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U. S. Digital AM and FM Radio Stations

Keith Greiner

August 9, 2020

This essay is about Digital AM and FM radio broadcasting in the U. S. Digital broadcasting in the AM and FM bands was one of several attempts to add flexibility and quality to the existing analog audio systems without disrupting the existing systems. By comparison, the television system was completely disrupted when the conversion was made from analog to digital. In July, 2017, it appears that digital AM and FM broadcasting has not caught on as a mass communications system due to the lack of low-cost AM and FM receivers and the availability of audio services via the internet. Additional information is provided below.

An Incomplete History of U. S. Radio Broadcasting

Radio broadcasting was a major influence in the 20^th century, and although some would predict its demise in the new internet world, there are indications that it continues to be an important part of American information and culture. AM (stands for Amplitude Modulation) broadcasting began in November, 1920 and was the simplest form to create in a transmitter and receiver. It could be created with a simple transformer in the output circuit of a CW transmitter, and could be received by a simple crystal receiver. FM broadcasting was first demonstrated in 1936 and was assigned the 88 Mhz to 108 Mhz frequency band in 1945. However, the system was not widely adopted until after some years of controversy over the frequency and type of FM signal to be used. FM stands for Frequency Modulation and is created with an audio connection to the frequency oscillator. It is more complex to receive, than is AM, and requires circuitry that an compare signals at two different frequencies. The post-WWII technology used a wide band FM signal having a maximum signal spread of 75 Khz. Although FM offered substantial noise reduction over AM broadcasting, many commercial broadcasters were slow adopt FM programming. Many, would simulcast the FM signal with the primary AM signal, while others would broadcast taped music with little or no advertising on the FM station.

When FM radio started to thrive, some predicted the demise of AM. When television started to thrive, some predicted the demise of both AM and FM. In 1968, one AM radio station owner predicted, to me, that by 2000, he thought it likely that neither AM or FM would exist, having been supplanted by local TV. While the services have had their challenges none of those fears has, to date, been completely true.

Through the last of the 20^th Century, manufacturers and broadcasters worked to innovate improvements to the systems. In the 1980s there were attempts to broadcast an AM stereo signal by placing one channel on the left sideband and another channel on the right sideband. In FM, a sub-carrier could be used to broadcast stereo data, and other services such as the MUZAK service that provided background music for businesses (often described as “elevator music”).

In 2009 all U. S. Television stations converted from the original analog system to a digital signal. This groundbreaking conversion meant that everyone in the nation had to have either a new digital receiver or a converter. AM and FM, broadcasting, however, adopted a system that allowed a digital signal to be added to the existing analog signal without disrupting the original signal. Digital audio broadcasting was approved in 2012. (see http://www.nbcnews.com/id/3078252/ns/technology_and_science-tech_and_gadgets/t/radio-going-digital/#.WXOtGK2ZO7Z)

Digital Audio Broadcasting

Now, in 2017, and according to my query and selection is from an FCC file, there appear to be 4,577 AM stations and 10,862 FM stations in the US, with some of them also broadcasting digital signals. Some variation in the counts may be due to the date the file was created and the interpretation of various codes that are in the file. Information from one station with AM and FM signals said that they once broadcast the digital signals but have since discontinued that service, even though they continue to be listed in the FCC's database.

Further data are needed to know whether the number of digital stations in increasing or decreasing. If there is a decline in the number of digital AM and FM stations, it is likely to be due to the high cost of receivers. Existing analog receivers cannot be converted, as TV signals could have been converted. Manufacturers are not being required to incorporate digital demodulation into their systems, and the cost of digital AM/FM receivers remains very high. Not even my relatively current software designed radio (SDR) receiver program includes an AM/FM digital demodulator program.

Below are plots of the locations of 239 AM digital stations and 1,907 FM digital stations that are listed in the FCC file. The plots give us a general overview of the distribution of digital audio stations in the Continental U. S.

AM Digital Stations

FM Digital Stations

AM and FM Digital Stations

Following are some links to additional information on this topic.

http://electronics.howstuffworks.com/hd-radio.htm

https://nabpilot.org/work/projects/all-digital-am-radio-testing/

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