Thanks for your comments. Not being too familiar with audio processing leads me to some intuitive leaps so bear with me if some of my thoughts are not clear on this. This is a good way for me (and I hope other readers) to learn.
Supposedly, the human voice is naturally assymetrical. I've never seen any tech data that says how much so we could relate it to some modulation level over 100%. Theoretically, if the human voice is assymetrical enough and the 222 allows assymetry to pass, then 125% pos peak modulation won't distort the waveform very much.
We should be careful using the word asymmetry. Though if you view a voice waveform on a scope you will see that the waveform above the 0 line differs from that below the line (asymmetry) this is not what you will see with a modulated AM RF signal out of a properly functioning transmitter. This waveform is symmetric about the 0 axis. Minor point but I didn't want anyone to assume the modulated RF waveform is asymmetric.
I believe I follow the thinking on this asymmetric processing but I can't connect it to being an advantage. We can introduce asymmetry in any waveform by non-linear processing yet from context it appears that there is a scheme to take advantage of the natural asymmetry in voice. Correct? It seems that we are trying to fit a waveform into a "slot" between 0 modulation and 125% modulation by adjusting offset and amplitude nearly instantaneously so the positive and negative peaks of the waveform touch the respective limits. This is processing based upon peak to peak amplitudes (some call this voltage clamping) and not upon asymmetry, thus the disconnect for me. I may be missing a key point here.
There is another way to view voice modulation. The ARRL Radio Amateur's Handbook has a chapter devoted to voice characteristics and processing for radio communication which might be interesting to those who want more information. This reference states that the peak to average power ratio for voice can be as high as 100:1, yet the working number for SSB is 3:1. It also discusses that voice signals can be highly compressed with little perceived loss of quality.
Clipping or compressing a voice waveform lowers this ratio and devotes more of the available sideband power to carrying the intelligence. Fortunately, according to the reference above, we can tolerate quite a bit of voice compression without adverse effect.
Your comment about the modulation level relating to the audio volume at the receiver is correct but another way to view this is that the audio output level of the radio is a function of the sideband power (which depends on the modulation level). More power in the sidebands, more audio volume, which I believe is equivalent to what you stated. Though my approach was directed at increasing range and yours at increasing audio output both would be achieved because the signal to noise ratio at the receiver should improve with 125% compared to 100% modulaton. (Noise power stays the same but intelligence power increases).
My opinion is that the increase in range is probably not worth the effort. Your objective, as I read it, is to increase the loudness at the receiver audio output. Would not advancing the volume control on the receiver do this, or is the idea to give the signal a "stand out from the crowd punch" by introducing audio distortion?
Your story about monitoring the IF signal is interesting but I wonder if the 0 clipping you observed is due to an artifact in the receiver rather than the transmitter. Perhaps the AGC was not tracking properly? It would be interesting to know if the station was causing splatter, though with IBOC it may be difficult to tell by tuning up and down and listening. Setting a transmitter intentionally to clip at 0 is not wise due to the interference created but also because the clipping robs power from the in channel signal and thus reduces range. Perhaps what you observed was a temporary malfunction or the result of the wrong people twisting the transmitter knobs.
Neil
All that I said before was "educated" speculation. I certainly wouldn't claim any expertise. I was kind of hoping that someone who really knows this stuff would step in.
I found a web site that presents a high level overview of what we have been discussing, including info about the Inovonics 222 asymetrical limiting.
http://www.icycolors.com/nu9n/am.html
There are probably other sites that have more detailed info. I didn't take the time to do a real good search. The technical presentation seems acurate to me, but then, info from the web is often not the best.
PhilB
Hmm, this thread has a slight feeling of déjà vu…
Some of this was covered earlier in http://part15.us/node/1247 , but a good review never hurt.
From an anecdotal point of view I can tell you asymmetrical limiting does make a difference in range. I have the Hamilton Rangemaster, and was using a Behringer 2 band limiter-compressor. When I added the Inovonics 222 my usable range did increase noticeably. If you can increase positive peaks from 100% to 125% then you can add almost 6db (I believe) of loudness to your signal. That extra few db will not make your signal go any further, but it will make it usable further towards the edge of your signal.
With 100mw of power, we have a lot of fringe in our signals. By punching up the positive peaks, we make much of that fringe usable. The extra 6db raises the modulation above the noise floor or the signal. As we get further to the edge of our signal, the signal (and volume) drop, until they sink into the noise floor. 6db can more than double your peak volume, and keep you above the noise floor much further out. This is not derived from a mathematical model, but by actually driving around and mapping the difference in where my signal can be heard with and without the Inovonics.
As was mentioned earlier, the Inovonics does not create positive peaks where none exist, it just passes them when they are present. Other (more expensive) processors do create asymmetry, and shape those signals for maximum modulation. But I have found that by running the Behringer hard into the Inovonics, I get a lot more “punch” and more range than I did before.
This does introduce some clipping and distortion, not a lot, and most of that distortion is in the high end which most AM radios do not pick up. For spoken word, (which most of my programming is) the distortion is un-noticeable. I have experimented with the 222 alone, no limiter-compressor in front of it, and you still get a boost in range, and the sound is more musical and less aggressive, and has almost no noticeable distortion.
Thanks Phil B for the link. Is is a good read, and though it did explain modulation and limiting, it failed to explain "asymmetrical" limiting or modulation. Asymmetrical processing was mentioned but not explained. It is probable that, due to my engineering background in which I have never encountered this term regarding limiting or modulation, that asymmetrical is a "buzz word" which is used to describe something else such as nonlinear and my lack of exposure to this causes my disconnect. Sort of like the "computing power" of a PC...I know what it means but it is not measured in watts nor in any basic terms of physics. It appears to be a problem of semantics for me and I can argue that using asymmetrical is not legitimate in this context, though I don't indend to do so beyond what I have just stated.
But, leaving that aside and accepting for now that both terms are equivalent, JGanley and I seem to be in agreement that a range advantage may be gained by 125% modulation (asymmetrical or non-linear....whatever you wish to call it) since it does improve the signal to noise ratio at the receiver and by raising the intelligence in the signal above the noise floor. I made this point in my last post and am glad we agree.
Now, according to your driving test, how much range increase did you observe and, in your opinion, is it worth the effort? It may well be so, but numbers would help in the evaluation of the cost/benefit ratio.
Neil
JGanley:
Ah yes, the old déjà vu again. I guess you might consider this thread as a continuation of that thread. After reading the old thread, I remember it, but forgot in the mean time!
Here is my summary so far:
1. Assymetric 130% limiting increases percieved range.
2. Asymetric limiting can add some distortion, but may not be really noticeable.
3. Asymetric limiting must be applied very carefully to balance increased percieved range vs. distortion.
4. Assymetric limiting seems like a good thing, unless one is a purist and is trying to achieve max fidelity.
As an interesting aside, I have read about commercial AM transmitters that are capable of dynamically increasing power on positive peaks. This is a neat thing to do because input power is typically measured with 0% modulation. I also seem to remember that the FCC frowns on this and maybe allows some dynamic power increase on peaks, but not to excess.
For part 15, this could be a great feature that has not been explored to my knowlege. A 100 mW transmitter will operate at 100mw with 0% modulation. At 100% modulation, the peak power is 400 mW. This is OK. But with dynamic power increase on peaks, the peak modulation power could be increased well beyond 400 mW and the 0% modulation power would still be 100mW. There is theoretically no limit to the dynamic power increase. I wonder what the FCC would make of this?
Any opinions?
Phil B
for commercial stations it's capped at +125%. I beleive they did this because beyond that you start to splatter adjacent channels. at least that is what i read somewhere.
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"As an interesting aside, I have read about commercial AM transmitters that are capable of dynamically increasing power on positive peaks."
I've been tinkering with the AM 100 circuit which you can find in the "PDF Reading Room" section of this board, and it does that if I understand the manual correctly.
Pages 7 and 8 of that pdf talk about modulation levels and the carrier control system and show what the o-scope readings would look like.
I don't have a scope that does rf range, I use my computer and some software for an af scope, so I can't say how accurate their depictions of the scope trace are. I *can* say I like the sound of it and it can get fairly loud without noticeable distortion and I haven't been running it with compression (basically because i don't like the sound of compression).
But if you want to tinker with the basic concept, that AM 100 circuit is pretty robust.
"There is theoretically no limit to the dynamic power increase. I wonder what the FCC would make of this?" >/cite>
I would *guess* that at least at some point they'd have a problem with it. Also there is a problem with driving the circuit hard to get more audio at fringe range.. It doesn't sound as good close in if you do. So there would definitely be a tradeoff in using it as a scheme for getting more audio level in the fringe range, I think.
Mostly it probably boils down to the usual.. If a licensed station complains then they'd look very close. Even though running "300% modulation on the positive peaks" might be *technically* legal, it'd be a very easy to just tell you not to do it and tell the licensed station that they told you not to overmodulate and that's that.
More interesting to me is that it is a good sounding alternative to compression or using a clipper other than a .6v diode "clamp" as a hard limiter in case of some accidentally/unexpectedly high transient or peak.
I can't really say how it affects actual range in the field, since the project is still at the bench level here. And currently on hold until I rebuild my bipolar power supply which I managed to fry one of the regulators and a couple resistors of during an embarassingly brain-dead moment.
I haven't done "real world" tests for range yet, and have just used a wire around 2 ft long and approximately vertical right off the circuit board, to emulate a fringe level signal at the other end of the house (maybe 50 ft) and the near range being about 10 ft away. No idea how it'll do outdoors with a decent antenna and something resembling an actual ground. Still considering options for actual antenna and case for outdoor mounting.
But what I saw when I was tinkering with it seems to indicate that pushing the "over 100% positive peak" concept too far has an adverse effect on the audio quality to a nearby receiver, though it does improve the legibility more to the fringes. So I'd say it's possibly a tradeoff and while there theoretically might not be a legal limit, there would be practical limits so far as actually putting out a good sounding signal. Yeah, it'd be physically possible to use a circuit similar to this to do 400% or 800% or whatever, but it might result in actually not sounding good for anybody *except* a fringe listener, and pushed enough, it might not even sound good on the fringe. When Keith designed his Rangemaster, I doubt he went for the possible 125% because he couldn't think of ways to get it any higher. Keith is a highly inventive individual and I'm sure he could think of ways to push it way past that. I'm guessing 125% was what he found to be a good compromise between getting better audio at the fringe *and* sounding good near.
Listeners expect stations they're listening to will sound good through the whole range they can receive it and will most likely tune to another station if it starts sounding crappy, and wouldn't care much about the technical reasons why it didn't sound good.
Back to the FCC aspect, I think I recall reading somewhere that (as you said) it's fairly standard practice but not when done to excess and that they particularly frown on it being used to make ads considerably louder than the usual program material. I think if it was routinely being used to *drastically* increase part15 ranges, they'd draft a new rule forbidding it or simply tell stations they can't do it. Part15 operation is permitted so long as it doesn't violate the rules, but at least as I understand it, we are not by any means *guaranteed* operation even if we stick to the rules if it causes any problems. If licensed stations complained because a theoretically legal (at unmodulated carrier power) part15 is getting enough solid range that the licensed station doesn't like it, I expect that in most cases the part15 station would simply be told not to do it.
Some tradeoff between sound quality and range is acceptable of course, but if all we wanted was to get maximum range.. a CW signal and using principles like the people doing LW beacons do could give great range. But who'd want to listen?
Daniel
I have actually used the Inovonics to push the modulation in two locations, one a rural area where I live and run my little station, and one in a denser more suburban area where my brother has his little station. As the ad men say, your mileage may vary… On average the gain in range is 100%, with some variations.
I test the range by using the same car radio and simply noting how far I can go before it becomes unlistenable – not totally scientific, but good enough.
In the rural area where I am, sort of a valley, with a ridge of stone and hills about 300 yards south, a lake directly north, and a hill of about 25 feet to the north east. The signal increase across the lake was most dramatic, with probably a 200% increase in range. The gain to the north east on the other side of the 25 foot hill was probably 75%. Surprisingly, the gain on the road that runs along the top of the stone ridge was very good, with a gain of 100%; doubling the distance the station could be heard.
At my brother’s location he is on a hill looking directly down on the town. He always had spotty coverage in the downtown, which became pretty solid with the 222, and out on the interstate his coverage probably increased from two miles to about four miles (which adds about 120 seconds of listening time at highway speeds…).
It is interesting to note that if you are in a car with a really bad am radio the range increase is less. Particularly if the radio is suffering from electrical noise from the car itself. No magic box will make up for really crappy receivers.
Is it worth it? I think so. I am sure some of the smarter folks on this board can probably homebrew a good asymmetrical limiter, but if you are not able to roll your own, a unit like this does make a difference. It is not an inexpensive option – A new Inovonics 222 is about $800 and used 222 on e-bay is about $300 to $500 when you are lucky enough to find one, and a used Optimod for AM is much more. BTW, these boxes, although intended for broadcasting, are also used by a few hams to increase their range and QRP’s.
But before you run out and spend the money, make sure all of the fundamentals are covered – properly tuned transmitter, best ground possible, etc.
JGanley,
Thanks for the report. It is always better to have fact (your observation) over opinion (mine) and as more experience is gained and reported on this we will have a better basis to decide whether to pursue it.
Neil