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- 18 Mar 2008
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Wireless speakers eliminate the need for unsightly wires and provide for an elegant rear speaker solution.
BY PHILIP LIU
Edifier Technology, Hong Kong, China
TAMIR SHAANAN
Infra-Com, Netanya, Israel
and JAMES TOAL
Vishay Semiconductors, Santa Clara, CA
The main feature that sets a home theater system apart from an ordinary television with stereo speakers is the side and rear speakers. A standard home theater system has three speakers in front, two side or rear speakers, and at least one subwoofer.
A majority of consumers don’t install the rear speakers because they don’t want wires running around the room or the expense of installing wires in walls and below carpets and floors. But they may not know what they’re missing. Movie soundtracks are recorded with multiple channels to produce a three-dimensional effect, and purists such as Theo Kalomirakis of Discovery’s “Digital Home” consider it a crime to eliminate the rear speakers.
Fortunately, the choice is no longer between having intrusive wiring and having a complete surround-sound experience. The side or rear speakers and subwoofer can now receive audio signals from the surround-sound receiver wirelessly.
RF technology drawbacks
The most widely used technologies for wirelessly transmitting data signals are based on the 2.4-GHz free RF band, which is used by Bluetooth and various types of Wi-Fi systems. High-quality digital audio transmission requires a bandwidth of 1.4112 Mbits/s for an uncompressed stereo (L/R) source of 44.1-kHz/16-bit CD-quality audio.
Bluetooth isn’t fast enough for this; the most it can handle is 1 Mbit/s. Wi-Fi has a bandwidth of 11 to 54 Mbits/s, but it presents a number of problems for home theater systems. The most significant of these is delayed transmission.
As Wi-Fi data packets are continuously being verified or checked for possible errors, the delay in actual processing of the data can vary from tens to several hundred milliseconds. This delay is long enough for the video and audio to be out of synch. Overcoming this phenomenon would mean employing hardware-intensive buffers to compensate for the delay.
The cost of the hardware pushes up the cost of the end product, which pushes away customers. Signals transmitted over Wi-Fi and Bluetooth can also travel through walls, which might potentially cause interference with other signals for example, from a wireless LAN network or a cordless phone operating in the 2.4-GHz band.
Diffused infrared
While less often associated with audio transmission, infrared light is commonly used to transmit data signals. Look no further than the TV remote control.
The remote contains an infrared emitter, which sends modulated bursts of light to the infrared receiver in the TV. Near -nfrared light includes wavelengths from 750 to 1,050 nm and has most of the physical properties of visible light, except that it is invisible. Infrared light, like visible light, will reflect off the ceiling, walls, and most other objects in a room in an omnidirectional pattern.
There are two main types of infrared transmission: direct line-of-sight and diffused. Direct infrared is characterized by the need for an unobstructed line of sight between the transmitting and the receiving devices. TV remote control units and IrDA-based technology fall into this category. Diffused infrared is non-line-of-sight and non-directional. It acts very much like light emitted from an incandescent bulb. It fills the room with light by reflecting off the walls and ceilings.
There are several features of diffused infrared light that make it particularly well suited for transmitting audio signals to side or rear speakers. These features are well illustrated by the IrGate system from Infra-Com, which is designed for indoor, high- data-rate wireless communication like that needed for high-end audio transmission in home theater systems.
Fig. 1. Diffused infrared is non-line-of-sight and non-directional. Figure a shows transmit circuitry while b shows receive.
How diffused IR works
As shown in Fig. 1a, the transmitter receives the audio signal from the surround-sound receiver. This signal is converted from analog to digital, framed, encoded, and modulated by a digital IC. This digital IC interfaces directly to an infrared-emitting diode (IRED) driver IC through a simple two-wire bus, forming a physical layer transmit chipset. The IRED driver connects to an array of high-speed infrared-emitting diodes. A single digital IC can drive several IRED driver ICs in a daisy-chain configuration. The transmitted signal is a baseband signal and not modulated on a carrier frequency.
The receiver (see Fig. 1b) uses an array of lensed PIN photodiodes to collect the emitted infrared 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 infrared receiver must exhibit a high sensitivity, up to 40 dB more, than a direct, line-of-sight infrared communication link. The infrared light carrying the modulated audio signal is converted to an electronic signal. Powerful analog and digital techniques are used to process the incoming signal and to extract the data from the ambient light in which it is immersed.
Link distance: 12 meters
The diffused infrared IrGate chipset can create communication links of up to 12 m (39 ft). The diffused infrared signal is emitted from an array of wide-angle IREDs, each with an emission angle of about ± 45°. This array is usually pointed toward the ceiling.
The opening photo shows Edifier’s Rainbow infrared wireless audio system – winner of the 2008 CES Innovations Award and the IF Product Design Award at CeBIT – with its cylindrical infrared window design for emitting the optical signal. The emitted signal creates “IR spots” on the ceiling and adjacent walls.
These surfaces are used as natural reflectors or diffusers to spread the infrared emissions all over the room. People walking about, or fixed objects in the room, will not interfere with ongoing communications. The transmitter can be set to emit various levels of optical power, which allows for different ranges.
Data rate and no latency
IrGate can currently 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 (PCM) 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 synch with RF transmission, diffused infrared employs enough bandwidth to use forward error correction (FEC), which allows the receiver to detect and correct sporadic errors without the need for retransmission. IrGate’s latency is an unnoticable 100 µs, keeping video and audio in synch.
Security
Infrared signals do not pass through walls. An “antenna” in an adjacent room can’t eavesdrop. While this is not a great concern for home theater applications, it is the key reason that infrared audio transmission is used in city and county courtrooms.
Those with impaired hearing or needing language translation will use infrared-enabled headsets. The lawyers, witnesses, translators, and judge(s) all wear microphones connected to a system that converts the audio to infrared and sends the signal out into the room. The infrared signals are secure within the room.
Regulation and cost
The IRED transmitter array uses Class 1 LED emitters similar to the emitter found in a TV remote control, which means it is eye safe. The optical bandwidth of 840 to 950 nm is unregulated, worldwide. There is no need for country-specific frequency allocations, special testing, or labeling. This is a significant advantage over RF-based systems that require costly local testing and approvals.
Infrared PIN photodiodes and receivers are relatively low cost. IrGate uses Vishay Semiconductor’s high-intensity TSFF5510 infrared emitter with a peak wavelength of 870 nm, intensity of 32 mW/sr with a ±38° angle of half intensity, and rise and fall times of 15 ns (also shown in opening photo).
Next generation
LCD TVs are getting thinner, with manufacturers targeting a width of 1 cm. It is very hard if not impossible to integrate decent speakers into these thin TVs. Soundbar speakers will likely be bundled with TVs in the near future to deliver high-performance sound that complements the high-definition screen. Even now, consumers may not be able to tell the difference between HD and regular TV, but they can easily hear the difference between good and bad speakers. Given consumers’ aversion to speaker wires, it is likely that not only the side or rear speakers will be wireless, but also the front soundbar speakers as well, where diffused infrared will serve as an effective solution.
BY PHILIP LIU
Edifier Technology, Hong Kong, China
TAMIR SHAANAN
Infra-Com, Netanya, Israel
and JAMES TOAL
Vishay Semiconductors, Santa Clara, CA
The main feature that sets a home theater system apart from an ordinary television with stereo speakers is the side and rear speakers. A standard home theater system has three speakers in front, two side or rear speakers, and at least one subwoofer.
A majority of consumers don’t install the rear speakers because they don’t want wires running around the room or the expense of installing wires in walls and below carpets and floors. But they may not know what they’re missing. Movie soundtracks are recorded with multiple channels to produce a three-dimensional effect, and purists such as Theo Kalomirakis of Discovery’s “Digital Home” consider it a crime to eliminate the rear speakers.
Fortunately, the choice is no longer between having intrusive wiring and having a complete surround-sound experience. The side or rear speakers and subwoofer can now receive audio signals from the surround-sound receiver wirelessly.
RF technology drawbacks
The most widely used technologies for wirelessly transmitting data signals are based on the 2.4-GHz free RF band, which is used by Bluetooth and various types of Wi-Fi systems. High-quality digital audio transmission requires a bandwidth of 1.4112 Mbits/s for an uncompressed stereo (L/R) source of 44.1-kHz/16-bit CD-quality audio.
Bluetooth isn’t fast enough for this; the most it can handle is 1 Mbit/s. Wi-Fi has a bandwidth of 11 to 54 Mbits/s, but it presents a number of problems for home theater systems. The most significant of these is delayed transmission.
As Wi-Fi data packets are continuously being verified or checked for possible errors, the delay in actual processing of the data can vary from tens to several hundred milliseconds. This delay is long enough for the video and audio to be out of synch. Overcoming this phenomenon would mean employing hardware-intensive buffers to compensate for the delay.
The cost of the hardware pushes up the cost of the end product, which pushes away customers. Signals transmitted over Wi-Fi and Bluetooth can also travel through walls, which might potentially cause interference with other signals for example, from a wireless LAN network or a cordless phone operating in the 2.4-GHz band.
Diffused infrared
While less often associated with audio transmission, infrared light is commonly used to transmit data signals. Look no further than the TV remote control.
The remote contains an infrared emitter, which sends modulated bursts of light to the infrared receiver in the TV. Near -nfrared light includes wavelengths from 750 to 1,050 nm and has most of the physical properties of visible light, except that it is invisible. Infrared light, like visible light, will reflect off the ceiling, walls, and most other objects in a room in an omnidirectional pattern.
There are two main types of infrared transmission: direct line-of-sight and diffused. Direct infrared is characterized by the need for an unobstructed line of sight between the transmitting and the receiving devices. TV remote control units and IrDA-based technology fall into this category. Diffused infrared is non-line-of-sight and non-directional. It acts very much like light emitted from an incandescent bulb. It fills the room with light by reflecting off the walls and ceilings.
There are several features of diffused infrared light that make it particularly well suited for transmitting audio signals to side or rear speakers. These features are well illustrated by the IrGate system from Infra-Com, which is designed for indoor, high- data-rate wireless communication like that needed for high-end audio transmission in home theater systems.
Fig. 1. Diffused infrared is non-line-of-sight and non-directional. Figure a shows transmit circuitry while b shows receive.
How diffused IR works
As shown in Fig. 1a, the transmitter receives the audio signal from the surround-sound receiver. This signal is converted from analog to digital, framed, encoded, and modulated by a digital IC. This digital IC interfaces directly to an infrared-emitting diode (IRED) driver IC through a simple two-wire bus, forming a physical layer transmit chipset. The IRED driver connects to an array of high-speed infrared-emitting diodes. A single digital IC can drive several IRED driver ICs in a daisy-chain configuration. The transmitted signal is a baseband signal and not modulated on a carrier frequency.
The receiver (see Fig. 1b) uses an array of lensed PIN photodiodes to collect the emitted infrared 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 infrared receiver must exhibit a high sensitivity, up to 40 dB more, than a direct, line-of-sight infrared communication link. The infrared light carrying the modulated audio signal is converted to an electronic signal. Powerful analog and digital techniques are used to process the incoming signal and to extract the data from the ambient light in which it is immersed.
Link distance: 12 meters
The diffused infrared IrGate chipset can create communication links of up to 12 m (39 ft). The diffused infrared signal is emitted from an array of wide-angle IREDs, each with an emission angle of about ± 45°. This array is usually pointed toward the ceiling.
The opening photo shows Edifier’s Rainbow infrared wireless audio system – winner of the 2008 CES Innovations Award and the IF Product Design Award at CeBIT – with its cylindrical infrared window design for emitting the optical signal. The emitted signal creates “IR spots” on the ceiling and adjacent walls.
These surfaces are used as natural reflectors or diffusers to spread the infrared emissions all over the room. People walking about, or fixed objects in the room, will not interfere with ongoing communications. The transmitter can be set to emit various levels of optical power, which allows for different ranges.
Data rate and no latency
IrGate can currently 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 (PCM) 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 synch with RF transmission, diffused infrared employs enough bandwidth to use forward error correction (FEC), which allows the receiver to detect and correct sporadic errors without the need for retransmission. IrGate’s latency is an unnoticable 100 µs, keeping video and audio in synch.
Security
Infrared signals do not pass through walls. An “antenna” in an adjacent room can’t eavesdrop. While this is not a great concern for home theater applications, it is the key reason that infrared audio transmission is used in city and county courtrooms.
Those with impaired hearing or needing language translation will use infrared-enabled headsets. The lawyers, witnesses, translators, and judge(s) all wear microphones connected to a system that converts the audio to infrared and sends the signal out into the room. The infrared signals are secure within the room.
Regulation and cost
The IRED transmitter array uses Class 1 LED emitters similar to the emitter found in a TV remote control, which means it is eye safe. The optical bandwidth of 840 to 950 nm is unregulated, worldwide. There is no need for country-specific frequency allocations, special testing, or labeling. This is a significant advantage over RF-based systems that require costly local testing and approvals.
Infrared PIN photodiodes and receivers are relatively low cost. IrGate uses Vishay Semiconductor’s high-intensity TSFF5510 infrared emitter with a peak wavelength of 870 nm, intensity of 32 mW/sr with a ±38° angle of half intensity, and rise and fall times of 15 ns (also shown in opening photo).
Next generation
LCD TVs are getting thinner, with manufacturers targeting a width of 1 cm. It is very hard if not impossible to integrate decent speakers into these thin TVs. Soundbar speakers will likely be bundled with TVs in the near future to deliver high-performance sound that complements the high-definition screen. Even now, consumers may not be able to tell the difference between HD and regular TV, but they can easily hear the difference between good and bad speakers. Given consumers’ aversion to speaker wires, it is likely that not only the side or rear speakers will be wireless, but also the front soundbar speakers as well, where diffused infrared will serve as an effective solution.
