Picture This: Cutting the Cord
Jul 1, 2004 12:00 PM, By Jeff Sauer
The InFocus LiteShow interface with the LP600 projector.
If you've been following the projector industry, especially personal travel-size models, you probably know that “wireless” is one of the day's hottest features. It's not hard to understand why if you imagine road warrior presenters attempting to tactfully carry on a conversation, set a proper mood for their pitch, and remain cool all while fumbling with setup and cables between their projector and notebook computer. Booting up and starting the show is potentially a lot more elegant and is a lot of what wireless offers.
It's no surprise, then, that several projector manufacturers have come out with wireless interfaces between computers and projectors. These generally consist of an 802.11b, or WiFi, PC Card in the computer talking to, most often, another PC Card inserted into a slot in the projector (though there are a few unique solutions out there).
WIRELESS SOLUTIONS
For example, InFocus's LiteShow interface plugs directly in to the projector's M1-DA connector. It's an intuitive solution because that's where the VGA cable would otherwise attach were you to traditionally tether the projector and computer together. About the size of a small cell phone, the LiteShow module is a little larger than a PC Card, but building the 802.11b receiver into an external module allows InFocus to use it with smaller travel projectors than those needing to house a PC Card slot. The LiteShow is an InFocus product that first and foremost augments the company's products, but it is not an exclusive solution and can work with other projectors that use the same M1-DA connector. However, though there are a few others, M1-DA — equipped projectors are a strong minority in the market.
OTC Wireless, on the other hand, makes a product called the WiJet, which is similar to the LiteShow except that it features a standard 15-pin RGB port instead of the M1-DA connector. The WiJet is also an external unit rather than a PC Card that would go into a slot, and it uses 802.11b as the communications protocol.
Of course, if you have followed the trend toward wireless connections, you probably also know that these 802.11b interfaces come with something of a performance penalty. Images tend to be a little softer. Color range sometimes isn't quite as full. But most obviously, latency causes projected images to appear as much as three to four seconds after they would appear on a local monitor, such as on the notebook's built-in display. Mouse movements typically show very little, if any, perceivable delay, and adding bullet points to PowerPoint slides generally happens within a second. However, advancing entire slides and especially opening new windows on the desktop can take from one to four seconds to appear, and that can be a little disconcerting to a presenter.
What's more, though 802.11b can reach up to 11 Mbps — more bandwidth than the average DVD movie — it is not yet an effective solution for video content. Indeed, some early 802.11b wireless solutions simply choked when trying to play motion video. The reasons are twofold: first, actual 802.11b throughput tends to be only around 4 to 5 Mbps; and second, the type of compression used to send a potentially enormous amount of onscreen data to the projector is not as efficient as the native MPEG format of DVD video.
MPEG-2 needs to compress “only” 30 frames/60 fields of 720-by-480 images every second compared with much of the higher resolutions and refresh rates of most computer displays. MPEG also uses highly optimized temporal compression with bidirectional motion estimation and motion prediction algorithms to do it. Typical wireless projector solutions tend to use Vector Quantization compression, which can be efficient on the larger blocks of identical colors that generally appear on a computer screen (like the white background of this page or the blue background of a PowerPoint slide). However, this compression method is far less effective with the constant interframe and intraframe variations that are the norm in video. Yes, wireless projector solutions do leverage temporal compression, sending, for example, only the area of mouse movements rather than the entire screen, but it is not as advanced as that of MPEG.
BETTER BIT RATES
What if those wireless solutions could move more data? It would be no surprise that performance would improve. At least a couple wireless companies are doing just that.
Avocent Technology LongView Wireless Extender
Avocent Technology, best know for its KVM switches, offers the LongView Wireless Extender ($995), a transmitter unit that has both a local monitor out and a wireless antenna bundled with a receiver unit for a remote display. (A similar, newly announced AutoView Wireless product adds one-to-many KVM switches.) It uses standard 15-pin ports to connect the computer, the local monitor, and the remote display, so it's a little like the aforementioned WiJet, though it also sends wireless keyboard and mouse data. However, instead of 802.11b, Avocent has built the unit around the much higher bandwidth 802.11a standard. That standard uses the underused 5 GHz frequency band rather than the increasingly crowded 2.4 GHz band of 802.11b.
With 54 Mbps (about 27 Mbps actual) of headroom rather than just 11 Mbps, the LongView Wireless Extender has a lot more data to play with than 802.11b solutions. The result is almost no visible latency (less than one second for even dramatic onscreen changes) on computer source material and a lot better performance with motion video.
Admittedly, motion video quality is still not what you'd get from a cabled solution, because Avocent must still recompress the analog RGB data coming from the computer in order to send it wirelessly to the receiver, even if the video started as MPEG-2. That's a compromise but one that allows the LongView Wireless Extender to function with straightforward computer data, motion video, or, as it often is the case, a combination of data and video or other moving images. For example, a presentation might have video in a window of a PowerPoint slide. Or perhaps a digital signage application might have an information scrolling ticker at the bottom of the screen or switch back and forth between video and data. That sort of application is likely one of the uses that Pioneer Electronics and Avocent have in mind for the recently announced LongView module that will slide into a Pioneer plasma monitor expansion slot, thus creating an effective “wireless plasma.”
OTC Wireless, by contrast, is about to ship another WiJet product, the WiJet.Video ($799), which uses a standard PC Card in the source computer that can effectively separate the Windows screen information for any MPEG video that might be playing in a window or at full screen. Similar to how Windows uses Active X controls to map motion video over a section (or all of) the desktop, OTC's WiJet software (installed on the source computer) sends native MPEG video over a different WiFi channel or port than the Windows VGA data. The WiJet device then reassembles the VGA and video data again on the remote display side, overlaying the video in the appropriate section of the screen. Using that approach means the video does not go through a cycle of being uncompressed and recompressed, thus reducing likely cascading artifacts and maintaining image quality.
Unlike the other WiJet hardware, the WiJet.Video uses the 802.11g protocol, which achieves a similar 54 Mbps theoretical bandwidth to 802.11a and an actual bandwidth of between 20 and 27 Mbps, depending on traffic (802.11g uses the same 2.4 GHZ as 802.11b). The WiJet.Video receiver includes an MPEG decode chip to allow the video to be displayed either on a data display or through a separate composite output (S-video or component with an adapter) to a video monitor, albeit without Windows desktop information.
LOOK MA, NO CABLES
All of these wireless solutions show that cutting the cord and eliminating the tether between the computer and the display have great appeal. Whether it's that hypothetical road warrior, a digital signage application, or just a clearer installation without unsightly cables, no one likes to be tied down.
Jeff Sauer writes the “Picture This” column for Sound & Video Contractor and is a contributing editor for Video Systems. He's a video producer, an industry consultant, and director of the Desktop Video Group, a video and computer products testing lab in Cambridge, Massachusetts. He can be reached at jeff@dtvgroup.com.
Acceptable Use Policy blog comments powered by Disqus
