Why does FM radio sound so good?

I can add to this. In the vinyl era, many radio stations also used broadcast tape cartridges for a lot of content: spots, promos, jingles, news actualities and music. These look like an 8-track cartridge, but differ because the rubber capstan isn’t part of the cartridge (it’s a part of the player) and they run at 7.5 ips. Typically, each cart had one track, whether it was a song or a jingle; many were stereo. The best of these cart players sounded very, very good, and they didn’t decay from wear like an LP or 45. There were stations that had all of their content on these carts. Carts were used on some radio stations well into the ’80s, and perhaps beyond.

Automation dates back at least to the early 60s, and early automation systems often relied on those carts. They were loaded into a carousel for spots, voice tracks and promos; the music usually came from reel to reels that contained cue tones to actuate the carts. Schafer was one of the manufacturers of these systems, and companies such as Drake-Chenault and Schulte produced content for them. They were extremely clunkly and imprecise compared to today’s automation.

lowrider57

Carts were clunky, but the later generation of tape decks used to rec/playback were first-rate. When making a promo or any recording, you would hit a button and a cue tone would be recorded into the tape.

Yup! The best cart machines had very high SQ. Carts also used a "tertiary" cue tone that could trigger a flashing light (very helpful on Top 40 stations to mark the vocal post or the approaching end of the track) or to trigger another machine.