I don't know why this just popped into my head, I've thought about before but apparently never mentioned it. The Nyquist limit is most accurately described on wikipedia, but here's a simple way to visualize it that I've always used to explain it.
Imagine a video camera, shooting 30 frames per second. You walk into the frame, and your actions are very accurately recorded. Now slow down the camera to 1 frame every 60 seconds. The camera takes a picture, then you run through the frame in 2 seconds. 60 seconds later, it takes a picture and it's like you were never there, even though you were. You were faster than the sampling rate of the camera, so it didn't capture you at all. It -can't- if you continue to move that fast. You have to slow down, or speed up the camera. That's why the hard limit.
Now, you if you vary your speed, you can play with the frequency of how often you show up. So let's say the camera it still running at 1 frame a minute, and your pacing back and forth, spending 5 seconds in the frame out of every 45 seconds. So your real rate of passing the camera is once every 45 seconds. So the camera starts, as do you, and your first pass comes at 45 seconds and you're gone. The camera fires, and there's no you. You pass by again at 1m:30s...then 2m:15s...then at 3m. The camera is firing then because it fires at 0s, 1m, 2m, etc...So you're caught on film. You're next pass is 3m:45s, then 4m:30s, then 5m:15s, and 6m, and you're caught on film again at the 6 minute mark.
So you're real rate of passing by is every 45 seconds. But according to the film, you're only passing by every 3 minutes. That's aliasing. Because the sampling rate isn't fast enough to capture your frequency, you are "aliasing" to a different frequency. From 45s to 3m. You can imagine when capturing music with samples that this would do baaaad things to the soundwaves, which is why digital audio systems filter out high-frequency sounds somewhere around the Nyquist limit, so they don't alias to the wrong frequencies and destroy everything.
This explanation isn't perfect of course, but I have found it useful to explain the concept.
This is Rob Meyer's weblog, a weblog focused on software development and system administration based on 10 years of experience. Want to explore further? You can find out more me or see the rest of my website.
Wondering if I've written on something in particular? Try searching:
You might want to take a look at some of the more requested postings (as judged by incoming traffic):
Want more? Subscribe to this site 
or contact me at rob at big dis dot com.
See my writings on: