What is Impedance?

[Chris Gohlke]

<div class='os_post_top_link'><a href='http://forum.ecoustics.com/bbs/messages/34579/131527.html' target='_blank'>http://forum.ecoustics.com/bbs/messages/34579/131527.html</a><br /><br /></div><i>"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."</i> <br /><br /><img src="http://www.digitalmediathoughts.com/images/gcomega.jpg" /> <br /><br />And no, impedance is not something that you take Viagra for. :wink:

[Darius Wey]

Oh dear, this takes me back to high school Physics classes years and years ago. :lol:

[Don Tolson]

I didn't read the whole article in any depth, since it was getting confusing.

But the main point I think the author missed is that impedance is a measure too, of the 'load' the output device sees, and that for the best transmission of the output signal (i.e. with little or no change from its original), it's important that the impedance of the cable and the terminator matches the impedance of the generator of the output signal.

If everything matches, maximum transfer of the 'complete' signal occurs from generator to output device.

You CAN transfer signal from a 75-ohm output through a 300-ohm cable to a 50-ohm output device, but the signal will be degraded at each junction of mis-matched impedances. Usually you'll see this as a loss of higher frequencies or a 'muddy' quality.

It's especially noticeable in audio, where people try to connect the headphone output of one device to the line-in of another device. The impedances at these junctures are usually VERY different (very low for headphones and very high for line-in), so the results are not what they expect.

/drt