Image mixer

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A Sony BVS-3200CP image mixer

A picture mixer (also called a trick mixer ) is a device used to switch between different video sources , mix them or sometimes add special effects , much like a mixer is used for sound. A video mixer is typically found in a television studio , television broadcast van, or at a linear editing suite . The term “vision mixer” can also refer to the person who operates the device.

Skills and commitment

A Panasonic WJ-MX20 vision mixer

In addition to hard cuts (direct switching between two image sources), an image mixer can also create a series of transitions, from simple cross-fades (mix or dissolve) to wiping . In addition, most mixers can fade in punches ( keys ) and generate color signals (mostly referred to as "mattes" in this context). Most video mixers are designed for professional use, so the older analog models have component connections and modern digital models have SDI connections. They are used for live broadcasts , recordings on video cassettes and for linear editing. In the editing area, however, they have largely been replaced by video editing software .

Older professional mixers also used composite signals as sources. There are still some video mixers for the amateur and semi-professional area that work with FBAS, S-Video or FireWire . They are often used for VJing , presentations, and smaller multi-camera recordings.



The basic concept of a professional video mixer is the bus , to put it simply, a series of buttons, each of which represents an image source. When a button is pressed, the corresponding signal is switched to the output of the bus. Older picture mixers had two buses of equal status (A and B, such a mixer is then called an A / B mixer ). One of these buses could then be selected as the output or program bus. Most modern mixers, however, have a bus that is always the program bus . The second bus is then used as a preview bus. Such a mixer is known as a flip-flop mixer , since the sources of the program and preview buses can be exchanged. Both the program and the preview bus usually have their own monitor .

Another noticeable feature of a picture mixer is the aperture lever , also known as the T-bar or fader bar . Similar to a sound fader, this lever creates a transition between two buses. In a flip-flop mixer, the position of the shutter lever is not an indication of which bus is active, because the program bus is always active (or hot ). Instead of moving the lever by hand, you can press a button (usually called “AutoTrans”) which will carry out the transition within an adjustable time. Another button, usually labeled “Cut”, swaps the two buses immediately and without a transition. The type of transition can be selected in the Transition Section area. Common transitions are diaphragms (analogous to crossfading in the tone area) and wiper diaphragms , so-called wipes .

The third bus of a picture mixer is the "key bus". A picture mixer can have several of these, but in this case they usually share a number of buttons. A signal for keying in can be selected here. The source of the image that will be seen is called the fill , while the mask that determines the transparency of the signal is called the key . In order to generate this mask signal, a further signal is fed to the so-called keyer in addition to the fill signal: key source . This is a normal video signal and often corresponds to the fill , but it can also be a completely different one ( split function ). The keyer uses the key source to generate the actual punch signal, the key, based on the type of key selected (luminance, linear, additive or chroma key) and the settings made (clip, gain, density, hue). The various key types and settings are made in the key area of ​​the mixer. Keying is also possible without a key source signal: with the help of the mostly built-in pattern generator. This provides its own punch signal, provided that the pattern key is selected as the type. If the fill and key source match (typically with a chroma key), both signals are selected simultaneously via the key bus, otherwise either the often available split key must be held and then the key source signal on the key - or Aux bus can be selected. Alternatively, there is often the Auto-Select option, in which the assignment of fill and key source is determined via an adjustable table of the mixer. Typically, a key is toggled on and off in the same way as a crossfade. For this purpose, the “transition section” can be switched from the program (or background ) to the key.

These three main buses together form the basic mixer module called Program | Preset or P | P . Larger mixers can have several modules of this type, which are then called Mix / Effects ( M / E for short , often mistakenly interpreted as “mixer level”) and numbered. Each M / E section can be selected as a source in the P / P section and in every other M / E , which makes the mixer much more versatile, as effects and keys can be created "offline" and then " on the air ”.

After the P / P section there is another key level called Downstreamkeyer | DSK . This is mostly used for text overlay from font generators or graphics (e.g. so-called belly bands ) and has its own "Cut" and "Mix" buttons. The advantage of the classification at the end of the mixer section is that before the downstream keyer there is a clean, i.e. H. Free image can be taken from all shipment-specific graphics, the so-called cleanfeed .

After the downstream keyer there is a last button, usually called FTB or Fade To Black . This acts as an "emergency button" in the event that an unrecoverable mixer state accidentally occurs and an unwanted signal is present at the output. This button then blends the output onto a black screen.

Modern video mixers can have additional functions, for example remote control of decks (via a general purpose interface ), aux channels for assigning sources to additional outputs, macro programming or saving the current mixer status ("snapshot").

All large mixers also have a so-called tally output for each source . This carries a signal as soon as the source (completely or only partially) is switched to the output of the video mixer. This signal is also used to control the red light on the cameras .

Construction and wiring

Since image mixers combine different image sources such as decks and video cameras , it is very important that all of these sources are properly synchronized. In professional facilities, a synchronous generator generates the clock signals for all other devices. Synchronicity can also be established by the vision mixer sending a clock signal to the connected devices ( genlock ). Signals that cannot be synchronized (either because they come from outside the facility or because the device does not support external synchronization) must go through a time base corrector . Some mixers have internal "frame syncs", if not, an additional device is required. If the mixer is used for video editing, the editing control (which usually controls the video mixer remotely) must also be synchronized.

Most of the larger image mixers separate the control panel of the image mixer from the actual device for reasons of volume, heat generation and because of the shorter cable runs. The control unit is in the control room, while the main unit, to which all cables are connected, is installed with the rest of the devices in the central equipment room.