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You have one light at your disposal and essentially two states, on or off. You can send information is a lot of different ways.
One way or another you control the length of time you keep the light on or off. This is not too dissimilar to Morse code with the dots and dashes.
Held on for longer
Held off for longer
Held off for different amounts of time
Off then on or on then off
The more the light is on the more power is used.
This type will give rise to long battery life
Compared to this one which has a long period at the beginning called the automatic gain control - used by the TV/DVD to adjust to the light level that it receives.
Could you vary the light level or adjust the frequency of the signal?
The diagarams above are not strictly accurate. The "On's" are actually bursts of on and off:
The remote chip will typically turn the LED light on and off 60 times for a long bit and 20 times for a short bit.
Whilst the diagrams were representative, they are what appears from the IR sensor in the TV.
To detect different frequcies a different sort of detector is required.
Adjusting the amplitude / light level is fine in theory, but would cause a lot of unreliability given that the light level received depends on how far the remote is from the TV.
An uncopiable remote control
Being in the business of copying thousands of remote controls, I have wondered whether a remote could be made that is very hard to copy. Some posh/fancy hifi systems do use very high frequency signals which standard IR components find hard to cope with.
A mixture of amplitude and frequency variation, along with difficult to predict encoding might make it too troublesome to make copies. Or make it too much effort in relation to the small profits made by selling replacements. Some manufacturers profit from selling replacement parts, but many opt to offer no aftersales service support beyond the standard guarantee whatsoever. The infrastructure and manpower needed to provide spare parts is far from cheap.