What is Impedance?

What is Impedance?

As you probably know if you've been shopping for cables, coax for most video applications is "75 ohm" coax–but what the heck does that mean, anyway? This article is a short, non-technical explanation of impedance, and its significance in video and other circuitry, for those who have always scratched their heads at this one.

Every signal input, and every output, has an impedance–this "impedance" represents the relationship between voltage and current which a device is capable of accepting or delivering. But it's hard to talk about electrons flying around in wire, and volts and amps, without making the whole subject seem abstract and obscure. Let's think about another situation with which we're all familiar: an automobile and its transmission.

An automobile engine can put out a certain amount of power–horsepower, we call it. But the engine, itself, isn't the only thing in the drive train. Between the engine and the road lies the transmission, and a transmission controls the relationship between the speed at which the wheels turn and the power which the wheels are capable of delivering at that speed. When a car is in first gear, it can't go very fast, but the rotational force (torque) delivered to the wheels is very high. For certain types of road, this is just what we need: climbing a 25% grade, slogging through mud, or starting from a dead stop, we don't want the wheels to spin fast, but we do want them to have enough power to move the car steadily along. When the same car is in fourth gear, it can turn the wheels very fast, but each turn contains less power; this works better for the open freeway, after we've built up speed, because if we stayed in first gear we'd have to waste a lot of energy–and move pretty slowly to boot–to get where we're going.

Electricity is all about the flow of electrons in wire, which may seem like a whole different kettle of fish, but it's not. "Voltage" is a measure of how hard the electrons are pressing to get through–it's like water pressure in a pipe, or like the rotational force in our car's wheels. "Current," measured in amps, is a measure of how fast the electrons are flowing–it's like the gallons-per-minute flow in a pipe, or the feet-per-minute rate of spin of the tires on our car. Total power delivery, in an electrical circuit, is measured in watts, which are simply the volts multiplied by the amps; in the same way, the total power delivered by the car in our example is the amount of rotational force delivered by the wheels, multiplied by the speed of rotation. A number of watts may represent a very high voltage with relatively low current (such as we see in high-tension power lines) or a low voltage with very high current (such as we see when a 12-volt car battery delivers hundreds of amps into a starter).

But a car engine can't respond to just any kind of load; if it's delivering 1000 rpm at some torque, we can't simply make it spin 1000 times faster, at 1000 times less torque, or 1000 times slower, at 1000 times more torque. When we try, we reach limits which are based on the physical capabilities of the engine. In the same way, an output circuit–for example, the composite video output of a VCR–can't supply just any combination of voltage and current we want. Instead, it's designed to deliver a signal into a specific kind of load ("load," here, simply meaning the device–such as the TV input–that the signal is being delivered to). The "impedance" of the load represents the opposition to current flow which the load presents.

The impedance of the load is expressed in ohms, and the relationship between the current and the voltage in the circuit is controlled by the impedances in the circuit. When a signal source, such as our composite video output, sees a very low-impedance circuit, it produces a larger than intended current; when it sees a very high-impedance circuit, it produces a smaller than intended current. These mismatched impedances redistribute the power in the circuit so that less of it is delivered to the load than the circuit was designed for–because the nature of the circuit is that it can't simply readjust the voltage to deliver the same power regardless of the rate of current flow. Imagine, again, riding in your car down the Interstate in first gear, flooring the gas pedal and going just as fast as you can. It's obvious, as you watch the cars zip past, that no matter how much horsepower you have under the hood, most of that horsepower isn't getting delivered to the road; instead, a lot of it is burning up in the engine as excess heat, and if you keep this driving up for long, you'll damage your engine. The same thing happens in an impedance mismatch between a source and load; power isn't being transferred properly because the source circuit wasn't designed to drive the kind of load it's connected to. In some electronic applications, this will burn out equipment just like it'll burn out your engine–a radio transmitter must be able to deliver its power into an antenna load that presents the proper impedance or it will self-destruct, and an audio amplifier can easily be destroyed by attaching it to speakers of the wrong impedance. Now, in the case of our car, we have a transmission, and the answer to the mismatch is to shift; but when we're dealing with video input and output circuits, we have no transmission (well, there are such things as impedance transformers–but that's another story). We can't drive on just any kind of road; we need to match the source and load if we're going to deliver the signal properly.
Transmission Line Impedance:

So, when we say that the input impedance of your TV's composite video jack is 75 ohms, that's what we mean. But what does it mean to say that the impedance of the cable between the VCR and TV is 75 ohms? Well, first, it doesn't mean that the cable itself presents a 75 ohm load. If it did, the total load would now be 150 ohms, and you'd have an impedance mismatch. Furthermore, if the cable itself constituted a 75 ohm load, that load would be dependent on length–so a cable twice as long would be 150 ohms, a cable half as long would be 37.5 ohms, and so on.

When we say that the characteristic impedance of a cable is 75 ohms–or 50, 110, 300, or what-have-you–what we mean is that if we attach a load of the specified impedance to the other end of the cable, it will look like a load of that impedance regardless of the length of the cable between. The object of a 75 ohm cable is simply to "carry" that 75 ohm impedance from point A to point B, so that as far as the devices are concerned, they're right next to one another. If we take a hundred feet of 300-ohm television twin-lead cable, solder it to RCA connectors, and stick that in between the TV and VCR, the load, as "seen" by the VCR, will not be 75 ohms. How bad the mismatch is, and what the consequences of it are, will depend on a variety of factors, but it's fair to say that this sort of mismatch needs to be avoided.

So, Why Is It Important?

Transmission line impedance is critical in some applications, and not so critical in others. In analog audio, particularly, impedance is basically a nonfactor–because at the relatively low frequencies involved in analog audio, and at anything approaching ordinary lengths, any reasonably designed cable will effectively "pass through" the impedance of the devices at either end–and the input and output impedances of line-level analog audio devices themselves are usually not critical. For analog audio cables, other design considerations like shielding and capacitance may be very important, but impedance really is not.

But the behavior of cables changes as signal frequencies increase. This is so because as frequency increases, the electrical "wavelength" of a signal becomes shorter and shorter; at video frequencies, signal wavelength is short enough to start causing problems. As the length of a cable becomes closer to a large fraction of the electrical wavelength of the signal it carries, the cable can resonate at that wavelength or a closely related wavelength, and an impedance mismatch can result in portions of the signal being lost. Basically, what happens is that an impedance mismatch reflects portions of the signal back down the cable, away from the load and toward the source; these reflected portions can set up what is called a "standing wave" in the cable–a signal that never leaves the cable to enter the load, but just sustains itself on a portion of the energy fed by the source. How much of the signal is lost in this way is highly dependent upon frequency and the length of the cable, and this can wreak havoc, because a complex video signal is not just a single frequency, but a whole range of frequencies spread across a large bandwidth.

This effect is dependent upon frequency, and that dependency makes it particularly relevant for digital signals. Where analog audio or video signals consist of electrical waves which rise or fall continuously through a range, digital signals are very different–they switch rapidly between two states representing bits, 1 and 0. This switching creates what we call a "square wave," a waveform which, instead of being sloped like a sine wave, has sharp, sudden transitions. Although a digital signal can be said to have a "frequency" at the rate at which it switches, electrically, a square wave of a given frequency is equivalent to a sine wave at that frequency accompanied by an infinite series of harmonics–that is, multiples of the frequency. If all of these harmonics aren't faithfully carried through the cable–and, in fact, it's physically impossible to carry all of them faithfully–then the "shoulders" of the digital square wave begin to round off. The more the wave becomes rounded, the higher the possibility of bit errors becomes. The device at the load end will, of course, reconstitute the digital information from this somewhat rounded wave, but as the rounding becomes worse and worse, eventually there comes a point where the errors are too severe to be corrected, and the signal can no longer be reconstituted. The best defense against the problem is, of course, a cable of the right impedance: for digital video or SPDIF digital audio, this means a 75 ohm cable like Belden 1694A or Canare L-5CFB; for AES/EBU balanced digital audio, this means a 110 ohm cable like Belden 1800F.

Choosing the Right Impedance Cable and Connectors

Fortunately, for most applications, it's very easy to choose the right impedance cable. All common home video standards use 75 ohm cable, as do coaxial digital audio connections. If you have balanced AES/EBU type digital audio lines, you'll want 110 ohm AES/EBU cable. There are a few others you may bump into, however, and it's good to be aware of them. RG-58 coax, such as is often used for coaxial computer network connections or for CB or ham radio antenna lines, is 50 ohms–not suitable for video use. Twin-lead cable–the two wires separated by a band of insulation that used to be the most common way to hook up a TV antenna–is a 300 ohm balanced line, also unsuited for home video interconnection, and if you need to hook a 300 ohm antenna line to a 75 ohm video jack, or a 75 ohm antenna line to an old two-screw antenna connection on your TV, you'll want a little impedance transformer/balun, readily available at any electronics shop, to link the two properly.

Connectors have impedance, too, and should be matched to the cable and equipment; many BNC connectors, especially on older cables, are 50 ohm types, and so it's important to be sure that you're using 75 ohm BNCs–like those from Canare–when connecting video lines. RCA connectors can't quite meet the 75 ohm impedance standard because their physical dimensions just aren't fully compatible with it, but there are RCA plugs–Canare, again, being a prime example–which are designed for the best possible impedance match with 75 ohm cable and equipment.

Transmission line impedance can be a bit confusing, and of course this discussion just scratches the surface; but we hope it's been helpful to you in understanding just what "impedance" means and why it's important in video and digital audio applications.

by Blue Jeans Cable (reprinted with permission)


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