First, FM radios were more complicated than AM — and more expensive. Meanwhile, television was being rolled out across the country, including in Pittsburgh, where construction of the city’s first station, WDTV (3), was underway.

Why would consumers invest in a more expensive radio when TV was on the horizon?

Second, FM promised clearer, high-fidelity sound than AM. But with the limited range of the audio circuit in a typical 1940s radio, many listeners didn’t hear much of a difference between AM and FM.

Finally, many historians point the finger at the legendary chairman of Radio Corporation of America (RCA), David Sarnoff, as being principally responsible for stunting FM’s growth in the 1940s.

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Here’s a highly simplified explanation of the difference between AM and FM. Every radio transmitter puts out what’s called a “carrier wave.” The audio signal is superimposed on top of the carrier wave.

In AM, or “amplitude modulation,” the loudness and softness of the audio wave makes the carrier wave get bigger or smaller. But static and other interference can also superimpose themselves on top of the wave, and they’re almost impossible for the receiver to filter.

In FM, or “frequency modulation,” transmitters, the carrier wave changes frequency slightly, depending on the frequency of the audio. Because decoding the audio isn’t dependent on the “amplitude” of the wave, it’s easier to prevent static from getting through to the listener.

. . .

Armstrong was already a highly respected radio engineer when he demonstrated his FM system. Soon, experimental FM stations were signing on across the country in the new 42 to 50 MHz FM band, and several manufacturers began making AM-FM radios. Armstrong created a network of FM stations up and down the East Coast.

Instead of embracing the new technology, Sarnoff decided it was a threat to RCA and its subsidiary, National Broadcasting Company (NBC). RCA would have to pay a fee to license Armstrong’s FM system, and NBC would have to spend millions to convert its stations to FM transmission.

As memorably recounted in Ken Burns’ 1991 PBS documentary “Empire of the Air” and the book of the same name by Tom Lewis, Sarnoff directed RCA’s attorneys to lobby the Federal Communications Commission to block the expansion of FM. He also told RCA’s engineers to find a way around Armstrong’s patents.

. . .

The first strategy proved successful when the FCC decided to move the FM band to the present 88 to 108 MHz, supposedly because the 42 to 50 MHz band was better suited to television. (Remember, one reason why analog TV is scheduled to disappear is to clear the bandwidth for two-way radios used by emergency services, but Internet and wireless phone companies also covet those frequencies.)

Moving the FM band instantly obsoleted the thousands of FM radios already sold (much as next year’s scheduled switch to digital TV will obsolete millions of existing TV sets).

The change also limited the range of Armstrong’s FM network, because higher frequency signals didn’t travel as far.

The second attempt landed RCA in federal court, after Armstrong sued for patent infringement. The courts eventually vindicated Armstrong and forced RCA to pay damages; by then, Armstrong, discouraged and worn down from years of legal proceedings, had committed suicide.

. . .

The Major’s ultimate vindication, of course, didn’t come from the courts. It came from the technological improvements of the 1950s and ’60s, which made FM radios cheaper and better.

By the late 1960s, no one disputed that FM sounded noticeably better. Within a decade, listeners who wanted to hear music were abandoning AM for FM.

And it all started 70 years ago this week at a 600 watt station in Alpine, N.J.!

Seventy years ago this week, a New York inventor turned the broadcasting world on its ear and made a very powerful enemy.

It would take a while for the rest of the world to realize how incredible his new invention was — but it’s hard to imagine the world of broadcasting without it.

The invention was FM radio, and 70 years ago this week, on April 10, 1938, Major Edwin Armstrong signed on the world’s first FM radio station.

Using the experimental call sign W2XMN, it broadcast with 600 watts at 43.7 MHz from a tower in Alpine, N.J., just across the Hudson River from New York City.

The first broadcast day lasted only five hours; regular broadcasting didn’t begin until July 1939.

Pittsburgh’s first FM station went on the air in 1940, when the Post-Gazette‘s WWSW (970) received its license for an FM station at 44.7 MHz. KDKA signed on its FM outlet at 47.5 MHz on April 4, 1942.

Ten years after Armstrong’s station opened, Pittsburgh had six FM stations, including WKJF-FM (93.7), KQV-FM (98.1), WJAS-FM (99.7) and WPIT-FM (101.5), which all broadcast for the first time in 1948.

All had successful AM sister stations except for WKJF, which billed itself as “the first and only exclusive FM station in the city.” Opened in July 1948 on Mount Washington, WKJF was owned by Agnes Reeves Greer, wife of the publisher of the Morgantown Dominion News and Post newspapers, which also owned (and still does) WAJR, Morgantown, and WDNE, Elkins, W.Va.

Other local FM stations included WMCK-FM (104.9) in McKeesport and WMBS-FM (105.7) in Uniontown. Wheeling and Youngstown had two FM stations each — WKWK-FM and WWVA-FM, and WKBN-FM and WFMJ-FM, respectively.

Frankly, none of them were attracting many listeners, and a few (like WMCK-FM and WMBS-FM) eventually were closed.

There are a lot of reasons why FM had a hard time catching on, and there are some parallels to the current transition from analog TV to digital TV.