I was listening to Jethro Tull’s “Thick as a Brick” this afternoon when I started wondering about rock radio and editing of songs. Top 40 stations have edited rock songs for decades (“Light My Fire”, “Beginnings”, etc.). However, I’ve always heard rock stations in the '70’s played the songs unedited–which for the most part is still the case on classic rock radio. Did rock stations really play songs as long as “Thick as a Brick” unedited? And when did rock stations start cutting songs for time considerations?
It used to be FM stations usually played the full album versions of songs while AM Top-40 stations played the edited single versions. This was back in the 60’s and 70’s when it was not uncommon for hit records to last longer than three minutes. Of course, since this was long before there was a media monopoly like Clear Channel setting rigid rules for programming, some AM Top 40 stations went ahead and played the full versions of songs like “Light My Fire.”
As for “Thick as a Brick”, the whole album is basically one cut so unless an FM album-oriented rock station was setting aside a whole hour to play the entire album (which they sometimes did), they generally played just one portion.
“Thick as a Brick” never showed up on commercial rock stations unless they had an “album show” where they played an entire albums.
BTW, it wasn’t the stations that edited “Light My Fire” (improving it considerably). It was the record company. They knew it was a potential hit, but was too long, so they released a single version with the dull organ solo cut out.
Generally, these cuts were done by the record company. You wanted the radio stations to be able to play the song without having to take any special action, and you also wanted the edited song to sound the same in all markets.
There were even “edits” of “Thick as a Brick” and “A Passion Play” – shorter sections of the entire piece – that were available for airplay, but they were rarely played, if at all. Jethro Tull was big on Album Rock stations, where cutting a song was seen as a sacrilege. The current “Thick as a Brick” for airplay is a five-minute condensation of the entire album, and was probably done by the record company.
Sometimes the artist made the decision: Pink Floyd divided “Atom Heart Mother” into named sections in order to try to get it to be played by the BBC, which had a limit to song length.
The first song I know for sure was edited for radio play was Light My Fire by the Doors. It was over 7 minutes on the album, and 3 or less on the radio.
Bingo! Radio’s have to submit playlists, and from those, performance royalities are calculated an paid. A shorter song can potentially mean more plays.
This was a bigger deal back when juke boxes were more common. One song playing for 7 minutes would mean that one or two other songs weren’t being played in that time frame.
But I’m not sure about the difference between AM and FM and why so many rock stations would play full album cuts.
'Cause FM was groovy, baby! 
Does anybody know what the deal is with songs like Steve Miller’s Band’s “Jet Airliner” and Chicago’s “Hard to Say I’m Sorry”? Some stations will play “Threshold” right before “Jet Airliner” and “Get Away” right after “Hard to Say I’m Sorry”, but other stations omit those important parts.
AM radio started out with hit songs; the top 40 format was everywhere, and by the late 50s, all AM ever played was singles (this included the “race music” stations, which played singles by Black artists).
And, of course, albums were only a collection of singles.
FM radio came along later and only began to hit its stride in the late 60s. At the same time, music artists were releasing albums meant to be taken as an entire piece of music, and writing music that weren’t just three-minute songs. College stations – which were often FM – looked down upon the singles format and began to play the longer cuts. Since FM had fewer listeners (FM wasn’t standard in car radios, and the signal didn’t carry as far as AM), there was less money to be made in FM, so it was better able to fill a niche market. You couldn’t make millions in FM, so you could play songs that were not as popular (there also were listener-supported FM station, on the PBS model, which aimed at playing music you couldn’t hear elsewhere*).
Thus, with fewer ads, you had more time to play the longer songs that musicians were beginning to record. Eventually, FM became more popular and moneymaking and were switched to a limited playlist. In addition, the independent FM stations often had trouble making ends meet – a niche audience is nice, but you have to be able to pay your bills – and either sold their license or switched format to something more popular.
*Our local PBS station now has a station that plays independent rock and is supported by donations: http://www.exit977.org/).