Figure 1. Simple monitor stack arrangement. Click here to see an enlarged diagram.
The growth in video and broadcast services over recent years has presented many challenges to broadcasters and service providers. One such challenge is the need to monitor and observe an increasing number of video sources within a limited space.
This has led to the emergence of virtual monitor walls. These walls consist of large-screen projection or flat-panel displays married to multiple image display processors to render a large number of video sources on a single wall. They offer an alternative to the traditional cathode ray tube (CRT) monitor stack, which requires a separate CRT for each source. (See Figure 1.)
The early driver for such systems was simply to save space by showing a large number of video sources in a limited space. (See Figure 2.) This is evolving to become a complete visual information management system, where the operator is presented with not just video but other information such as tally, Under Monitor Display (UMD), audiometers and alarm status on the same display and in an integrated fashion. The analog clock on the wall is even becoming “virtual” through incorporation within the display wall.
Figure 2. Equivalent virtual wall display. Click here to see an enlarged diagram.
It is this type of flexibility that has helped its popularity as a monitor stack replacement. With its ability to be reconfigured at a button press, the virtual monitor wall is flexible when compared with a CRT monitor stack. There are additional benefits in terms of space used, reduced power consumption and heat generation, as well as cost. The ability to add more sources and to easily upgrade the display wall or to enlarge one of the virtual monitors is useful to a facility that has changing monitoring requirements. The traditional CRT monitor stack would otherwise require the addition of more shelving along with new CRT monitors.
While the concept of the virtual display wall seems to be a panacea to all visual monitoring problems, it also presents its own challenges, and it should be evaluated against an installation's requirements.
Issues that affect the quality of the display wall depend on the type of display used, as well as the processing that is performed on the source. Important aspects in choosing a particular display are pixel resolution, size, color fidelity, contrast ratio, serviceability, lifetime, power consumption and cost.
The most popular displays that are in use for these types of applications are flat-panel displays (FPDs) and rear projection. They offer several advantages over CRT such as flicker-free display, minimal geometric distortion and power savings, although CRT remains vital for quality monitoring. FPDs save space and are ideal for space and cost -critical installations. They are based around Plasma or Liquid Crystal Display (LCD) technology, with LCD offering long life and resolutions up to 1920×1080 on a 46-inch display.
Plasma, on the other hand, has the benefit of lower cost, up to 65-inch diagonal and a mature technology. However, the technology suffers from burn-in over time, and native resolutions are limited to a maximum of 1366×768. Rear-projection technology is generally based on LCD, Liquid Crystal on Silicon (LCOS) or Digital Micromirror Device (DMD). These offer the advantage of large display size, high resolution and the ability to be stacked almost seamlessly. Each of the underlying technologies has its own advantages, and the choice usually comes down to user preference and cost.
The control room of Spain’s Retevision features 10 Zandar FusionPro MultiViewer systems, which display more than 160 video signals and four VGA input signals.
Reliability and serviceability
The display processor (sometimes referred to as a multi-viewer) lies at the front end of the monitor wall. This typically accepts a variety of analog or digital video sources along with computer sources and processes each source for rendering to one or more displays.
The processor also incorporates functions such as audio meters, UMDs, tallies and alarm condition monitoring to the display to give a data-rich visual environment. The important considerations when choosing a display processor are the number and type of inputs supported, reliability, serviceability, image quality, flexibility, ease of use and compatibility with the display.
Video window display processors and accompanying displays have MTBF rates comparable to other broadcast equipment. However, the possible failure of even one display will result in the loss of multiple picture sources. It is important to build in redundancy in mission critical applications. Single-point-of-failure analysis needs to be carefully thought out, and backup strategies through redundancy or dynamic reconfiguration need to be considered.
Image quality considerations
There is inevitably a tradeoff between the redundancy designed into the wall vs. the cost. Multiple processors and displays can be used to re-direct sources to an alternate display through dynamic reconfiguration of the wall. Preset layouts can be used in the event of a processor or display failure. Many topologies can achieve varying degrees of redundancy while minimizing or eliminating single points of failure, although the number of risk factors rises significantly when considering processors with multiple inputs and outputs.
Projection systems require the occasional lamp replacement, with some projectors supporting redundant lamps. With FPDs, the design of the installation should allow easy replacement.
To give the best image quality, it is important that the display device is driven at its native resolution. Otherwise, the FPD may scale the image internally, giving an immediate loss of quality, as well as potential distortion of the picture aspect ratio.
Control and operation
Another key consideration in 50Hz regions is how the displays handle 50Hz frame rate sources. It is important that the display genlocks at 50Hz where 50Hz video sources are displayed. Some displays will internally convert the frame rate to 60Hz. This causes undesirable frame rate conversion artifacts. Most digital displays operate in progressive scan mode, yet most video sources are interlaced. Here, the window display processor must de-interlace each video source and do so without introducing visible distortion. As there are many de-interlacing algorithms, both good and bad, it is a key quality consideration, as is the quality of scaling applied to each individual source.
The ideal number of windows to view on a single display is a subjective judgment, with ergonomic factors such as size of display, resolution and operator viewing angle to be considered. For example, on 40- or 42-inch FPDs, it is common to display 12 to 16 sources and still observe a reasonable degree of detail within each source.
The control of a virtual monitor wall is by means of PC-based control software and/or a remote panel. The software generally allows editing of the wall layout to control video window orientations and what other data is to be displayed. During operation, the wall configuration can be recalled from presets that were created offline. The control software varies between manufacturers but shares many common features in terms of window editing, setup and layout recall, and alarm configuration.
The virtual monitor wall is an exciting and evolving solution to the problem of monitoring the increasing number of sources and accompanying services. It provides multiple benefits in applications such as OB trucks and master control rooms, where space is limited and flexibility is required.
The future should see improvements in the technology with higher quality and resolution, as well as adding intelligent management to the virtual wall and tighter integration with other equipment. More features and functions normally found in external equipment are finding their way into the processors to detect and measure anomalies in the video and data sources. The concept is broadly accepted, and the benefits are widely recognized by broadcasters who need to maximize the quality of service for the facility.
Killian O'Sullivan is technical director for Zandar Technologies.