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other meeting reports
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12/19/01 Meeting Highlights
by Giles Davis
Wireless Telephone Design Considerations.
On December 19th 2001 at the Holiday Inn North Shore in Skokie, Bob Zurek of Motorola gave a presentation entitled "Wireless Telephone Design Considerations."
As wireless phones have matured, the expectations on performance and size have led to new acoustic challenges. The desire to reduce the size of modern cell phones has made the incorporation of an earcup in the device impractical. Without an earcup, a decent seal usually assumed with a landline phone is not possible. The environment that a cell phone is used in contributes to the lack of seal in the system. Users will often set the volume to its maximum setting and vary the amplitude by moving the phone towards or away from their ear. A system that is designed to accommodate this varying impedance is often referred to as leak-tolerant.
The push to reduce the overall size of the unit has created system echo problems that do not exist in full size landline systems. Shortening the length of the phone increases the microphone-to-mouth distance, affecting signal-to-noise ratio and microphone sensitivity. A reduction in the internal volume coupled with an increased distance between the mouth and the microphone tend to increase receiver to microphone feedback. This feedback coupled with digital system delay (as much as 200ms) creates significant echo problem for the far end user. To reduce the problem, consider that the best case coupling from receiver to microphone is the external air path (but is rarely the case). Phone housings and PC boards act as ducts, directing energy down the phone toward the microphone. This ducted energy can be reduced by creating partitions and using grommets around microphone and speaker. Alternate acoustic paths, such as leaking through jacks on the side or bottom of the phone, also need to be sealed. Separately rotating housings eliminate both the directly ducted energy and alternate acoustic paths, and also decreases the microphone-to-mouth distance.
Audible Alerts or ringers have historically been able to produce only a few tones at a very high level for their size and input voltage. Armature alerts are typically used to produce these tones. However, consumers are now asking for programmable musical tones and audio for games from these same devices. Since space is very limited for these devices, new system configurations are being implemented to get as much SPL out of as little volume as possible. Increasingly micro-loudspeakers are replacing the armature alerts to provide more musical alert tones along with speakerphone operation. Since volume and board space are at a premium on cellular systems, components are now being used for multiple uses. Several companies are now marketing multifunction transducers, which typically include vibrator (silent alert), alert, and receiver.
New techniques are constantly being developed to overcome the challenges being presented by new trends in portable communications devices. Coupled with consumer demands for greater integration of entertainment devices, new devices may eventually change our lives in the same way the first cellular phones have in the 30 years since their inception.
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