(Pictured: record producer Mickie Most in 1975 with a Teac A-3300 tape machine, once a fixture in radio stations and recording studios.)

If you visit the American Top 40 archive at Charis Music Group, you can see the original cue sheets for the shows from the very first one in 1970 to the end of Casey’s run and beyond. For the earliest shows, you see something even more interesting—the rundown sheets used to plan and execute the show. The one for the show dated October 30, 1971, has handwritten calculations of the show’s running time. It also includes master log sheets, which indicate that each hour of the program was delivered on two reels of tape, with the audio on each tape preceded by a series of tones to help radio stations calibrate the output of the tape machines on which the show would play.

Syndicated shows that were delivered on tape often had to be returned to the syndicator so that the tapes could be reused. If that seems a little bit crazy, it probably was, although we didn’t think much about it back then. In an analog world, sound quality was as good as we could make it, and that didn’t necessarily mean it was pristine. Shows like American Top 40 were high-speed duplicates—they were not mastered in real time—and that meant a certain loss of audio quality. And while there were certainly methods of quality control to make sure damaged tapes weren’t reused, it was not unheard-of for a station to receive a copy of a syndicated show that had a splice in the tape, or a distortion from a slightly stretched tape.

At some point in the 70s, it became feasible for shows such as AT40 to be pressed on to vinyl discs. These often had calibration tones, and some shows were lock-banded, meaning that at the end of each segment, the needle would not advance to the next segment without being manually moved.

(In my personal library, I had a vinyl copy of some old show with calibration tones, and I used it to set levels on my cassette deck before making mix tapes. There are people reading this post right now who are nodding and thinking, “Yup, I did that.”)

It was not until the turn of the 90s that it became common for shows to be delivered on CD, although vinyl and even tape delivery continued, depending on what the syndicator chose to do. Today, shows are delivered via an audio server; for American Top 40, you log onto a password-protected website and download it. Some shows are even delivered automatically, directly to whatever automation system a station is using, at a predetermined time.

But back to the handwritten rundown sheets: one of the endearing things about the early days of AT40 was that it was basically a live radio show captured on tape. If Casey flubbed his last line of a segment, they’d go back and record the entire segment, which meant sitting through the records and revoicing the other lines too. (This maybe explains some of the oddball errors and near-mistakes that slip through: “aw hell, it ain’t that bad, leave it in.”) The on-the-fly nature of the early shows is also seen in the handwritten notes on the rundown sheets. They were up against the clock every hour. On 10/30/71 and other shows, rundown sheets have one title scratched out and replaced with another, either for purposes of time or for some other spur-of-the-moment reason.

And there’s also this: during the first year or two of AT40, Casey would sometimes play a track from the #1 album of the week. During the week of October 30, 1971, it was John Lennon’s Imagine. Based on the rundown sheet, it looks like the AT40 staff didn’t decide what cut to play from Imagine until they’d started taping the show. They may have picked “Oh My Love” because it was the right length to help fill out the first hour.

(However they chose it, “Oh My Love,” which credits George Harrison on guitar, is the deepest cut I’ve ever heard on AT40.)

We’ve heard from AT40 staffers who say that at the time, they weren’t thinking about any of the minuscule stuff we notice here—they were just making a radio show. I’ve said myself that the shows were never intended to be examined on a molecular level. But, as you know from your own obsessions, nuts and bolts can take on an incredible fascination. So it is with the most popular syndicated radio music show of all time, and nerds such as we.