What do tv stations use to put text on the screen?

Answer 1

The device is called a caption generator, normally shortened to "CG" in broadcast speak. In their most basic form, they produce a white image on a black background to create a full screen with the text in the position it is required. Early versions were not much more than a typewriter that produced the text on a video image but they have become far more flexible since those days - more on the later versions below. Once the caption has been generated, it is added to the main signal using a keyer. A keyer does two things. First, it detects where the text is on the caption image and cuts a hole in the background image. What this means is that the main image now has black where the text is to be placed rather than the original image. The second step is to take the caption image and insert it on the background. The image may be exactly the same signal coming from the caption generator but often, it can draw from a "matte", a separate signal that is noting more than a screen of a single color. Rather than plain white text, the matte can be used to generate any color and any shade of text to fill the black holes left by the hole cutter. Keyers are normally part of the switcher or vision mixer although they can also be a separate unit from the switcher. The switcher is the large control panel that can be seen in television control galleries. CGs and keyers have advanced a great deal since the days of the single color CG. Most modern CGs generate two signals, one for the hole cutter known as a the "key" or sometimes called an alpha channel. The other is the "fill" channel which has the content to be displayed. The key channel is often just black or white, with the white areas cutting the hole for the caption. However, if the key channel uses shades of gray, the hole that is cut in the background image will still have the image showing. The darker the gray, the more of the background image is shown. When the fill channel is added, the two will be blended together and the caption appears to be transparent. Fill and key techniques are used to display not only text, but transparent station idents, still or moving images as used frequently in news broadcasts - the small image that appears over the shoulder of the news presenter for example uses fill and key signals. Similarly, scrolling text and information panels at the bottom of the screen are inserted in the same way. News broadcasts rely heavily on keyers to generate the final image. It is quite common for 8 or more keyers with captions generators to be used. The techniques described are known as linear or luma keys. There is another keyer technique known as Chroma Keying. It is also referred to as "blue screen" or "green screen". For a brief descripton of chroma keying, take a look at this answer: http://wiki.answers.com/Q/What_is_a_tech_term_called_blue_picture