Once you've got an HDTV set, you'll need something to pull in HD signals and pass them onto your television. Learn what to look for, and what to buy.
Set top box, or STB, is a term that can include any type of accessory that can connect to the HDTV. Common STBs are satellite receivers, cable TV receivers, OTA receivers, DVD players, VCRs, and so on. Generally, though, an STB is a device that pulls in a high-definition signal, instead of just pushing pictures through to your TV.
Choosing a set top box is like choosing any other component; you need to find one that has the right functionality, and more important, the right connections for your gear. For an STB, that means ensuring that both the video and audio it receives can be passed on to the rest of your gear.
Unfortunately, a single universal standard for unit-to-unit video connections doesn't exist. Eventually, through competition, the best of the following will survive. Any STB you acquire probably will have more than one of these output connectors. When you buy an HDTV and an STB, try to select units that can connect to each other directly. Otherwise, you will have to pay for a transcoder or a video switch box. Here are your basic connection options.
- CH3/CH4 output
ATSC output via a preselected channel on the TV is one of the oldest connection methods. Obviously outdated, this almost never shows up in modern components.
- Composite video
This one-wire standard, in use for many years, conveys complete video images. It is designed for NTSC and can't transport HDTV images.
- S-Video
This two-wire standard is an improvement over composite video. But it was designed for NTSC and can't carry anything else.
- Component video
This three-wire standard, originally designed for DVD players, can carry HDTV via three wires with phono plugs. The three wires carry analog raster (image-scanning) signals, either red/green/blue or Y/Pr/Pb (Y=intensity, Pr=Y-red, and Pb=Y-blue). Some units can handle either color scheme. You must verify that both units can use the same scheme.
- VGA
This five-wire standard, originally devised for computer monitors, carries HDTV raster signals, usually red, green, blue, Hsync (horizontal sync), and Vsync (vertical sync). However, in some units, Y, Pr, and Pb can substitute for the color channels. Usually the five wires are bundled into a single cable.
The connector can be a 15-pin VGA connector or five BNC connectors.
- DVI (Digital Visual Interface)
This connector conveys HDTV raster-like signals in binary data form. The data rate is very high (1.65 Gb/s). Monitors other than CRTs, such as plasma, LCD, DLP, LCOS, and others, prefer binary data. DVI comes with a decryption option called HDCP (High-bandwidth Digital Content Protection), which will decode encrypted programs such as first-run movies. However, there is a serious problem here: the motion picture industry might try to require distributors (HBO, Cinemax, etc.) to use HDCP encryption on all high-definition movies. HDCP decryption hardware is proprietary, and any hardware manufacturer must sign a contract to include it in his product. That contract forbids high-definition analog output (VGA or component video) when encryption is enabled, and allows HDCP decryption to take place only in the monitor. This is an attempt by Hollywood to prevent unauthorized copying and distribution of high-definition material. However, it means that millions of HDTVs already sold that have only analog inputs could become useless, except for viewing whatever sitcoms or dramas the networks allow. The FCC hasn't yet ruled on this, and doesn't seem to be in any hurry to step into the issue.
- HDMI
This new miniature connector is intended to replace DVI. It is backward compatible with DVI, and an adapter will connect it to a DVI unit. It has 19 pins and carries DVI, plus digital audio. It also has a control line that allows the STB to sense the monitor's state and native formats.
- IEEE 1394
Also called FireWire or i.link, this is a high-speed bus common in computers. IEEE 1394 is fast enough to carry compressed MPEG-2 video data plus audio and controls. There is an encryption standard for IEEE 1394, called DTCP (Digital Transmission Content Protection, and sometimes called 5C copy protection). But because IEEE 1394 is an open standard, Hollywood has less control over it. Because it is a two-way bus, it could allow units to control each other. This holds out the promise of eliminating the need for 5 or 10 handheld remotes to control the home theater. IEEE 1394 is just a connector definition plus a software shell. Additional software is required for the units to talk to each other.
If the STB has a CH3/CH4, composite, or S-Video connector, it is for standard-definition images only. When a high-definition program is being received, these connectors are either disabled or carry an image that has been down-converted to NTSC.
Neither VGA nor component video is superior to the other. For a cable length of six feet, VGA is more convenient. For longer runs, component video is usually more convenient.
When using DVI, VGA, and component video, very few sets will draw both 1080i and 720p. If you feed the set a mode that it can't draw, you will get either a blank screen or garbage.
Tip
The law requires a set to receive all 18 modes. However, the law only regulates tuners, not these intermediate inputs.
An exception to this is fixed-pixel displays that will redigitize component video.
DVI and 1394 are presently competing for the hearts and minds of the manufacturers, but which will win is unclear. A third possibility is that both will be adopted, DVI for video and 1394 for audio and control. All record devices likely will use 1394.
DVI was originally developed for computer monitors, but has been adopted by HDTV. DVI comes in different versions. All versions use the same 29-pin connector. Sometimes you can tell which version you have by seeing how many of the 29 pins are missing.
- DVI-D
This is the version most commonly used for HDTV. The five large pins usually are missing. There is a single-link version of this that uses only 12 of the 24 small pins. Single link will work properly with all HDTVs.
- DVI-I
This version uses all 29 pins. The 5 large pins pass analog VGA signals. Presently, the computer industry primarily uses DVI-I, but front projector HDTVs, from a number of makers, support DVI-I. DVI-to-VGA adapters and adapter cables are available for these units. Front projectors from a couple of makers accept component video signals through their DVI connectors. These companies provide DVI-to-component adapter cables. However, this is nonstandard.
These adapter cables work only with DVI-I. In most cases, if you want to connect a DVI unit to a VGA or component unit, these adapters won't work. That would require a transcoder circuit that can convert between analog and digital signals.
- HDMI
HDMI is a single-link DVI plus digital audio and a control line in a miniature connector. It carries no analog signals.
Unfortunately, you can't avoid risk completely. When you select an STB, you must decide among DVI, 1394, or analog (VGA and component video are considered analog). There is no way to tell which will become the long-term winner. Presently, Hollywood doesn't want any DBS or cable set top box to have a 1394 connector passing MPEG-2 data. They even consider analog to be a piracy threat. If the DVI interface catches on big, Hollywood could order all DBS and cable companies to disable all STB analog or 1394 video outputs whenever a high-definition movie is showing. Some people think the FCC would delay that order by 10 years to allow depreciation of the millions of HDTV sets that would become OTA- or SD-only as a result.
I believe Hollywood will not carry out its threat anytime soon. What Hollywood is most concerned about is movie piracy via the Internet. Currently that is not practical at high definition because it takes too long to download huge movies in this resolution. However, if it should become practical and piracy proliferates, Hollywood will try to shut down those STBs that contribute to it. The FCC certainly will side with Hollywood if movie piracy makes movie-making unprofitable. This is not all bad, though, because it guarantees home access to first-rate films.
An STB is likely to provide one or more of the following audio outputs:
Six-channel audio (six wires with phono plugs)
Coaxial digital audio (one wire with phono plug)
Optical digital audio (one TOSlink fiber optic line)
IEEE 1394 audio and video
DVI audio and video
Again, it is wise to plan for audio connectivity before buying. You could be in a real bind if the TV and STB don't have compatible connectors for video, or if your receiver/processor and STB can't speak audio to each other.
âKenneth L. Nist
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