A TV remote control sends infrared signals modulated at 40 KHz – Bit streams code different commands

 
Notes based on this link ARRL - the National Association for Amateur Radio
 
The TV remote control has a light emitting diode (LED) which is an infrared transmitter and the TV has another one which is an infrared receiver. The remote control uses infrared light at the frequency of 980 nm to send the commands.
 
However, the sun sends infrared light along with the other frequencies of the light spectrum. House lights emit infrared as well but that light is turned on and off at a rate of 120 Hz (US) or 100 Hz, the harmonic frequency of household AC (current). Note that infrared light from the sun is sent as a continuous wave.
 
How does the TV receiver distinguishes between sunlight or house lamps and the remote control?
 
The answer is that the remote control infrared signal is modulated at a frequency of 38.000 Hz (approximately 40KHz).
 
Let us now consider how to code the information to change channels e.g. how to code the command to set channel 2 (button 2) or 3 etc. We will use the SONY TV code.
 
The code for channel 2 is : 000 0001
The code for channel 3 is : 000 0010
 
How do you code a 0? With 0.8ms of infrared signal on.
How do you code a 1? With 1.2ms on.
How do you separate digits? With 0.68ms off.
A start signal is needed: That would be 2.4ms on.
 
A final signal that constitutes the device identifier code is needed (also called address). It allows the TV to verify that the code is intended for it. In its absence the code will be ignored.
 
The picture on the left illustrates the bit stream sent when key 0 is pressed. The picture on the right illustrates a different bit stream from this link http://www.sbprojects.com/knowledge/ir/sirc.php.
 

 

 

 

Li-Fi

 
Excerpts from purelifi.com
 
"LiFi, meaning Light Fidelity, is a term coined by pureLiFi co-founder and CSO, Professor Harald Haas, when he demonstrated LiFi for the first time at a TED talk in 2011. LiFi is high-speed, bidirectional, networked and mobile wireless communication using the light spectrum, to provide a user experience similar to traditional wireless systems. However, LiFi offers thousands of more pathways for data to travel, which means it offers unprecedented Data and Bandwidth. The answer to our digital future."
 
Excerpts from white paper "Shedding Light on Li-Fi"
 
"How Does It Work?"
"Standard LED light bulbs are controlled by a driver that turns the LED on and off, or dims and brightens it. With Li-Fi enabled LED light bulbs, the driver is used to transmit encoded data by controlling the LED light. An optical sensor is used to receive the data, which is then decoded. This is conceptually similar to Morse code – but at rates of many millions of times a second, which is imperceivable to the human eye. The receiver has optics, and is fast enough to ‘see’ the light dimming and brightening, smart enough to decode the Li-Fi data, and then deliver it to the attached device such as a laptop computer. Devices can include both a transmitter and receiver to enjoy two-way communications."
 
"What is Visible Light Communications (VLC)?"
"Visible Light Communications is a very generic term that suggests any form of data communications via visible, primarily white, light – in contrast to Infra Red (IR), the communication used on most consumer remote controls. In addition to data communications, Li-Fi addresses advanced networking capabilities, including data roaming, hand over and multiple access."
 
 
"The History of Li-Fi"
"Alexander Graham Bell is most famous for inventing the telephone, but he also demonstrated the first VLC system in 1880. In fact only 4 years after inventing the telephone, Bell demonstrated the world’s first wireless telephone call. He did this with an apparatus called a Photophone that used light, not radio. Bell had to use daylight for transmission, which severely limited its practical use. Professor Harald Haas and his research team first developed the modern concept of Li-Fi at the University of Edinburgh, ironically in labs within the Alexander Graham Bell Building (Bell was born in Edinburgh and attended the University). Li-Fi was developed as a solution to the growing radio spectrum congestion problems. Haas demonstrated the technology by streaming live video for the first time at TED Global in July 2011, and the term ‘Li-Fi’ was coined by him during this talk. The company pureVLC (www.purevlc.com) was created in 2012, in order to commercialise Li-Fi."