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AM Broadcast Spectral Components

G

George

Jan 1, 1970
0
Hi all, and Happy Holidays:

In the Los Angeles area several local AM broadcast stations appear to contain signals at between 100 Hz and 300 Hz away from the carriers, below the main program audio. I have called a couple stations to ask what these signals are, but they haven't called me back.

I'm wondering what they might be. In Europe there's the AMSS service that's used for low-speed remote signaling, and there's OFDM, but I didn't thinkthey are in use in the US. There's also a Nielsen TV rating signal that'ssometimes used in the US.

But does anyone have any idea what the signals probably are. A couple of spectrum plots are posted here:

https://dl.dropbox.com/u/2087421/carrier 870 spectrum - narrow.pdf

https://dl.dropbox.com/u/2087421/carrier 710 spectrum - narrow.pdf

Thanks
 
J

Joerg

Jan 1, 1970
0
George said:
Hi all, and Happy Holidays:

In the Los Angeles area several local AM broadcast stations appear to
contain signals at between 100 Hz and 300 Hz away from the carriers,
below the main program audio. I have called a couple stations to ask
what these signals are, but they haven't called me back.

I'm wondering what they might be. In Europe there's the AMSS service
that's used for low-speed remote signaling, and there's OFDM, but I
didn't think they are in use in the US. There's also a Nielsen TV
rating signal that's sometimes used in the US.

But does anyone have any idea what the signals probably are. A
couple of spectrum plots are posted here:

https://dl.dropbox.com/u/2087421/carrier 870 spectrum - narrow.pdf


https://dl.dropbox.com/u/2087421/carrier 710 spectrum - narrow.pdf

Quitle likely plain old hum :)

Seriously, local stations can generate a lot of level on house wiring,
get modulated at all sorts of cheapo power supply rectifiers and then
re-radiated. Had it many times. In the US you'd mostly see 60Hz, 180Hz
and so on. In Europe 50Hz and 150Hz.

You can't really have much in terms of little sub-audio carriers because
most AM radios would make that annoyingly audible. Also, the audio
frequency response of an AM channel is something like 40Hz to 5kHz anyhow.
 
R

Ralph Barone

Jan 1, 1970
0
George said:
Hi all, and Happy Holidays:

In the Los Angeles area several local AM broadcast stations appear to
contain signals at between 100 Hz and 300 Hz away from the carriers,
below the main program audio. I have called a couple stations to ask
what these signals are, but they haven't called me back.

I'm wondering what they might be. In Europe there's the AMSS service
that's used for low-speed remote signaling, and there's OFDM, but I
didn't think they are in use in the US. There's also a Nielsen TV rating
signal that's sometimes used in the US.

But does anyone have any idea what the signals probably are. A couple of
spectrum plots are posted here:

https://dl.dropbox.com/u/2087421/carrier 870 spectrum - narrow.pdf

https://dl.dropbox.com/u/2087421/carrier 710 spectrum - narrow.pdf

Thanks

Up here in Canada, we call frequencies in the 100 to 300 Hz range "bass".
 
R

Robert Baer

Jan 1, 1970
0
George said:
Hi all, and Happy Holidays:

In the Los Angeles area several local AM broadcast stations appear to contain signals at between 100 Hz and 300 Hz away from the carriers, below the main program audio. I have called a couple stations to ask what these signals are, but they haven't called me back.

I'm wondering what they might be. In Europe there's the AMSS service that's used for low-speed remote signaling, and there's OFDM, but I didn't think they are in use in the US. There's also a Nielsen TV rating signal that's sometimes used in the US.

But does anyone have any idea what the signals probably are. A couple of spectrum plots are posted here:

https://dl.dropbox.com/u/2087421/carrier 870 spectrum - narrow.pdf

https://dl.dropbox.com/u/2087421/carrier 710 spectrum - narrow.pdf

Thanks
Huh??
I see nothing unusual about either of those spectrum plots.
 
M

miso

Jan 1, 1970
0
HD (IBOC) is in the guard band, i.e outside the main AM signal. IBOC
stands for In Band On Channel, but really it is on the channel of
another station that used to be protected.

I don't know if it still lives in cyberspace, but there was a weird
signal coming out of the Nevada Test Site in the early 90's or maybe
late 80's. When it was traced to the NTS, the signal stopped. No
explanation was given. IIRC, it was in the 800 to 900 range. Art Bell
mentioned it on Coast to Coast, so that might pin down the time frame.
You could hear it on the skip in California.

I was thinking it might be a weird plasma etcher of some sort,but they
use ISM frequencies. Usually 13.56MHz. When Triangle Machinery and RA
Associates were still in business, these ISM RF linears were on the
surplus shelves. This predated RFID, but still, what would you do with one?
 
G

George

Jan 1, 1970
0
Quitle likely plain old hum :)



Seriously, local stations can generate a lot of level on house wiring,

get modulated at all sorts of cheapo power supply rectifiers and then

re-radiated. Had it many times. In the US you'd mostly see 60Hz, 180Hz

and so on. In Europe 50Hz and 150Hz.



You can't really have much in terms of little sub-audio carriers because

most AM radios would make that annoyingly audible. Also, the audio

frequency response of an AM channel is something like 40Hz to 5kHz anyhow..



--

Regards, Joerg



http://www.analogconsultants.com/

I wondered about it being hum. But the envelope looks more like the shoulders you expect from data modulation rather than line frequency spikes, and they do not occur at exactly 60 Hz or multiples. And -35 dBc is pretty loud for incidental hum. Also they don't appear to vary with the main audio modulation either, based on repeated observations.

George
 
M

Mark

Jan 1, 1970
0
I wondered about it being hum.  But the envelope looks more like the shoulders you expect from data modulation rather than line frequency spikes, and they do not occur at exactly 60 Hz or multiples.  And -35 dBc is pretty loud for incidental hum.  Also they don't appear to vary with the mainaudio modulation either, based on repeated observations.

George

well what does the station SOUND like?

you would easily be able to hear this

Mark
 
P

Phil Allison

Jan 1, 1970
0
"Mark"

well what does the station SOUND like?

you would easily be able to hear this



** Hey - don't spoil George's perfectly good troll with a sensible
question.





..... Phil
 
J

Joerg

Jan 1, 1970
0
George said:
I wondered about it being hum. But the envelope looks more like the
shoulders you expect from data modulation rather than line frequency
spikes, and they do not occur at exactly 60 Hz or multiples.


In one plot it looks like 60, 180 ... Hz, the other looks like 120Hz and
harmonices which you'd have with older single-dide rectifiers. Because
that results in even-order harmonics. The spectral width depends on the
load. For example, if this comes from uncle Leroy's old TIG welder while
he is welding it'll be pretty wide.

... And -35
dBc is pretty loud for incidental hum. ...


Pretty normal in some situations. Often it is louder on local stations
than on distant ones.

... Also they don't appear to
vary with the main audio modulation either, based on repeated
observations.

Hum modulation usually doesn't. Its amplitude and spectral constellation
depend on the load that's on the offending rectifier. In the old days it
was easy to find out the offender by unplugging one device after the
other. Today that's almost impossible because there is so much of it and
some stuff needs elaborate re-programming after unplugging, such as the VCR.
 
J

Joerg

Jan 1, 1970
0
John said:
George wrote:
[...]

Hum modulation usually doesn't. Its amplitude and spectral constellation
depend on the load that's on the offending rectifier. In the old days it
was easy to find out the offender by unplugging one device after the
other. Today that's almost impossible because there is so much of it and
some stuff needs elaborate re-programming after unplugging, such as the VCR.


That's an interesting component, a dipole or other antenna that's
periodically shorted at the feed point, by a diode or some other sort
of switch. It becomes a modulated reflector. Of course, any house is
full of them.

European shutters could drive people crazy that way. The expensive kind
is not plastic but aluminum. Crackle .. ka-crackle ..

In the days of NTSC you could get some really interesting effects when
placing an ancient metal-blade fan in the path. With DTV you'll just get
the usual frozen Picasso or blue screen.
 
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