interference to FM radio broadcast by other electromagnetic radiation

Thread Starter

PeteHL

Joined Dec 17, 2014
253
Is FM radio broadcast in stereo more susceptible to interference from other signals than the monaural signal? In listening to a local FM radio broadcast, switching my receiver from monaural to stereo reception results in me initially hearing no static, then hearing lots of static.

Thanks if you know,
Pete
 

Papabravo

Joined Feb 24, 2006
12,401
What you are hearing on the FM band from an FM receiver is not the kind of broadband static you hear on an AM radio, like from lightning. The signal is frequency modulated for exactly the purpose of eliminating the broadband static, typical of AM signals. What you are hearing is the difference between a weak signal and a useable signal for the current position of the tuning dial, assuming the lack of an automatic fine tuning feature in the receiver.
 

Thread Starter

PeteHL

Joined Dec 17, 2014
253
What you are hearing on the FM band from an FM receiver is not the kind of broadband static you hear on an AM radio, like from lightning. The signal is frequency modulated for exactly the purpose of eliminating the broadband static, typical of AM signals. What you are hearing is the difference between a weak signal and a useable signal for the current position of the tuning dial, assuming the lack of an automatic fine tuning feature in the receiver.
Given what you say, I'm wondering whether or not you agree with what is stated in the article that I'm providing a link to.

https://www.northcountrypublicradio.org/about/reception.htm
 

Reloadron

Joined Jan 15, 2015
4,508
Given what you say, I'm wondering whether or not you agree with what is stated in the article that I'm providing a link to.

https://www.northcountrypublicradio.org/about/reception.htm
I tend to agree with the article but there are several considerations. How well and clear you receive a FM station depends on a few unknown things. How far are you from the transmitter site? How good is the quality of your FM receiver? How good is your antenna? A $20 FM table top FM receiver is not going to function as well as a high quality FM receiver. Next is the transmitting station power and what is the terrain between you and the transmitter site? All in all, without going real deep there is likely a dozen variables in this.

I grew up on Long Island NY with primarily AM stations and yes, I am that old. At night when the local small stations went off the air the large clear channel stations increased power and signal quality, with a quality receiver was great. Life was great right till a thunderstorm was within 50 miles at which point life was reduced to static and noise. As FM grew it gained in popularity and the noise which plagued AM was gone. However, what I mentioned above applies.

Today I live in Cleveland, Ohio USA. I frequently drive down to Columbus, Ohio which is 130 miles south. With my truck radio I start losing my Cleveland FM stations at about 50 miles south. While a good radio my truck radio is not quite great and nor is my antenna on the truck. So all the variables need weighed in. Do you get noise on all stations or only the station linked to? Again, consider the terrain in upstate NY and the surrounding areas.

Ron
 

Thread Starter

PeteHL

Joined Dec 17, 2014
253
@Reloadron

The change from no static to substantial static when switching from mono to stereo occurs when listening to different stations. What I was hoping for was to narrow down the cause of the static by pointing out that switching to stereo reception from mono results in static.

Thanks,
Pete
 

Thread Starter

PeteHL

Joined Dec 17, 2014
253
What you are hearing on the FM band from an FM receiver is not the kind of broadband static you hear on an AM radio, like from lightning. The signal is frequency modulated for exactly the purpose of eliminating the broadband static, typical of AM signals. What you are hearing is the difference between a weak signal and a useable signal for the current position of the tuning dial, assuming the lack of an automatic fine tuning feature in the receiver.
My understanding of RF is poor. But, as I understand it in a simplified way, the carrier is frequency modulated by the (L+R) signal. A sub-carrier that is amplitude modulated by the (L-R) signal also frequency modulates the carrier. So switching the receiver from mono to stereo reception I would think involves some additional processing of the carrier, not changing the tuning to receive the carrier. So then the receiver is doing the same tuning whether deriving the (L-R) signal or not, that is, reception of the stereo broadcast compared to monaural doesn't require any finer tuning of the carrier. If the signal is good in monaural, then the signal should also be static-free when receiving the stereo broadcast.
 

Papabravo

Joined Feb 24, 2006
12,401
My understanding of RF is poor. But, as I understand it in a simplified way, the carrier is frequency modulated by the (L+R) signal. A sub-carrier that is amplitude modulated by the (L-R) signal also frequency modulates the carrier. So switching the receiver from mono to stereo reception I would think involves some additional processing of the carrier, not changing the tuning to receive the carrier. So then the receiver is doing the same tuning whether deriving the (L-R) signal or not, that is, reception of the stereo broadcast compared to monaural doesn't require any finer tuning of the carrier. If the signal is good in monaural, then the signal should also be static-free when receiving the stereo broadcast.
OK so the information is not contained in the carrier signal. It is contained in the upper sideband of the carrier. Refer to the following picture

http://hyperphysics.phy-astr.gsu.edu/hbase/Audio/radio.html (Scroll down to FM broadcast band)

If the mono signal is in the L+R band, then to switch to stereo, the L-R band, which also FM modulates the 38 kHz. sub-carrier, has to be decoded as well, and it takes a finite amount of time for the switching to take place. What is changing is not the tuning of the main carrier, but the tuning of the 38 kHz. sub-carrier. In the actual receiver, the carrier and the sub-carrier are suppressed by filters.

I can't remember if the lower sideband with the L+R information, which is an identical copy of the L+R information in the upper sideband, is suppressed by the transmitter, or is suppressed in the receiver.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
3,870
@Reloadron

The change from no static to substantial static when switching from mono to stereo occurs when listening to different stations. What I was hoping for was to narrow down the cause of the static by pointing out that switching to stereo reception from mono results in static.

Thanks,
Pete
Probably what you are hearing is not static, just noise. There are lots of different kinds of interference around.
Since there is more information in a stereo signal it takes a higher signal strength to have good sound than a mono signal. That is why it sounds better when you switch to mono.
 
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