I Am a Strange Loop

And why does it always give off that same high-pitched screeching sound? Why not a low roar? Why not the sound of a waterfall or a jet engine or long low thunder? This has to do with the natural resonance frequency of the system — an acoustic analogue of the natural oscillation frequency of a playground swing, roughly once every couple of seconds. An amplifier’s feedback loop has a natural oscillation frequency, too, and for reasons that need not concern us, it usually has a pitch close to that of a high-frequency scream. However, the system does not instantly settle down precisely on its final pitch. If you could drastically slow down the process, you would hear it homing in on that squealing pitch much as the rolling soccer ball seeks the bottom of the gutter — namely, by means of a very rapid series of back-and-forth swings in frequency, almost as if it “wanted” to reach that natural spot in the sonic spectrum.

What we have seen here is that even the simplest imaginable feedback loop has levels of subtlety and complexity that are seldom given any thought, but that turn out to be rich and full of surprise. Imagine, then, what happens in the case of more complex feedback loops.

Feedback and Its Bad Rap

The first time my parents wanted to buy a video camera, sometime in the 1970’s, I went to the store with them and we asked to see what they had. We were escorted to an area of the store that had several TV screens on a shelf, and a video camera was plugged into the back of one of them, thus allowing us to see what the camera was looking at and to gauge its color accuracy and such things. I took the camera and pointed it at my father, and we saw his amused smile jump right up onto the screen. Next I pointed the camera at my own face and presto, there was I, up on the screen, replacing my father. But then, inevitably, I felt compelled to try pointing the camera at the TV screen itself.

Now comes the really curious fact, which I will forever remember with some degree of shame: I was hesitant to close the loop! Instead of just going ahead and doing it, I balked and timidly asked the salesperson for permission to do so. Now why on earth would I have done such a thing? Well, perhaps it will help if I relate how he replied to my request. What he said was this: “No, no, no! Don’t do that — you’ll break the camera!”

And how did I react to his sudden panic? With scorn? With laughter? Did I just go ahead and follow my whim anyway? No. The truth is, I wasn’t quite sure of myself, and his panicky outburst reinforced my vague uneasiness, so I held my desire in check and didn’t do it. Later, though, as we were driving home with our brand-new video camera, I reflected carefully on the matter, and I just couldn’t see where in the world there would have been any danger to the system — either to the camera or to the TV — if I had closed the loop (though a priori either one of them would seem vulnerable to a meltdown). And so when we got home, I gingerly tried pointing the camera at the screen and, mirabile dictu, nothing terrible happened at all.

The danger I suppose one could fear is something analogous to audio feedback: perhaps one particular spot on the screen (the spot the camera is pointing straight at, of course) would grow brighter and brighter and brighter, and soon the screen would melt down right there. But why might this happen? As in audio feedback, it would have to come from some kind of amplification of the light’s intensity; however, we know that video cameras are not designed to amplify an image in any way, but simply to transmit it to a different place. Just as I had figured out in the calm of the drive home, there is no danger at all in standard video feedback (by the way, I don’t know when the term “video feedback” was invented, nor by whom; certainly I had never heard it back then). But danger or no danger, I remember well my hesitation at the store, and so I can easily imagine the salesperson’s panic, irrational though it was. Feedback — making a system turn back or twist back on itself, thus forming some kind of mystically taboo loop — seems to be dangerous, seems to be tempting fate, perhaps even to be intrinsically wrong, whatever that might mean.