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Basics of IR remote control

Posted: 15 Dec 2015     Print Version  Bookmark and Share

Keywords:IR remote control 

A multitude of IR remote control communication schemes have been integrated in commercial equipment for the last forty years. Even though the schemes use different timing, data organisation and length, they can be dismantled into discrete pieces which form the building blocks of the final IR pattern. The example below briefly describes how a button press is encoded into an IR pattern and transmitted to a piece of consumer equipment.

Depressing a button on a modern Consumer IR, CIR, remote control causes a 940nm LED to blink. As the LED toggles a pulse stream of infrared light is generated. The pulse stream is a pattern which contains data to be received by consumer equipment. Typical remote control protocols derive a pattern of IR pulses based on Pulse Width Modulation, such as Philips RC-5/6, or Manchester Encoding, such as Sony SIRC. Regardless of the encoding method, patterns can be deconstructed into a small number of fundamental pulse widths. Virtually all CIR patterns have fewer than 16 fundamental pulse widths.

Pulse widths with no IR emission are called Spaces and pulse widths with IR emission are called Marks. During a Mark, the IR LED typically is not solidly illuminated. To improve signal to noise ratio, the LED is modulated by a carrier frequency. Typical carrier periods are in the range of 10µs to 66µs. The duration of a Mark or Space can be measured in time, however, it can more easily be quantified as the number of carrier periods that fit within the pulse width. Figure 1 depicts the aforementioned nomenclature.

Figure 1: Basic building blocks of a CIR pattern.

Figure 2: 12bit SIRC Pulse Stream.

The remote control must be in the direct line of sight when the IR is blast to the receiver. The pulse stream shown in figure 2, has four parts: Lead-in, Command, Address, and Lead-out (not shown). The Lead-in has five bit-times in total, four with LED modulation and one without. The receiver uses the lead-in to adjust the gain of the photodiode AFE. With the gain adjusted, the carrier frequency is removed using a notch filter. The command and address data is sent LSB first within the digital pulse train and is then interpreted by equipment such as a Television or Cable Box. Some equipment may require more than one pulse stream repetition separated by a lead-out to verify successful reception and/or proper equipment selection.

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