The Role of Cabling in a Top-Quality Home Theater Installation

Many of our customers are involved in doing a thorough home theater installation, and have questions regarding what sorts of cables they’ll need for that home theater installation. It’s not unusual for cabling to be the very last thing that comes to mind — after drywall, after equipment placement, after acoustic treatment. While it’s hard to get a complex job organized in a completely logical sequence, cabling certainly should be among the first subjects that gets addressed in planning a home theater installation, rather than being among the last.

Why is that? Well, the cleanest installations, and the most professional-looking, are generally those where a good job has been done to conceal such things as cables from view. But concealed installation of cables, in walls or otherwise, is easier to do early in the job when large parts of the job haven’t been completed. If you’re doing drywall, acoustical treatment, or even just a coat of paint, it’s easier to get cable cleanly installed before, not after, your finishes are all on and your walls are all closed up. As an added benefit, a well-thought-out home theater installation will tend to eliminate the need to go back and add more cabling at a later date.

At the outset, to be able to approach cable planning in a logical way, one has to consider the layout of the home theater installation. Sometimes, all equipment is well-contained on a single rack, with the display on top, which presents very few cabling challenges; other times, the equipment may be on a rack and the display in another location; cables may be run in surface-mount raceways, or through walls or ceilings, to the display, and this may present issues for cable concealment. Also, it’s important to consider how and where signals enter the room; lines from outside sources, such as a satellite dish, an off-air antenna, or a CATV system need to be taken into account in planning for routing and concealment. Lastly, consider speaker wires; will they be run along baseboards and behind furniture, or will you need to run them inside of walls or in other concealed locations?

What Cables Do I Need?

In doing a home theater installation, it’s important first to consider the placement and installation of those cables that will be difficult or impossible to install at a later stage of the job. Patch cables reaching from one component to another on an equipment rack are easy to deal with at any time, all the way up to the day equipment is put in place; but in-wall runs to displays, speakers and the like are best dealt with early.

The most puzzling problem for many people at this point in planning a home theater is deciding what sorts of cables to run to the display. Obviously, you’ll need to determine what types of signals — component, DVI, composite, HDMI, s-video — you’ll be running to the display. A couple of tips here:

• It’s important to keep in mind that certain equipment does not, and likely will never, support component video, DVI or HDMI output. If you’re running a standard VHS deck, for example, the output will be available as composite or RF modulated video only. An S-VHS deck will ordinarily support either composite or s-video; and many satellite receivers do likewise. If you’ll need to be able to play VHS tapes, or run other composite or s-video lines to the display, you’ll want to make sure that the appropriate cabling.
• DVI and HDMI cables, due to the relatively large connector size, are particularly difficult to install after-the-fact. Even if your current display doesn’t use these signals, you may want to consider installing cable in the interests of future-proofing.
• Does your home theater display have speakers, and will you ever use them? If you’re running cable to a display with built-in speakers, but never plan to use them, you can just run video, and no analog audio. However, if you’re only going to use your surround receiver with some sources and not others, and want your display’s speakers to work when you’re just catching a little morning news, you’ll want to run right/left analog audio to the display.

Conduit or No Conduit?

Many people do the wiring for a home theater installation by first running a lot of conduit in walls, and then pulling cable in as needed. As often as not, this turns out rather badly. It’s easy to underestimate the size of conduit required, and we have had countless calls from people who suddenly, at the peak of their installation work, need to pull a large amount of cable through an undersized conduit. If you’re going to use conduit, we recommend installing the largest conduit your wall cavity will accommodate; 3/4 inch conduit will give you lots of grief unless your cabling needs are exceedingly modest.

The best use of conduit in most home theater installations is as a future-proofing device rather than as a primary means of installing cable. Rather than installing cable in conduit, consider installing cable and conduit. As long as you have access to the space where the conduit will go, it’s generally easier to install the cable alongside, and the conduit then provides some assurance that, in the event that you need to run new types, or duplicate runs, of cable, you’ll have a convenient way to get them in.

If, however, you need to run conduit and then pull cable through it, there are a few tips that will make life easier:

• Again: install the largest conduit your wall cavity will accommodate. If you’re having this work done by an electrician, and he balks, assure him that this is what you want. Electricians rarely, in residential work, need to install large conduit, and are often skeptical of the need for it — but they’re usually not dealing with cables with limited pull strength, large dimensions, and pre-installed connectors.
• Don’t use bundled cables if you can avoid it. Cables like the “structured wiring” products found in home improvement stores, or the multi-coax bundles from Belden (e.g. 7710A) aren’t flexible enough to be installed in conduit, especially if there are bends in the line.
• When pulling cable, be sure to stagger connectors, so that there isn’t one wide “blob” of connector bodies at the leading edge of the pull.
• Always have someone able to “feed” the cable at the source as it’s being pulled. Coaxial cables won’t twist easily, and so it’s important to be sure that they’re being fed straight into the entry rather than, say, being left in a coil on the floor and being pulled through the conduit in a twisted fashion.

NEC Fire Code Ratings for Home Theater Cable Installation:

Code compliance in home theater cable installation is another important consideration; many local governmental jurisdictions use the National Electrical Code, which sets requirements for fire safety for wiring to be installed in buildings. These requirements don’t apply to patch cords between devices, because those are not deemed to be “installed” wiring, but anything that will be run through walls, behind baseboards, under floors or over ceilings needs to be NEC-rated for the installation.

As a general rule, any of the following NEC ratings — which are usually printed on the cable jacket — are suitable for home wiring: CMP, CL3P, CL2P, CMR, CL3R, CL2R, CM, CMG, CL3, CL2, CMX, CL3X, CL2X, CATV, CATVR, CATVP, CATVX. There are some limitations and special circumstances, however; for more detail, see our article on NEC ratings.

Pass-through Wall Jacks: Neat Entry and Exit Points

Another question to be considered is how cable will come out of a wall. There are basically two choices: one can run cables out of an open hole in a wall, straight to the device being connected, or one can use a pass-through wallplate jack, and connect the device to the jack with a cable. The latter, of course, is a familiar solution — most cable tv installations are handled in just that way.

There are advantages to the passthrough wallplate jack that are often overlooked, and so we’ll touch on some of those here. First, since many different applications can use the same cables, it’s possible to have an extremely versatile installation by using passthrough jacks. For example, one Belden 1694A cable can accommodate digital audio, composite video, a CATV input, or a satellite antenna line; two can take stereo audio or s-video in “breakout” form; three can take component video; and so on. In our own theater room at home, we have ten 1505A cables running about fifteen feet from a couple of wallplates down to a sort of “patch panel” setup in the basement, so that any of the ten BNC jacks can be assigned any function just by plugging in the appropriate cables and devices. All ten were measured to the same length prior to installation, so that there can be no issue of mis-timing. We use some to bring in satellite signal, one to bring in cable TV signal, one to carry out a mix of RF-modulated signals so that we can watch content from any source at any location in the house — and we have more in reserve should we want to route, say, component video and digital audio out of that location to another display elsewhere in the home.

The passthrough jack arrangement is also remarkably flexible in terms of equipment placement; if cables are run directly out of the wall to where devices are, Murphy’s law holds that sooner or later, you’ll realize that you want to move all of the devices another five feet away, and that can be a mess. If instead you’ve used passthrough jacks, it becomes just a matter of buying a somewhat longer set of patch cords to go from the wall jacks to the equipment.

Passthrough jacks are available in a variety of connector types, and that raises the question of what sorts of connections you’ll want to make if you use passthroughs. For some applications, it’s fairly obvious; for speaker connections, for example, you’ll probably want binding posts rather than any other connector type. But for other applications, what people tend to do is use the connector types they’re most familiar with — F for RF connections, RCA for audio and baseband video, mini-DINs for s-video. The best approach generally is to simplify the situation by using only one connector type for all of your coaxial-type passthrough jacks, and what we recommend is to use nothing but BNCs. The BNC has the best locking characteristics of any of the commonly available connector types (RCAs don’t lock at all, and F connectors sometimes need to be tightened quite far to make solid and dependable connection). Since half of your connections are actually inside the wall and inaccessible, it’s very nice to know that you don’t have to worry about connections coming loose, and only the BNC provides really good assurance on that point. We can build any patch cord for any coaxial-type cable with BNC connectors at one end, so it’s not difficult to procure the patch cords (and adapters, should one need them, are readily available).

We’ve found in talking to our customers that a lot of people have a fundamental discomfort with passthrough jacks. They have heard that every connector in the signal path “degrades” the signal, and they worry that running one cable from the source to a wallplate, another inside the wall from wallplate to wallplate, and another from the wallplate to the destination, will cause some sort of loss of signal information. We don’t know what the source of this anti-adapter view originally was, but the concern is vastly out of proportion to the issue. Basically, the only concern with adapters is that they should be mechanically sound (so that the connections made with them don’t suffer from intermittency) and, for video and digital audio signals, they should be impedance-matched to the cable (that is, they should be 75 ohm impedance adapters if possible). 75 ohm passthrough BNCs are readily available; and even a 50 ohm BNC passthrough isn’t going to cause enough impedance mismatch to get concerned over. Adapters, if they make for a good mechanical and electrical connection, do not degrade signal.

Cable Color — a Handy Option for Low-Cost, Low-Effort Concealment

If boring lots of holes in walls, floors and ceilings for a home theater installation is something you’d like to avoid, here’s another simple cable concealment suggestion which often works well: cable color choice to match your room. Many of our cables are available in a variety of jacket colors, which can be selected by using the dropdown option boxes when shopping. If you have white baseboards, white cable sometimes will be almost invisible to a casual observer when run closely along the baseboard — especially if the baseboard itself is sometimes obscured by furniture. Although color match can be an issue, the same of course goes for the various other cable jacket colors.

Have Fun!

Above all, of course–have fun. In the middle of cable installation, that’s always not easy to do; there can be a lot of scuttling about in crawlspaces, sawdust in the eyes, and minor frustrations along the way. When you’re in the middle of those sorts of little challenges and difficulties, it’s always heartening to know that you’re doing your home theater installation right the first time–which, of course, means that you might not have to do it again for a long while, and that there’ll be lots of bags of popcorn in between now and then.

This article has been reprinted with permission

There was a time, not so long ago, when television hookups were pretty simple. There were two screws on the back of the set, which went to two spade lugs on a 300-ohm twinlead antenna line, and that was it. Apart from plugging the set into the wall and waiting for the tubes to light up, there wasn’t a whole lot else to know about connections.

That all changed with the advent of a series of technologies: cable television; the VCR; the S-VHS VCR; the LaserDisk and DVD players; the satellite receiver; the PVR; the Home Theater PC; and High-Definition TV. Now, instead of video coming into the home and being handled in a single run of twin-lead antenna line, we have a huge assortment of video standards to handle, and a variety of cable types, terminations, and configurations to deliver them.

One of the most common sources of confusion in all of this is the tendency to mix terms when talking about three distinct concepts: signal formats, cable types, and connector types. Often-asked questions like “can I get a cable to go from RCA to s-video?” or “can I go from component to BNC?” suggest that the mixing of these terms hasn’t aided understanding.

Connectors are one thing; internal cable structures are another; and signal types are yet another, and it’s important to separate out these concepts in order to understand what can, and can’t, be hooked together, and how. It’s almost always possible to fabricate a cable which will physically join two components; but whether that cable, once installed in the system, will actually successfully convey a signal from point A to point B is another question, which has more to do with signal types than with connector types.

A cable is a transmission line; its function is to get signals from point to point without meaningful alteration. Consequently, when one has incompatible source signals and destinations, a cable won’t solve the problem. One can’t simply wire up a cable with an F-connector at one end and a DVI-D plug at the other and expect to pull digital video out of an antenna. What’s more…and more confusing: connections that look perfectly compatible with one another can be completely incompatible. A device with red, green and blue jacks running sync-on-green RGB can be plugged into a device expecting Y/Pb/Pr component video, but the two can’t make sense of each other. But two devices both running Y/Pb/Pr component video, one through BNCs and the other through RCAs, can be hooked together with a cable and will work fine, despite the dissimilarity of connector types.

A Whirlwind Tour through the World of Signal Types:

Because the best way to understand what can, and can’t, be hooked together is to understand just what kind of signal is running through the line, here’s a quick description of the common signal types–some you’ll see on almost every piece of gear, and some are not so common. If you have two pieces of equipment, one putting out and the other receiving the same signal type, they can talk to one another as long as you can come up with a cable to join them.

1. RF (Radio Frequency) Modulated Television:

RF, or Radio Frequency, is the type of signal that comes through the air by antenna or through a cable tv connection. In standard-definition broadcast and analog cable, a composite video signal and accompanying audio are mixed, at the transmitting end, with high-frequency radio waves, and are broadcast through the air or distributed through a cable system. To be viewed on a display, these signals have to be separated from the other channels in the line and converted to unmodulated “baseband” video and audio signals using a television tuner (found in any conventional television set or VCR). RF is used as a distribution medium because (1) it propagates through the air very well, making it suitable for over-the-air broadcast, and (2) many video signals can be modulated at many different frequencies, it’s possible for us to have many “channels” available simultaneously without having them interfere with one another.

2. Composite Video:

Composite Video is a single signal which carries both the chrominance (color) and luminance (brightness) components of a video signal, along with sync information, on a single wire. Unlike an RF signal, a composite video signal does not need to be demodulated to be understood by a video display. Like other baseband video formats, a composite video signal does not carry any audio content, which must be handled separately.

3. S-Video:

S-video is a format which splits the chrominance and luminance out onto two separate lines, “C” and “Y,” each requiring its own cable; the sync pulses are carried on the luminance line. Why, then, does an s-video cable usually look like just one cable rather than a pair of cables? We’ll get to that further below.

4. Component Video:

“Component Video” is an unfortunate sort of name, in that other formats have used this name over the years, leading to some potential for confusion; but today, the expression “component video” ordinarily refers to “Y/Pb/Pr,” also known as “YUV,” video. In Y/Pb/Pr Component Video, there is a luminance channel, “Y,” which carries the luminance along with the sync pulses, and two color-difference channels, which carry signals representing Blue minus Luminance (B-Y, or Pb) and Red minus Luminance (R-Y, or Pr). From these signals, the display device separates out the sync information and reconstitutes the red, green and blue components of the picture. Just as s-video requires two signal-carrying wires instead of one, component video requires three to convey the whole signal.

5. RGB and its variants: RGsB, RGBS, RGBHV:

The original “component video” was RGB, which appears in three principal varieties, each requiring a different number of connections. The most common type is RGBHV, with five lines: one for red, one for green, one for blue, one for the horizontal sync and one for the vertical sync. RGBHV is the standard used in VGA and other analog PC computer monitors. RGBS, having four connections, differs from RGBHV in having the vertical and horizontal sync combined on a single channel, while RGsB, or “sync-on-green,” places the sync information on the green channel–not unlike, but still not compatible with, Y/Pb/Pr component video.

6. DVI and its several flavors: DVI-D, DVI-A, DVI-I

DVI is a tad confusing because the term is identified both with more than one signal type and more than one connector type. “DVI-A” is nothing but RGBHV in a funny connector, and isn’t digital at all. “DVI-I” isn’t really a signal type, but refers, as we’ll review later, to a connector type which combines DVI-A and DVI-D. DVI-D is a parallel digital standard–a nasty little tangle of wires in a nasty little plug–which consists of up to seven balanced lines (all other common video standards are run unbalanced) carrying the video itself, and five miscellaneous conductors carrying other information. Because this is a digital rather than an analog signal, it can only be converted to another format through a device that is equipped to decode the digital bitstream and render it in analog form. Similar to DVI is HDMI, a standard intended to be backward-compatible with DVI and employing the same encoding/decoding scheme.

7. SDI:

SDI is serial digital video, run in an unbalanced line unlike DVI, and used primarily in professional production environments. You’re unlikely to see it in a conventional home theater application, but we can always hope…

Cable Types:

The cables for the applications above differ, but not so much as one might think. All of the unbalanced analog and digital standards, from RF down through SDI, are run in 75 ohm coaxial cables. This fact, in itself, seems to confuse people; it is widely assumed that “coax” is something used for RF, or for SPDIF digital audio, and that composite video or component video are run in a different type of cable suited particularly for those formats. In fact, the only differences are small; RF is frequently, but not always, run in cables using copper-coated steel conductors for higher strength and lower cost; SDI is generally run in “precision” video cables because its wide bandwidth requires very tight impedance tolerance; but these cables are all “coax.” Even s-video is only apparently an exception. A round s-video cable is just a round jacket over two miniature coaxes, one carrying luminance and the other chrominance.

What makes a coax a coax is simply that the signal and ground conductors are “co-axial,” that is, they share a common axis. At the center of a coax is a wire; at an even distance from that wire, surrounding it and separated from it by an insulating dielectric, is a shield. Because the axis of the cylindrical shield is the same as the axis of the center conductor, the structure is said to be coaxial.

DVI and HDMI are run in cables which are particular to their own applications. The Digital Display Working Group, which designed the DVI standard, chose to run high-bitrate parallel digital video through a set of twisted-pair balanced lines, which by spec are supposed to be 100 ohms plus or minus 10 percent. Running high-bitrate information through tiny parallel twisted pairs with no possibility for error correction is something of an invitation to disaster, and the poor design of the standard has much to do with the uneven reliability of DVI cables in general. Simply running the signals unbalanced and using coaxial cables, with their far tighter impedance tolerance (+/- 2% as spec’d, much better in actual practice) and consequently better return loss performance, would have resulted in a far more robust standard capable of longer runs.

Connector Types:

As we’ve pointed out, it’s always possible to hook up two devices that employ the same video signal type, regardless of whether they use the same connector; the only problem is in making sure you’ve got the connectors you need at both ends. When trying to puzzle out a connection problem, therefore, the important issue is always, first, ensuring that you’re really sending a signal of type A into an input of type A. There’s no such thing as “RCA video,” but there is such a thing as composite video coming out of an RCA jack, or component video coming out of three RCA jacks.

Here are some common connector types, and what they are commonly used for:

1. The RCA Plug and Jack:

The RCA is the most common connector type on consumer gear for composite and component video, as well as for both digital and analog audio. It’s not a very good connector, as connectors go, but as it’s what equipment manufacturers have given us, it’s what we often have to use. RCA jacks color-coded yellow on a device usually are composite video inputs or outputs. If you’ve got a single RCA jack on the back panel, labeled “video” or something similar, that’s almost certainly composite. Component video is usually represented by three RCA connections color-coded green (Y, or Luminance), blue (Pb) and red (Pr). RGBHV will usually, though not always, be color-coded red, green, blue, yellow (horizontal sync) and white (vertical sync). Some devices will have labeling for both RGBHV and Y/Pb/Pr; this signifies that the device is capable of supporting either RGBHV or Y/Pb/Pr, using all five or only three connections; read your manual for details.

2. The BNC Plug and Jack:

The BNC is the standard connector for most video signals on professional gear, and is showing up increasingly on high-end consumer gear as well. It will be labeled similarly to the RCA, indicating composite video (one connection), Y/C s-video (two connections), Y/Pb/Pr (three connections), or one form or another of RGB. The most common confusion with BNCs, in our experience, is that people often assume the female connector is a male; the problem is that both the male and female connectors have what looks like a pin in the center. On closer inspection, however, you’ll see that a female BNC’s “pin” is actually a receptacle for the male pin. A panel-mounted BNC will ALWAYS be female; a cable-mounted BNC will almost always be male, though there are exceptions (such as our breakout adapters, which have female BNCs to join with standard cable-mount male BNCs).

3. F-Connectors:

The F-connector is the screw-on type connection used for most antenna and cable TV connections. F-connectors are rarely used for anything other than RF; the one notable exception being that they were used as digital audio connectors on some laser disk players.

4. The 4-pin mini-DIN Plug:

The common s-video plug on consumer gear is a four-pin mini-DIN plug, and is, frankly, an awful choice for video. It has a tendency to unplug itself at the slightest urging, and its small profile mandates the use of tiny video cable to allow two coaxes into the cable entry hole. It does, however, at least have the merit of being readily recognizable.

5. The HD15 / mini dSub 15 / VGA connector:

An increasing number of devices are showing up with 15-pin connectors; there are about as many names as pins for this connector, which is well known as the plug used with most PC computer monitors and consequently is often called a “VGA” plug. Since VGA is an RGBHV-type video signal, however, this usage is a bit confusing; this same plug is used not only for RGBHV, but for RGBS, RGB sync-on-green, and Y/Pb/Pr Component video. Because the plug can be used with so many different video standards, it’s very important, when you want to use a 15-pin connector on a device, to be sure you know what sort of video it can put out or take in. Many projectors currently on the market, for example, can accept either Y/Pb/Pr component video or RGBHV through a 15-pin plug, but some will accept only RGBHV. Fortunately, the “pinout” is the same either way; a cable designed to carry RGBHV will carry Y/Pb/Pr on the Green/Blue/Red lines, respectively, so that all one needs to do is match up the color-coding on the plugs.

5. DVI Connectors:

DVI Connectors come in a few types; the most important, in general, are DVI-I and DVI-D. The difference between the two is that a DVI-I connector has extra pins at one end, which carry most of the analog video signal. A DVI-I cable can be used either for a digital or analog signal, because it contains both the digital and analog pins. But a DVI-D socket, being designed to take a DVI-D plug, will ordinarily lack any place for the analog pins on a DVI-I plug to go; accordingly, it’s important to be sure that the cable you buy will actually plug in to the equipment you own.

So, What Do I Do if My Signals Are Incompatible?

The above may help you figure out whether your connection problem can be solved just by buying a cable to link two devices together, or whether the problem is deeper, involving a difference in signal types. If you do need to connect dissimilar signals, what can you do?

In general, the answer to this question is that you need a device capable of converting one signal format to another. First, let’s look at some simple cases. If you’re trying to convert an RF signal into composite video and analog audio, any tuner will do; even a VCR with a broken tape transport can be pressed into service here. If you’re trying to do the opposite, a VCR can be used to modulate a composite video and analog audio input into an RF signal, typically only on channels 3 or 4; the one drawback being that, if you’re running a Macrovision copy-protected source, the modulator will work very poorly and produce a picture you won’t want to watch. Alternatively, one can buy a cheap RF modulator–commonly available because of the lack of RF outputs on most DVD players–or, for a bit more money, an “agile” modulator which will put out a signal not just on channels 3 or 4, but on any of a range of channels. If you’re trying to convert s-video to composite, or vice versa, a simple passive converter can be had for a few dollars.

Beyond those simple conversions, it gets dicey. The most common request we see is for a device to convert Y/Pb/Pr component video to RGBHV, to run a DVD player through a computer monitor, or the reverse–RGBHV to component–to run a computer through a TV display. For complicated tasks like that, one needs a device called a “transcoder,” and these range in price from a hundred dollars or so up to thousands, depending on the flexibility and signal quality required. In many applications, it turns out to be less expensive to simply replace the incompatible device than to buy an outboard transcoder to solve the connectivity problem. Devices like these are generally available online from broadcast-industry supply houses, such as Markertek.

In Conclusion:

We hope this tour through the subject of signal types, cable types, and connector types hasn’t been too confusing, and that we may have answered a question or two. If you have a connectivity problem you can’t work out, feel free to give us a shout.

This article has been reprinted with permission

As DVI and HDMI connections become more and more widely used, we are often asked: which is better, DVI (or HDMI) or component video? The answer, as it happens, is not cut-and-dried.

First, to clear away one element that can be confusing: DVI and HDMI are exactly the same as one another, image-quality-wise. The principal differences are that HDMI carries audio as well as video, and uses a different type of connector, but both use the same encoding scheme, and that’s why a DVI source can be connected to an HDMI monitor, or vice versa, with a DVI/HDMI cable, with no intervening converter box.

The upshot of this article–in case you’re not inclined to read all the details–is that it’s very hard to predict whether a digital DVI or HDMI connection will produce a better or worse image than an analog component video connection. There will often be significant differences between the digital and the analog signals, but those differences are not inherent in the connection type and instead depend upon the characteristics of the source device (e.g., your DVD player) and the display device (e.g., your TV set). Why that is, however, requires a bit more discussion.

What are DVI, HDMI and Component Video?

DVI/HDMI and Component Video are all video standards which support a variety of resolutions, but which deliver the signal from the source to the display in very different ways. The principal important difference is that DVI/HDMI deliver the signal in a digital format, much the same way that a file is delivered from one computer to another along a network, while Component Video is an analog format, delivering the signal not as a bitstream, but as a set of continuously varying voltages representing (albeit indirectly, as we’ll get to in a moment) the red, green and blue components of the signal.

Both DVI/HDMI and Component Video deliver signals as discrete red, green, and blue color components, together with sync information which allows the display to determine when a new line, or a new frame, begins. The DVI/HDMI standard delivers these along three data channels in a format called T.M.D.S., which stands for “Transmission Minimized Differential Signaling.” Big words aside, the T.M.D.S. format basically involves a blue channel to which horizontal and vertical sync are added, and separate green and red channels.

Component Video is delivered, similarly, with the color information split up three ways. However, component video uses a “color-difference” type signal, which consists of Luminance (the “Y”, or “green,” channel, representing the total brightness of the image), Red Minus Luminance (the “Pr,” or “Red,” channel), and Blue Minus Luminance (the “Pb,” or “Blue,” channel). The sync pulses for both horizontal and vertical are delivered on the Y channel. The display calculates the values of red, green and blue from the Y, Pb, and Pr signals.

Both signal types, then, are fundamentally quite similar; they break up the image in similar ways, and deliver the same type of information to the display, albeit in different forms. How they differ, as we’ll see, will depend to a great extent upon the particular characteristics of the source and display devices, and can depend upon cabling as well.

Isn’t Digital Just Better?

It is often supposed by writers on this subject that “digital is better.” Digital signal transfer, it is assumed, is error-free, while analog signals are always subject to some amount of degradation and information loss. There is an element of truth to this argument, but it tends to fly in the face of real-world considerations. First, there is no reason why any perceptible degradation of an analog component video signal should occur even over rather substantial distances; the maximum runs in home theater installations do not present a challenge for analog cabling built to professional standards. Second, it is a flawed assumption to suppose that digital signal handling is always error-free. DVI and HDMI signals aren’t subject to error correction; once information is lost, it’s lost for good. That is not a consideration with well-made cable over short distances, but can easily become a factor at distance.

So What Does Determine Image Quality?

Video doesn’t just translate directly from source material to displays, for a variety of reasons. Very few displays operate at the native resolutions of common source material, so when you’re viewing material in 480p, 720p, or 1080i, there is, of necessity, some scaling going on. Meanwhile, the signals representing colors have to be accurately rendered, which is dependent on black level and “delta,” the relationship between signal level and actual as-rendered color level. Original signal formats don’t correspond well to display hardware; for example, DVD recordings have 480 lines, but non-square pixels, and they have color recorded in color-difference format, while HDMI ordinarily runs in RGB colorspace. Few displays correspond very well to any common output resolution; instead of 720 lines or 1080, they often will have 768, or 1024, or some other number of lines. What all of this means is that there is signal processing to go on along the signal chain.

The argument often made for the DVI or HDMI signal formats is the “pure digital” argument–that by taking a digital recording, such as a DVD or a digital satellite signal, and rendering it straight into digital form as a DVI or HDMI signal, and then delivering that digital signal straight to the display, there is a sort of a perfect no-loss-and-no-alteration-of-information signal chain. If the display itself is a native digital display (e.g. an LCD or Plasma display), the argument goes, the signal never has to undergo digital-to-analog conversion and therefore is less altered along the way.

That might be true, were it not for the fact that digital signals are encoded in different ways and have to be converted, and that these signals have to be scaled and processed to be displayed. Consequently, there are always conversions going on, and these conversions aren’t always easy going. “Digital to digital” conversion is no more a guarantee of signal quality than “digital to analog,” and in practice may be substantially worse. Whether it’s better or worse will depend upon the circuitry involved–and that is something which isn’t usually practical to figure out. As a general rule, with consumer equipment, one simply doesn’t know how signals are processed, and one doesn’t know how that processing varies by input. Analog and digital inputs must either be scaled through separate circuits, or one must be converted to the other to use the same scaler. How is that done? In general, you won’t find an answer to that anywhere in your instruction manual, and even if you did, it’d be hard to judge which is the better scaler without viewing the actual video output. It’s fair to say, in general, that even in very high-end consumer gear, the quality of circuits for signal processing and scaling is quite variable.

Additionally, it’s not uncommon to find that the display characteristics of different inputs have been set up differently. Black level, for example, may vary considerably from the digital to the analog inputs, and depending on how sophisticated your setup options on your display are, that may or may not be an easy thing to recalibrate.

The Role of Cable and Connection Quality

Cable quality, in general, should not be a significant factor in the DVI/HDMI versus Component Video comparison, as long as the cables in question are of high quality. There are, however, ways in which cable quality issues can come into play.

Analog component video is an extremely robust signal type; we have had our customers run analog component, without any need for boosters, relays or other special equipment, up to 200 feet without any signal quality issues at all. However, at long lengths, cable quality can be a consideration–in particular, impedance needs to be strictly controlled to a tight tolerance (ideally, 75 +/- 1.5 ohms) to prevent problems with signal reflection which can cause ghosting or ringing.

DVI and HDMI, unfortunately, are not so robust. The problem here is the same as the virtue of analog component: tight control over impedance. When the professional video industry went to digital signals, it settled upon a standard–SDI, serial digital video–which was designed to be run in coaxial cables, where impedance can be controlled very tightly, and consequently, uncompressed, full-blown HD signals can be run hundreds of feet with no loss of information in SDI. For reasons known only to the designers of the DVI and HDMI standards, this very sound design principle was ignored; instead of coaxial cable, the DVI and HDMI signals are run balanced, through twisted-pair cable. The best twisted pair cables control impedance to about +/- 10%. When a digital signal is run through a cable, the edges of the bits (represented by sudden transitions in voltage) round off, and the rounding increases dramatically with distance. Meanwhile, poor control over impedance results in signal reflections–portions of the signal bounce off of the display end of the line, propagate back down the cable, and return, interfering with later information in the same bitstream. At some point, the data become unrecoverable, and with no error correction available, there’s no way to restore the lost information.

DVI and HDMI connections, for this reason, are subject to the “digital cliff” phenomenon. Up to some length, a DVI or HDMI cable will perform just fine; the rounding and reflections will not compromise the ability of the display device to reconstruct the original bitstream, and no information will be lost. As we make the cable longer and longer, the difficulty of reconstructing the bitstream increases. At some point, unrecoverable bit errors start to occur; these are colloquially described in the home theater community as “sparklies,” because the bit errors manifest themselves as pixel dropouts which make the image sparkle. If we make the cable just a bit longer, so much information is lost that the display becomes unable to reconstitute enough information to even render an image; the bitstream has fallen off the digital cliff, so called because of the abruptness of the failure. A cable design that works perfectly at 20 feet may get “sparkly” at 25, and stop working entirely at 30.

In practice, it’s very hard to say when a DVI or HDMI signal will fail. We have found well-made DVI and HDMI cables to be quite reliable up to 50 feet. But because the ability to reconstitute the bitstream varies depending on the quality of the circuitry in the source and display devices, it’s not uncommon for a cable to work fine at 30, 40, or 50 feet on one source/display combination, and not work at all on another.

The Upshot: It Depends

So, which is better, HDMI or component? The answer–unsatisfying, perhaps, but true–is that it depends. It depends upon your source and display devices, and there’s no good way, in principle, to say in advance whether the digital or the analog connection will render a better picture. You may even find, say, that your DVD player looks better through its HDMI output, while your satellite or cable box looks better through its component output, on the same display. In this case, there’s no real substitute for simply plugging it in and giving it a try both ways.

This article has been reprinted with permission