Technology Showcase: Video Format Converters

Jan 1, 2004 12:00 PM, By David Riffle

A video format converter (VFC) is a utility tool designed to convert video from one format to another. It is simple enough in concept, but because video formats are intended for a multiplicity of applications and usually differ substantially from one another, performing an acceptable conversion is quite demanding. Video formats differ in line and pixel count, frame aspect ratio, color space, signal levels and dynamic range, as well as in other aspects. A high-quality VFC is expected to translate each signal parameter of one format into an accurate representation of that parameter in another format.

WHY CONVERT?

However accurate and seamless this translation may be, each format has its own display or transmission characteristics, and therefore any video image will have a different appearance when it is converted to another format. It will seldom look better as it is passed through a conversion, even if that conversion is perfect. That is because a video image is normally designed to look its best in its original format. The best that any conversion process can hope is to maintain the design appearance of the source video and not to introduce any picture or signal degradation.

Why convert video signals then? A general rule is: don't. Design your graphics and video elements, shoot programs, edit, and obtain video material in the format you need. Usually, it is the display method that will determine the format requirements, but many times it will be the method of signal transport or the playback options at your disposal. Whichever it is, you should make a decision as to which will be your video medium and keep all of your source and master material in that format.

If you are fortunate enough to control the process of video origination, editing, distribution, and display, as well as the equipment involved in each step of this process, then you should have little difficulty selecting and maintaining the appropriate video format. This can be the case in a television studio environment, for example. Usually, though, you will find that you have a variety of video source material comprising several formats and that you need to distribute this material for use in a selection of formats or maybe to a selection of display types. When this is your situation, a VFC is what you will need.

WHAT ARE VIDEO FORMAT CONVERTERS?

Video format converters encompass a broad category of hardware and software. This discussion will focus on hardware rather than software. Conversion from one format to another of greater resolution is referred to as up-conversion, and true up-converters perform just that: up-conversion. Devices that are intended to up-convert, cross-convert, and sometimes down-convert to a limited extent are known as video scalers. The primary purpose of those devices is to adjust the format characteristics of the video source material into the proper format needed to match the characteristics of a particular display device. This article will discuss the aspect of video format conversion that pertains to scaling source video for proper compatibility with a selected display.

With the proliferation of digital video signals and formats in the past decade, especially during the past five years, the issue of video and display resolution has become increasingly prominent. Analog signals are inherently unlimited in resolution, having limits set only by the record and playback devices as well as the transmission and display mediums. But digital signals contain finite units of information, and digital video images can be mapped onto the screen of a display device rather than “painted” onto the screen as is the case with analog signals. For this reason, it is important to map every video source onto the screen in the same manner and to choose a single display format that matches the design parameters of the display device. In doing that, you will ensure that your display is as good as it can be. You must also ensure that the conversion of each source format to your chosen display format is as accurate as is reasonably possible.

DISPLAY BASICS

If you look first at the display devices available, there will basically be two types: analog and digital. An example of an analog device is a CRT TV monitor, which has analog component or composite video inputs, or digital inputs, which will be converted to analog signals internally. This type of monitor will process the input format into a form suitable to drive a CRT display, and then it will paint the images on the screen in a mostly continuous manner. The CRT has a grid of phosphors across its screen that are illuminated to create the displayed images. The phosphors have a finite size and dimension and are not continuous, so in this way, they resemble a digital display. The number of phosphors on the CRT must be great enough to faithfully represent the amount of detail in the source video, but there is no theoretical limit to their number beyond that point. The resolution of the video format itself is limited in the number of vertical scan lines, which will become more visible as the screen size is increased. An analog input is a smooth, continuous flow of information and can be painted onto whatever size display is available.

In contrast, a digital signal has distinct units of image data and a limit to the amount of information in each data unit. A digital display has inputs for digital component or analog signals but then converts each into a digital data stream suitable to be loaded into the display screen. This type of display can be thought of as sending video data directly to each pixel element, which is similar in function to each phosphor in a CRT screen. Examples of digital displays are plasma and LCD monitors. The optimum image a digital display can produce will result when the video aspect ratio matches the display screen dimensions and when the input video signal can be mapped directly onto the screen without any loss of data owing to an incompatible number of screen pixels and video data “pixels.” If a digital display is presented with a video format that has a different frame aspect ratio and pixel resolution, it will not be able to map the image pixel for pixel and will produce a distorted display image, if any at all. Video scalers are designed to avoid this predicament by scaling the source video format image in whichever direction required to match the display screen resolution. This may involve some aspect ratio conversion and frame rate adjustments, but the end result will be an image that will display without distortion.

GENERALLY SPEAKING

In many applications such as boardroom or trade show presentations, the source video material will likely be an assortment of computer graphics, component video from various types of players, and video played directly from computer sources. Each of these sources will likely have different resolutions, video levels, and aspect ratios. Therefore, a video scaler that can take in each source, process and equalize the video levels, and then output a single resolution and format that is matched to the pixel resolution and aspect ratio of the intended display is just what is needed. Ideally, that video scaler would also be easy to configure, have a clear menu structure, have the most common type of connectors on its inputs and output, and possess the capability for a wide range of input formats. If you want to apply it to different types of displays at other times, then it should also possess the capability for a wide range of output formats. It may also need to transition between the sources rather than cut from one to the other, and it may need to store various configurations in memory.

Begin by determining whether your need is in a fixed application such as a home theater or a flexible application such as a trade show and then take an inventory of the source material, player formats, connector types on each, and especially the type of display you have, along with its display resolution and aspect ratio. If you have a fixed application, you can limit the input source compatibility range to those that you currently have, and you can limit the output resolution of the video scaler, as well. In this type of application, you probably will not require the ability to transition between sources, but you may need to be able to process the source video levels. You will know exactly which types of input and output connectors are needed. Make some allowance, however, for expansion or for equipment replacements as new technology manifests itself.

If your requirement is for a flexible application, then you must carefully consider the input and output format compatibility ranges and the number of inputs and outputs that you expect to encounter. Source transitions will probably be necessary, as well as aspect ratio adjustments. The video scaler will need to provide you with a lot of flexibility in an easy-to-manage user interface. This type is one of the universal scalers. Essentially, you may want to acquire a video scaler with as many features as your budget will allow so that you will be ready for any contingency.

SPEAKING OF FEATURES

There are several features that video scalers can include that will have a significant impact on the displayed image quality. Line doubling, tripling, or quadrupling; deinterlacing and 3-2 pull-down removal; and motion compensation and video processing have pronounced effects. Many video scalers are actually matrix video routers, but most are multi-input, single- or dual-output video switchers. Usually you will need audio to follow your video switching, and in the case of live event applications, you may need a mic source or a way to key a logo over the output. Depending on the display application, you may need to rotate the image and sometimes display multiple images simultaneously. Remote control is important in many cases, and sometimes this will need to be done from a computer application rather than an IR controller. It will be important to consider whether you will want to store your scaler configuration settings for later recall or for an interruption in line power. Similarly, it may be necessary to upgrade the feature set on your scaler so that you can correct shortcomings or adjust to the developments that new technology will bring. Each video scaler on the market has some or all of these features, so just as with any other major equipment purchase, put a lot of thought into the evaluation of your expected application and future needs. In every case, reliability is essential.

You should evaluate your video scaler carefully and judge its output signal quality with a variety of video material displayed on the type of monitor or projector you intend to use. Look at material with sharp-edged computer graphics, scrolling and rolling text, nature scenes with both rich and subtle colors, and scenes with sharp detail. Verify that all the source video formats you need are managed to your satisfaction and that you have all the adjustment range you feel is appropriate for your choice of output format, such as video processing, image zoom, and aspect ratio adjustments.

The manufacturer of your video scaler should be responsive, easy to reach, and ready to provide technical support if needed. Remember, the video scaler is what will make your source material look the way you intend, every bit as much as the display itself.

MANUFACTURER RESOURCES

Video scalers are available in three primary categories: home theater, presentation, and staging and rental. There is some overlap, of course, but generally, a home theater will not require as much flexibility from a video scaler, whereas a live event will probably require as much as possible.

A small-scale, single-purpose scaler can be compact and finely tuned for a limited number of source and output formats, and it does not need elaborate configuration interfaces. These devices include plug-in modules for direct insertion into plasma displays and compact standalone units that can be remotely switched from one input to the next. Altinex, Analog Way, Communications Specialties, Extron, Folsom, Kramer, and RGB Spectrum market devices of this sort, most of which scale analog composite, S-video, and component video inputs to analog component HD video resolutions of 480p, 720p, or 1080i and VGA, SVGA, XGA, or SXGA resolutions, providing for video level and aspect ratio control in the process. Remote control is included in many models. Aurora Multimedia makes four types of direct plug-in scaler cards for Pioneer and Hitachi plasma monitors. The inputs to these cards include DVI and output into the plasma at 720p or 1080i. The ASR-44 has a picture-in-picture (PiP) generator and audio-follow-video capability, and it can rotate, stretch, and crop multiple images at once. It also features significant video signal-processing controls, as well as audio propagation delay control to keep the audio in sync with the video as it is processed. The X-Tune+ includes RF inputs, dual tuners, motion compensation, and DVI connections. Key Digital Systems makes the KD-PIO768P direct plug-in card for the Pioneer plasma monitors, which includes DVI and SDI video signal inputs. Focus Enhancements' CenterStage line, the CS-1/2/HD, includes half-rack-size scalers that have digital input and remote control and are compatible with Crestron and similar controllers. Silicon Optix markets the Image AnyPlace, a scaler that has sophisticated keystone correction circuitry, which can manipulate the output for use with a projector display. Although the scaler is not limited to projector displays, this feature allows a projector to be used at an angle of as much as 40 degrees horizontally and 30 degrees vertically from the display surface. The Image AnyPlace can correct for a number of optical distortions, including those produced from the effects of surfaces that are curved or irregularly shaped.

Slightly larger and with more inputs and format choices are the video scalers designed primarily for presentation applications. These scalers will often feature multiple outputs and are usually more flexible in their ability to process images, especially when it is necessary to control conversion artifacts. Among these are the universal video scalers. Astro Systems, Communications Specialties, Extron, Folsom, and RGB Spectrum each have high-performance video scalers, well suited to presentation applications. Analog Way has the OCP-802, an 8-input, audio-follow-video with mic input universal scaler that can input analog PAL, SECAM, and NTSC composite video, S-video, component, and RGB and VGA through SXGA signals. It also has two boosted HD15 outputs scaled from 640 by 480 to 1280 by 1024, along with stereo audio. The TSC-550 and TSC-432 are similar, but less of universal switcher/scalers. Aurora Multimedia's DIDO has PiP, stretch, rotate, image crop, and audio-follow-video capability. The DIDO has audio propagation delay, motion adaptive image deinterlacing, and readily upgradeable firmware capability. FSR has the MAS-4100A, which is a 4-by-1 audio-follow-video switcher with an integrated scaler. It is capable of scaling analog composite, component, and S-video to user-selectable output resolutions up to 1080i HD, and it includes remote control. Key Digital Systems has the KD-DH12, which includes a digital tuner and distribution amp with 12 separate outputs. The KD-HD1080P is a digital in and out SD/HD video scaler/cross-converter. TV One makes the CSC-1600HD, a high-performance scaler capable of analog composite and component through HD 1080i input resolutions. The output can be to HD15 or BNC connectors, and the unit provides for deinterlacing, motion compensation, and signal-level adjustments. It has a variety of local and remote control possibilities, and like many of this type, it uses an onscreen display menu for configuration. The CSC-1100A is lower-cost model and has fewer features.

In the staging and rental category, several scalers are designed for the particular needs of live event switching and scaling. These units are usually much larger than any in the previous categories, and they typically include more inputs and multiple outputs. Often they add a separate mic input and the means to key video source material. Altinex, Focus Enhancements, FSR, Folsom, and RGB Spectrum market devices in this set. TV One's C2-770, for example, is a universal switcher/scaler that includes a frame synchronizer, TBC, aspect ratio converter, image genlock, PiP generator, chromakeyer, and lumakeyer.

THE MISSING LINK

Video scalers are a vital link between source video and digital display devices. In fact, any display larger than 38 inches is large enough to benefit from the image enhancement tools that a video scaler can offer. But with digital displays like plasma and LCD monitors and newer projection systems, the best display can be realized only if you carefully match the resolution and aspect ratio of the source video to the display characteristics. Many of the source video formats will benefit from image processing during the up-conversion process, as well. With the right video scaler, you will be very pleased with the display results!

David Riffle is the CTO for Video Arts, a San Francisco video post-production facility. With more than 20 years experience in the video industry, Riffle has worked in broadcast and post-production. He can be reached at Driffle@vidarts.com.