So, I think I have a fairly decent handle on what causes audio feedback, some kind of never-ending amplification that instantly produces Jimi Hendrix songs. If you’d like to clarify, I’d love to hear a more scientific analysis.
My real question is, is there any sort of equivalent of feedback in video? I seem to remember playing with a camcorder in Sears and turning it on the TV it was hooked up to, and nothing really happened beyond a Norman Rockwell-y tunnel of Sonys. If there is an equivalent, what is it and under what conditions does it occur? If not, how come?
There is video feedback, but it’s not as common. The problem is that the small picture of the camera in the TV, which is then picked up by the camera, etc., etc., isn’t exactly the same each time- it’s getting smaller. This difference keeps it from going out of control. It is possible to set the camera so it’s the same size each time- I’ve done that with analog cameras (not digital CCD cameras). What happens then is pretty cool- a psychodelic display of colored lines and circles appear in the center of the screen, moving outward to the edges, while more patterns are produced in the center again. You can get all sorts of 60’s style psychedelic patterns by playing with the camera controls. If you deviate too far, though, it’ll break down and you be back to looking at a picture of the camera again. This generated endless hours of fun in college
To try it, start by playing with the zoom and focus of the camera. CCD cameras might not be able to sustain it due to the quantized nature of the CCD (not enough pixels). I haven’t tried it recently, though, so you might get some results with a modern high-end CCD video camera.
I think I’d like to clarify… I’m simplifying greatly and I’m sure I’ll catch some grief from folks who would have said it differently, but here goes…
The amplification is not never-ending. In fact, the amplification is constant, though feedback can start from soft sounds (at some fixed amplification) and grow to very loud sounds (at the same amplification). At some point the amplifier is incapable of properly amplifying the input signal and a phenomenon known as clipping occurs. If you could look at the waveforms on an oscilloscope you’d see that what this means is the once smooth, roundish signals are chopped off on the tops. This is also known as distortion. Distortion is always present, but recursive amplification of the distortion is what leads to interesting effects. In the audio realm distortion can add harmonics (pseudo-tones). If managed in a controlled fashion, a la Jimi Hendrix, feedback can yield some rich, warm sounds from your guitar. If unmanaged, you can get that screeching sound that you often hear in school gymnasiums…
Absolutely. First, feedback is simply the mechanism where all or part of the output signal is ‘fed back’ into the input. What you really want to ask is there some sort of equivalent distortion in the video domain. The answer to this question is also, absolutely. The difference is the nature of the distortion. As you’ve already noted, you get that ‘tunnelling’ effect (I would more likely associate it with Salvidor Dali than Norman Rockwell, but that’s a different story). If you look carefully at the tunneling, you’ll see that straight edges start to round over and look wavy or twist. This is one type of video distortion. The images are often ‘squished’, which is another type of distortion. You’ll also notice that generally there is color distortion (each recursive copy doesn’t do as good a job at reproducing the original color). Most camcorders actually diminish the amount of light from the original, so you generally see a diminishing brightness, which again is another kind of distortion. However, if you want to see a kind of video distortion that looks more like the ‘runaway’ distortion that you might tend to associate with screeching audio feedback, this too can be obtained. You need to find a camcorder with a brightness adjustment. Aim the camera at it’s monitor and raise the brightness. Eventaully you should see the image ‘wash’ out as the CCD saturates (note really too different than what happens when you aim a camera at a bright light). There are, of course, other kinds of video distortion that can be exploited with feedback.
So bottom line: All amplifiers introduce distortion. When you feed back the output signal this distortion is further distorted which can lead to sometimes plesant, sometimes interesting, and sometimes distateful results.
