Many people spend a lot of time selecting their gear with barely a thought to how they're going to hook everything up — until it's unpacked in the living room. In a modern A/V system, however, making the right connections can have a big effect on what you see and hear.

That doesn't mean you have to spend a fortune on cables. But you can do yourself a real favor by figuring out which types of audio and video connections are optimal for the equipment in your system and ensuring that the cables you use to make those connections are well-constructed and have the proper electrical characteristics. This can buy you not only visual and sonic improvements but also the peace of mind that comes with not having to troubleshoot annoying buzzes, hums, or flat-out signal failures.

Cable Fundamentals
Aside from speaker wires, which are normally simple insulated (but unshielded) pairs of copper conductors, there are three types of cables: twisted-pair, coaxial, and fiber-optic. Both twisted-pair and coaxial cables carry signals along copper wires, while fiber-optic cables carry signals modulated as pulses of light along strands of glass or transparent plastic. Twisted-pair cables, used mainly in phone networks and Ethernet connections, have pairs of independently insulated wires twisted around each other, which helps fend off noise from adjacent wires (crosstalk) as well as outside electrical interference. Coaxial cables have a core wire (called the conductor) surrounded by an insulating material (called the dielectric), plus a layer of electrical shielding and a plastic or nylon outer jacket. Fiber-optic cables consist of a glass- or plastic-fiber core, cladding (which helps keep the light pulses within the fibers), and an external jacket.

Twisted pair — used in Cat-5e cables, for instance — is relatively inexpensive and easy to install, but it's susceptible to electromagnetic and radio-frequency interference (EMI and RFI, respectively). Coax can support a wider frequency range and has better shielding against interference, but it can still pick up EMI or RFI in difficult environments or induced hum from nearby power cables. Fiber-optic cable has great bandwidth and is completely immune to EMI and induced hum, but it's costlier and more fragile.

Anatomy of a Cable
A cable's basic job is to convey a signal from a source to another component in as near its original form as possible. To understand how a better cable differs from a cheap one, it's useful to know how a cable works. Since probably 95% of all interconnects are coaxial, we'll focus on how a coax cable is constructed. (Also see Inner Workings: Inside a High-End Audio Cable.)

The conductor normally is copper, though some high-end cables add a silver coating, which slightly increases conductivity at the extremely high frequencies required for video and digital audio. That's because signals at those frequencies tend to travel along the outside of the conductor rather than throughout it, a phenomenon called "the skin effect." The copper may also be specified as "oxygen-free," meaning that it contains very few copper oxide impurities.

The dielectric material electrically insulates the conductor and separates it physically from the wire-mesh shield, which acts as a barrier against EMI and RFI. In unbalanced cables, the type normally used in home systems, the shield also acts as a second conductor (essentially a ground — balanced cables have a third, separate conductor for this purpose). Because the mesh contains small gaps, most high-quality cables have a second, overlapping mesh shield, sometimes augmented with a foil wrap for additional RF protection.

A coaxial cable's impedance — its resistance together with its inductance and, especially, capacitance — plays a role in its performance. Every conductor has some resistance, which impedes signal flow equally at all frequencies. Capacitance, which stores energy in an electrostatic field, arises when two conductors are close together but separated by a dielectric. A coaxial cable is thus a capacitor. Cable capacitance interacts with other impedances in the connection to create a low-pass filter that gradually attenuates the signal above a certain frequency. This is mainly a concern for analog audio connections, where a combination of high output impedance in the source component, high cable capacitance, and a long run could result in audible treble loss. This sort of problem is rare in modern audio and home theater systems, however.

For video and digital audio signals, which have extremely short wavelengths, what's known as the characteristic impedance is important. (It doesn't matter for analog audio connections.) Determined by the ratio of inductance to capacitance, the characteristic impedance should be the same for the output, the cable, and the input at the other end to prevent reflections that can cause signal cancelations. The standard characteristic impedance for video and digital audio connections is 75 ohms.

The consequences of cable losses are different for analog and digital signals. Analog signals tend to deteriorate gradually as the loss increases, whereas digital connections fail abruptly when the receiving device can no longer distinguish the pulses that comprise the signal well enough for it to be reconstructed accurately.

A cable's terminations are as important as its construction. Cheap cables usually have cheap connectors, which can lead to intermittent or failed connections, EMI pickup, or hum. The connectors on high-quality cables are made of a solid, conductive material, typically brass plated with nickel or gold to resist oxidation and corrosion. Connectors are joined to the cable using solder joints or compression fittings. Good cables have clean, complete solder joints or tightly compressed, very flat fittings and connectors that fit securely, but not too snugly, into the jacks to assure maximum surface contact and to prevent them from pulling out to easily.

Audio Cable Types
The standard analog-audio interconnects are coaxial cables terminated with RCA jacks. The advent of digital audio sources such as CDs and DVDs created a need for digital audio cables. There are two main types: coaxial and optical. Coaxial cable for digital audio is like any other but must maintain wide bandwidth and the standard 75-ohm impedance. Most sold specifically for digital audio also have gold-plated RCA connectors, but composite-video cables also work fine for this application. Some high-end components use the more secure bayonet-style BNC connectors for coaxial hook-ups. (BNC connectors also have the benefit of maintaining a true 75-ohm impedance, which RCA plugs and jacks, at closer to 40 ohms, can't.) Another alternative occasionally encountered is the professional AES/EBU interface, which requires balanced interconnects with three-prong XLR connectors.

Optical digital-audio interconnects, also called TOSlink cables (for Toshiba, which created the system), carry signals as pulses of light. Since they don't incorporate electrical conductors, they're immune to induced hum and electromagnetic and radio-frequency interference. Most use the small, squarish TOSlink connector, but there's also a smaller optical mini-connector that looks like a standard mini-plug.

Video Cable Types
Video interconnects come in a number of flavors. Anyone with a cable box will recognize coaxial RF cable — also called RG-6 — which is most often used to send RF-modulated audio and video from your cable or satellite company into your home. (The slightly thinner RG-59 is also used sometimes.) RG-6 is usually terminated with a round pin F connector (which never seems to thread properly onto the connector on the back of your TV). Composite video — the cable with the familiar yellow RCA plug — is the lowest-quality pure video cable, since it sends all the video information via a single wire. An S-video, or Y/C, connection, which has a circular 4-pin plug, will give you better quality than composite video since it keeps the luminance (brightness) and chrominance (color) information separate.

But composite- and S-video cables can't carry high-def video. For that, you'll need a component (a.k.a. YPbPr) or RGB connection. Component video uses three coax cables, with one carrying the luminance signal (Y), which indicates brightness, and the other two carrying "color difference" signals (blue minus luminance, or B — Y, and red minus luminance, or R — Y). Almost every HDTV has at least one or two analog component-video inputs. RGB video, used mainly in front projectors and monitors, is typically terminated with a 15-pin VGA connector, although RCA or BNC connectors are options. The RGB+H/V format used in home theater systems has five cables: three in the manner of component video, plus one each for horizontal (H) and vertical (V) sync data.

HD Goes Digital
The main types of digital video cables are HDMI, DVI, and FireWire (a.k.a. IEEE 1394; Sony and some others call it i.Link). FireWire is a high-speed (400 Mbps) interconnect that can carry digital audio, video, and control data on a single cable. But because it does not have enough bandwidth for uncompressed digital video, it's mainly used for transferring camcorder footage and digital photos.

A copyright-protected version of DVI (Digital Video Interface) called DVI with HDCP (High-bandwidth Digital Content Protection) quickly became the primary, if short-lived, digital TV connection. While capable of very high bandwidth, DVI is limited to just 15 feet for a standard run. Many monitors and some TVs still have DVI connectors.

DVI was quickly superseded by HDMI (High Definition Multimedia Interface), which, unlike its predecessor, can carry both digital audio and video signals on a single cable. It's also theoretically capable of supporting cable runs of up to 50 feet. An HDMI cable carries its signals on four twisted pairs — one each for red, green, and blue (which also carries the sync information), and one for carrying a digital "pixel clock," used to time the data stream.

HDMI is backward-compatible with DVI and DVI with HDCP, but requires an adapter to handle its smaller 19-pin connector. (A 29-pin Type B version has been approved for even higher-resolution displays.) HDMI has the bandwidth to support 1080p 60-Hz high-def video, plus eight channels of uncompressed 192-kHz/24-bit audio. The latest version, HDMI 1.3, doubles the bandwidth to 10.2 Gbps and adds support for 30-, 36- and 48-bit color depths and a new "xvYCC" color-space standard, as well as the new Dolby True HD and DTS-HD Master lossless audio formats available on some Blu-ray Discs and HD DVDs. (For more on HDMI 1.3, see HDMI 1.3: The Missing Link.)

Given the variety of choices available, you'll find no shortage of ways to get your rig up and running. Just remember to use the best available connection types to achieve the best possible performance.

For more on cable construction, see Inside a High-End Audio Cable.

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