The Production Switcher — Vision Mixer

Whenever you have several video sources (e.g. cameras,VTRs) you need a production switcher/vision mixer in order to select and blend their pictures. (The sound from these sources is routed separately, to the sound-control desk.)

The basic switcher is simple enough to operate, but designs range from rudimentary, to highly sophisticated devices incorporating various video effects. There are even automated switchers that can be preprogrammed.

Where the director is preoccupied with guiding talent,camera moves, cuing, etc., and switcher operations become complex (video effects, combined shots), it is better for a separate person to concentrate on its operation— usually the switcher/vision mixer, or the technical director.

Switcher operation

• The simplest switcher has a row of cut buttons which enables you to switch between various video sources or ‘channels’ (cameras, VTRs, film, slides, etc.).

The button you select on this program bus or bank will light up when pushed, and put that source ‘on air’.At the same time, its picture appears on the main color monitor (termed line, master, transmission or studio out) The output of the switcher goes to be videotaped — or transmitted if the show is live.

Interswitching produces cuts between picture sources.

• In most switchers, there are two identical rows of these cut buttons. Which row Is operative, ‘ A-Bus’ or ‘B-Bus’, is determined by a large communal fader lever (bus-fader), that cross-fades between them. If this fader is over-to ‘A-Bus’, ‘B-Bus’is in‘standby’mode.

Punch up Cam-1 on A-Bus, and Cam-2 on B-Bus, and as you move the fader from A-Bus to B-Bus position, the screen will mix (dissolve) from Cam-1 to Cam-2’s picture. Stop part way, and you have a superimposition,

• A further row of buttons (Preview bus) allows the picture from any source to be checked on a color switchable preview monitor. (The individual picture monitors for each source are usually monochrome.) Switching on this preview bus does not affect the studio output in any way.

These are just the basics, but even the most complex switchers are really variations on this theme.

Additional features

Larger switchers also include many refinements such as:

• Effects buses, or mix banks to control wipes, insets, etc.

• Interswitching or mixing groups of sources.

• Color synthesizer selection.

• Picture manipulation;e.g. negative/positive, inversion,mirror, ripple.

• Title edging (‘edge generator’).

• Title insertion (‘downstream keying’).

• A chromakey (CSO) system.

• Remote control for VTRs, film, slide sources.

The production switcher–basic operations

MIX (DISSOLVE) between Cams 2 and 3. .Select 2 on A-bus. It is now on line (on air) as the bus fader is at A. Prepare for a mix, by pressing 3 on B-bus.

Push down bus fader(s) from A to B, and output mixes from Cam 2 to 3. Move fader to A-bus: output mixes back from Cam 3 to 2.

SUPER CAMERAS 2 and 3: Either as for a mix, stopping when superimposition strength is-as required, or split faders, moving appropriate lever towards the bus to be added.

FADE OUT CAM 2 (to black screen).Either split faders pushing A-bys lewer to OUT (B-bys is already faded out); or where the system would lose chroma using this method, select special black-level button in the other bus, and mix to black.