Adult contemporary is essentially a continuation of the soft rock style that became popular in the '70s, with a few adjustments that reflect the evolution of pop-music production techniques. Adult contemporary has the same lush, soothing, highly polished qualities as soft rock, and it works the same "middle of the road" territory — in other words, it largely lacks the grit of even the slicker varieties of rock or soul music, but it's too indebted to those original sources to qualify as traditional pop or easy listening. The main dividing line between soft rock and adult contemporary is that the latter commonly features synthesizers (and other electronics, such as drum machines) as an important component of the music's smooth studio sheen. Both as a style and a popular radio format, adult contemporary was heavy on romantic ballads, but there was also room for catchy pop/rock (often updating styles — early rock & roll, smooth soul, Beatlesque pop — that adult contemporary's main audience had grown up with) and thoughtful singer/songwriter sensibilities. Some artists stayed in adult contemporary territory with pretty much everything they did, but the format also provided a home for ballads by dance-pop and urban soul singers (and, sometimes, those styles' most popular artists could land some AC airplay with their more typical work). Adult contemporary was usually melodic enough to hold listeners' attention, but agreeable and inoffensive enough to work equally well as background music; most critics reviled it as sonic wallpaper or overly safe, sentimental glop, but that didn't prevent it from becoming one of the most popular radio formats of the '80s. Adult contemporary's visibility dwindled just a bit in the '90s, mainly because the rigidly compartmentalized state of radio and the decline of Top 40 programming during the decade meant that AC artists couldn't necessarily rely on as much hit-making exposure outside of their core adult audience. Still, there continued to be a heavy demand for the kind of ambience adult contemporary radio provided, and it remained a well-entrenched style into the new millennium. Some of adult contemporary's core artists over the years include Elton John, Billy Joel, Sting, Air Supply, Michael Bolton, Phil Collins, Lionel Richie, Chicago, and Celine Dion.