Home Theater Cables
Types of Cable, and Important Attributes, for Home Theater Use
As late as the 1970s, it was possible to hook up any piece of video equipment a consumer was likely to own with a screwdriver; the typical back-panel of a television set had a two-screw connection for a twinlead antenna, and the only connectivity question one was likely to face was "flathead or Phillips?"
Now, of course, home theater cables are a good bit more complicated. Twinlead has largely disappeared in favor of coaxial cable (better suited to CATV systems), but apart from RF-modulated video over an antenna or cable TV system, there are now a whole range of types of video signals which our home theater devices are capable of sending or receiving, and a range of types of cables for them to travel down. These video signal types are broadly classified as "baseband" video signals, which is just a fancy way of saying that they haven't been modulated--that is, attached to a radio-frequency signal for broadcast or CATV distribution.
Meanwhile, where once upon a time there was just mono analogue audio, and then stereo analogue audio, to contend with, now there is digital audio over coaxial cable, digital audio over fiber cable, and a range of multichannel analogue formats--Dolby 5.1, 6.1, 7.1--to contend with.
This article is intended to provide a quick guide to these different digital and analogue signal formats, and to identify the general principles which govern the suitability of home theater cables for each application. As we'll see, home theater cables for one application are sometimes freely substitutable for cables built for entirely different applications--and sometimes not.
Video Cables for Home Theater:
75 Ohm Analog Video Cables:
The world of analogue video cable for home theater is simpler than one might imagine. There are a slew of signal standards: Composite Video, S-Video, Component Video, RGBHV ("VGA"), RGBS, RGsB, and good old-fashioned RF-modulated video/audio. Yet all of these signal types are designed to run on the same cable: 75 ohm video coax. In some cases it's not obvious that the cable structure is coaxial; s-video typically involves two miniature coaxes in a single round jacket, while RGBHV is often run in a five-coax miniature bundle, terminated with a VGA ("HD15", or "mini D-sub 15") plug. The cable may be miniaturized, bundled, and ambiguously labeled--but it's still 75 ohm video coax.
Regardless of whether the signal is composite, component, or one of those other standards, the technical characteristics that make a good 75 ohm video cable are the same. Those attributes are: (1) impedance consistency--the degree to which the cable impedance stays at, or close to, 75 ohms (for an explanation of what "impedance" in cable is, see "What is Impedance, Anyway?"); (2) shield effectiveness--which determines how well the cable keeps out noise from external sources; and (3) attenuation--which is how much energy the signal loses along the way. Generally speaking, attenuation isn't a major concern in runs of ordinary lengths--but as runs become longer, it can become an issue.
Those are the technical characteristics which distinguish an excellent video cable for home theater use; however, there are other considerations which may come into play in a particular application. The most-ignored of these, perhaps, is flexibility. Because impedance control is a matter of controlling precisely the physical dimensions of the cable, and because those dimensions alter as the cable flexes, there is something of an inverse relationship between cable flexibility and impedance tolerance. However, in ordinary applications, at reasonable lengths, the best high-flex cables will perform equally well (in observable terms) as the best high-precision cables. Inflexible cable will tend to have superior specs, but can be frustrating to work with if your equipment setup won't accommodate it.
Parallel Digital Cables: DVI and HDMI
DVI and HDMI, unlike the various signal types discussed just above, are digital formats. In fact, they're the same digital format; there is some misinformation floating around to the effect that HDMI will support 12-bit colour where DVI will not, but in fact the encoding schemes for these two formats are identical. HDMI carries a digital audio signal as well as video, while DVI carries video only.
Inside, DVI and HDMI cables are completely different from analogue video cables (apart from the rarely-used DVI-A, an analogue signal type which is just VGA in a funny plug). The structure of a DVI or HDMI cable is balanced, which is to say that it's a twisted-pair cable (akin to, for example, CAT5 or telephone cable). This was an unfortunate design choice, and the result is that DVI and HDMI are rather vulnerable to catastrophic signal loss over fairly modest lengths. Where analogue component video cable can be run hundreds of feet without trouble, the limit for DVI or HDMI varies with the cable and the equipment, and is generally around 50 feet, but sometimes shorter, because the impedance tolerance of twisted pair cable is inherently much inferior to that of coax. For many home theater cable installations, this isn't a factor--but if you're trying to distribute from room-to-room, or need to take a roundabout path to a ceiling-mounted projector, it can be a real problem.
Audio Interconnect Cables:
Analog Audio--from mono on up:
Most consumer electronic gear supports analogue "unbalanced" audio. Once upon a time, practically all sources were monophonic, and were carried in a cable with RCA connectors; then stereo took over the world. After quadrophonic audio failed to catch on, one might have thought that multichannel audio would never emerge, but now we have it--in the form of Dolby 5.1, 6.1, and 7.1. These are confusing names, in that 5.1 is actually a six-channel system, 6.1 a seven-channel system, and 7.1 an eight-channel system. The "point-one" in each of these cases refers to the subwoofer or LFE channel, which is a restricted-bandwidth channel, carrying only low-frequency sounds.
These multichannel formats often are carried only as a digital audio signal, about which more later; but sometimes they need to be run as discrete analogue channels. The two most common scenarios where that's required are (1) use of SACD or DVD-A, where outputs can be run only analogue, not digital, or (2) use of a multichannel preamp with a main amp, with surround decoding being done in the preamp for amplification by the main amp.
Whether one is running stereo, mono, or what-have-you, the attributes of analogue audio cable that are most important are (1) shield effectiveness, especially at audio frequencies (much lower than radio frequencies), and (2) capacitance. Capacitance, in a cable, is its ability to store some of the energy of the signal without delivering it to the destination; it causes loss in the high audio frequencies, often referred to as "rolloff." In short runs, high capacitance tends not to be a very significant issue; as the run becomes longer, however, capacitance piles up and is increasingly likely to present a sound quality problem. For this reason, it's desirable, when possible, to avoid long analogue audio interconnect runs.
Some equipment supports "balanced" analogue audio--this is the form in which analogue audio is generally run in professional environments. Balanced audio is usually run in three-conductor cable, with a twisted pair in the center, enclosed by an outer grounded shield. Sometimes one will see cables with RCA plugs marketed as "balanced," but they aren't, and can't be; the attributes of a balanced circuit can't be duplicated by a cable design, so if your gear has RCA jacks, the signal is unbalanced, regardless of the internal structure of the cable hooked up to it. Balanced audio offers better noise rejection than unbalanced audio--not a huge factor in most installations, but it certainly can be in some.
Digital Audio:
Digital Audio is the usual mode for running multichannel audio from one device to another. The consumer standard, SPDIF, is nearly universal, and is incompatible with the professional standard, AES/EBU audio. In a digital audio cable, all of the audio for a number of channels is combined, digitally encoded, in a single bitstream. This may be conveyed from device to device either in an optical (Toslink) cable, or in a coaxial cable, so home theater cables for digital audio may take either of those forms.
Coaxial digital audio cable is, very simply, 75 ohm video cable. The same considerations apply; tight impedance tolerance and good shielding are desirable characteristics. SPDIF signals in coaxial digital audio cable can be run, without trouble, for hundreds of feet.
Optical digital audio cable is an odd beast. It is commonly assumed that the optical signal is more robust, because most people are familiar with the notion of fiber optic cable being used to run signals with huge bandwidth over vast distances; if a whole bundle of phone lines can be condensed into one hair-thickness optical fiber and run loss-free for miles, then certainly Toslink ought to be able to run for similar distances--right? Well, Toslink fiber is plastic, and, unfortunately, isn't at all the same thing as glass optical fiber, and there are technical problems with using glass fiber to do the job of Toslink fiber (notably, that the aperture of Toslink fiber is enormous compared with the size of glass fiber). Problems with Toslink can appear on runs as short as fifteen feet, though high-quality plastic fiber may extend that limit out considerably. When selecting a digital audio cable for home theater, if it's possible to run coaxial rather than optical, that's usually the best option, especially if significant distance must be covered.
There is also balanced digital audio--AES/EBU audio is conventionally run balanced, though it's not uncommon to see it converted to unbalanced form for long runs, and run in video coax, because of the tighter impedance tolerance of the video cable.
Speaker Cables: Not Much More Than Wire
Speaker cables, like analogue audio interconnects, carry analogue audio; however, because they carry a low-impedance, high-power signal rather than the line-level, low-current, high-impedance signal in an interconnect, the characteristics that govern speaker cable quality are quite different from those which apply to interconnects.
First, shielding isn't a factor; the signal level in speaker cable is so large, and the current flow so high, that entry of noise into the cable can't (barring some truly extraordinary level of electrical noise in the environment) cause alteration of the sound output. On those rare occasions when shielded speaker cable is used, the purpose of the shield is not to protect the speaker signal from outside noise, but to protect other circuits from noise induced by the speaker cable.
Second, the impedance of speakers is both low and frequency-dependent. Speakers draw a high current, and because their impedance is low (typically 8 ohms, sometimes as low as 4), the resistance of speaker wire can be a significant factor in the circuit (unlike an interconnect, where a few ohms of resistance is nothing compared to perhaps 50K ohms of input impedance). That means that, if speaker wire gauge is too small, a fair amount of the power is used up in the speaker cable to generate heat rather than being delivered to the speaker to make sound. And because the speaker impedance varies with frequency, the effect will be different for high frequencies than for low frequencies, throwing the overall frequency response off.
Third, capacitance isn't usually a factor. We say "usually" because there are some exceptionally high-capacitance exotic speaker cable designs out there (the most popular being a DIY design made by braiding together a large number of strands of CAT5 cable), and these can cause high-frequency rolloff or, in a severe case, can cause the amp to oscillate. Barring such an odd design, however, the rule is that ordinary differences in capacitance between one type of speaker wire and another will not make an audible difference to the signal, because in a low-impedance circuit there isn't as much tendency for signal to be stored in that capacitance.
So, what does that mean for speaker wire quality? Basically, it means that wire gauge is the overwhelming consideration. The bigger the wire, the lower the resistance, and the lower the resistance, the lower the likelihood of uneven frequency response resulting from speaker cable resistance. 12-gauge wire typically is plenty big enough for just about any residential application, but people often like to go to 10-gauge for a little extra margin.
Conclusion:
There are a lot of home theater cables, and there are a lot of home theater cable vendors, which can make cable selection a bit confusing. Keeping in mind the desirable attributes of the various types of home theater cables, and scrutinizing products for the relevant specs, can simplify the selection process considerably. The best way to stay informed about the products you're examining is to get detailed spec sheets from manufacturers and use those numbers to compare.