วันจันทร์ที่ 25 พฤศจิกายน พ.ศ. 2556

How Do Modern Wireless Headphones Work?

An ever increasing amount of wireless products such as wireless headphones and wireless headphones is causing increasing competition for the precious frequency space. I will look at some technologies which are used by today's digital audio products to see how well these products can work in a real-world environment.The popularity of wireless devices has caused a rapid increase of transmitters which broadcast in the most popular frequency bands of 900 MHz, 2.4 GHz and 5.8 GHz and thus wireless interference has become a major concern.Traditional FM transmitters usually operate at 900 MHz and don't have any particular way of coping with interference but changing the transmit channel is a way to deal with interfering transmitters. The 2.4 GHz and 5.8 GHz frequency bands are used by digital transmitters and also have become quite crowded lately since digital signals occupy more bandwidth than analog transmitters.Simply changing channels, however, is not a reliable solution for avoiding certain transmitters which use frequency hopping. Frequency hoppers such as Bluetooth devices or many cordless phones will hop throughout the entire frequency spectrum. Thus transmission over channels will be disrupted for short bursts of time. Audio can be considered a real-time protocol. As such it has strict requirements regarding reliability. Also, low latency is essential in many applications. Therefore, more sophisticated methods are required to guarantee reliability.One approach is called FEC or forward error correction. This method will allow the receiver to repair a corrupted signal. For this purpose, additional data is sent by the transmitter. The receiver uses an algorithm that uses the additional data. If the signal is corrupted during the transmission due to interference, the receiver can filter out the erroneous data and recover the original signal. This method works if the amount of interference does not exceed a certain threshold. Transmitters using FEC can transmit to a large number of wireless receivers and does not require any feedback from the receiver.Another approach uses bidirectional transmission, i.e. each receiver transmits data back to the transmitter. This approach is only practical if the number of receivers is small. It also requires a back channel to the transmitter. The transmitters includes a checksum with each data packet. Each receiver can determine whether a particular packet was received correctly or disrupted due to interference. Then, each wireless receiver will send an acknowledgement to the transmitter. If a packet was corrupted, the receiver will alert the transmitter and request retransmission of the packet. As such, the transmitter needs to store a certain amount of packets in a buffer. Similarly, the receiver will need to maintain a data buffer. Using buffers causes a delay or latency in the transmission. The amount of the delay is directly related to the buffer size. A larger buffer size increases the reliability of the transmission. Video applications, however, require the audio to be in sync with the video. In this case a large latency is problematic. Wireless systems which incorporate this method, however, can only transmit to a limited number of wireless receivers. Usually the receivers have to be paired to the transmitter. Since each receiver also requires transmit functionality, the receivers are more expensive to manufacture and also consume more power.Often a frequency channel may become occupied by another transmitter. Ideally the transmitter will recognize this fact and switch to another channel. To do so, some transmitters continuously monitor which channels are available so that they can immediately switch to a clean channel. Since the transmitter has a list of clean channels, there is no delay in trying to find a clean channel. It is simply selected from the list. This method is often called adaptive frequency hopping spread spectrum.



Get additional information about wireless headphones from Amphony's website.

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