During black & white, monochrome days, the commercials were supplied on 16mm spot reels, these were wound off and temporarily spliced with sticky (splicing) tape into the film program, or if a video-tape was the program, they'd be spliced together as a commercial reel. Spacer would be included to separate the individual commercial breaks. At the end of the night the films would be "broken down" meaning returning the commercials back onto their respective spot reels and joining the film programs back together, again with temporary splices, ready for the next station to splice in their commercials. With colour came a distinct problem. Each time the temporary tape splice was removed, it would pull off part of the film emulsion, the result left being usually a green colour. To avoid the green, the frame would be cut off. The outcome of this was as film programs were sent around their built in fades to black were being cut off. Also the film commercials were being reduced in length. This rapidly brought on the distribution of commercials on 2" video-tape. As at the time we only had 2 x VR-1200 2" machines, we had to make up an "edit reel". This was done at the end of the night, all commercials for the next day compiled together as required, taking into account use of the 2 x 2" machines for program replay. If video-tape programs were back to back, we'd use the 1" machine for replay of the station id. Often the "edit reel" would not be wound off a machine, instead supply and take up reels with the tape dangling between, lifted off and put onto the other 2" vtr. After about a year, the ACR-25 cart machine arrived. This was thanks to the partnership called Great Eastland Television, which consisted of us, NRN11 Coffs Harbour and NEN9 Tamworth. One ACR-25 was purchased for each station. With this machine we still made a back up, being an "edit reel" but it was easy, the ads were played out of the ACR. The idea of the back up lasted about a week as the ACR was just so good. At times it did pack up, usually one of the two transports, but the machine was extremely fast, we could play ads off the one side with about 5 seconds black in between.
I should also mention in the early days some ads were put to air as a static 35mm slide and audio cartridge. It was essential when making a new recording on an audio cart, to bulk erase it. The audio carts were endless loops, instigating the record put a start cue pulse on them. If they were not bulk erased, they could cue on the older recording, or worse still, if the voice over talent fluffed it, then another record was done, you'd have one good cue for the ad, but the next time it would cue for a replay of the fluff, which could be an expletive. Slide and cart commercials were usually put first in the break when we were running relay programs, as they were 10 seconds and we needed an 8 second roll on our video-tape commercials. The ACR-25 gave us instant roll, so we could have tape commercials first in the break.
Today, servers in the main playout centres do the job with little human intervention.