Infrared Signaling and how it works.
IR remotes operate by modulating (turning on and off) an infra red (IR) light source. When the IR light source (the IR emitter) is "on" it is actually turning itself on and off thousands of times per second, too fast for the human eye to follow. The rate at which this occurs is called the carrier frequency. The terminology comes from the metaphor that the "carrier" carries the "information". This is done to provide a better transmission system and allow the overall IR system (transmitter and receiver) to operate in noisy (with respect to light) environments. It is important to understand that the IR receiver for a given remote is tuned to IR "carrier" frequency for that remote and will effectively not see IR signals sent on a different carrier frequency such as from other remotes. [Note: The human eye can never see an infrared transmission, so the concept of on and off is not with regards to visible light. Some equipment has a "telltale", a little red light that visibly flashes when the equipment receives IR signals. That is what we can see]
The "information" is placed on the "carrier" using several different techniques. The most common technique is Pulse Width Modulation. In Pulse Width Modulation the duration of the ON (carrier present, light flashing thousands of times per second), or Off (no light at all coming out of the IR emitter) periods is made to vary. Lets assume, because this is what is done, that we wish to send numbers representing what key has been pressed (and perhaps even what device this key is for). We first need to simplify the problem so that we don’t have deal with too many "Pulse widths". We can easily do this by representing the number in base 2, or binary. (I apologize if this now gets a little technical, but in reality it already has). In binary there are only two digits to worry about not ten as in decimal. Therefore we only need to have two distinct "pulse widths". If you think about it, the periods of on and off will need to alternate. If they didn’t it would be hard to judge their width. [Note: Other modulation schemes in particular RC5 do not use PWM. RC5 uses Phase modulation. Luckily for us we never have to decode or figure out the RC5 patterns because Philips has provided them as pure clean data.] Only one of the widths needs to vary. Either the width of the ON period or the Width of the Off period.
In summary, IR transmission most often takes place by varying the on off times of an IR emitter to represent binary numbers according to some well established pattern.
[Note: At this point I am going to assume that the reader has a basic understanding of the binary numbering system. Not detailed enough to add, subtract or multiply them, but enough to be able to form the decimal value of a binary number.]
Each manufacturer has the option of deciding just how big a number he wishes to send to his equipment, and what meaning is given to that number (or numbers) when they are received. Remember the environment through which the IR signals are passing (the air) is noisy in a light sense. Bright sunlight, Fluorescent lights, all contribute to the noise. Some manufacturers add additional "redundant" information such as sending the numbers twice to ensure that they get to the equipment correctly. Some do not. I will discuss those details when I discuss some of the more common manufacturer’s products.