Jumat, 21 Maret 2014

How Have Today's Wireless Speakers Evolved Recently?

By Mike Heller


A constantly expanding amount of cordless gadgets including wireless speakers is bringing about increasing competition for the precious frequency space. I'll evaluate a few systems that are used by today's digital sound gadgets in order to determine how well these solutions may work in a real-world environment.

The popularity of wireless products including wireless speakers is mainly responsible for a rapid increase of transmitters that broadcast in the preferred frequency bands of 900 MHz, 2.4 Gigahertz and 5.8 Gigahertz and thus wireless interference has turned into a major concern.

Customary FM transmitters normally work at 900 MHz and don't have any particular way of dealing with interference yet changing the broadcast channel can be a solution to deal with interfering transmitters. Digital audio transmission is generally utilized by more contemporary audio gadgets. Digital transmitters typically function at 2.4 GHz or 5.8 GHz. The signal bandwidth is higher than 900 MHz transmitters and thus competition in these frequency bands is high. Only switching channels, however, is no dependable remedy for staying away from specific transmitters which use frequency hopping. Frequency hoppers like Bluetooth gadgets as well as several cordless telephones will hop through the whole frequency spectrum. Hence transmission on channels will likely be disrupted for brief bursts of time. Audio can be viewed as a real-time protocol. As such it has stringent needs concerning stability. Also, low latency is vital in numerous applications. As a result more innovative methods are required to assure stability.

One of these approaches is known as forward error correction or FEC in short. The transmitter is going to transmit additional data besides the sound data. By using a few advanced algorithms, the receiver is able to restore the information that may in part be corrupted by interfering transmitters. Subsequently, these systems can transmit 100% error-free even when there's interference. Transmitters making use of FEC can transmit to a large number of cordless devices and doesn't require any kind of feedback from the receiver.

Another method uses bidirectional transmission, i.e. every receiver sends information to the transmitter. This strategy is only helpful if the quantity of receivers is small. Additionally, it needs a back channel to the transmitter. The information that is broadcast includes a checksum. Using this checksum the receiver can detect whether any particular packet was received correctly and acknowledge. In cases of dropped packets, the receiver will alert the transmitter and the dropped packet is resent. As a result both the transmitter and receiver require a buffer to store packets. This will introduce an audio latency, also known as delay, to the transmission which may be a dilemma for real-time protocols including audio. Typically, the larger the buffer is, the greater the robustness of the transmission. A large latency can generate problems for many applications however. Particularly when video exists, the sound should be in sync with the video. Additionally, in multichannel applications where a number of speakers are cordless, the cordless loudspeakers ought to be synchronized with the corded loudspeakers. Products which incorporate this particular procedure, however, are limited to transmitting to a small number of receivers and the receivers consume more power.

So as to better overcome interference, several wireless speakers will monitor the available frequency band as a way to determine which channels are clear at any given moment in time. If any specific channel becomes congested by a competing transmitter, these systems can switch transmission to a clean channel without interruption of the audio. The clean channel is picked from a list of channels which has been determined to be clean. A modern technology that employs this kind of transmission protocol is referred to as adaptive frequency hopping spread spectrum or AFHSS




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