Science Fair Project Encyclopedia
A vision mixer (also called video switcher or production switcher) is a device used to select between several different video sources and in some cases composite (mix) video sources together and add special effects. This is similar to what a mixing console does for audio. Typically a vision mixer would be found in a professional television production environment such as a television studio, cable broadcast facility, commercial production facility or linear video editing bay. The term can also refer to the person operating the device.
Vision mixer and video mixer are almost exclusively European terms. In the United States, the common name for a device of this kind is (video) production switcher, and most US video professionals will only know it by this name.
Capabilities and usage
Besides hard cuts (switching directly between two input signals), mixers can also generate a variety of transitions, from simple dissolves to pattern wipes. Additionally, most vision mixers can perform keying operations and generate color signals (called mattes in this context). Most vision mixers are targeted at the professional market, with newer analog models having component video connections and digital ones using SDI. They are used in live and video taped television productions and for linear video editing, even though the use of vision mixers in video editing has been largely supplanted by computer based non-linear editing.
Older professional mixers worked with composite video inputs. There are still a number of consumer video switchers with composite video, S-Video or even FireWire available. These are often used for VJing, presentations, and small multi-camera productions.
The main concept of a professional vision mixer is the bus, basically a row of buttons with each button representing a video source. Pressing such a button will set the video out of that bus. Older video mixers had two equivalent busses (called the A and B bus, such a mixer is known as an A/B mixer). One of these busses could be selected as the main out (or program) bus. Most modern mixers, however, have one bus that is always the program bus, the second main bus being the preview bus. These mixers are called flip-flop mixers, since the selected source of the preview and program busses can be exchanged. Both preview and program bus usually have their own video monitor.
Another main feature of a vision mixer is the transition lever, also called a T-bar. This lever, similar to an audio fader, creates a transition between two buses. Note that in a flip-flop mixer, the position of the main transition lever does not indicate which bus is active, since the program bus is always the active or hot bus. Instead of moving the lever by hand, a button (commonly labeled "Mix") can be used, which performs the transition over a user-defined period of time. Another button, usually labeled "Cut", directly swaps the busses without any transition. The type of transition used can be selected in the transition section. Common transitions include dissolves (similar to an audio crossfade) and pattern wipes.
The third bus on a vision mixer is the key bus. A mixer can actually have more than one of these, but they usually share only one set of buttons. Here, a signal can selected for keying into the program. The image that will be seen in the program is called the fill, while the mask used to create the keys' translucence is called the source. This source, e.g. chrominance, luminance, pattern (the internal pattern generator is used) or split (an additional video signal similar to an alpha channel is used) and can be selected in the keying section of the mixer. Note that instead of the key bus, other video sources can be selected for the fill signal, but the key bus is usually the most convenient method for selecting a key fill. Usually, a key is turned on and off the same way a transition is. For this, the transition section can be switched from program (or background) mode to key mode.
These three main buses together form the basic mixer section called Program/Preset or P/P. Bigger production mixers may have a number of additional sections of this type, which are called Mix/Effects (M/E for short) and numbered. Any M/E section can be selected as a source in the P/P stage, making the mixer operations much more versatile, since effects or keys can be composed "offline" in an M/E and then go "live" at the push of one button.
After the P/P section, there is another keying stage called the downstream keyer. It is mostly used for keying text or graphics, and has its own "Cut" and "Mix" buttons. After the downstream keyer is one last stage that overrides any signal with black, usually called FTB or Fade To Black.
Modern vision mixers may also have additional functions, such as machine control , aux channels for routing signals to other sources than the program out, macro programming and DVE (Digital Video Effects ) capabilities.
Since vision mixers combine various video signals such as VTRs and video cameras, it is very important that all these sources are properly synchronized. In professional facilities a sync generator feeds all the equipment. Sync can also be achieved by sending out a black burst signal from the switcher (this method is called "genlock" ). Signals which cannot be synchronized (either because they originate outside the facility or because the particular equipment doesn’t accept external sync) must go through a frame store synchronizer. Some vision mixers have internal “frame-syncs” or they can be a separate piece of equipment. If the mixer is used for video editing, the editing console (which usually controls the vision mixer remotely) must also be synced. Most larger vision mixers divide the control panel from the actual circuitry because of noise, temperature and cable length considerations. The control panel is located in the production control room, while the main unit, to which all cables are connected, is located in a machine room alongside the other hardware.
Manufacturers of vision mixers
- "Ampex Production Switcher." CSU-Pueblo Television Production Handbook. Accessed on February 28, 2005.
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