Distribution systems have evolved since the earliest days of broadcast television, and they have historically imposed many system parameters and constraints that ripple upstream to content creation systems. Experimental television developments were a hot technology topic of the 1930s and development efforts were taking place around the world. By the late 1930s, receiver manufacturers started to agree on early standards. In analog television systems and their cathode ray tube displays, it was extremely beneficial and economical to relate the key system parameters to the power line frequency. The development of commercial broadcast television networks and stations followed the established model for radio broadcasting. In the early 1980s, the technology to digitize video signals started to become practical. Initial digital standards for video simply involved digitizing the analog composite signals using a sampling rate that was an integer multiple of the color subcarrier frequency. Live productions typically provide a contribution feed to the Network Operations Center (NOC).