Infrared Still Dominates Short-Range Wireless Communications

Feb. 23, 2011
Infrared (IR) technology continues as a viable short-range wireless option.

IR signals

More than 700 million infrared (IR) receivers were shipped in 2010. While most of these devices are used for familiar remote-control applications in set-top boxes (STBs), DVDs, TVs, game consoles, air conditioners, projectors, digital cameras, and notebook PCs, IR technology is finding life in several emerging applications.

First-generation active-shutter glasses for 3D television are being synchronized to the TV using IR signals (see the figure). Home theater installation is being simplified with the use of IR to transmit audio signals to rear/side speakers. And, IR has become the platform for protocols being launched by Universal Electronics to enable two-way communication between a handheld device like a remote control and an STB.

There are two main types of IR transmission: direct line-of-sight and diffused. Direct is characterized by the need for an unobstructed line-of-sight between transmitting and receiving devices. Diffused is non-line-of-sight and non-directional, acting like light from a bulb.

Interoperability Challenges
Active 3D shutter glasses for home theater create a stereoscopic effect by opening and closing the left and right apertures in synchronization to images displayed on the monitor (see “Don’t Let Battery Life ‘Drain’ On The 3D Parade"). IR is used to get the synchronization signal from the TV to the goggles.

When these goggles first appeared, the industry had yet to settle on a physical layer or communication protocol standard for their use, and many solutions resulted. Some first-generation 3D synchronization systems used wavelengths of 850 nm. Others used 940 nm. A number of first-gen systems adopted a transmission protocol using a carrier frequency, and others used a non-modulated signal.

Because some 3DTVs used the same IR wavelength as remote-control receivers, typically 940 nm, interference problems cropped up. During 3D movies, the synchronizing emitter is often “blinking” on and off. If someone, say, pauses the movie, adjusts volume, or turns on subtitles, the remote signal has to squeeze through the onrush of IR to the remote-control receiver, which automatically adjusts gain in response to the IR from the synchronizing emitter.

The Consumer Electronics Association’s work group for 3DTV synchronization is producing a communication standard allowing glasses to be interoperable. The goal is to ensure that the IR synchronization signal from the 3DTV to the active glasses is unaffected by the remote signal and other ambient optical-noise sources, as well as for remote signals to be unaffected by the synchronization signal.

Optics For Home Theater Sound
While interoperability and interference are the major challenges of remote control and 3DTV synchronization, the transmitted signal is focused or directed toward the receiver. But the diffused IR signal is non-directional. The emitted signal is weakened or attenuated as it reflects off surfaces before being received. The challenge of diffused IR systems is to be able to receive and process this weak signal.

The side and the rear speakers set a home theater apart from a TV with stereo speakers. With most surroundsound systems, there are three speakers in front, two side or rear speakers, and at least one subwoofer. Movie soundtracks are recorded with multiple channels to produce a 3D audio effect, and purists such as Theo Kalomirakis of Discovery’s Digital Home consider it a crime to eliminate the side and rear speakers.

Near-IR light includes wavelengths from 750 to 1050 nm and has most of the physical properties of visible light. IR or diffused light will reflect off the ceiling, walls, and most objects. Infra-Com’s IrGate system illustrates the features of diffused IR light that make it well suited for transmitting signals to rear and side speakers.

The transmitter receives the audio signal from the surroundsound receiver. The signal is converted from analog to digital, framed, encoded, and modulated by a digital IC, which interfaces to an IR emitting diode (IRED) driver IC that drives an array of high-speed IREDs. The diffused IR IrGate chipset can create communication links of up to 12 meters (39 feet). The emitter array is usually pointed upward, creating “IR spots” on the ceiling and walls.

The receiver uses an array of lensed PIN photodiodes to collect the emitted IR transmission. The transmitted signal is heavily attenuated through absorption and multiple reflections, so only a small part of the transmitted signal power reaches the receiver. Therefore, the diffused IR receiver must exhibit a high sensitivity, up to 40 db more than a direct, line-of-sight IR communication link. The IR light carrying the modulated audio signal is converted to an electronic signal.

IrGate can support data rates up to 25 Mbits/s in half-duplex. This high bit rate allows the simultaneous delivery of up to four uncompressed, pulse code-modulated digital audio channels. It automatically supports 44.1/48/88.2/96-kHz sampled audio at up to 24-bit sample length. Where sound and video could be out of sync with RF transmission, diffused IR employs enough bandwidth to use forward error correction.

Two-Way Interface For Remote Controls
The interface controlling the display is evolving. Touchscreen remotes are compelling but expensive compared to most of today’s remote controls. Gesture controls and RF-based devices are also gaining traction in the remote market. Meanwhile, most remote controls are likely to remain IR for a long time. This doesn’t mean the handheld unit won’t be changing, though.

Bidirectional communication has been implemented in many STBs and accompanying remotes. Making use of the Universal Electronics XMP-2 protocol, these remotes can receive data from the STB that can be used for software updates. This same bidirectional channel can also be used for more advanced communication between the TV or STB and the remote. The amount of information being exchanged is well within the capabilities of infrared communication.


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