Tom's Tech Corner: On the Matter of the Wayward Remote

A reader writes:

*"Dear Tom, my TV remote works fine when I point it right at the TV, but if I aim it off to the side, or if I'm lying on the couch with my arm resting funny, it doesn't do anything. Is something wrong with it? — Gary, Cedar Rapids, Iowa"*

Gary, what a wonderful question, and I want to say up front: nothing is wrong with your remote. Nothing at all. What you have stumbled into, quite by accident, from the comfort of your couch, is a full-body encounter with the discipline of applied electromagnetic geometry, and I am delighted to walk you through it, assuming, of course, standard atmospheric transmissivity and a television manufactured after 1998.

Let's begin at the beginning, which is to say: light. Your remote control communicates with your television using infrared light, which is of course simply visible light's shy cousin, sitting just below the red end of the spectrum where the human eye, mercifully, cannot follow it there to gawk. The remote's emitter — the little dark bump at the front, which I assume you've never looked at directly through a phone camera, though I encourage you to try it sometime purely as an act of contemplation rather than diagnosis — pulses this infrared light in a specific timed pattern. This pattern is not, as one might assume, a simple "channel up" signal shouted into the void. It is a modulated carrier wave, typically oscillating around 38 kilohertz, upon which your actual command is encoded as a series of on-off pulses, not unlike Morse code if Morse code had been designed by a committee of very patient engineers in Japan in the early 1980s and had to survive the electrical noise of a room lit by incandescent bulbs. (See Figure 3 for the full pulse-train diagram; I won't reproduce it here as it wants the two-color printing to be legible.)

Now, here is where your couch posture enters the picture, Gary, and I promise this is not a tangent, merely necessary scaffolding. Infrared light of this wavelength is what we call, in the trade, "quasi-optical" — meaning it behaves mostly like light, traveling in straight lines, but it is forgiving enough to bounce, faintly, off of walls, ceilings, and, in your case, presumably a coffee table. This is why your remote sometimes works even when you're not pointed directly at the sensor: you are relying on what's called multipath reception, where the television's receiving diode is catching not the primary beam but a secondary reflection of it, degraded in intensity by the inverse square law and by the absorptive properties of your particular ceiling texture. When you rest your arm "funny," as you put it, you are very likely altering the angle of incidence past the receiver's acceptance cone — typically about 30 degrees off-axis for a standard consumer photodiode, assuming the diode hasn't been miniaturized past spec for cost reasons, which, regrettably, in this economy, it often has.

I want to pause here and address something that is, I know, not what you asked, but which I would be doing you a disservice to skip: the receiving diode itself. This is a photodetector, typically a PIN photodiode, tuned via a bandpass filter to reject ambient infrared noise — sunlight, incandescent bulbs, and, notably, the older style of fluorescent ballast, which flickers at a rate that can, in rare and frankly fascinating cases, produce false triggers in poorly shielded receivers. This is a lovely little detour into the history of lighting technology, which I won't take here in full, except to note that the entire consumer infrared remote industry owes an unacknowledged debt to fluorescent lighting engineers of the 1970s, whose flicker problems forced the modulation standards your remote still uses today. Appendix B covers this lineage in more detail, including the transition from 30kHz to 38kHz as the de facto carrier frequency, which happened, essentially, by consensus rather than decree.

But back to your couch, Gary. There's a second factor at play, which is signal attenuation over distance, governed — as all radiative phenomena are — by the inverse square law: double your distance from the television, and your effective signal strength drops to a quarter. This is the same principle, incidentally, that governs how faint a satellite's signal becomes by the time it reaches your dish from geostationary orbit, some 35,786 kilometers up, a fact I mention not because it's strictly relevant to your remote but because I think it's worth sitting with, for a moment, how the same elegant law governs both your living room and the heavens. It's the kind of thing that, once you see it, you cannot unsee it. Of course, your remote is only traveling a few meters, not tens of thousands of kilometers, so the attenuation itself is not your problem — I mention it purely for the architecture of understanding, so that when you next find yourself gesturing futilely at the television, you can appreciate that you are, in a small way, doing orbital mechanics.

So: why does it fail when you're off to the side? Three factors are stacking against you simultaneously, and I think laying them out plainly will bring you real peace. First, the angular attenuation of the emitter itself, which does not broadcast infrared uniformly in all directions but in a cone, typically 20 to 30 degrees off boresight, per the emitter's own radiation pattern (again, Figure 3, which I really do wish I could show you here). Second, the acceptance cone of the receiving diode, narrowing your effective envelope further, the two cones overlapping in what I like to call the "usable ellipse" of remote-to-television communication. And third — this is the one people forget — your own body. The human body is, from an infrared standpoint, a remarkably effective absorber and occluder, given that we ourselves radiate in the far infrared and our tissue readily attenuates near-infrared wavelengths at the intensities a remote control produces. When your arm rests "funny," you are very possibly interposing a limb, a cushion, or the family dog between emitter and receiver, and no amount of multipath reflection is going to save you from a direct obstruction, though I'll note the dog is, infrared-wise, a more effective blocker than the cushion owing to fur density. This is not, I should say, a fault of your remote's design, but simply the physics of the room asserting itself, as physics is known to do.

I should also mention, briefly, battery voltage sag, since a remote's infrared LED brightness is directly proportional to forward current, which itself degrades as battery voltage droops below the diode's optimal forward voltage — but I suspect, Gary, this is a story for another letter, and I don't want to overwhelm you.

What I want you to take from all this, Gary, is not a set of instructions — I'd never presume to tell you what to do with your own remote, and frankly the theory is the interesting part anyway — but simply the deep and settled understanding of why the couch matters, why the angle matters, why your arm resting funny is, in its own small way, a geometry problem playing out in your living room every evening. You are not fighting a broken device. You are standing, gloriously, at the intersection of quantum electronics, orbital law, and interior decorating, and I think that's rather beautiful.

Glad I could clear that up!

— Tom