AM vs. FM radio and their content

When I was a kid in the 60s (and for all I know now), AM radio had all the pop and–in the South–country music. FM had all the arty stations: classical, jazz, and college stations.

Question 1: How cum?

Question 2: On cars, why was AM only available?

FM was a stepchild of the radio industry. There were technical reasons why it was more difficult to do the technology (frequency modulation rather than amplitude modulation.) Most transistor radios didn’t even get the FM band. Without an audience there was no call for advertising, and all media live only for advertising. Even though FM allowed for better musical quality than AM, most FM stations up through the 60s were owned by AM stations and just rebroadcast the material to the small listener base. Other stations were owned by the kind of people who couldn’t afford to buy into AM: educational, religious, and special interest broadcasters, the types that mostly didn’t need to be supported by advertising.

By the end of the 60s, though, conditions changed. Music was shifting from singles, which could be listened to on tinny transistor radios without too much loss, to highly produced stereo albums. People started spending big bucks for high end stereos and wanted the same quality in radio. Plus, AM couldn’t do stereo at all at the time.

Nobody thought that playing album music had an audience, except for a few people in a few large cities. They had true freedom, though, because no real money was at stake. FM pioneers played “free-form” radio, crossing genres and styles, although lots of 60s classic rock was a mainstay. They showed there was an audience. The whole industry rushed to FM in the early 70s.

FM has another technical problem. It’s available only through line of sight to the transmitter, like television (which also uses FM sound). That limits the coverage for any station to around 50 miles. AM signals can bounce off the atmosphere. The big “clear-channel” radio stations - the ones that had a bandwidth to themselves and broadcast at 50,000 watts - could be heard literally all over the country, especially at night. Lying awake and trying to amass AM stations from around the country was a big thing in my high school in the 60s. Between the technical issues that meant that AM stations were much easier to pick up as you drove outside a city and the fact that the demand was limited in the first place, there wasn’t much incentive for car manufacturers to include FM. When the demand appeared they rushed to cover it, just like the rest of the radio industry.

Answer 1. Bandwidth, and in general sound quality. The broadcast AM service allows each station to transmit (at most) 10.2 kilohertz of audio (occupying 20.2 kHz of RF bandwidth) which is plenty good for speech but not terribly good for music. The broadcast FM service allows a higher bandwidth signal, 15 kilohertz, and in addition a second 15 kilohertz “difference” channel to allow for stereo. In addition, FM stations use frequency modulation, instead of amplitude modulation; this allows for better signal recovery by the receiver (due to what is called the “FM capture effect”), at the expense of using more RF bandwidth. As a result, channels in the FM service occupy roughly 180 kHz of RF bandwidth, or about nine times that of an AM station.

Answer 2. AM receivers are much simpler in design than FM receivers, and can be built much more cheaply. This was even more true in the 60s, when transistors were still young; an AM radio can be built with only one transistor (for the audio amplifier; the product detector can be done with a diode or even a crystal) while an FM is going to need three or four at least. Most consumer FM receivers in the 60s would have been tube-based, but obviously the use of tubes in automobile electronics is not practical. FM broadcasting was still relatively young and not widespread in the 1960s; the added cost of making an FM-capable radio for a car would not have made market sense at that time.

AM broadcasting operates in what’s called the MF (“medium frequency”) band; in this band there are actually three modes of propagation: line-of-sight, “sky wave”, and “ground wave”, and all three are significant. Unless you live right under the tower, any AM station you get is likely to be received via ground wave propagation (see this article for an explanation of ground wave propagation) during the day. At night, when the D layer calms down, it becomes possible for signals that don’t become entrained in the ground to bounce off the higher E and F layers and reflect back down to distant locations. It’s possible, but relatively unlikely, for an AM station to be heard “round the world”, although that would require double skip (where the signal bounces off the ionosphere, then off the earth’s surface, then off the ionosphere again, and is then received by a very distant station), which is quite rare in AM because the earth tends not to be a good reflector of signals at these frequencies.

The band the FM broadcast service is in is actually capable of skywave propagation but this is a relatively uncommon event, unlike the extremely predictable propagation of the MF band that AM broadcast uses. It’s not uncommon for stations in Illinois to be able to pick up VHF stations in the Caribbean, with some patience and some understanding of the mechanisms at work.

Thanks, Kelly. I used to know all the technical details, but that was back in the early 70s. I know the socio-cultural info well today and the technical stuff gets remembered just well enough to hand wave. (A ubiquitous broadcasting mechanism at short distances. :))

I note that my first answer to question 1 was incomplete. FM receivers and especially FM transmitters are much more expensive than their AM counterparts. A broadcaster would only move into FM if the listener density in their region was high enough to cover these costs, or if the station could justify operating at a loss for some other reason. Also, the fact that VHF propagation is much shorter range than MF propagation means that a broadcast FM station, to be profitable, needs a large number of listeners in a relatively small area. This effectively limited FM operations to urban areas (which might possibly have the high density required) and to situations where breaking even wasn’t a major factor (donor-subsidized classical stations and college stations). The fact that ownership of an FM radio was limited to more well-to-do people probably also impacted which programs were likely to be successful.

It took the widespread availability of transistorized FM radios to break the barrier; that combined with the development of a “standardized” way to do FM stereo in the late 60s that started the stampede: there was finally enough listener density for a station to have a good shot at being profitable even at the higher operating costs, and the quality boost offered by stereo, combined with FM’s greater noise immunity (due to the FM capture effect), was enough to push most everyone playing music to FM.

Talk, sports, news, and religious programming by and large never moved, because these formats don’t gain much of anything from the higher fidelity offered by FM.

Giving rise to my standard complaint about talky “morning shows” on my favorite music stations:

“If I wanted to hear people talk, I’d listen to AM!”

Is this still true, and if so, why do some MP3 players have the capability to receive FM but not AM radio?

Tell that to Motorola. ALL my cars in the 1950’s had tube radios. Transistors were not available in cars until the 1960’s at the earliest.

And you got a cite for your claim that AM radio circuitry is less complicated or costly to build than FM?

My take on the OP, question 2, is that in its infancy, FM was poorly utilized. There were dozens of AM stations, but only 2 FM stations in the market I grew up in, and one of the 2 FM stations was only a simulcast of its AM twin. People didn’t want or need FM radios cause there was nothing to listen to, and stations didn’t spring up cause there were no listeners. Chicken/egg and all that.

Also AM Radio was the “TV” of it’s day

From the late 20s till the early 50s, AM radio was, what “TV” is today.

After TV came into fashion and became affordable, AM radio had to find another source of programming.

This involved a shuffle of sorts.

AM was best suited when radio served what “TV” does today. Since it was mostly talking shows, it needed minimal fidelity with a large service area. AM does this great.

FM was blocked by AM radio, who coincidently were not so keen on TV, as were the movies not so keen on it.

After the shuffle of the early 50s and TV won out, AM had to find a niche. Since most people still had receivers of AM, the natural market was music.

But AM was not well suited for music. So it took a decade or so for music to shift to the FM band. Remember AM radio stations had a huge investment going back to the late 20s. They weren’t going down without a fight.

But with FM stereo and more FM stations being built it was obvious AM would eventually lose out. Most AM stations handled this simply by buying or affiliating with FM stations of their own.

But this left a gap, what to do with AM Radio? It was still there. Well News, Sports and Talk eventually filled this gap.

But it took awhile, as I said, no one was going down without a fight. I recall in a big city like Chicago, even into the mid 80s we still had AM radio stations playing pop music

Define “blocked”. Opposed?

It also needs to be mentioned that the FCC didn’t settle on a standard for FM stereo broadcasting until April, 1961. That was the final hurdle – no one would invest in FM technology (either from the business side or the programming side) until the standards were set.

After that came the drop in the price of transistors and the development of less-expensive FM radios in the mid-60s. The migration of music from AM to FM started shortly afterward.

Yes, it’s still true. There are any number of reasons the designer of an MP3 player might choose to not include an AM radio, but my guess as to the most common reason is this one: size.

AM broadcast uses very long wavelength radio waves, about 300 meters. An “ideal” antenna (for reasons I don’t really want to get into here) needs to be about a half wavelength long. For AM broadcast, this is about 150 meters. Now, obviously nobody is going to carry around a 150 meter long antenna with a piece of portable electronics. You can use a coil to “electrically lengthen” a shorter antenna, but the size of this coil is also dependent on wavelength: longer wavelength, larger coil. If you look inside most portable AM radios, you’ll find an iron bar wrapped with wire. This coil is, for all practical purposes, the AM antenna (known as a “bar antenna”). That coil has to be the size it is (it could be made somewhat smaller by using a core made of very high permittivity materials, but then you’d be spending more on that core than for the rest of the radio) in order for the radio to work, and that coil alone is larger than most MP3 players.

The short of it is that while you could put an AM radio into a device the size of an MP3 player, it would have extremely poor reception because of the lack of a suitable antenna system. This isn’t as much a problem with FM broadcast because FM broadcast uses radio waves of about 3 meters in length; a functional antenna for this wavelength can be made small enough to tuck inside the case of an MP3 player of reasonable size.

There’s still music on AM in the United States, but it’s generally not pop music. in my hometown, AM stations broadcast old-school country music, gospel, and polkas (Polish, not Mexican). Spanish norteno and ranchero music is common on AM in parts of the country with a larger Mexican population. Country is common in more rural areas, but it tends to be more “classic” than the newer glurge-filled country that dominates AM. Radio Disney is common on AM. I’ve also heard some nostalgic-oriented stations on AM, with programming and disk jockeys that attempt to recreate 1950s/early 1960s broadcasting.

I’ve heard pop and rock stations on AM in Canada.

With the wider availability of the Internet, are high-powered radio stations that broadcast nothing but crop prices at various markets and elevators still common in the rural Midwest?

It’s obviously possible to put tube radios into a car (we put tube radios into airplanes during WW2, after all) but there are all sorts of difficult issues involved. It does occur to me that many of the hard problems with tube radios in mobile environments are limited to transmitters, which generally require high voltages. I’d be curious, though: did those radios carry their own batteries, or were they driven from the car’s electrical system?

This is just basic RF electronics. You can detect and recover an AM signal with a single diode and an reactive tank for tuning and filtering; this requires basically three components, although the recovered audio signal will be very weak, requiring an amplifier of some sort to drive more than a tiny earpiece. Product detection in FM requires more complicated circuitry; the simplest method that works decently well (slope detection “works” but not reliably or well) I know of requires two diodes and a specially-made transformer and a whole gaggle of other components, and typically needs a preamplifier as well in order to work well. (See here for more on FM detection.)

In addition to what KellyM said, another problem is that the AM radio band is around 1 MHz frequency, while the FM band is around 100 MHz. That was pretty high frequency back in the 60s, and required more expensive components.

I remember when I was a little kid in the late '60s, my older sister got a radio that had a switch with three positions: AM, FM, and Off. I thought that the FM setting was equivalent to Off, because the radio never made any noise when the switch was set there. There were no FM stations around.

It’s been interesting to watch as the “upper working frequency” of transistors has moved steadily upwards over the years, and is why we’ve seen public service radio migrate from 36 MHz to 154 MHz to 460 MHz and now to 800 MHz as this “ceiling” has been pushed up by steady advances in semiconductor fabrication technology.

In addition to requiring more expensive components (which remains somewhat true) the manufacturing tolerances in VHF are much stricter than they are in MF. At the low frequencies used by AM broadcast, the length of a wire in your radio design is not going to matter much; it can vary by quite a lot and the circuit will still work. This is not at all true at VHF; in these frequencies the length of the lead of a component on the circuit board can significant alter the performance of the device. Surface mount technology does a lot to mitigate these problems, but that didn’t become widely available until the 1980s.

We take for granted today a lot of development in basic electronics that took place in the 60s, 70s, and 80s. The world is a lot different place now than then.

The Doors’ “Light My Fire”, on the first album, lasts about seven minutues. When it was released in early 1967, it became an FM hit. That is to say, it probably reached a small audience of heads and music freaks perhaps not too different from the college radio audience today. Even so, according to the story, AM DJs and station managers found that their listeners were clamoring for the song–but they couldn’t play a seven minute song in the AM format. It simply wasn’t done. So the Doors’ producer edited out the guitar and keyboard solos, and that became the AM single. The band hated the amputation of their song, but had to admit it was a deft job of editing. At any rate, there was the long FM version and the short AM version of the same song.

This is because on AM you had to “sell soap” by running ads every five minutes or you wouldn’t cover your operating costs, but on FM the record company paid you to play the album, so you didn’t need to run advertisements.

Not saying that every FM station in the 50s and 60s took payola, but I bet a lot of them did.

Yes, because farmers still don’t have the Internet in their tractors, combines and pickup trucks. Here’s the farm programmingfor one of my long-time favorite high-powered Midwestern stations.

But those stations were never 100% crop prices and weather. They always had some other programming on between crop reports. These days, it’s usually the same talk radio you hear in the big city.