Adult Contemporary format in 1979 and a Rhythmic Top 40 format in 1980. Other FM competitors like oldies station WCBS-FM, soul station WBLS, and album-oriented rock stations like WPLJ and WNEW-FM all did well in the ratings, but none rivalled WABC's success. AM competitor WNBC also never came close to WABC's audience during this period. WNBC then had a format similar to 99X playing Adult Top 40. In 1977, WNBC tried sounding younger and moved its format musically closer to WABC. Then by 1979 they tried sounding older and somewhere in-between. Until 1978, WABC remained dominant. WABC's ratings strength came from its cumulative audience, what the radio industry calls "cume". Most listeners did not stay with WABC for long periods of time, as the station had some of the shortest "time spent listening" (or TSL) spans in the history of music radio—an average listener spent about 10 minutes listening to WABC. It was the price paid for a short playlist, and numerous commercials between songs (the large number of ads being due to WABC's large audience), but what WABC lacked in TSL it more than made up for with its sheer number of listeners. By 1975, WABC tried becoming more music-intensive, reducing commercial breaks to three per hour. It began playing 3 to 5 songs in a row, still mixed with talk and personality, but done in a tighter manner. Fed up with the short playlist, Cousin Brucie left in August 1974 to defect to rival WNBC. Rick Sklar was promoted in 1976 to vice president of programming for ABC Radio, and his assistant program director Glenn Morgan became WABC's program director. The station's influence could be found in odd places: Philip Glass' 1976 opera, Einstein on the Beach, has as part of the background a recitation of WABC's DJ schedule in the 1960s. "Disco" era The end of the 1970s found FM radio beginning to overtake AM music stations in most markets. In June 1975, an FM station on 92.3, owned by the San Juan (Puerto Rico) Racing Association flipped to Soft Rock and became known as Mellow 92 WKTU. That station had very low ratings and had no effect on WABC. But on July 24, 1978, at 6 PM, WKTU abruptly dropped its Soft Rock format in favor of a disco-based top 40 format known as "Disco 92". By December of that year, WABC was unseated, as WKTU