[poulsbobill]

I will offer this up as i have been researching all of this.

https://www.pavekmuseum.org/Lein.html He talks about a 37-116.

Just so i am clear the standard broadcast can be 5khz each side so 10 total correct?

Thanks

Bill

[morzh]

Yes, 10kHz is the step between AM stations.

[BrendaAnnD]

Mike,

10 KHz is the STEP between frequencies in our ITU zone, it is not the limit on bandwidth. Stations in a given area are assigned a minimum of 30 KHz apart. This leaves plenty of room for a 10 KHz audio bandwidth (10K either side of the carrier, a total of 20 KHz) still leaving a guard band of 10KHz. The current "mask" being used is not mandatory.

[mikethedruid]

I have been following the discussions here with some interest, and am a bit puzzled. Here we are talking about AMPLITUDE MODULATION, not FREQUENCY MODULATION, if I am not mistaken. Nor are we talking about digital AM, but simple AM. Now, were we discussing frequency modulation, I could certainly understand why an increased bandwidth would be important to improve fidelity; but with amplitude modulation, I do not see how a wider bandwidth is necessary to provide this. With amplitude modulation, all that needs to vary is the intensity, the strength, of the electro-magnetic waves reaching the radio's antenna. Even with a very narrow bandwidth, a signal which varies in intensity with the audio frequency it carries, should reproduce that audio frequency once detected at the receiver. All that a wider bandwidth really does is allow a station to "splash" all over adjacent stations, and make reception of them difficult or impossible. This was actually proved some years ago by experiments done by WLW (700AM, Cincinnati) and WOR (710AM, New York), both 50,000 watt, clear channel stations.They cut back their bandwidth to 10KC max so that neither stepped on the other. Listeners reported no discernible difference in audio quality between their normal and restricted bandwidth broadcasts, but did report that they could now receive both stations where under their normal bandwidth they could only receive one of them clearly.
https://www.radioworld.com/tech-and-gear...idth-on-am

[BrendaAnnD]

Mikethedruid,

You need to understand that AM is much more complicated than just varying the strength of a carrier wave. There's lots of math involved, much of which even I have trouble with. The short story is, all information in AM is carried in the sidebands, and there is a direct mathematical correlation between the audio bandwidth (frequency response) and the width of those sidebands. If you only want 4 KHz of audio bandwidth, you only need 8 KHz modulation bandwidth (say, you have a carrier on 1000 KHz, you would have sidebands out to 996 and 1004 KHz.) For more audio bandwidth, you need correspondingly more modulation bandwidth. For a 10 KHz audio bandwidth, you would create sidebands out to 990 and 1010 KHz. You can't get something for nothing.

FM is complex in a different way. To get a 15 KHz audio bandwidth on FM, you COULD use 15 KHz deviation either side of the carrier (30 KHz total), but this gives limited signal to noise ratio, so for broadcast purposes, a deviation of 75 KHz either side of the carrier (150 KHz total) was chosen. In this case, the audio bandwidth remains the same, but other information is placed onto the carrier at certain frequencies and deviation levels (like the 19 KHz pilot subcarrier and the 38 KHz L-R subcarrier) and allows for a "louder" modulation with better signal to noise ratio.

Digital (IBOC, or as I call it, I-BLOCK in the US) is another whole kettle of fish. It use fixed sidebands with neither AM nor FM modulation, but Pulse Width Modulation and a codec that allows for information other than the L+R modulation of a standard AM signal. These include a L-R and metadata information.

[rfeenstra]

After considerable reading, I found several references that do indeed indicate, as Brenda said, that stations are allowed a wider frequency response because of the 30khz spacing in a given local. It was also stated that some of those stations must limit their frequency response at night lest they interfere with distant stations do to skip. I was always under the impression that the FCC limited each AM station to 5khz audio. It's nice to be enlightened!! Thank you Brenda.

poulsbobill: Interesting scope trace of the frequency response of the 37-116 IF system. I have done a similar thing using a spectrum analyzer to compare the IF response from very narrow to wide. (See https://philcoradio.com/phorum/showthread.php?tid=20135 - post 109) It definitely changes the selectivity and the frequency response of the audio signal. Its's fun to play with.

Mike: The AM signal is actually made up of the carrier frequency plus the audio frequency and minus the audio frequency - the 2 sidebands. The more the width of the sidebands are suppressed, the lower the frequency that can be carried, even in AM.

I had read that same article and the explanation for people not hearing the difference is probably that most AM radios produced since the 1960's and many before have a very narrow bandwidth. This increases selectivity but decreases frequency response. Most AM receivers' IF system cuts off or severely attenuates the higher frequency portion of the AM signal. Many AM receivers built are only able to reproduce up to 4 or 5khz. Thus, reducing the transmitted frequency response from, say, 7khz to 5khz would not be heard by people listening on these radios. This includes most car radios. You will notice that the article was advocating for all AM stations limiting their audio bandwidth to 5khz except those that are music intensive. For those, he was proposing 6khz. Since most AM radio is talk radio these days, and most AM receivers have limited frequency response, his position was that there is no need for wideband AM except for those rare stations broadcasting music. I think he's trying to make way for HD AM!

[mikethedruid]

After doing considerable searching and reading through both the books in my own engineering library and also searching on line, I FINALLY have a decent explanation as to how the sidebands on either side of the carrier are generated. The books all too often just say "This is so" and give some mathematical formula, but don't give any clear explanation as to the HOW these sidebands come about. There is a simple, one word explanation, HETERODYNE. It is the same principle used to generate the IF in a superheterodyne radio. Just as when you mix two RADIO frequencies, you produce image frequencies both above and below the tuned carrier frequency by the sum and difference of the local oscillator frequency; when you combine an AUDIO frequency with a carrier frequency, you produce image frequencies both above and below the carrier frequency by the sum and difference of the two frequencies. Since speech and music consist of MULTIPLE frequencies, each of these frequencies which mix with the carrier frequency produce images above and below the carrier. This multiplicity of images forms what are known as the upper and lower sidebands. If you just remember the word HETERODYNE, and what it means, then sidebands make sense.

[rfeenstra]

Yes! 

[morzh]

Brenda


If the #18 post was directed to me, yea I know that. Thus is why I wrote #15.

I've been trying to find out what band our station, the call of which I listed there, has.