It's All About Connections
In addition to source quality, the quality of different connection types has a huge impact on the overall image (or audio) quality you ultimately see (or hear). In other words, even with a high definition video source, if you use a lower quality video connection standard (such as composite video—the normal "yellow" RCA video connector), the picture could be worse than a lower resolution video source that uses a higher quality connection type (such as component video).
For example, referring back to progressive scan DVD players, one of the first things you often find out about these devices is that the only way to get access to the higher-quality progressive scan signal is with a three-headed component video cable and the component video (sometimes called Y/Pb/Pr or Y/Cb/Cr) connection. The reason for this is that only component video cables have the "bandwidth," or signal-passing ability, to deliver a progressive scan analog vide display (there are digital alternatives that I will discuss momentarily). Though many people are not aware of this, the type of video connection and accompanying cable that you use to connect devices does have an impact on the quality of the final picture. I'm not referring to the brand or quality of cable used (although that, too, can make a difference), but the type of connection. Most people are familiar with the regular, yellow-colored RCA video connectors technically called composite video mentioned earlier, for example, but not everyone knows that the round, 8-pin S-Video connectors that many video devices also include carry a slightly higher quality version of the same signal. In other words, if you plugged them both in and compared them by switching back and forth between the different inputs on a display, you would see that the S-Video signal is higher quality.
The reason for this quality difference is due to the makeup, or structure, of video signals. Simply put, video signals are made up of different component parts and by keeping those parts separate, you can maintain a higher quality image. In the case of composite video and the older RF connection system, which uses cable TV-type coaxial cable, all components of the signal are combined into one and the result is a lower quality picture. With RF, both the audio and the video are mixed together on a single cable, which gives it the lowest image quality of all connection types. With S-Video, the chrominance (color information) and luminance (light/dark information) are kept separate, resulting in a better picture quality, particularly for things like color accuracy and color separation in certain images. In the case of component video, the color information is further subdivided into separate parts, resulting in even better color accuracy and an overall improvement in video quality (as well as three separate connectors on each end of the cable). In addition, as mentioned above, any higher-definition content, including progressive scan DVD as well as high-definition television signals that are sent in analog form can only be sent via component video cables. Please note, however, that simply using component video cables doesn't make everything a progressive signal—the original source has to be set to be output in progressive mode. Otherwise you'll just get an interlaced signal sent over a high-quality progressive "capable" cable.
Higher-definition can also be sent digitally from device-to-device (in some cases), using one of several digital connectors, most notably DVI (Digital Visual Interface) and HDMI (High-Definition Multimedia Interface), but also IEEE 1394 (sometimes called Firewire or iLink). DVI, which is also used to connect some LCD monitors to PCs, delivers a very high-quality digital version of the video signal. DVI with HDCP (High Definition Copy Protection), sometimes referred to as DVI/HDCP, is a slightly refined version of the interface that provides the same high-quality digital video signal support, but also includes support for copy-protected digital content. Hollywood interests, having seen what happened to digital copying of music CDs, are very concerned about avoiding that problem when it comes to digital video and so pushed for the inclusion of hardware-based copy protection technology such as HDCP into consumer electronics products. As of now, no high-definition content (that is, shows being broadcast in high-definition) requires HDCP support in order to view it, but it is technically possible that high-definition signals that are "flagged" as being copy-protected could not be carried on a plain DVI connection. In other words, if you tried to watch a copy-protected signal over a connection that didn't support the copy protection, you'd see a blank screen or a message on the connected display about how the material is copy-protected and requires a copy-protected interface. In this case, the issue isn't with the cable or the DVI interface but with circuitry on the sending and receiving end of the link. Thankfully, most current consumer electronics products with DVI also include support for HDCP—which would allow you to view even copy-protected video content—but it's still worth checking to be sure.
HDMI, which is compatible with DVI via a simple cable adapter, includes the same digital video signal as DVI but also adds support for digital audio. Best of all, HDMI does so in a smaller, more consumer-friendly connector than DVI. HDMI completely supports HDCP, as well, making it compatible with any copy-protected content. As a result of this and integrated digital audio support, HDMI has started making a big splash in consumer electronics products (and PCs, for that matter), if for no other reason than because it can greatly reduce the cabling spaghetti mess the typifies the back ends of many home theater systems.