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Set Top Box (STB) : HDTV Connections

“Set top box” is a term that can include any type of accessory that may connect to HDTV.
Common STBs are satellite receivers, cable TV receivers, OTA receivers, DVD players, VCRs, and so on.

STB video output options

The problem is that a single universal standard for unit-to-unit video connections does not exist. Any STB you acquire will probably have more than one of these output connectors. When you buy an HDTV and a 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.

CH3/CH4 output ATSC output by this method is very rare.

Composite video This 1-wire standard, in use for many years, conveys complete video images. It is designed for NTSC and cannot transport HDTV images.

S-video This 2-wire standard is an improvement over composite video. But it was designed for NTSC and cannot carry anything else.

Component video   This 3-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. Neither scheme is better than the other.

VGA This 5-wire standard, originally devised for computer monitors, carries HDTV raster signals, usually red, green, blue, Hsync, and Vsync. However in some units Y, Pr, and Pb can substitute for the colors. Usually the 5 wires are bundled into a single cable. Five separate cables are advised for runs longer than 12 feet. The connector can be either a 15-pin VGA connector or five BNC connectors. Warning: some HDTVs have VGA inputs that only accept computer formats, such as 600x800 and 720x1024. Many makers use the term RGB in place of VGA despite the confusion that causes.

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). Binary data is preferred by monitors that are not CRTs, such as plasma, LCD, DLP, LCOS, and others.

DVI comes with a decryption option called HDCP (High-bandwidth Digital Content Protection), which will decode encrypted programs such as first-run movies. There is a serious problem here. The motion picture industry is going to try to require distributors (HBO, etc) to use HDCP encryption on all high-def movies. HDCP decryption hardware is proprietary, and any hardware manufacturer must sign a contract in order to include it in his product. That contract forbids hi-def analog output (VGA or component video) on any device with a DVI interface, and allows HDCP decryption only in the monitor. This is an attempt by Hollywood to prevent unauthorized copying and distribution of hi-def material. But it means that millions of HDTVs already sold that have only analog inputs could become useless (or low-def) except for viewing whatever sitcoms or dramas the networks allow. The FCC has not yet ruled on this.

IEEE 1394 Also called Firewire or i.link, this is a high-speed serial 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, also called 5C copy protection). But since IEEE 1394 is an open standard, Hollywood has less control over it. Since 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 hand held 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. HAVi (Home Audio Video Interoperability) is such software. HAVi allows plug-and-play recognition of devices, interoperability, and brand independence.
HDMI HDMI (High Definition Multimedia Interface) is a specification that combines video and audio into a single digital interface for use with digital versatile disc (DVD) players, digital television (DTV) players, set-top boxes, and other audiovisual devices. The basis for HDMI is High Bandwidth Digital Content Protection (HDCP) and the core technology of Digital Visual Interface (DVI). HDCP is an Intel specification used to protect digital content transmitted and received by DVI-compliant displays. HDMI supports standard, enhanced, or high-definition video plus standard to multi-channel surround-sound audio. HDMI benefits include uncompressed digital video, a bandwidth of up to 5 gigabytes per second, one connector instead of several cables and connectors, and communication between the video source and the DTV. HDMI development is overseen by the HDMI Working Group that includes Sony, Hitachi, Silicon Image, Philips, and Toshiba as members.

For options 1-3 above: If the STB has any of these connectors, it is only for standard definition images. When a high-def program is being received, these connectors are either disabled or carry an image that has been down-converted to NTSC. (They are usually disabled when the STB is actively driving a high-def set.)

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.

For DVI, VGA, and component video: Very few sets will draw both 1080i and 720p. If you feed the set a mode that it cannot draw, you will get either a blank screen or garbage. (The law requires a set to receive all 18 modes. But the law only regulates tuners, not these intermediate inputs.)

DVI and 1394 are presently competing for the hearts and minds of the manufacturers. Possibly both will be adopted, DVI for video and 1394 for audio and control.


S-video connector	BNC connector

IEEE 1394	DVI

RCA audio / video	VGA

How can I avoid risk?

You can’t. You must decide among DVI, 1394, or analog (VGA and component video are analog). There is no telling which will become the long-term winner. Presently, Hollywood does not 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 hi-def movie is showing. Most people think the FCC would delay that order by ten years to allow depreciation of the millions of HDTV sets that would become OTA or SD only.

What Hollywood is most concerned about is movie piracy via the Internet. That is currently not practical at high definition. But if it should become practical and piracy proliferates, Hollywood wants to shut down those STBs that contribute to it. (Hollywood would then not release films to distributors who enable STBs with piratable outputs.) This is not all bad since it guarantees home access to first-rate films.

STB audio output options

An STB is likely to provide one or more of the following audio outputs:

6 channel audio (6 wires with phono plugs)
Coaxial digital audio (1 wire with phono plug)
Optical digital audio (1 TOSlink fiber optic line)
IEEE 1394 audio and video
DVI audio and video

Again, it is wise to plan this before buying. You could be in a bind if the TV and STB do not have enough of the right connectors. Some optical/coaxial converters are available :

Fostex COP-1 $50

Midiman CO2 $60

More about DVI

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 is the version most commonly used for HDTV. The five large pins are usually missing. There is a “single link” version of this that uses only 12 of the 24 small pins, but I don’t think this is compatible with HDTV.

DVI-I uses all 29 pins. The five large pins pass analog VGA signals. Presently, DVI-I is used mainly by the computer industry, but front projector HDTVs by a number of makers support DVI-I. There are DVI-to-VGA adapters and adapter cables available for these units. Front projectors from a couple makers will accept component video signals through their DVI connectors. These companies will provide DVI-to-Component adapter cables. However this is nonstandard.

These adapter cables only work with DVI-I. In most cases, if you want to connect a DVI unit to a VGA or component unit, these adapters will not work. That would require a transcoder circuit that can convert between analog and digital signals.

Video Transcoders (adapters)

The following device will convert component video into VGA.

Key Digital KD-CTCA2 $300 YPrPb to RGBHV -3dB @ 110MHz

The following devices will convert VGA into component video.

Audio Authority 9A60 $129 RGBHV to YPrPb -3dB @ 250 MHz

Key Digital KD-VTCA2 $200 RGBHV to YPrPb -3dB @ 110 MHz

RCA VHDC300 $130 RGBHV to YPrPb

The following devices will convert between DVI and VGA. However they were developed for computer formats. Whether they will work between an HD monitor and an HD STB is not clear. (The manufacturer claims they will, but they have not been reviewed for this.)

RAM Electronics EXT-DVI-2-VGA DVI-to-VGA $400

RAM Electronics EXT-VGA-2-DVI VGA-to-DVI $300

There are no transcoders for IEEE 1394.

VGA to Component video adapters

Component video to VGA adapters

These can be found on the Internet for $25-$50. In most situations they will not work. VGA is a 5 wire standard with Hsync and Vsync on the 4^th and 5^th wires. For component video, these two sync signals are multiplexed onto the green wire (Y in a YPrPb system). But some STB makers add the syncs to the green wire of a VGA port, and some monitors will look for the syncs on the VGA green wire. A menu item or a back panel switch may enable these actions, or they may be always enabled. Often these features are completely undocumented, and It is harder still finding out before you buy if these features are present. If they are then the adapter will work and you can avoid buying a transcoder.

Video Switch Boxes

The current generation of A/V Receivers will switch composite video. The next generation, some of which are already in stores, will switch component video. If you have too many STBs, one of these new receivers might serve as a switch for your STBs. Otherwise you will need a video switch box such as one of these:

Name plate Inday Audio Authority Zektor Inline

model number RGB4X 1154 HDS4 IN3582

configuration 4-to-1 4-to-1 4-to-1 2-to-1

component video yes yes yes yes

VGA no no yes no

bandwidth 230 MHz -3dB at 60 MHz 250 MHz 280 MHz

coaxial digital audio no yes yes yes

optical digital audio no yes yes no

connectors (per device) 3 phono 6 phono + TOSlink 6 phono + TOSlink 4 BNC

pushbutton switched yes yes no no

activity switched no yes no yes

IR remote switched yes no yes no

price $157 $194 $300 $260

availability now now now now

The activity switched units allow only one source to be on all of the time. The other sources must be disabled, which may require powering them off. The 1154 has pushbutton override of the activity switching.

The author recommends switches with a bandwidth that is flat to 100 MHz or -3 dB at 200 MHz. (Notice there are two ways of specifying bandwidth.) If you buy a switch that is flat to 30 MHz, -3 dB at 60 MHz :

There will be no degradation of the image if your TV’s horizontal resolution is only 1280.
There might be a barely detectable degradation if your TV’s horizontal resolution is 1920.
Some pixels will be altered if your monitor is digital (plasma, DLP, etc.) but it might not be noticeable.