The biggest feature that distinguishes a home theater system from an ordinary TV with 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.
The vast majority of consumers do not install rear speakers because they are reluctant to allow wiring to cover the entire room, and the cost of routing wires in walls or under carpets and floors is too high. But they did not realize what they had lost. The soundtrack of the movie is recorded in multiple channels to produce a three-dimensional effect. Therefore, enthusiasts of Carlo Mirax Home Theater believe that it is a crime to cancel the rear speakers.
Fortunately, people no longer have to choose between complicated wiring work and having a complete surround sound experience. Side / rear speakers or auxiliary woofer can wirelessly receive audio signals through a surround sound receiver.
The commonly used wireless transmission technology is based on the 2.4GHz free RF band, which is mostly used by Bluetooth and a variety of different Wi-Fi systems. In practical applications, a 44.1kHz / 16-bit CD-quality uncompressed stereo signal requires 1.4112Mb / s bandwidth for transmission. The speed of Bluetooth cannot meet this requirement, and its maximum rate is 1Mb / s. Although Wi-Fi has a bandwidth of 11 ~ 54Mb, but it has a lot of problems in home theater. The biggest trouble is the transmission delay.
When Wi-Fi packets are constantly verified or checked, the actual data transmission delay will change from 10 milliseconds to hundreds of milliseconds. This delay will make the audio and video signals no longer synchronized. Solving this problem will mean using hardware-enhanced cache to compensate for the delay. However, the use of hardware will increase the cost of end products and keep them away from consumers. In addition, signals transmitted through Wi-Fi or Bluetooth can also be transmitted through the wall, which may cause interference with other signals. For example, a wireless LAN device or a cordless phone operating at 2.4GHz.
Diffuse infrared light
Compared to rarely used to transmit audio signals, infrared light is often used to transmit data signals, for example, TV remote control. The remote control has a built-in infrared transmitter, which can transmit the modulated infrared beam. In general, infrared light has a wavelength of 750-1050 nm, and has most of the physical characteristics of visible light, except that it is invisible. Infrared light, like visible light, is totally reflected from ceilings, walls and other objects in the room.
There are two types of infrared transmission: direct transmission and scattering. The characteristic of direct infrared is that a non-blocking path is required between the transmitter and the receiver.
TV remote controls 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 the incandescent lamp, illuminating the entire room through the reflection of the wall and ceiling.
The scattered infrared light has several characteristics suitable for transmitting signals to the side / rear speakers. Infra-Com's IrGate system explains these features well. IrGate system can be used for indoor high-speed wireless data communication, and is also suitable for high-end audio signal transmission in home theaters.
How diffuse infrared light works
As shown in Figure 1 (a), the transmitter receives audio signals from a surround sound 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 LED driver chip through a two-wire bus. One digital chip can drive multiple IRED driver chips in a 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 PIN photodiode array with a lens to receive infrared signals. The transmitted signal is attenuated by absorption and multiple reflections, so only a small portion can reach the receiver. Therefore, the scattering infrared receiver must have high sensitivity, with a minimum of 40dB. The modulated audio signal carried by infrared light is converted into an electronic signal, which is processed by powerful analog and digital techniques to separate it from the ambient light signal.
Connection distance: 12m
The diffuse infrared IrGate chipset can create a communication connection of up to 12m. The scattered infrared signal is transmitted through a wide-angle IRED array, where the emission angle of each tube is ± 45 °. This array usually points to the ceiling.
Figure 2 is the winner of the Edifier's "Rainbow" infrared wireless audio system-the 2008 CES Innovation Award and CeBIT IF Product Design Award. Its cylindrical infrared glass is used to scatter light signals, and the emitted signal will create "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 intensity, so that it can communicate at different distances.
Figure 2 "Rainbow" infrared wireless audio system
Data transfer rate and no delay characteristics
The maximum data transmission rate that IrGate can currently support is 25Mb / s (half duplex). Such a high data transmission rate allows simultaneous transmission of 4 uncompressed pulse signal modulated digital audio channels. It can automatically support 44.1kHz / 48kHz / 88.2kHz / 96kHz sampling frequency and 24bit sampling length. Where the audio and video signals are not synchronized with the RF transmission, the scattered infrared light can perform the forward error correction function with sufficient bandwidth, which allows the receiver to detect and correct incidental errors without retransmission. The waiting time of IrGate is 100μs, it is easy to synchronize audio and video.
safety
Infrared signals cannot pass through walls, nor can antennas in nearby rooms eavesdrop. Although this has little to do with home theater applications, it is a key factor in the use of infrared audio transmission in urban courts. Those who have hearing impairments or need language translation will use infrared headphones. The headphones worn by lawyers, witnesses, translators and judges are all connected to the same system, which converts the sound signal into an infrared signal and sends it back to the house. In the house, the infrared signal is very safe (the above paragraph describes the situation in the United States).
Specification and cost
The level 1 LED transmitter used by the IRED transmitter array is similar to the transmitter on the TV remote control, which means it is harmless to the eyes. Moreover, the optical bandwidth of 840 to 950 nm is unregulated worldwide, and does not require frequency allocation, special experiments or marking in each country. This is an important advantage for RF systems that require local testing and approval.
Infrared photodiodes and receivers are quite cheap. IrGate uses Vishay's high-sensitivity TSFF5510 infrared transmitter with a wavelength of 870nm, a light intensity of 32mW at a 38 ° emission angle, and a period of 15ns.
Next generation products
For LCD TVs to become thinner, manufacturers aim for a thickness of 1cm, making it difficult to integrate advanced speakers. It is possible for the strip speakers to be integrated with the TV to provide high-performance audio functions. Even now, consumers still cannot distinguish the difference between high-definition TV and ordinary TV, but they can clearly distinguish the quality of the speakers. Assuming that consumers are not interested in wired speakers, not only the side or rear speakers may be wireless, even the front-end sound bar can be wireless, and in these applications, diffuse infrared will become a very effective means.
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