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I decited to create this topic in order to be able to compare with others some MPX signals from FM stations .
I have a big big big …but simple question for all of you and of course..Leif and anyone else who knows..
Lets compare 2 MPX signals..
The first is mine . ( breakaway fm with mpx clipping..)
The second is from another station ( omnia with mpx clipping..)Why the second radio station has so big signals.. ( at the red note..)
?!?!?! why is that so high ?!!?!?!?? what does this mean? is this good for that radio station? who is the "healthiest" MPX fm modulation? the first one(mine) or the second one..? what are the advantages of the first or the second, and what are the disadvantages if there..are any?
please help! check the first and the second and tell me about the..red note..
[attachment=1:2n5lb05p]mystereostation.JPG[/attachment:2n5lb05p]
[attachment=0:2n5lb05p]otherstereo.JPG[/attachment:2n5lb05p]It depends on the program material. That peak means they had a lot of low frequency stereo at the time. Playing piano will usually do it!
If you’re using the ITU BS.1268 limiter in BBP, that peak will completely go away, and be level with the rest of the subcarrier.
That peak probably means the station would be difficult to receive cleanly in stereo in a high-multipath environment!
By the way, look, I know some people love building audio-processing chains, but seriously, don’t put any other clipper after BBP. You will at most gain 0.24dB ( see http://www.indexcom.com/papers/loud2.html ) but for that imperceptible loudness advantage you lose the careful, distortion-free way in which BBP clips its output. Any tiny little bit of clipping of the output will be immediately audible on difficult program material. Composite clipping, to be effective, has to be done as part of the processor — not as an afterthought.
///Leif
Camclone,
You need to be very careful in making off air comparisons because it is easy to come to the wrong conclusions.
First, you are really not comparing apples to apples in your measurements. Did you sample exactly the same piece of audio from both signals for the measurement?
Did you have exactly the same RF signal levels for the measurements, free from multipath or synchronous AM modulation artifacts?
My conclusion is you did not, which is understandable, it’s hard to do. However if you look at the "other" stations signal sample it becomes obvious there is little or no main carrier audio modulation.My impression is what you are seeing are a couple of possibilities.
1. You may be seeing synchronous AM artifacts at the receiver causing phase distortion. This type of receiver distortion generates higher second harmonic energy (from 19kHz pilot) internally whereby giving you the illusion of "higher" 38kHz energy. Proper tuners for off air modulation meter are very carefully engineered to be wideband and have a very high degree of phase linearity to accurately measure signal levels. Consumer tuners are not nearly this well designed.2.The other station’s Omnia MPX modulator has a phase imbalance issue whereby the 38kHz subcarrier is not completely nulled out. Remember the 38kHz subcarrier is a double sideband suppressed carrier. This means that in an ideal system if there is no audio energy on the main carrier, there should be zero 38kHz energy present in the subcarrier spectrum.
3. The other station’s transmitter has inter-stage coupling problems. As I once wrote about in a previous forum post to you, synchronous AM artifacts of this type are common problems with multi stage transmitters. In this situation if the coupling is skewed (or tilted) the 19kHz pilot tone is getting distorted creating a rise in second harmonic energy in the 38kHz spectrum, regardless if the MPX modulator is perfectly nulled out.
I highly agree with Sparky’s explanation about the signal. It can cause really weird readouts and especially multipath is a killer.
You can only do a quick measurement that way. The only way to properly measure it is by coupling the receiver directly to the transmittor using the correct filters. But you can’t do that with a commercial radiostation, so as close as possible and inline of sight (watch the Fresnel zone) of the antenna will do the trick for a correct measurement.About your own modulation. Turn down the RDS deviation to about 4% and the stereo one to 7%
You are all the BEST radio engineers i have ever seen 🙂 I mean it! You know a lot lot more than me!
Thanks!My radio station is ok , the only "problem" was the high RDS levels… ( not so high..as others..)
The MPX performance of my station is excellent becuse of BREAAAAKAWAAAYYYY FM !
thanks everyone! excellent work , thanks for your precius time!
Leif, the website link you gave was ..wow!! .
..finnaly..:
make that f@@@ing MPX clipper 😯 🙂 You will become rich if you make all these…hardware…!
just doooooooooo it 🙂 🙄 🙄 🙄
Hello camclone,
Where can I get this tool "FM ANALYZER TOOL" ?
Thanx! 😀
Hey Cam… i think you have to work hardo on the linearity of your STL-TRANSMITTER system… most of the problem in this configuration are inside the stl receiver.. ( rf and Lf filters… and quadrature demodulators)
You CANNOT solve nothing with a composite clipper , if first you have not a copletely linear tansmission chain.. FROM MODULATOR TO ANTENNA ( including ros problems, bad matched armonic filters end others… ).
Composite Clipping give you a spectrum "energyzed" version of the audio given by an audio processor… but this "energyzed" signal is the most fragile signal you can have to treat.
Normally the most of the clipping , in normal audio processing , is in the bass region , due to the energy of the bass spectrum , and in the 3-15 Khz region , due to the preemphasis…. this can be handled by an audio processor…. Mpx Clippers take all this shit and make a bigger mess!!! clipping again all togheter!What you need ( i think) is an "Oveshoot compensation cure" , first matching all your transmission chain , and then an overshoot compensator at the transmitter site.
Overshoot compensator ( if correcly used ) can give you more REAL modulation , and less overshoot… this is one of the keys to be LOUD AND CLEAR..
If you need more PROCESSING , tune your processor , don’t destroy the Mpx spectrum…
Ciao from ItalyAdriano.
and re: stages in a processor to perform MPX clipping… Leif’s doesn’t clip L/R first. It ONLY has MPX clipping, yet you get more advantage than any other method out there. Download the Freakdag MPX recordings if you want to hear it, from a cheap soundcard no less.
http://mpxtool.com/site/meetdag2009.htmlwow! Great MPX recordings !!
I think that the year 2009 will be the year of the creation of the "loudest" and legal.(75Khz).FM ..MPX signal!
… that would be … my New York preset, with Leif’s MPX backend. 😉 It is indeed impressive, but I’m sure with a full control version, people will figure out how to push it harder without getting a "worse" sound than my NYC preset. There’s not much room there left over, but when considering a specific genra or sound, it’s possible to get a hair or two louder.
[quote author=”Cleiton”]Hello camclone,
Where can I get this tool "FM ANALYZER TOOL" ?
Thanx! 😀[/quote]
[quote author=”JesseG”]… that would be … my New York preset, with Leif’s MPX backend. 😉 It is indeed impressive, but I’m sure with a full control version, people will figure out how to push it harder without getting a "worse" sound than my NYC preset. There’s not much room there left over, but when considering a specific genra or sound, it’s possible to get a hair or two louder.[/quote]
Hey Jesse, I will make it sound like the French radiostations in 80’s. I’m about 99% close at the moment with that preset.
I shape the input in a way and it seems to be working.
Megacool ‘worse’ preset 😀
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