Girth rites

When it comes to huge screen diagonals, your dollars go further with a rear projection TV. With technology advances, rear pro now offers large, flat screen sizes in a cabinet with a surprisingly small footprint, writes Stephen Dawson.

When choosing a rear projection TV (RPTV), there are a number of competing technologies to consider. The original RPTVs use old-fashioned CRT tube projectors to create an image on the screen. Today most RPTVs are moving towards projectors based on digital LCD (Liquid Crystal Display) or DLP (Digital Light Processing) technology, with new LCoS (Liquid Crystal on Silicon) technology also emerging as an alternative in the future, albeit an expensive one.

While RPTVS may use different technologies to produce and image, they all share the same basic  design. A projector located in the bottom section of the TV?s cabinet shoots the image through special lenses and onto a mirror, which then reflects the image forwards onto a onto a translucent screen. And that?s the picture you see … from the other side of course.

Until a few years ago, all RPTVs used an internal cathode ray tube (CRT) projector. These types of projectors, however, necessitate deep cabinets, and require periodic maintenance. Specifically, the red, green and blue guns of the projector that must be ?converged? precisely at the screen in order to create accurate colours and sharp pictures could be bumped out of alignment. CRT-type rear projection sets also perform poorly in brightly lit rooms, but perhaps their biggest shortcoming is their comparatively narrow viewing angle. The zone of maximum brightness is designed for the couch-seated viewer. If you stand up, the brightness drops off very sharply, so much so that the picture appears to fade away. It?s also not particularly good for kids sitting on the floor up close to the screen.

Another problem is their tendency to produce visible scan lines (the lines that make up the picture) if you sit too close. And with the large size of the screen, the eye more readily detects the flicker inherent in our 50 hertz video system. To explain, every television picture is produced by a fast electron beam of half pictures. This happens 50 times per second with a conventional 50 hertz television, and is slow enough to be detected by the human eye. Models that feature 100 hertz processing overcome this by projecting the half pictures 100 times per second on the screen, fast enough to no longer be perceptible to the human eye. Sets with 100Hz processing cost more. There are also models that support high-definition video signals, but when operating in this mode they again tend to flicker visibly.

On the upside, CRT rear projection systems are relatively inexpensive, offer better black-level performance than the new digital projection systems, and will still give good picture quality in a proper environment with proper fine-tuning and set-up.