|
 |
Signal Types and Connector Types;
Keeping them Straight
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.
7. HDMI
HDMI is much more straightforward than DVI: only one type of signal, and only through a 19-pin connector (though there are mini and micro versions of the connector). HDMI is all digital and runs through a bundle with four primary signal pairs (originally 3 for video and one for clock, but in later versions all four are used for video). An additional pair is used if Ethernet connectivity is provided, though few devices use it.
8. 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.