An ever expanding number of cordless gadgets including wireless speakers is causing increasing competition for the valuable frequency space. I’ll examine a number of technologies which are used by the latest digital sound gadgets to discover how well these products can operate in a real-world situation.
The buzz of cordless devices such as wireless outdoor speakers is mainly responsible for a quick rise of transmitters which transmit in the preferred frequency bands of 900 MHz, 2.4 GHz and 5.8 GHz and therefore cordless interference has turned into a major problem. FM type audio transmitters are generally the least reliable in regards to tolerating interference because the transmission does not have any method to deal with competing transmitters. On the other hand, those transmitters use a fairly limited bandwidth and changing channels can frequently steer clear of interference. Modern sound gadgets employ digital sound transmission and frequently operate at 2.4 GHz. Those digital transmitters transmit a signal that takes up more frequency space than 900 MHz transmitters and thus have a greater possibility of colliding with other transmitters. Just switching channels, on the other hand, is no reliable remedy for avoiding specific transmitters that use frequency hopping. Frequency hoppers such as Bluetooth products or many cordless telephones are going to hop throughout the full frequency spectrum. Hence transmission on channels is going to be disrupted for short bursts of time. Audio can be regarded as a real-time protocol. Consequently it has strict needs regarding stability. Furthermore, low latency is essential in most applications. Thus more advanced techniques are required to ensure dependability. A frequently utilized technique is forward error correction where the transmitter transmits supplemental information along with the audio. Because of this additional information, the receiver can easily restore the original data whether or not the signal was damaged to a certain degree. FEC is unidirectional. The receiver does not send back any information to the transmitter. Thus it is often used by equipment similar to radio receivers in which the quantity of receivers is big.
A different approach makes use of bidirectional transmission, i.e. every receiver transmits information back 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 packets have a checksum from which every receiver can determine whether a packet was received properly and acknowledge proper receipt to the transmitter. In situations of dropped packets, the receiver is going to notify the transmitter and the dropped packet is resent. Therefore both the transmitter and also receiver have to have a buffer to keep packets. This will create an audio latency, also called delay, to the transmission which is often a challenge for real-time protocols like audio. Generally, the bigger the buffer is, the greater the robustness of the transmission. However a large buffer can result in a large latency which may bring about issues with loudspeakers not being synchronized with the video. One constraint is that products where the receiver communicates with the transmitter can usually only broadcast to a small number of cordless receivers. Also, receivers have to incorporate a transmitter and usually consume additional current
Often a frequency channel can get occupied by a different transmitter. Ideally the transmitter is going to realize this fact and change to yet another channel. To do so, a few wireless speakers consistently monitor which channels are available so that they can quickly switch to a clean channel. Considering that the transmitter has a list of clean channels, there is no delay in trying to find a clear channel. It is simply selected from the list. This strategy is usually called adaptive frequency hopping spread spectrum.