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May 2008

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Getting Video from Here to There: Switchers and Scalers

by Kent Morris

Production switchers are open to more creative transitions and a blending of multiple sources at once. Thus, a church video booth may contain both a production switcher for blending many cameras into a visual story and a presentation switcher to feed into the house video projectors.

Flown from the ceiling on a spider mount, the gleaming new projector is a marvel of technology. Just as impressive is the recently installed retractable wide-format screen. But, still, the image leaves something to be desired. As much as high lumen output and contrast help an image, they matter little if the source material is left un-scaled and un-switched. When individual cables are sent from each video device to the projector, the operator must use the remote control to scroll sequentially through the inputs until the correct source is located. The projector’s internal scaling engine must then adapt the various signals (DVD, camera, and computer) to the projector’s preferred display level. In even the most advanced projectors, the internal scaler is slow and ungainly. An outboard video scaler takes a low-level image, such as an SD (Standard Definition) camera signal, and raises it to the native resolution of the projector, improving the signal’s quality dramatically. Native resolution refers to the projector’s comfort zone, where the incoming signal matches the projector’s display structure perfectly. So, for example, if a scaler is used to increase an SD camera’s output to match the XGA (1024 x 768 pixel) panels of an LCD projector, the projector can simply pass the signal through without working to conform it to its native format. With an external scaler, the image will then be as clear as possible and the camera will appear to be of higher quality than it is.

Switching involves sending different signals to the projector from a single point, as in a live multi-camera shoot, where one moment the entire sanctuary is onscreen and then the image fades into a close-up of the pastor speaking. In a live camera environment, I-Mag (Image Magnification) serves to bring the congregation closer to the minister and smooth switching among the camera shots is vital to accomplish the goal. Camera switchers, with their familiar T-Bar transition device and numerous wipes and fade effects, are distinct from presentation switchers where the need is for simple selection among inputs. Camera switchers, also known as production switchers, are able to blend multiple images at once and offer creative opportunities for artistic expression.

A presentation or AV switcher for the projector, on the other hand, is typically only populated with a series of pushbuttons for the inputs. Presentation switchers can be thought of as clear-cut decision makers, with an image from DVD playback switching to the speaker’s Power Point slides. Production switchers are open to more creative transitions and a blending of multiple sources at once. Thus, a church video booth may contain both a production switcher for blending many cameras into a visual story and a presentation switcher to feed into the house video projectors.

A scaler with an integrated switcher will improve the final image by allowing the operator to change the selected input without annoying glitches or an unattractive “blue screen.” Switchers can range in quality from modest to exceptional, with lower cost units generally offering only three inputs and a “seamed” approach to changing images. As the images are selected, the scaling switcher must synchronize each output to maintain a stable image. When cost constraints force the use of a seamed switcher, the output is usually blanked or faded to black until the sync is stable, a period of perhaps one to two seconds, then the image is displayed. Higher end units will sport seven or more inputs and deliver a seamless transition from input to input, meaning there is no fade out or vertical roll when a different source is selected. The change from one input to another is dissolved or wiped in perfect lock.

Unlike audio, where termination connections are universal across a few connector types, video signals can use any of a dozen or more terminations in several formats. For instance, from lowly composite signals to high resolution YPbPr analog component video, the RCA connector is used, but component video can also flow down individually terminated BNC (British Naval Connector) lines or be bundled together inside an HD-15 headshell connector, commonly termed a VGA (Variable Graphics Array) since it is found on computer display terminals. In between composite (one cable to carry all video information) and component (separate lines for each aspect of the video signal) the S-video connection, with its four-pin DIN (Deutsche International Normal) terminator foregoes the color separation of RGBHV (Red, Green Blue, Horizontal and Vertical sync) found in component runs in favor of chroma (color) and luma (brightness) divisions on the internal cabling. Some scalers attempt to include all the possible terminations on its rear face while others offer a series of HD-15 connections along with several breakout adapters for the interfaces.

With these numerous input types and format varieties, a scaling switcher must be able to accept and recognize the signal, route it internally and then display it at its optimum level. Though a scaler appears simple, it must perform a myriad of functions without introducing latency, a delay based on the time needed to compute and generate the processed signal. In video, latency is often measured in frames, with about 30 frames equaling one second of display. With image magnification (IMAG), I/O (Input to Output) latency of more than five frames can cause a visible decoupling of the image onstage and onscreen, so a scaler with three frames of latency or less is preferred if the pastor’s waving arms on the screen are to remain connected to the congregation’s view of the arms on stage. As part of its duties, a scaler multiplies the number of lines of a video image to create a denser, more appealing display. The increase in resolution and brightness offered by a line doubler or quadrupler is impressive but must be balanced with the need for low latency. Generally speaking, a scaler with a stronger scaling engine will exhibit lower latency. Computer signals received at the scaler will pass through unaltered, assuming the resolutions match. Automatic film to video conversion, also known as 3/2 and 2/2 pulldown, is included in most mid-range and higher scalers along with some provision for wide format images, such as 16:9, on both the input and output sides of the processing. Depending on the model, the scaler can take a standard NTSC (National Television Standards Committee) 4:3 format TV image (four lines vertical by three lines horizontal) and convert it, though not perfectly, to a movie-style image.

Scaler/switchers may also include audio inputs and outputs, usually on bare wire Phoenix-style connections. For DVD clips and tape playback of announcements, the audio-follow-video feature is essential to segue smoothly from one segment to another without patching each video source’s audio through the mixing console and forcing the audio operator to fade the signals in and out at the correct time. Additionally, some upmarket scalers boast a titling program designed to add words over the video sources as they travel outbound to the projector. Though not as effective as a stand-alone lyric display program, the internal titler is useful for name display and nursery alerts.

Scaler/switchers can make the difference between a marginally acceptable image and a stellar display of all the camera, DVD and computer images flowing deftly through the video operator’s control. When combined with a reasonably strong projector, the scaled visual message will be conveyed with power and effectiveness.