I will investigate how modern audio transmission technologies which are used in today's wireless speakers and wireless headphones work in real-world environments with a large amount of interference from other wireless devices.
The most popular frequency bands which are used by cordless gadgets are the 900 MHz, 2.4 GHz and 5.8 GHz frequency band. Mostly the 900 MHz and 2.4 GHz frequency bands have started to become clogged by the ever increasing number of gadgets such as wireless networks (WLANs), cordless phones etc.
The least expensive transmitters typically transmit at 900 MHz. They operate similar to FM radios. Since the FM signal has a small bandwidth and thus only occupies a small fraction of the available frequency space, interference can be avoided by changing to another channel. Digital audio transmission is usually employed by more modern audio products. Digital transmitters usually work 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.
Frequency hopping devices, however, will still cause problems since they will disrupt even transmitters using transmit channels. Real-time audio has fairly strict requirements regarding reliability and low latency. In order to offer these, other mechanisms are required.
An often used method is forward error correction in which the transmitter sends additional data along with the audio. From this additional information, the receiver can recover the original data even if the signal was corrupted to a certain extent. FEC is unidirectional. The receiver does not send back any information to the transmitter. Thus it is often used for products like radio receivers where the number of receivers is large.
Another method uses receivers which transmit data packets back to the transmitter. The data packets include a checksum from which each receiver can determine if a packet was received correctly and acknowledge proper receipt to the transmitter. Since lost packets will have to be resent, the transmitter and receivers need to store data packets in a buffer. This buffer causes an audio delay which depends on the buffer size with a larger buffer increasing the robustness of the transmission. A large latency can be a problem for certain applications however. Especially if video is present, the audio should be in sync with the video. Also, in surround sound applications where some speakers are wireless, the wireless speakers should be in sync with the corded speakers. Systems which incorporate this mechanism, however, are limited to transmitting to a small number of receivers and the receivers consume more power.
To avoid crowded frequency channels, some transmitters monitor clear channels and can switch to a clean channel as soon as the current channel becomes occupied by another transmitter. This method is also known as adaptive frequency hopping.
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