Q19: I've read in the magazines that 75 Ohm characteristic impedance matching is important in my system and my cables. What does that mean? What IS characteristic impedance? How does it affect my System? Is it really all that crucial?
A19: Video and coaxial digital signals and equipment are specified by industry agreement to have a characteristic impedance of 75 Ohms. Although "Ohms" is the quantifying unit for resistance, resistance isn't what is being referred to in this case. Because video and digital signals are always either very high frequency AC or very rapidly occurring digital pulses, they are affected not only by resistance but also by capacitive and inductive reactance. The combination product of all three of these factors is called "impedance", and impedance is to AC and pulsed signals essentially what simple resistance is to a DC current. The "characteristic" impedance of a circuit, a cable or a connector refers to its characteristics when used as a transmission line for high frequency or pulsed signals and the most important thing about characteristic impedance is not its specific value, but whether the characteristic impedances of all the elements to be used together (components, cables, connectors) match.
Matching characteristic impedances in low impedance circuits is always a good idea, and the Hi-Fi and Home Theater magazines are right when they say that a 75 Ohm source and a 75 Ohm load should always be matched to a 75 Ohm cable terminated with 75 Ohm connectors. The problem is that even components, connectors or cables that fall within the normal range of variance allowed by their specifications can still be 20% apart from each other , and XLO has actually measured input and output impedances on High End components that were claimed to be "75 Ohm" but were really as low as 3 Ohms or as high as 200.
The point of all this is that, regardless of what your equipment's claimed characteristic impedance may be, it may still be impossible to match it correctly. Does this matter? Maybe. It all depends on the frequency of the signal that you want to pass and the length of the cable that you want to pass it through.
The reason for matching characteristic impedances is that in a perfectly matched transmission line, where all of the characteristic impedances of all of the components, cables and connectors is identical, all of the signal energy that is put in at one end of the line will be passed through and come out at the other. In an IMPERFECTLY matched transmission line, though, some of the signal energy will not be passed through, but will hit a point of mismatch and be reflected (bounced) back to its source.
It's this reflected energy that creates problems. Heading back down the cable like a driver going the wrong way on a one way street, the reflected signal energy "heterodynes" (adds algebraically to form a new signal) with the energy coming in the opposite direction and produces -- just as one example - the black bars or diamond shapes that appear in the picture of a video system using mismatched components or a too-long S-video cable.
Heterodyning artifacts are "noise", in that they add something to the signal, and they can be a serious problem in a video or digital transmission system. Here's how they come about:
All reflected energy can be described by its frequency of reflection and by its relative amplitude, as compared to signal level. The frequency of reflection in a cable is easily calculated by simply dividing the length of the cable in meters into 300 million, the speed of light expressed in meters per second. Doing this, it's easy to see that the frequency of reflection in a one meter cable will be 300 megahertz (300,000,000 meters ) 1 = 300,000,000); that the frequency of reflection in a 2 meter cable will be 150 megahertz (300,000,000 ) 2 = 150,000,000), and so on, the longer the cable, the lower the frequency of reflection.
As reflected energy passes back along the line at its frequency of reflection, that frequency adds to or subtracts from the incoming energy to create a new "beat" frequency (a heterodyne) at a frequency which is the average of the frequency of reflection and the frequency of the incoming signal.
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