I am going to examine exactly how contemporary sound transmission technologies that are employed in current bluetooth loudspeakers work in real-world conditions with a great deal of interference from other wireless devices.
The increasing popularity of wireless consumer products such as wireless speakers has started to cause difficulties with numerous products competing for the limited frequency space. Wireless networks, cordless phones , Bluetooth as well as various other devices are eating up the precious frequency space at 900 MHz and 2.4 GHz. Wireless sound products ought to ensure reliable real-time transmission within an environment having a large amount of interference.
The most cost effective transmitters generally transmit at 900 MHz. They operate just like FM stereos. Considering that the FM signal uses a small bandwidth and therefore only occupies a small part of the free frequency space, interference is generally eliminated by changing to another channel. Digital sound transmission is usually used by newer audio products. Digital transmitters normally operate at 2.4 GHz or 5.8 Gigahertz. The signal bandwidth is higher than 900 MHz transmitters and thus competition in these frequency bands is high.
Frequency hopping systems, however, are going to continue to cause problems because they will affect even transmitters working with transmit channels. Audio can be viewed as a real-time protocol. Therefore it has stringent requirements concerning stability. Furthermore, low latency is important in most applications. Consequently more advanced strategies are necessary to ensure stability.
A frequently used method is forward error correction where the transmitter sends supplemental data combined with the audio. By using several sophisticated algorithms, the receiver may then fix the information which may partly be damaged by interfering transmitters. Because of this, these products may transmit 100% error-free even when there’s interference. FEC is unidirectional. The receiver does not send back any data to the transmitter. Thus it is frequently employed for systems just like radio receivers in which the number of receivers is large.
In cases where there’s only a few receivers, commonly a different method is used. The wireless receiver sends information packets back to the transmitter to confirm correct receipt of data. The information packets include a checksum from which each receiver can decide if a packet was received properly and acknowledge proper receipt to the transmitter. Given that lost packets will have to be resent, the transmitter and receivers have to store data packets in a buffer. This buffer causes an audio delay which will depend on the buffer size with a bigger buffer improving the robustness of the transmission. Video applications, however, need the sound to be synchronized with the video. In cases like this a big latency is problematical. Wireless products that incorporate this approach, however, can only transmit to a small quantity of wireless receivers. Usually the receivers have to be paired to the transmitter. As each receiver also requires transmit functionality, the receivers cost more to make and in addition use up more power.
So as to better handle interference, a number of wireless speakers is going to monitor the accessible frequency band in order to decide which channels are clear at any given time. If any certain channel gets congested by a competing transmitter, these products can change transmission to a clean channel without interruption of the audio. The clear channel is picked out from a list of channels that was identified to be clear. A modern technology which makes use of this particular transmission protocol is called adaptive frequency hopping spread spectrum or AFHSS
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