March 2003

Connections and Interfaces: Part One

One of the hardest things about home theater is getting everything connected. It’s almost impossible to get the straight story at a store. You invariably find out when you get home that something you wanted won’t work unless you get some other cable. In this series of articles I’ll deal with all the home-theater connections, and help you understand which ones to use to get what you want. Another frequent question is, "Which connection is better?" I’ll explain that as well. This month is video oriented. Already, I see the picture is getting clearer.

Antenna, cable, and satellite connections

Before satellite TV, there was cable, and before cable, you used an antenna or you didn’t get TV at all. There are two basic types of antenna connections: 300 ohm and 75 ohm. 300-ohm antenna wire is OK for short runs. It has two small, copper wires in a semi-transparent, flat cable. The most common place you see 300-ohm cable these days is on "rabbit ear" dual-mast antennas that go on top of a TV, or single-mast antennas that are built-in to some smaller TVs. Many TVs have eliminated connections for 300-ohm antenna wire. If you need to use a 300-ohm connection, you can purchase a low-cost 300-ohm-to-75-ohm converter called a "balun." Every TV made today has at least one 75-ohm connection for an antenna or cable TV.

These connections accept the round, black or white coaxial cable used for cable TV, many satellite-dish installations, and even some of the larger broadcast antennas.

75-ohm coaxial cable, when carrying video signals, is fitted with "F-connectors" on the ends. These thread onto the mating connector for a secure fit that won’t come undone by accident. You probably have at least one 75-ohm connection in your home-theater system such as a cable-TV or satellite-TV decoder box.

In the case of TV, cable, and satellite, the 300-ohm or 75-ohm cable carries both audio and video. The tuner, wherever it resides, has to separate the audio from the video and send each onto the proper processing path.

Monitor connections: analog

TVs have come a long way from the days when they had only 300-ohm or 75-ohm antenna connections. In those days, there were no VCRs or other sources to connect to your TV. With the dawn of VCRs and video games, it became clear that TVs were going to need more connections -- more ways to get other video signals to the TV to be displayed. The first "new" video connection was called composite video.

Composite cable carries all three elements of the video signal on a single cable. Most of the time it is a 75-ohm coaxial cable with RCA connectors on both ends. Yellow became the standardized cable color for composite video. People found rather quickly that even the lowly VCR produced better images when using the composite video connection than it did when using the 75-ohm antenna connection that carried audio and video on a single cable.

The advent of Super-VHS and Super Beta VCRs brought a significant improvement in picture quality -- so much so that the lowly composite video cable was now limiting the picture quality you could get. The response to this was the addition of S-video.

The S-video cable uses the diabolical, round, four-pin DIN connector that is annoying to use for most people. Inside the S-video cable are actually two mini 75-ohm coaxial cables. The video signal is split into two separate signals: the black-and-white portion of the image, called the luminance signal or "luma" for short, and the color portion of the image, called the chrominance signal or "chroma" for short. Separating the two parts of the video signal improves picture quality to a degree.

With the arrival of DVD players, and especially high-definition TV, it was clear we needed to have an analog video interface that was better still. Component video was the result. Component-video connections maintain separation of the three elements of the video signal from the source, like a DVD player, to the monitor. The three video-signal components are named luminance, chrominance 1, and chrominance 2.

TVs have two chrominance signals; both are necessary to deliver all the information needed to display images using the red, green, and blue phosphors, or light colors used in video display devices to produce images. Keeping the three signals separate from each other gives the best possible image quality. These cables are essentially identical to the old composite video cable, except that there are three of them. Each is a 75-ohm coaxial cable with RCA connectors on the ends. Because yellow was already used to identify composite connections, the consumer electronics industry settled on red, green, and blue to signify the three component-video connections. Some people assume these cables carry red, green, and blue signals because of those color designators, but that’s not true. Computer displays and some high-end home-video products do accept red, green, and blue video signals, but consumer products are luma, chroma 1, and chroma 2 -- very different and incompatible with R, G, and B signals.

Luma, chroma 1, and chroma 2 video signals are usually designated by the letters Y, Pr, and Pb or Y, Cr, and Cb. There used to be an actual difference between Pr/Pb and Cr/Cb (Pr/Pb could handle high definition, Cr/Cb was standard-definition only), but so many manufacturers have misused them for so many years that it is unlikely that the designations will ever be corrected.

Don’t confuse the "red" (right channel) stereo audio connection with the red component-video connection! If you use a red audio cable for the red component-video connection, your picture quality will suffer greatly. Likewise, the three-cable set that you may have gotten with a receiver some years ago probably has a yellow, red, and white cable. The yellow cable is the only video cable in that set. The red and white cables are audio cables and are not suitable for carrying video signals.

Monitor connections: digital

This is the part of the story where consumers like us get the shaft from the industry: TV manufacturers, movie studios, not to mention cable TV and satellite TV providers along with their pay-per-view operations. Because high-definition TV signals are digital, it only makes sense that you’d want to move signals from one place in the system to another while the signals are still in digital format. This requires a new type of connection: digital video. Until recently, there has been no digital video connection on consumer products. Today, many, if not most, new digital TVs will have at least one digital interface.

There are three digital interfaces making life difficult for us consumers: DVI, the lame-duck digital interface; HDMI, which will replace DVI as early as this year or next; and Firewire (also called IEEE 1394 or iLink, depending on which company’s information you are reading).

DVI is an uncompressed digital video interface. The good thing about it is that it carries uncompressed video and therefore will have the best possible image quality. The bad thing about DVI is that last year it was being hailed as the greatest thing to happen to consumer video in decades and this year it’s about to be replaced by HDMI.

HDMI transmits the same uncompressed video signal as DVI, but audio can accompany the video signal so you can connect components to each other with a single digital cable. Now this is a real boon to the home-theater enthusiast. It’s a major step towards getting rid of the often-ridiculous tangle of wires needed for a modern home-theater system. The bad thing about HDMI is that not every manufacturer is endorsing it. Much of this comes from copy-protection fears from the movie studios and content owners; they want to control what you can do with their video signal. Pay-per-view operators don’t want you to be able to record their digital signal. Movie studios don’t want you to be able to record their DVDs, or high-definition movies from HBO-HD. Getting "content protection" into HDMI is proving to be problematic. Each time there is some progress, someone shows how easy it is to defeat.

HDMI has an "enemy" in the Firewire interface that several companies are promoting.

Firewire connections cannot move as much data as DVI/HDMI, and because of this high-definition video has to be compressed. Compressing the signal always causes some visible degradation of the image. That means that if you use Firewire, the images you get won’t be quite as perfect as DVI/HDMI images. You probably wouldn’t be able to tell there was a difference unless you saw the DVI/HDMI image side by side with the same Firewire image. Just knowing there is a difference is often enough to drive videophiles to distraction, whether the difference is noticeable or not. Note that there are at least two different shapes of Firewire connector. If you end up with equipment that has Firewire connections, be sure you know which connectors need to be on both ends.

As of winter 2003, a larger number of TV manufacturers favor HDMI over Firewire. That could change, but it isn’t likely. Typically, the consumer-electronics manufacturers won’t jump ship until the last minute when they realize there is nothing they can do to stop the inevitable. Because of the indecision and incompatibility of the two digital interfaces, consumers are going to be forced to make a choice. Those who make the wrong one this year or next might end up with an obsolete-before-its-time HD video display.

 ...Doug Blackburn
db@hometheatersound.com

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