Infrared light and signals t6e10eh
Since a long time the consumer electronics industy has been employing infrared remote controls for the control of television, VCR's and many other products. This same technic is popular In industrial- and office applications like control your pc to eliminate expensive keypads and cables. It's easier to use the infrared light than cables who always crossing each other.

We humans can't see infrared light because the wavelenght of infrared light is below the visible spectrum(Some animals can see it and use it as a target for killing the prey). Because we don't see ir light we use it for remote control purposes. In remote controls the light source is a led but around us there are many objects who are emitting infrared light. The brighest source is the sun. In fact everything that radiates heat is also emitting ir light like flames, light bulbs, and even our body.
Decoding infrared signals
The only hardware you need for decoding IR signals is an infrared receiver. There are two main principles of ir remote controle: using a modulated beam at a frequency about 40kHz or the second is an non-modulated beam of IR light and has a restricted range.
Transmitting and receiving IR lights
Transmitting IR light is fairly simple done by using an infrared led. Like normal leds emmiting light waves in the visible ranges, ir leds are doing this with a light invisible for human eyes. But when you will see the ir beam, take a webcam and point the remote control to the cam and you will see a flashing light.

Receiving and decoding the signal is a lot harder. The ir light is captured by an ir sensitive photodiode with the highest spectral sensivity around 950nm.

The output signal is optical filtered, amplified and passed through a signal filter tuned to the frequency of the emitted signal frequency. The best and most easy to use are the integrated 3 leg ir receivers like the Vishay TSOP1836 and the Siemens SFH505A. The output of these devices is TTL compatible, the optical filter, amplifier with automatic gain control, tuned filter, and demodulator are integrated.
Infra Red Remote Controls
All Infra Red (IR) remote controls use some kind of IR signal. The remotes transmit pulses of IR light to send the signal to the receiver. These IR LEDs transmit light in the frequency range of 30 kHz to 40 kHz. These high frequencies were chosen so that other light sources would not interfere with the receivers ability to correctly receive the transmitted signals. These signals are transmitted by the IR LED in some type of binary code. It turns out that for most consumer electronics this coding is the same. The binary signal varies in length for both time and bit length.
The physical communication layer is formed by the emmiter (a photo diode) that emits the signal and a receiver (another photo diode) that receives the signal.
The data link layer is implemented by using binary pulses. There are really only three different ways that manufacturers choose to code these signals. This coding is usually based on varying the length of pulses, varying the length of spaces between pulses or altering the order between spaces or pulses.
1) Pulse-Width-Coded Signals vary the length of pulses to code the information. In this case if the pulse width is short (approximately 550us) it corresponds to a logical zero or a low. If the pulse width is long (approximately 2200us) it corresponds to a logical one or a high.
Used by SONY remote controls.
2) Space-Coded Signals vary the length of the spaces between pulses to code the information. In this case if the space width is short (approximately 550us) it corresponds to a logical zero or a low. If the space width is long (approximately 1650us) it corresponds to a logical one or a high. orREC-80
Used by Panasonic remote controls
3)Shift-Coded Signals vary the order of pulse space to code the information. In this case if the space width is short (approximately 550us) and the pulse width is long (approximately 1100us) the signal corresponds to a logical one or a high. If the space is long and the pulse is short the signal corresponds to a logical zero or a low.
Also known as RC-5 . Used by Philips remote controls
This coding holds information such as the address to the machine that is using the remote and the command that the machine must follow. The address is very important because without it the signal would be processed by another IR receiver in the area.
When a button on a remote is pushed it sends a string of signals. The first piece of information in the string is called the Header. The Header usually contains a burst of highs that alerts all the IR receivers in the area to the string of data being sent. Following the burst of highs is the address to the specific machine to receive the next piece of data, the command. As long as the button is held down (depressed) the command will continue to repeat over and over. When the button is released a string of code called the stop is transmitted. As you may have guessed it the stop tells the machine to stop its executing the command.
The network layer is implemented by dividing the code into 2 parts.
Address : specifies the target device
Command : specifies the command to be sent to the device.

2. Panasonic's infrared remote protocol
The protocol is similar with the RECS-80 protocol but it use more bits than the RECS-80 protocol. For the datatransmission Panasonic uses the pulse-place modulation. For the communication a pulse is used with a fixed length, followed by a pause wich represents The logical state of the bit. 2048 codes are defined in this protocol, divided in 5 bits of custom code and 6 bits of data code. The custom code is a value wich represents the manufacturer code and the data code is a value wich represents the pressed button on the remote control. The full transmitted code is 22 bits: First a header is sent then the custom code (5 bits), then the data code, followed by the inverse of the custom code and the invers of the data code, And to terminate a stopbit is added to the code. The invers transmitted bits are very usefull for the error detection. Each first part of a bit is always a high level with a fixed time and is followed by a low level where the time defines if the bit is a logic 1 or a logic 0.
Timing diagram:
T =420 µs to approx 424 µs in the USA and Canada
T=454 µs to approx 460 µs in Europe and others
The header is 8T high and 8T low
A 1 is coded 2T high and 6T low
A 0 is coded 2T high and 2T low
Panasonic IR remote control

IR decoding program analysis
The RECS 80 code uses pulse length modulation. Each bit to be transmitted is encoded by a high level of the duration T followed by a low level of duration 2T representing a logical '0' or 3T representing a logical '1'.

T 2T T 3T T 2T
_ _ _
| | | | | |
_| |__| |___| |__
0 1 0

Notice that a '1' takes more time to be transmitted than a '0'. The RC 5 code instead has a uniform duration of all bits. A transition in the middle of the time interval assigned to each bit encodes the logical value. A '0' is encoded by a high to low transition and a '1' by a low to high transition. Therefore we need additional transitions at the beginning of each bit to set the proper start level if a series of equal bits is sent. We don't need this additional transition if the next bit has a different value. This is also called a 'biphase' code.

|1.Bit|2.Bit|3.Bit|4.Bit|
__ __ __ __
| | | | | |
|__| |_____| |__|
0 0 1 1

The RECS 80 code uses high pulses of uniform length while the low pulses differ in length. If there are high and low pulses of two different lengths it might be RC5 code. The receiver may invert the levels.
In RC5 code every command is encoded by 14 bits. The first two bits S are startbits to allow the receiver to adjust the automatic gain control and to synchronize. Next a bit T follows, that toggles with every new keystroke. Next is the address A of the device which shall respond to the command. At last the command itself follows.

| S | S | T | A4 | A3 | A2 | A1 | A0 | C5 | C4 | C3 | C2 | C1 | C0

Girder is the most powerful and feature rich Windows automation tool available. Girder controls programs like WinAmp and Windows Media Player using infrared and wireless remote controls so you can sit back and enjoy your digital library from your couch. In Home Theater PC's, Girder controls Windows programs (media/DVD players) and hardware devices (sound cards, X10 and serial devices such as projectors, and receivers). Girder is the behind the scenes glue providing full control of your multimedia experience.

Here’s Just a Sample of the Many Things You Can do With Girder:
· Use Girder with your remote to play music from your digital media library, select playlists, crank the volume and play digital DJ for your next party!
· Control your home theater PC’s and hardware- from start up to mid-movie play & pause to shut-down, Girder can do it all.
· Use Girder to automate your home. Control your lighting, security system, climate settings and more. Plus, save hundreds of dollars by using Girder instead of more expensive home automation packages!
· Control your CarPC.

Computer Interfacing Solutions
The 3 possible input ports that could be used to connect the receiver are
· IRDA port
· Serial Port
· Parallel Port
IRDA port
The IRDA port on the computer can be interfaced directly to the receiver, thus minimizing requirements for intermediate hardware.
In brief, the IRDA standard is based on a 3 layer protocol..
· Physical Layer / Data link : SIR
· Data link / Network Layer : IrLAP
· Transport Layer : IrLMP

Serial Port
The Serial Port on the computer receives asynchronous data at suitable speeds for our data. This, however, requires an extra hardware/logic interface between the IR receiver and the port. Some of the main incompatible features between the data links are.
· The serial port expects asynchrounous data, while the remote control sends self-timed data.
· The serial port can only accept words of length 5 or 7 bits. It also expects start and stop bits. The remote control has 12 bit words with a header.

To overcome these incompatibilities, we need hardware such as a demodulator and logic that encodes the IR signal as 5 bit / 7 bit words perhaps sending 1 data signal from the remote control at a time. Thus, a word received on the serial port would represent the data signal equivalent to a time period of T in the remote control pulse code.
After this, specific software needs to be written in order to decode / record the pulse code and interpret it as application commands.
Parallel Port
The Parallel Port on the computer is more flexible. The only extra hardware used is a high speed (109 kHz) timer, which is connected to Pin 11 as a clock to sample Pin 12, which is connected to the IR receiver output. This ensures that the data is sampled constantly through the parallel port.

However, low level software needs to be written, that would collect these samples from the pararllel port and store them for further use. After this, the application software has to take the responsibility of decoding the pulse code and appropriately interfacing the commands.