Frequency modulation (FM) distortion?

There is an almost local station, KCSN, over the hill from me. As a consequence, the signal is relatively weak. Nevertheless, the signal is
mostly good.
When listening to piano music in particular, there is distortion on most of the notes. It is a rattling and buzzing sound. What causes that? Is it some multipath phenomenon? What would the theory be?
Bill
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Salmon Egg wrote:

How long is a piece of string?
Do you get the same effect on a different receiver at the same location?
Do you get the same effect with the same receivers at different locations?
What type of receivers are they, eg pll/ afc, etc?
At a rough guess, your receiver is having difficulty locking on to the transmitter frequency and the effects you describe are the side effect of the attempts of the afc. If so, an "old fashioned" manually tuned receiver, or turning off the afc, if that is possible, should perform better.
--
Sue

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Palindrome wrote:

KCSN's web site invites you to ask them:
If you have specific questions regarding your reception situation, please call (during regular business hours) or e?mail snipped-for-privacy@kcsn.org
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Bill there are several possibilities. The most likely that comes to mind is it use of multiple compression schemes in the broadcast chain i.e. the music is stored on HD as MP3 then compressed and uncompressed again in a digital path to the transmitter(s).
KCSN use a 370 watt ERP transmitter in Northridge @ 501 meters HAAT and a 1.28 kW transmitter in West Los Angeles at 0 meters HAAT. with a booster arrangement such as this there is a zone of interference at the point where the 2 transmitters are heard with equal signal strength. If the 2 transmitters are not "frequency locked" the problem will be aggravated in the interference zone.
Another possibility is that the source material was poorly recorded (dubbed) into the computer.
Still another factor is your being "ofer the hill". FM boradcast (VHF) is a line of sight medium, however ther are a few effects such as knife edge difraction tha t makes it seem to go around corners or over mountains. With knife edge difraction small amount of power 'bends' downward over a hill or mountain.
Another problem with weak signals is co-channel interference from another station. In this case a distant station may have enough signal to distort the 38 kHz stereo L-R signal in the upper sideband of the carrier.
Your issue could be a combination of effects. The problem could be enhanced by poorly adjusted or malfunctioning audio processing, lack of clipping protection before STL (link) transmitters or digital encoders. The vintage of transmitter and exciter can make a surprising difference in sound quality.
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Of the responses I have received, this is the most useful.
I did not know that KSCN had a transmitter in West Los Angeles. A height of 0 IS low. Do you know where the WLA antenna is? On the other hand a height of 500 meters is significant. 370W ERP seems low, especially since it is not "effective" in my direction.
I am using an Eaton E5 digitally tuned dual conversion receiver. I presume that it uses a typical discriminator detection system. I bought it so that I could get precise tuning of KPCC and KUSC not afforded by analog tuning. Since KMZT disappeared as far as I am concerned, I look toward KCSN as a substitute.
What I do notice that may be consistent with two signals is that the receiver can remain quiet (when there is enough carrier to lock out receiver noise) in the absence of modulation. That is, the distortion shows up only in the presence of signal. Piano music in particular, with distinct percussive notes, seems to give distortion on the audio peaks but is quiet between the notes
I must admit that even with all the time I spent studying radio over the years, I really do not know how to analyze the effect of interference in FM radio. Armstrong really deserves being called an inventor.
Bill
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A booster must operate within the contour of the of the of the stations coverage area. It cannot extend the range.
FROM: http://www.fcc.gov/mb/audio/translator.html#WHATIS 'FM Translators and Boosters comprise a low power service on the FM broadcast band (88 to 108 MHz) which complements the primary FM service. This service was first created in 1970 to allow FM stations to provide supplementary service to areas in which direct reception of radio service is unsatisfactory due to distance or intervening terrain barriers (e.g., a mountain). "
Booster Stations are essentially translator stations on the same frequency as the main station. Booster stations must be owned by the licensee of the primary FM station. Booster stations are also restricted in that the service contour of the booster may not exceed the protected or service contour of the primary station at any azimuth
In this case the booster in on a tower with the center of radiation 79 meters above ground.

um.... it's a nice little radio but are you sure it can reproduce music to the standards you need?

FM receivers exibit something called capture effect. http://en.wikipedia.org/wiki/Capture_effect This is fortunate in the implimentation of boosters an "short spaced" FM stations.
FM receivers have a specification known as modulation acceptance. If the modulation bandwidth of the transmission exceeds the modulation acceptance of the receiver the resultant distortion will be exactly the effect you described. I have in the past seen this occur in "foreign" car radios and heavily processed music stations... I got pretty tired of retuning the radio station every time the PD got a new car. (:

I might speculate your location to be in the Palmdale / Lancaster area (because of the hill reference) I spent 2 or 3 years doing maintenance on KTPI and others in that area.
Further study shows that KCSNs main antenna is directional. with it's exact location being here http://maps.google.com/maps?f=q&hl=en&q4.31944,+-118.55417 +(KCSN-FM)&om=1
it claims a coverage area of: http://www.radio-locator.com/cgi-bin/pat?call=KCSN&service=FM&status=L&hours=U
The booster location is here: http://maps.google.com/maps?f=q&hl=en&q4.05361,+-118.40278 +(KCSN-1-FM)&om=1
It claims a coverage area of: http://www.radio-locator.com/cgi-bin/pat?call=KCSN-1&service &status=L&hours=U
The "fringe" contour is also known as the 50/50 contour. That is where 50% of receivers can hear the signal 50% of the time.
Low ERP does not nessessarily equate to low coverage. KATJ on Quartzite Mountain has but 270 W ERP yet has more then enough coverage to service it's area. (licensed to George, CA which does not exist)
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Thanks again for a thoughtful post. I have snipped a lot for my comments.

This too terse for me to understand.

I use it while away the time when insomnia strikes. It is not what I use for serious music appreciation.

It is not clear to me how the bandwidth would be increased by multipath. Moreover, the distortion often shows up during relatively soft passages when the frequency deviations should be well within the passband.

I do picture that the sidebands adding to the carrier can undergo serious interference from multipath, but without a mathematical model, I do not lnow how to understand the effects.

Actually. I am located near LAX. From the information you provided below, I should be getting little signal from the main antenna. The booster location is much closer with higher ERP. I am located between the local and distant contours.

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Booster Stations are essentially translator stations on the same frequency

Sorry, the booster's transmitting antenna is 79 meters above ground. The nearby hills make the higth above average terrain 0.
The center of radiation is the physical center of a phased array of antenna bays (as oppose to the top of the antenns for example)

The above refered to heavily compressed music and narrow band radios. In major markets the demand by thoese who consider themselves in charge is to "be the loudest thing on the dial".
I occasionally get a "warbeley sounding" effect on digitally encoded STL paths (studio to transmitter microwave links) when the decoder is on the verge os comming unlocked. This can be from excessive interfereance at the reciever or equipment malfunction.

Dealing with multipath especilly in your area can be pretty involved. One of my aquaintences recently explained in great detail the engineering associated with optimizing the county's law enforcement communications system.
The basic discription of multipath is here: http://en.wikipedia.org/wiki/Multipath .

If the problem is not consistant it may be that tropospheric ducting effects are comming into play.
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It is interesting that you mention this. I have occasional problems with digital television reception. In the absence of true knowledge I attribute it to scintillation. That is, signal amplitude will follow a log normal distribution.
Suppose the average signal level is considerably more than necessary to give good S/N, signal-to-noise based upon receiver noise. In the presence of turbulence, the log normal distribution causes dropouts that preventing the digital processing from working well.
Bill
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wrote:

Scintillation is a pretty esoteric effect and one which i have little familiarity with.
Ducting, however, was conspicuous by its absence in the high desert. I would ecpect it would be regular occurance in your area.
That is, signal amplitude will follow a

I'll give this some thought although for equipment I deal with it simplest explainations are usully the culpret. In one case the theory was advanced that group delay of ganged passband cavities before the receiver was contrubuting to a decoder problem. By using a triple conversion receiver with a passband optimized for digitaly encoded I was able to forgo the cavities in one instance. another receiver fed by the same antenna required them to make it work. Different frequency, more RFI. This is at a location that has problems similar to LA's Mount Wilson or Black Mountin in Las Vegas. In short these are RF jungles where there are so many signals comming in to and going out of that no practical method of predicting all the combinations of mixing products is feasible. It's just RF 'soup'.
Quit a few broadcasters are now using some form of audio over IP for STL applications. I have one on a backup system which uses a spread spectrum RF link. The delay method produces is pretty significant, 8 to 10 seconds IIRC.
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To summarize simply, normal distributions arise when fluctuations arise from a sum of separate independent fluctuations (the central limit theorem). For turbulence, transmission goes through a lot of non-lossy blobs. Each blob multiplies the signal by a factor close to one. The log of this product is a sum of the logs of the individual transmission factors. The central limit theorem applies to this sum. Thus, the log of the transmission is the random sum of the individual logs. That is, the log of the transmission is normally distributed.
At this point, that is more than I know.
Bill
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Salmon Egg wrote:

It could indeed be multipath, as a consequence of the refraction over the hill you may be getting a combination of direct and reflected waves.
For example, the weak direct signal at your antenna, might be combined with a signal reflecting from a nearby water tower or even some distant buildings. When two or more signals arrive at your receiver that have traveled different paths, have significant delay spread between them. This would be most apparent at the higher audio frequencies and would also affect the stereo sub-carrier. (try turning receiver to mono or "blend"). The destructive interference could be quite audible. To rule out a problem with the transmitter and the studio audio chain, try driving close by the transmitter while listening for similar effects. If the transmitter is "clean", try installing a directional yagi antenna on your receiver at a height sufficient to overcome the path obstructions.
--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"
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Your response is fine as far as it goes. Do you have a simple mathematical model that can illustrate these effects?
Bill
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