In the late 1950s more and more radio stations were adopting programming formats that were fast-paced requiring tight cuing of short segments of audio. There was a great need for some type of playback equipment that could handle these segments easily. This led to the development of the endless loop tape.
One of the first high-quality units of this type was the MacKenzie Repeaters from MacKenzie Electronics of Los Angeles. It had five magazines which were made up separately and could repeat individually. The unit had a single motor and a long belt driven shaft that drove all five slots. Each magazine had a pinch roller and could be started by its own push button switch. The recordings were made on any standard reel to reel tape recorder on a specially lubricated tape that was cut to length and loaded into a metal cartridge and then spliced to form an endless loop. The MacKenzie Repeater was the forerunner to the “cart” machines that became standard in radio for several decades.
The MacKenzie Repeater magazines, or cartridges, could contain station jingles, news intro's, commercials, sound effects or any short audio bite. The magazines could be changed in about 5 seconds. The start up was instant and the tape would play through and stop at the beginning ready to be played again at any time. The cuing was done by a silver sensing tape that was applied to the beginning of the audio segment. The conductive tape would close an electrical contact that stopped the tape loop.
The MacKenzie units were used in many applications requiring fast-paced sound effects, short music segments, and announcements and found use at Disneyland among other venues. I toured the CBS key station KNX in Hollywood in 1959 and saw a MacKenzie used in the studio used for the CBS Radio show “Gunsmoke” starring William Conrad. The machine was used to play short bridging music segments as well as sound effects in the show. KHJ in Hollywood also was one of the first stations to utilize the MacKenzie Repeaters to handle their “Radiant Radio” format that started in 1959. KHJ had been the Mutual Don Lee network key station in Los Angeles but had changed to a completely independent, music-news type operation.
With the development of these quick-cue audio playback devices...the handling of short segment audio was changed for many decades...until the coming of digital computer audio.