of the approach used by CBS Laboratories with the 4300 Volumax.
In the past, Bob Orban has pointed out: In the PWM rigs that the "forcing" method drives the transmitter's power output regulator nuts. He also mentions that going the pure symmetrical route and adding the "phase scrambler" results in heavier modulation. Decades ago he demonstrated at an NAB convention that asymmetric modulation:
1. Did not make the station sound louder
2. Added audible distortion
3. Had no benefits
At Continental Electronics, they seriously discourage other than symmetrical modulation on their big shortwave rigs. With marginal benefits, the cost can get outrageous for the added voltage rating on the parts including the insulators in the antenna system. +125% modulation can be expensive when you take into account the voltage ratings of the various components.
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I'll add:
If the system can allow natural assymetry of a complex wave to peak above 100 %, with the negative peak at 100%, that waveform should be passed to the receiver and speaker without distortion.
Artificial assymetry means taking already processed symmetrical audio and then limiting the negative peaks to 100% and positives to 125, specifically to force assymetry to the allowed standard. The effect would be similar to taking a sine wave and doing the same thing. Result: no longer a sine wave.
Assymetrical modulation at high levels used to be called "supermodulation", and at first there was no limit specified in the rules, then at some later time, the rules were changed to 125% max. Why?
In the recording studio we use a phase scrambler to make lopsided, assymetrical vocals fill up the wave editor display. The scrambled signal doesn't sound louder initially, but with a lower peak to average ratio it can be amplified a few db more without clipping usually. It's a very low distortion way of increasing the vocal power.
This would make a good Powerpoint presentation, I think it would be easier to grasp visually. NC
The last thing I read about asymmetrical modulation was in the manual for the new Orban processor I installed at the commercial station I work for. Our old processor was set up for the typical 100 negative 125 positive (an old CRL setup) and I was told to be sure to set up the new Orban the same way -- which would have required getting into the setup software more than I wanted to, and Orban advised against it thusly:
"Asymmetry
While the physics of carrier pinch-off limit any AM modulation system 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% modu- lation. Many 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 to correct this. Sometimes, RF drive capability to the final power amplifier must be increased.
Voice, by its nature, is substantially asymmetrical. Therefore, asymmetrical modula- tion was popular at one time in an attempt to increase the loudness of voice. Tradi- tionally, this was achieved by preserving the natural asymmetry of the voice signal. An asymmetry detector reversed the polarity of the signal to maintain greater posi- tive 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 multiband distortion-canceling clipper. Asymmetrical clipping slightly in- creases loudness and brightness, and can produce dense positive peaks up to 125%. 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, if time dispersion has been applied to the audio, produces a considerably louder and cleaner sound than does asymmetri- cal 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 fur- ther distortion."
So, following Orban's suggestion we are NOT assymetrical. However, Iron Range Country here in Bovey is quite nicely chugging away in Part 15 land at 100- and 125+ with the processing that's built into my Procaster.
Tim in Bovey
And FWIW I get quite a bit better range running 100- and 125+, than limiting modulation to 100 on both sides. Don't know about "loudness" but I could demonstrate quite easily the range increase. When I was first setting things up I did all the experimenting, including shutting off the processing in the Procaster and running with none at 100% peaks, and also using an regular audio compressor/limiter, which did make it louder but still symmetrical. The 100/125 setup increased range at about the same volume near as I could tell as the outborad processor. So, that's how I set it up, with the built in processing at 100/125 and there I've left it for the last nearly 2 years and it's just fine.
Tim in Bovey
With all the talk about the 222, I decided to do an experiment.
I pulled mine out, along with a Talking Sign transmitter. I've used the latter previously and gotten good results.
I first set up the Talking Sign (with its wire antenna) and determined the maximum distance that I could reasonably hear my signal at. Now, it wasn't very far, as I have a really less than ideal location for AM (in a townhouse complex, surrounded by other 3 story units, and my studio being on the ground floor). Plus, I can't even extend the wire antenna to its full length vertically - I have it in an L shape around a window frame. When I got decent range with this transmitter, I had the wire antenna out in the open, well above most obstructions.
Anyway, I then connected the 222, with everything else being equal. The range was significantly greater, although I didn't attempt to quantify it - my goal was to prove that the 222 does increase Part 15 AM signal range. And that I did.
I also noticed that the audio was 'punchier' - it made the 50s and 60s oldies that I was playing sound better than without it in the audio chain (my opinion, anyway).
"The range was significantly greater, although I didn't attempt to quantify it..."
The range was significantly greater? If you did not quantify it, how do you know?
Last time I checked, "the range was significantly greater" was not a quantitative phrase (i.e., I didn't measure exactly what the difference in the 2 ranges were). However, I did post that the signal extended a lot further (note - "lot" is again not a quantitative phrase).
To put it simply, I drove past the first range marker (where the Talking Sign alone faded out completely) and could still hear the signal with the 222. I continued to drive for a period of time and still could hear it. I didn't measure the difference in ranges, but it was significant (i.e., not just a few feet).
I don't understand the issue with the 222 (the 'authoritative voices' of Part 15 also don't have many positive things to say about it, even though the head voice has admitted he's never used one) - it does make a noticeable difference in Part 15 AM range, and it can make your sound 'punchier'.
And anything that can improve range legally is OK in my books.
It's sometimes hard to keep it all straight when discussing complex technoogy that is able to provide several functions all at once.
The high powered licensed stations (opinion) may not be so much interested in range as they are in percieved loudness so they stand out on the dial locally.
Part 15 low power radio has both problems... weak signal and sometimes weak audio, with the range being a significant advantage of using Innovonics 222.
In another thread John WDCX informed us that the StereoTool by Hans Van Zutphen can accomplish exactly the same AM processing using a computer, and I gave it a successful try processing the audio into the AMT5000.
I canceled the asymetric AM modulation within StereoTool because the same audio chain was feeding my internet audio streams, and asymetric processing is terrible when applied to internet audio streaming.
No loss to me, because range isn't important in my operation... I need to cover the immediate yard and nearby locations, and it's mission accomplished for that without special modulation.
Depends what you're doing.
I stand corrected.
In a previous post I stated that the 'head voice' on another Forum had posted that he had never used the 222. That apparently was an error on my part. There were a flurry of disparaging posts over there about the 222, and I obviously misinterpreted something that was said.
I, at least, admit my errors.
I still stand behind my thoughts on the 222. Carl is right, in that there are different reasons for getting into Part 15 broadcasting. But I suspect that many are interested in maximizing range legally, and the 222 is a good vehicle for that. It is relatively inexpensive (as far as audio processors go), is pretty simple to use (even without a modulation monitor) and does the job. Is there better equipment around? Probably. But that equipment also comes with a much higher pricetag (the Inovonics is still pretty expensive for hobbyist use, at around US$400 used - if you can find one).
When comparing various types of processing, be it assymmetrical, one processor vs another, etc. you can't really make ANY determination unless you know what your modulation is. If you pull one processor and substitute another and declare one louder, how do you know? Did you adjust the levels to be identical? And you can't do this siply by watching the blinking lights or whatever on the processor. There are too many variables. You need an actual modulation monitor -- a device that will actually show you your modulation levels in the on air signal itself, NOT in the audio feed going to the transmitter.
Not only does maximum modulation make your station louder, it has a dramatic effect on the RANGE of your signal. Peaking at 70% compared to 120% makes a big difference in how far and how well your station can be heard. You can't determine this without some type of actual off air modulation monitor. No VU meter on a mixer, or blinking LED's on a processor, is going to tell you what your actual modulation is, and you don't know if you're getting maximum range and loudness with out knowing your modulation.
When I first put Iron Range Country on the air I simply cranked up the audio till I could detect distortion then backed off until I was happy with the sound. I got it pretty darn close, but with the addition of a state of the art mod monitor, I know. Plus, I can check on the status of the station I work for and others in the market 🙂
A processor is a key element in making sure you're modulating as much as you want, as consistently as you want. Note that the processor in my Procaster gives me asymmetric modulation at 100-/125+ without the need to purchase any outboard device. One of the advantages that influenced me to choose this transmitter. Of course the processing can be adjusted, or even bypassed -- but I'm coming up on two years on the air and have felt no need to try anything else.
Tim in Bovey
I followed the procedure that you described, but would like to obtain a modulation monitor to check (I'm obviously in the ballpark as evidenced by the results). Can you recommend a good, but relatively inexpensive, one?