The assassination of President John F Kennedy led to a change in policy at CBS News and in other newsrooms. In the early 1960s, TV cameras took about 20 minutes to warm up to the point that they could produce pictures, so for the first 20 minutes, the news reports read by Walter Cronkite were voice-only, accompanied by a caption slide. Since then, newsroom cameras at CBS were left switched on 24/7 in case they were needed at short notice.