The distinguishing feature of home theater systems from ordinary stereo speakers is the side and back speakers. A standard home theater system has three front speakers, two side and rear speakers, and an auxiliary woofer.
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Most consumers don't install rear speakers because they don't want the wires to fill the room, and the cost of the steps inside the walls or under the carpet and floor is too high. But they didn't realize what they had lost. The soundtrack of the movie is multi-channel recorded to produce a three-dimensional effect, so the enthusiasts of the Carromy's home theater think that it is a crime to cancel the rear speaker.
Fortunately, people don't have to make trade-offs in complicated wiring and having a complete surround sound experience. The side/rear speakers or subwoofer can wirelessly receive audio signals through the surround sound receiver.
The commonly used wireless transmission technology is based on the 2.4 GHz free RF band, which is used by Bluetooth and many different Wi-Fi systems. In practical applications, the 44.1 kHz/16-bit CD quality uncompressed stereo signal requires 1.4112 Mb/s of bandwidth for transmission. The speed of Bluetooth does not meet this requirement, and its maximum rate is 1 Mb/s. Although Wi-Fi has a bandwidth of 11 to 54 Mb, it has a lot of problems in home theater. The biggest trouble is the transmission delay.
When Wi-Fi packets are constantly being verified or checked, the actual data transmission delay will change from 10 milliseconds to hundreds of milliseconds, which will cause the audio and video signals to be out of sync. Solving this problem would mean using hardware-enhanced caching to compensate for latency. But the use of hardware increases the cost of the end product and keeps it away from consumers. In addition, signals transmitted via Wi-Fi or Bluetooth can also be transmitted through the wall, which may interfere with other signals. For example, a wireless LAN device or a cordless phone operating at 2.4 GHz.
Scattered infrared light is rarely used to transmit audio signals, and infrared light is often used to transmit data signals, such as television remote controls. The remote control has an infrared emitter that transmits the modulated infrared beam. In general, infrared light has a wavelength of 750 to 1050 nm and has the physical properties of most visible light, except for invisibility. Infrared light, like visible light, is totally reflected from the ceiling, walls, and other objects in the room.
There are two types of infrared transmission: direct transmission and scattering. Direct infrared is characterized by a non-blocking path between the transmitter and receiver.
TV remotes and IrDA-based technologies fall into this category. The scattered infrared light is non-linear and has no fixed reflection angle. It is like the light emitted from an incandescent lamp, illuminating the entire room through reflections from walls and ceilings.
Scattered infrared light has several features that are suitable for transmitting signals to the side/rear speakers. Infra-Com's IrGate system is a good example of these features. The IrGate system can be used for indoor high-speed wireless data communication as well as for high-end audio signal transmission in home theaters.
How the scattered infrared light works As shown in Figure 1(a), the transmitter receives the audio signal from the surround receiver. This signal is converted from analog to digital and then channelized, encoded and modulated by the digital chip. The digital chip is directly connected to the infrared light emitting diode driver chip through a two-wire bus, and one digital chip can drive multiple IRED driver chips in the daisy chain configuration. The final output signal is a baseband signal that is not modulated on the carrier frequency.
The receiver (see Figure 1(b)) uses a lensed PIN photodiode array to receive the infrared signal. The transmitted signal is attenuated by absorption and multiple reflections, so only a small fraction can reach the receiver. Therefore, the scattering infrared receiver must have high sensitivity, with a minimum of 40dB. The modulated audio signal carried by the infrared light is converted into an electronic signal that is processed by powerful analog and digital techniques to separate from the ambient light signal.
Connection distance: 12m
The scattering infrared IrGate chipset creates a communication connection of up to 12m. The scattered infrared signals are transmitted through a wide-angle IRED array, where each tube has an emission angle of ±45°. This array usually points to the ceiling.
Figure 2 shows the winners of the "Rainbow" infrared wireless audio system - the 2008 CES Innovation Award and the CeBIT IF Product Design Award. Its cylindrical infrared glass is used to scatter light signals that emit "infrared spots" on the ceiling and adjacent walls. These surfaces are used as natural reflectors or diffusers to spread infrared light throughout the room. People walking or repairing objects in the house will not interfere with the communication. The transmitter can emit different levels of light so that different distances can be communicated.
Figure 2 "Rainbow" infrared wireless audio system
Data transfer rate and no delay characteristics
The maximum data transfer rate that IrGate can currently support is 25Mb/s (half duplex). Such a high data transmission rate allows simultaneous 4-channel uncompressed pulse signal modulation digital audio channel transmission. It automatically supports sampling frequencies of 44.1 kHz / 48 kHz / 88.2 kHz / 96 kHz and a sampling length of 24 bits. Where the audio and video signals are not synchronized with the RF transmission, the scattered infrared light can perform a forward error correction function with sufficient bandwidth, which allows the receiver to detect and correct sporadic errors without retransmission. IrGate's waiting time is 100μs, which makes it easy to synchronize audio and video.
The security infrared signal can't pass through the wall, and the antenna near the house can't be eavesdropped. Although this has little to do with home theater applications, it is a key factor in the use of infrared audio transmission in town courts. Infrared headphones are used by those who are hearing impaired or who need a language translation. Lawyers, witnesses, translators, and judges bring headphones to the same system. This system converts the sound signal into an infrared signal and sends it back to the house. In the house, infrared signals are very safe (the above paragraph describes the state of the United States).
Specification and cost
The Class 1 LED transmitter used in the IRED Transmitter Array is similar to the transmitter on the TV remote, which means it is harmless to the eyes. Moreover, the optical bandwidth of 840 to 950 nm is unmanaged worldwide, without the need for frequency allocation, special experiments or markings in each country. This is an important advantage for RF systems that require local testing and approval.
Infrared photodiodes and receivers are quite inexpensive. IrGate uses Vishay's high-sensitivity TSFF5510 infrared emitter with a wavelength of 870 nm, a light intensity of 32 mW at a 38° emission angle, and a period of 15 ns.
The next-generation LCD TVs have to be thinner, and the manufacturer's goal is 1cm thickness, making it difficult to integrate advanced speakers. Strip speakers are likely to be brought together with the TV to provide high-performance audio capabilities. Even now, consumers still can't tell the difference between HDTV and regular TV, but they can clearly distinguish the quality of the speakers. Assuming consumers are not interested in wired speakers, not only can the side or rear speakers be wireless, but even the front strip speakers can be wireless, and in these applications, scattering infrared will be a very effective means.
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