How Do Today's Wireless Speakers Deal With Interference?

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By Mike Heller


Wireless audio is becoming popular. A multitude of consumer products including wireless speakers are eliminating the cable plus promise ultimate freedom of movement. I am going to examine how newest wireless systems are able to address interference from other transmitters and how well they will perform in a real-world scenario.

Customary FM transmitters usually operate at 900 MHz and don't have any certain way of coping with interference but switching the broadcast channel can be a approach to cope with interfering transmitters. Modern sound systems employ digital sound transmission and frequently operate at 2.4 GHz. Such digital transmitters broadcast a signal which takes up far more frequency space than 900 MHz transmitters and thus have a greater chance of colliding with other transmitters.

The least expensive transmitters generally transmit at 900 MHz. They work just like FM stereos. Since the FM signal uses a small bandwidth and thus only uses up a tiny part of the available frequency space, interference is generally eliminated simply by changing to a different channel. Digital audio transmission is generally employed by more modern sound 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.

A frequently utilized method is forward error correction where the transmitter transmits extra information with the sound. The receiver utilizes a formula which uses the additional data. If the signal is damaged during the transmission as a result of interference, the receiver can easily filter out the incorrect information and recover the original signal. This technique will work if the level of interference does not rise above a specific limit. Transmitters making use of FEC can transmit to a great number of wireless receivers and doesn't require any kind of feedback from the receiver.

One more technique makes use of receivers which transmit data packets back to the transmitter. The data packets incorporate a checksum from which every receiver can easily determine if a packet was received properly and acknowledge correct receipt to the transmitter. In situations of dropped packets, the receiver will notify the transmitter and the dropped packet is resent. As a result both the transmitter as well as receiver have to have a buffer to keep packets. Employing buffers causes a delay or latency in the transmission. The amount of the delay is proportional to the buffer size. A bigger buffer size improves the reliability of the transmission. A large latency can be a problem for several applications nonetheless. Especially if video exists, the audio must be in sync with the movie. Additionally, in multichannel audio applications where a number of loudspeakers are cordless, the cordless loudspeakers ought to be in sync with the corded loudspeakers. Wireless systems which incorporate this technique, nonetheless, are only able to broadcast to a limited number of cordless receivers. Usually the receivers have to be paired to the transmitter. Given that each receiver also requires transmit functionality, the receivers are more pricey to manufacture and also use up more power.

In order to avoid crowded frequency channels, several wireless speakers monitor clear channels and can change to a clean channel as soon as the current channel gets occupied by another transmitter. This approach is also referred to as adaptive frequency hopping.




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