09-25-2021, 01:07 PM
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!
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!
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