A broadcast video router is the equipment that connects hundreds of video and audio sources in a TV studio to hundreds of destinations, and switches any source to any destination at the press of a button. The sources are cameras, recorders, reporters' live feeds, graphics computers and replay machines. The destinations are monitors, recorders, the vision mixer that the director uses to cut between cameras, and the playout system that sends the channel out. The router is also called a video matrix, or a routing switcher.
A router is often described by its size. A 288 × 288 router has 288 inputs and 288 outputs, and can connect any input to any number of outputs at once. The switch is made exactly between two frames of video, so that viewers never see a broken picture.
For decades, routers were large boxes joined by special video cables, called SDI cables. Each cable carried one video signal, and each source needed its own cable to the router. Now the same signals travel as data packets over ordinary computer networks, as described in video over IP. In such a studio, the router becomes two things: high-speed network switches, and control software that tells each switch which streams to send where. Every device must also keep the same exact time, so that video from different cameras can be switched cleanly. Adding a new camera then means plugging it into the network, not laying a new cable.
Take, for example, a Hindi news channel's studio in Noida at 9 pm. The router has 120 sources and 80 destinations. The anchor is in the studio, on camera 2. A reporter in Patna is waiting to go live from outside a counting centre. Her feed arrives over the internet and appears as source 64. A production assistant presses a button on a control panel. The router sends source 64 to input 7 of the vision mixer, and to one window of the director's monitor wall. When the anchor introduces the reporter, the director cuts to input 7. The same feed is also sent to a recorder, so that the report can be repeated in the next bulletin. The whole change takes less than a second.
Routers sit at the centre of every control room. They feed every camera to the instant replay system and to the screens that the broadcast monitoring team watches. They send the finished programme to playout. In remote production, the feeds from a stadium in one city arrive at a router in a control room in another city. Carrying uncompressed video over a network needs a great deal of capacity, and very little delay; see bandwidth and latency.