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The Problems with the Utilization of Wireless Techniques for Surround Channels




We all hate cables running all over the floor, especially when they are not

easily hidden.  We just invested in a top-notch home theater system.

We have a great video monitor, an audiophile quality home theater

receiver, and some great speakers. So far, so good. 

Now it’s time to connect the rear channel surround speakers. 


Agh!  Our system look was just degraded from Star Trek to Mad Max. 

This is an obvious application for wireless, right?


Actually, there are some serious problems with using wireless techniques

for surround sound channels.  




Time Lag:


Wireless surround systems have a processing/transmission time lag that makes the sound from the rear channels out of sync with the audio from the front speakers.  The inherent time required to generate the signal, transmit it to the speaker, decode the signal, and finally, to amplify the signal for the speaker causes the time offset. Minimal audio processing latency is required for effective surround sound. 


The sound from the different speakers should be heard at the same instant.  For most installations, the rear channels are closer to the listener than the front channels.  For this reason, Dolby Digital and Dolby Surround Pro Logic systems apply a delay of a few milliseconds for the surround sound channels.  The timing is that critical.


When humans hear the same sound from different directions, the illusion is created that the sound's source is a phantom location between the sound's sources.  Recording engineers use balance controls to simulate sounds across the sound field, allowing them to locate sounds to different perceived locations across the soundstage.  Timing is critical between surround channels where depth and location information are to be determined by the ears, and ultimately, by the brain.





The second problem with wireless surround systems is interference.  There are multiple technologies used for wireless surround, and the source of interference is specific to the technology. 


Both Bluetooth and WiFi devices operate within the 2.4 GHz band. Bluetooth is the technology that is used frequently for wireless keyboards and mice, but the capability of Bluetooth systems is marginal for surround sound applications.  A Bluetooth signal power is only about 1 milliwatt.  Dolby Digital requires a bandwidth of about 400 Kbps and Dolby DTS uses a bandwidth of about 450 Kbps for a 5.1 system.  Bluetooth has a maximum data transfer rate of 433 Kbps, which is cutting it pretty close.  


Another alternative for surround sound transmission is WiFi 802.11b (also in the 2.4 GHz band, as is 802.11g ).  An 802.11b signal is capable of 54 Mbps maximum throughput (30 Mbps in practice), which is more than sufficient for this application. 


 However, there are significant sources of interference in this 2.4 GHz spectrum from cordless phones, other WiFi equipment like wireless-enabled laptops, wireless game controllers, fluorescent lights, and even microwave ovens. Even bad electrical connections can cause broad Rf spectrum emissions. 


Bluetooth attempts to avoid interference by “frequency-hopping”.  If the signal is receiving interference, it will (hopefully) only be for a fraction of a second, because the system will identify the interference, and switch to a different frequency.  WiFi also has the ability to avoid interference by changing channels, and by redundant loops of information transmission   Many WiFi products claim to mange interference problems. Unfortunately, the products are limited by the capabilities of the 802.11 chipsets, and the 802.11 protocol itself.


IR (infrared), which is most often used by remote controls, is yet another technology sometimes used for wireless surround.  IR has a bandwidth capability of more than 100 Mbps.   IR for data transmission was popular on laptop computers a few years ago.  More recently, it has been replaced by other wireless technologies such as Bluetooth, because they don't need a direct line of sight (LOS).  IR transmission is lost whenever there is a loss of LOS between the transmitter and the receiver, and, in addition, the signal is subject to interference from IR remote controls and ambient light. 





A third problem is that the rear speakers will require a power source if they are receiving the signals wirelessly.  They will need power not only for the actual speakers, but they will also need power for an on-board amplifier.  If our goal is to eliminate all wires, this is a problem.  Powercast (Pittsburgh, PA) has a technology, also at 2.4 GHz, that will wirelessly supply power to small appliances like cell phones, but the maximum rate is about 10 watts, not enough for surround channels.    


On a final note, if you decide to purchase a wireless surround system, read the product attributes carefully.  I recently assisted on an installation where a DVD/Receiver system was bought with wireless capability.  However, in this case, the wireless capability was for distribution to a second room for the primary channels, not a wireless capability for surround channels.











 About the Author:


Brian Bradshaw is an InfoComm Certified Technical Specialist (CTS), Microsoft Certified Technology Specialist (MCTS), and CompTIA A+ Computer Technologist.  Areas of expertise include Video, HDTV, Audio, Computation, SATCOM Systems, and Communications.

Website:  http://bradshaw-vacuum-technology.com/