digital video mixer theory

[relicmarks]

Whats the different between a video mixer VS a optical printer?
What does the bussing and "aux" channels do on the video mixer?
What can i use the aux sends/returns for on the video mixer?

[RFBurns]

Howdy!

An optical printer uses a laser beam and optics to print or scan, a video mixer takes multiple video sources and allows you to do effects between the video sources such as transition from one source to another without loosing syncronization between the sources, perform effects like wipes, fade to black, hot-punch and chroma-key effects, which provide a way to insert backgrounds and such, much like a weather reporter as they stand in front of a blue or green colored wall. That is how the weather maps appear behind the reporter.

The bussing and aux connections allow you to take those video sources and channel them through external equipment and back again to the mixer. The external equipment can range from video processors such as TBC'c, frame sync's, AD-DA converters etc. Think of an audio mixer, the video mixer does the same functions as its counterpart, allows mixing of the different sources.

[relicmarks]

Can u please give more examples of how to use digital video mixers
and what they are used for please?



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[relicmarks]

What kind of extenal processor or gear would i use in the AUX channels?

[olyteddy]

What kind of extenal processor or gear would i use in the AUX channels?

Keyers and Compositors mostly.

[relicmarks]

What kind of keyers and compositing processors?

What would u use keyers and compositing for and how using a digital mixer?

[RFBurns]

What would u use keyers and compositing for and how using a digital mixer?

Keyers are a way to place backgrounds and foregrounds in a video signal. For example, when you see text on the screen such as credits at the begining or end of a program while there is still active or still video in the frame, the text is "keyed" over the video, so that the text appears "in front" of the video. It also works in reverse, where as you can place the text behind video.

A good example is a weather reporter standing in front of a weather map pointing out the clouds and temperatures. The weather reporter stands in front of a painted pannel, usually green since green gives a better key than blue, but keying can work with either color. Red is not used since red is such a "hot" color, keying can become difficult especially when trying to balance the key signal with whatever kind of set lighting is present. And its much easier to "key" in flesh tone colors with a green or blue chroma key board than red since red is close to normal flesh tone colors. If we used red, the weather guy or gal will fade or "mesh" in with the bacground weather map image, as if the person being keyed is almost like fading out or being "teleported" out of the picture.

Keying can take place either "upstream" or "downstream", meaning that a signal can be "keyed" or inserted before or after. For example, lets take our weather guy there and put him in front of his "chroma-key" board. The director then punches up the camera on a "key-buss" on his switcher while also punching up the weather map. As the weather guy stands in front of the green or blue "chroma-key" board, the director adjusts the "key clip" control on the switcher/mixer so that the weather map now appears behind the weather guy. When adjusted, it will appear that the weather guy is standing in front of a large map on our tv screens. This is "upstream" keying.

Lets also put text at the bottom of the screen that gives us the weather guy's name. Since we dont want to disturb our previous keying adjustment we will insert his name on another "key-buss" on the switcher/mixer. The text usually is generated on a CG or Computer Graphics machine. If we were to just look at the CG's output directly, it will be white text on a black background. Since we are not interested in the background from the CG machine, we adjust the "key-clip" on the 2nd "key-buss" till the white CG text appears. The text will look like it is in "front" of our weather guy and his weather map. This is known as "downstream" keying.

Send and return lets us make changes to a source all by itself via an outboard piece of equipment. For example, lets insert a still background image of a sunset on the horizon. We can put that on input 1 of our mixer. However the picture needs a bit of adjustment such as brightness or color enhancement. We can use the input 1 aux send/return to send this image to a video or image processor to "tweak" it for brightness and color. Then we take that processors output and send it to the mixer's input 1 return port, and WALA, we now have an improved image at the mixer's input 1 while not changing other signals on the other inputs of the mixer.

If we only want to adjust or change input 1's signal and leave the other mixer input signals as they are, then we dont want to put our processor at the mixer outputs just to change the signal at input 1 while trying to keep all the other inputs as they are. Thus the send/return feature lets us change that signal on input 1 and not affect the others or the mixer's output.

Hope this helps.