Audioclinic (Q and A) (Oct. 1972)

by Joseph Giovanelli

AM Bandwidth

Q. Under what circumstances can AM stations broadcast to frequencies higher than 5 kHz? If a 5 kHz tone will produce significant sidebands at plus or minus 5 kHz relative to the carrier frequency, it seems to me that such instances of "high fidelity" broadcasting must be rare. On the other hand, twice in recent readings I have come across passages which suggest that such a situation may not be as rare as I think it is.

-Gary Roboff, Worcester, Mass.

A. The broadcast band is broken down into segments, each of which is 10 kHz wide. While we have come to think of a broadcast station as being assigned to a specific operating frequency, this station actually occupies a spectrum of frequencies lying above and below this assigned carrier frequency. As you indicated in your question, the reason for this is that the audio frequencies produce what are known as sidebands. If a 5 kHz tone is broadcast, the AM station will be transmitting its assigned frequency plus two additional frequencies, one 5 kHz above, and one 5 kHz below the station's assigned carrier frequency. (These outer frequencies represent the sidebands.) If we consider two broadcast stations assigned to frequencies 10 kHz apart, and if each one is transmitting tones of 5 kHz, the lower sideband of the station assigned to the higher frequency and the upper sideband transmitted by the station assigned to the lower frequency will each produce the same frequency side band signal.

For example, one of the stations might be transmitting on 600 kHz; the other one on 610 kHz. With each station transmitting a 5 kHz tone, the station broadcasting on 610 kHz will be producing sidebands of 605 and 615 kHz; the station transmitting on 600 kHz will produce sidebands of 595 and 605 kHz. Note that both have sidebands of 605 kHz.

If now, we take our same two transmitters and transmit a 10 kHz tone on each, the situation will become even more interesting. The station transmitting on 600 kHz will be transmitting, in addition to this frequency, sidebands of 590 kHz and 610 kHz. The station transmitting on 610 kHz will transmit frequencies of 620 kHz and 600 kHz.

Notice that the sidebands from the 600 kHz station exactly fall on the center frequency of the transmitter assigned to 610 kHz. Notice, too, that the 610 kHz station will be transmitting a sideband on 600 kHz. If, therefore, these two stations were assigned to the same market area, very severe interference would be produced if audio frequencies out to 10 kHz were transmitted by each station. However, the FCC never assigns stations on adjacent channels to one market area. Stations would be spaced 30 or more kHz apart. Thus, the side bands of which we spoke would not result in interference between two stations within a given market area. Of course, good radio receivers can and do detect signals on adjacent channels. However, this is not a consideration of the FCC. Many of us have come to think that AM stations are assigned an audio bandwidth not to extend beyond 5 kHz.

This is not the case at all. In fact, the rules of the Federal Communications Commission which cover AM radio broadcast transmissions say that a broadcast station shall not depart from a flat response by more than 2 dB between 100 and 5 kHz, based on a mid band frequency of 1 kHz. This does not mean, therefore, that the broadcast station must curtail all frequencies above 5 kHz or all frequencies below 100 Hz.

The only stipulation the FCC makes with regard to the upper limits of audio response is that the band of frequencies between 15 and 30 kHz must be attenuated 25 dB with respect to a signal of 1 kHz. Frequencies beyond 30 kHz must be additionally attenuated.

Therefore, if it were possible to design a filter so sharp that within the space of 1 kHz a 25 dB attenuation could take place, the AM stations could produce frequency response flat right out to 14 kHz. Of course, 15 kHz would then have to be attenuated by 25 dB, placing stringent demands on such a filter.

You can see from all of this that a broadcast station is free to broadcast audio frequencies considerably in excess of 5 kHz; probably a realistic limit is 10 kHz.

(Audio magazine, Oct. 1972)

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