Article, June 2007
Buying an HDTV
Recap of the SWIPCUG February 2007 Meeting
By Pim Borman, Webmaster, with notes by Kay Anne Peake,
Member of SW Indiana PC Users Group, Inc.
firstname.lastname@example.org (click to email author)
Obtained from APCUG with the author's permission for publication by APCUG member groups.
Robert D. Covington was the featured presenter. He explained that he does not sell TVs but learned a lot about them while advising a variety of companies as CTO of Rhysome, a provider of Enterprise Software.
Bob explained how TV broadcasters are in the process of changing from analog to digital technology. The change will result in more efficient use of the available transmission frequencies and improved image quality. The US Government also had a $14 billion windfall from auctioning off the available frequency bands to interested parties. As long as there is adequate signal strength digital TV presents a sharp picture, free of snow and distortions. When there is poor reception an analog TV might still present a recognizable picture, but the digital TV won't show any picture at all. It is easier to manipulate digital transmissions, and to make and store perfect copies. The downside is that digital data can be compressed to reduce bandwidth requirements, resulting in loss of quality, and that they are wide open to all sorts of Digital Rights Management (DRM) restrictions that so far have caused problems and remonstrations.
February 17, 2009 is the currently expected date when broadcasters will stop sending analog signals. TV sets will continue to receive programs from cable and satellite providers who will supply conversion boxes to their customers. TV receivers connected to an antenna will no longer work, unless provided with a converter box and a suitable antenna to receive the digital signals. Starting in 2008, the National Telecommunications and Information Administration (NTIA) will provide up to 2 coupons per household, worth $40 each, towards the purchase of the converter boxes. The required antennas are similar to those used for UHF reception on analog TVs, but they are more sensitive and can be placed almost anywhere in the house. For more details see http://www.ntia.doc.gov .
Concurrent with the change to digital broadcasting, the aspect ratio of the picture is being changed from the current 4:3 to 16:9. The 4:3 ratio was originally chosen because it was similar to the 35 mm slide format. The 16:9 ratio is the format used for most current Hollywood movies and is therefore preferred for the best display of movies on a TV screen. Many TV programs and commercials are already being produced in the new format and show up on current televisions as a “letterbox” across the screen, unless the receiver has the new, wider format.
Digital broadcasts will use different protocols depending on the transmission medium. Over-the-air transmissions to antennas will use ATSC (8VSB), satellite broadcasts use QPSK, and cable systems use QAM (“kwam”). When buying a converter box it is important to make sure it supports the correct protocol. Cable and satellite providers are likely to provide the correct boxes, but over-the-air (antenna-based) receivers will need an ATSC-capable converter. Newer TVs may also have built-in QAM conversion. In that case the cable provider only has to install a circuit card to configure the connection.
As with all things digital, what used to be simple and good for a lifetime of use gets complicated and subject to early obsolescence with digital TVs. The buyer will be faced with many choices to make in addition to the screen size he prefers and can afford. The next most important choice involves the type of display. Bob described 3 kinds, current Standard Definition TV (SDTV), Enhanced Definition TV (EDTV), and High Definition TV (HDTV).
SDTV and EDTV both use a screen with 480 lines of 640 pixels wide, but EDTV also provides a choice in screen dimensions with 4:3 ratio or 16:9 ratio, and progressive or interlaced scanning. As Bob explained, with interlaced scanning as currently used each frame is generated in 2 steps, first painting the odd-numbered lines followed by the even-numbered lines. If each step takes 1/60th of a second, the resulting frame rate is 30 fps. In progressive scanning all the lines of each frame are painted in a single step. In fast-moving scenes, such as sports broadcasts, interlaced scanning can result in a slightly distorted picture on large monitors and the (more expensive) progressive method is then preferred.
HDTV is available in 2 screen resolutions, 720 lines of 1280 pixels progressive only, or 1080 lines of 1920 pixels with a choice of interlaced or progressive. How's your headache so far? But wait, there's more! With EDTV and HDTV you also have a choice of frame rates, usually 24, 30, or 60, depending on the other parameters chosen. Not many broadcasts are of the highest possible quality, as the broadcasters may prefer instead to use their allotted bandwidth to transmit several different programs at the same time, using various compression levels and display parameters. The difference in quality may not even be noticeable except on the largest displays. But there is no point in paying for the best quality if no matching broadcasts are available.
Bob discussed what to look for when you buy a digital TV. One of the specifications is the luminance (brightness) of the display in candelas per square meter (cd/m2). An LCD or plasma screen should have a minimum of 250 cd/m2 while a projection system should have at least 1200 cd/m2 . The other display specification is the contrast ratio between white and black areas, at least 800:1.
To support digital connections between digital devices, new cable standards have been developed to make connections between units of a digital home system. The new Direct Video Interface (DVI) standards are labeled DVI-D for carrying digital signals only, and DVI-I for carrying digital-to-digital and analog-to-analog signals. A DVI-D plug will fit in a DVI-I socket, but not the other way around. Converter boxes usually carry a complete set of outlets compatible with DVI as well as the older RGB, 3-wire composite, and S-video connections. A more recent standard, High Definition Multimedia Interface (HDMI), carries both digital video and sound but is only useful if your whole home theater is newly digital and supports the cables.
CRT displays are rapidly disappearing because of their bulk and difficulty in forming a sharp color image across a large screen. Liquid Crystal Displays (LCD) are currently the most economical but suffer from limited contrast, brightness, color depth, slow refresh rates, and fixed screen resolution. Plasma displays depend on micro fluorescent light sources to provide bright images with excellent color, a wide viewing angle, and high contrast. They are still expensive and limited in size and have a limited lifetime. [However, when they burn out they have probably become obsolete already anyway, and a new unit of higher quality may be available at lower cost – Pim.] Finally, for large sizes projection displays have been used. They are based on Texas Instrument Digital Light Processing (DLP) technology that uses a chip with thousands of tiny mirrors that can flip up and down to direct light beams to the screen. They use an expensive, bright light bulb of limited lifetime.
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