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I have to say that I’ve experimented with AM Part 15 a reasonable amount, and all things being equal, the biggest range and performance boost came from adding an Inovonics 222 to the audio chain (after a Symetrix 421). Radials made little difference, at least in my installation. Height makes a significant difference (in spite of the discussions that obstructions aren’t really relevant at these frequencies). But this – listenability was greatly increased at the outer edges of the half mile range that I typically get, and I could still hear the signal (although I wouldn’t want to listen to it for long periods of time) well over a mile out. And this is in an area that is heavily forested and fairly hilly.
If you can afford one (they’re pricey), I’d highly recommend it. The downside is that it’s not straightforward to adjust the unit for best results; I’m still working on a slight distortion issue. But the added punch to the signal is well worth the headaches.
Total posts : 45366If you can get your hands on an oscilloscope, you will find the job of removing most of the distortion is a little easier. Listening to the received audio and seeing the wave form is real valuable.
Remember, over loading the front end of a reasonably sensitive AM receiver sometimes shows up as distortion in the audio. With an O-scope, you will be able to see some of the distortion, even if you look at the receiver audio.
My 222 runs best with:
Peak Limiting: ON – peaking at -2
Low Pass: ON
Pre-Emphasis: ON
High Freq G/R: no great than -6
Positive Peaks: near 120% peaksPlus, with the O-scope, I can check to make sure the stereo audio chain that is combined to make high quality mono, is in phase.
My Rangemaster/222 combo has been helped by the addition of a mufti-octave audio equalizer to boost the low end frequencies on the transmitter.
Total posts : 45366Yeah, one of my problems is not having an oscilloscope handy right now – just following the instructions in the 222 manual along with those on the Hamilton website. I pretty much get the same readings that you have on the 222 (I can’t see what the +ve peaks are, of course), and I have to keep the Hamilton audio right at about 1/2. I’ve had to reduce the target output level on my 421 a little, and the distortion appears to have gone (I think some of it was overloading the receiver I was listening to – it’s gone in the car radio).
While I still don’t know if the setup is operating optimally, I can’t argue with the results. I will get hold of an oscilloscope and fine tune it, hopefully shortly.
Total posts : 45366What is the best way to observe or measure 120% with an o-scope?
Total posts : 45366The way I accomplish this task is to modulate the transmitter with a 1 KHz tone to approximately 100 percent negative peaks, then with a piece of tape mark the 100 percent positive peak level on the CRT screen. You can also adjust the display to match one of the horizontal graticule lines. Then determine a line 20 percent above (or 20 percent of space between graticule lines). This process will supply an estimation of peak modulation. Establish a mark for the 20 percent. It will not be exact. Remove the tone. Supply program audio and adjust program audio peaks to touch the 120 percent mark/graticule.
Be sure to reduce the negative peaks to 95 percent or so, first, to keep from distorting the signal and causing harmonics in the transmitter. Also make sure you check the modulation levels with the “low pass” and “pre-emphasis” functions turned on.
This procedure can only be accomplished on an audio processor used for AM broadcasting, like the Inovonics 222.
Total posts : 45366We currently have two good threads going regarding audio and modulation: this one and
so my comments could apply to both.
It was mentioned to monitor or check your audio using a receiver and headphones. This is a good idea if you use quality headphones. Ear buds and some of the cheaper phones distort audio and can give a false positive for distortion and in my experience attenuate the bass frequencies. I use a $20 enclosed ear headset which is a world apart in sound quality from the buds and loosely fitting small foam covered types. When I switched to these I discovered hum in my FM signal which I had not heard before (another topic).
It was mentioned that negative over modulation causes harmonics and it is worth explaining that these are audio harmonics which expand the bandwidth of the AM signal. A check on this can be done without an oscilloscope by tuning the receiver several tens of kHz off the carrier signal. If you hear “bleed over” or “splatter” from your modulation you may be over modulating (or overloading the receiver). A digital storage scope is a great way to capture and examine the modulated RF but in lieu of this a careful listen off frequency is useful.
I have observed that some audio compressors which operate in real time do not prevent over modulation because of the 5 to 20 millisecond “attack” time. By the time the compressor AVC has kicked in the event has happened and over modulation has occurred, especially with vocal audio. This may not apply to the Ivonics types since I am not that familiar with them and cannot comment but it applies to some of the simple and built in compressors.
Neil
Total posts : 45366This procedure can only be accomplished on an audio processor used for AM broadcasting, like the Inovonics 222.
Total posts : 45366Some reasons for processor enabled modulation on AM only:
*The processor (and transmitter) must be capable of asymmetrical modulation, meaning the positive and negative peaks are at different levels. Asymmetrical modulation adds distortion to the audio package fed to the transmitter. However, greater positive modulation increases the signal to noise ratio for the listener’s receiver. 120+ percent modulation can produce a couple db in signal improvement. Often, asymmetrical modulation processors also use audio envelope phase shifting to maximize modulation. This is done because most vocal audio and some music are asymmetrical by nature, and in the wrong direction for 120+ peaks for AM. Most of this technology was developed by an engineering acquaintance of mine, Grant Fiekert (sp).
*The “low-pass” function of the Inovonics 222 complies the the NRSC voluntary standard established for AM broadcasting several years ago. This to reduce unneeded sideband energy and adjacent channel noise thereby raise the signal to noise ratio.
*Pre-emphasis along with “look ahead” peak limiting can produce audio fidelity closer to that of Fm in even cheap receivers. This is also part of the voluntary NRSC AM broadcast standard.
I have been told the compressor in the 222 is a “soft-knee” processor with slow release timing. The attack and release are adjusted by the machine automatically by the “look ahead” function of the processor.
The cleaner and more spectral balanced the audio, the better the performance of the processor; remember GIGO.
As a final note; audiophiles will find most modern-day AM broadcast processing too aggressive and awful sounding because of the inherent distortion added to the final audio for the sake of improvement in the signal to noise ratios. The caveat is that if you can’t hear the low finite strains of music or speech for the atmospheric noise and in-band interference, then the compromise is to process the audio.
Total posts : 45366Interesting post from Virtual Engineer forum. This post appeared in the thread at: http://www.broadcastengineering.info/viewtopic.php?f=2&t=2585#p16660
Re: Confusion on Inovonics 222
by Relay » Mon May 31, 2010 4:54 amI thought this reponse to the original question here might be of interest. This response was received from John Wood at Inovonics and it explains the process very well.
> The 222 does not create asymmetry, nor should any processor. If it
> does, it is, by definition, introducing 2nd harmonic distortion. Any
> sound, save a pure sine-wave tone, is generally asymmetrical… a
> trumpet note, the human voice, even a ‘blended’ orchestra. The 222
> simply allows naturally-occurring positive-going program peaks to
> assume a value up to 125% of the negative ones, as allowed by the FCC
> in AM broadcasting practice. The advantage, in terms of a perceived
> loudness increase, depends somewhat on the asymmetrical source, but
> it’s on the order of 1dB. Big deal, eh?Cheers
Total posts : 45366John Wood is technically correct. However, his explanation is an over simplification and genuine double speak.
The introduction of the second harmonic distortion is actually audio distortion. Audio compression and limiting, by process, distort the original source audio wave form; the more processing the greater the distortion. By controlling only the negative peaks, the original waveform has to be changed or distorted. Just enough processing is used to improve the signal to noise ratio of the signal, just short of creating fatigue in the listener. It is a balance.
In the real world, set the input so that the processor supplies just enough peak limiting and compression, then set the output level to 95-98 percent and finally adjust the positive peaks so that the occasional 120-125 percent peak appears on the monitor or O-scope.
Mr. Wood does mention the asymmetrical audio of certain sources, but fails to mention the auto audio phase reversal adjustment function of the 222 unit to maximize the asymmetrical qualities and consistency of the program audio. Thereby producing a more listenable louder audio signal.
The actual “peak” signal improvement is more on the order of near 2 dB, where as average or RMS improvement would be about 1 dB, depending on the density of the final processed audio..
Has anyone tried to send the processed audio output of a single 222 AM processor over a studio-transmitter-link system (microwave, Wifi, digital data circuit) and directly into several transmitters? If this could be done, then only one 222 could process audio for an entire network of transmitters at once. Then, field set up of a transmitter would entail transmitter/antenna tuning and setting the audio level for full modulation. Easy squeazy.
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