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For the most part, remote-control consumer electronic equipment still uses IR wireless communication. It's simple to design in, robust, cheap to manufacture and yields a controller that can run for months on two to four AAA 1.5 V cells.
But IR is showing its age. Originally designed in the late seventies to replace ultrasonic devices when a greater range of functionality was required, IR remotes are inconvenient to use when navigating the complex multi-layered menus typical of today's digital electronics. (See sidebar "A short history of the remote control.").
Users must point the remote directly at the IR receiver unobstructed by furniture, pets and people. The technology is almost always a uni-directional (bi-directional communication is possible, but it's expensive and prone to interference from other light sources).
Today's consumers also prefer to have a user interface on their remote offering intuitive instructions and information about the media they're listening to or watching.
Fortunately, a new generation of RF remotes promises to finally match the convenience of IR—namely design simplicity, low cost and long battery life—while providing consumers with wireless connectivity that can support the more advanced menu-based browse facilities now common to home entertainment devices.
Better yet, the silicon vendors are making great strides to simplify the traditionally tricky RF design process by offering industry-proven transceivers plus reference designs, customised protocols and RF expertise to assist inexperienced designers.
IR basics
IR is electromagnetic (EM) radiation of wavelengths longer than visible light, but shorter than RF spanning three orders of magnitude between 750 nm and 1 mm. IrDA, the Infrared Data Association, champions IR in the electronics sector and most offerings adhere to the organization's standards, aiding interoperability.
IR remote controls use IR LEDs to emit radiation that's focused by a plastic lens into a narrow beam. Data is encoded by modulating the beam to provide immunity from other IR sources such as fluorescent lights. The receiver uses a silicon photodiode to convert the IR radiation to a current for decoding by the receiver's MCU as shown in Figure 1.
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Figure 1: Schematic illustrating IR modulation and demodulation.
IR doesn't penetrate walls—although it can be reflected by walls and ceilings—and so generally does not interfere with other devices in adjoining rooms.
A simple IR remote comprises a keypad to input instructions, a resonator to provide a reliable clock base, an 8-bit MCU to detect key presses and modulate the IR signal and an LED to generate the IR.
There are many modulation protocols but most are frequency or format variations of a few base protocols. Examples include amplitude modulation, frequency modulation or pulse modulation.
For example, with pulse distance encoding, pulses remain the same length, while intervals between are either long or short (representing "0" and "1" respectively) as shown in Figure 2.
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Figure 2: IR remote pulse distance encoding protocol .
This protocol is favored by Japanese consumer electronics companies and features a data payload of 8 bits address and 8 bits command, sent twice for reliability.
In this example, a 9 ms train pulse precedes the data, followed by a 4.5 ms mark, then around 54 ms for the address and command information. IR communication is typically one way.
That means the remote has now way of knowing if the signal has been received. The remote will dumbly repeat the command as long as a button is pressed. This example protocol provides repeat frames every 110 ms, meaning the IR remote control is transmitting for perhaps 90 ms during a half-second key press key press (See Figure 3.).
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Figure 3 Pulse distance encoding full sequence structure.
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