How does a downstream keyer work with a video switcher?

Answer 1

Keyers are used for titles and effects in television programmes. All keyers work in a similar way. Take a name of a news presenter as an example. It needs to be placed on the image of the presenter without covering the whole of the image. The keyer takes the signal that makes up the name, only a small part of the whole screen area and does two things. First, it cuts a "hole" in the background image so the background now has black wherever the title needs to be placed. Second, the title is inserted into the image in the black area. The resulting image has the news presenter with his name over the top of the image. Most video switchers (known as mixers in Europe) have several keyers to allow title bars, over the shoulder images, news crawls etc to be displayed. The keyers are placed in various points in the signal chain, depending on how they are to be used. Getting to the downstream keyer (DSK), this is just another keyer, that works in the same way as the rest, but its position is downstream of every other keyer and effect in the switcher. After all the backgrounds, titles and other effects have been inserted, the DSK can add one last title, station ident or logo. In most cases, the DSK is built into the switcher but there are also external DSK units. These take the output from the switcher, add the keyed image and pass the signal on to the rest of the transmission chain. The description of keyers above is greatly simplified. Modern switchers offer a huge number of facilities in the keyers, such as different ways to mix the images together, resizing and positioning of the keyed image to name but two.