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Echo Cancelling Gives Full Duplex Speech
You may be familiar with the situation - you want to look up information on the PC, find a folder, make notes - all while you are talking on the phone having only one hand free. The solution is a speakerphone. Most of us have tried one, but the result is disappointing. This because an ordinary loudspeaking phone "chops up" the conversation. You can not talk and listen at the same time and the voice direction is determined by who is speaking the loudest. One has to be very disciplined to make sure the speech directions switches with the person speaking. In addition the sound quality may be poor, which makes the experience even worse.
When using the handset, both parties can speak and listen at the same time - this is called full duplex or open duplex. Full duplex conversation is possible because the speaker volume is relatively low and the microphone is placed away from the speaker, i.e. only a small fraction of the sound travels from the loudspeaker back to the microphone. On a speakerphone, most of the sound travels from the speaker to the microphone resulting in oscillation and howling - if not blocked one way or another. Half duplex is one way to solve this problem, but this technique introduces drawbacks. The voice switching may not be fast enough or it may incorrectly switch direction due to background noise. It is - in other words - impossible to interrupt the participants at the distant end when they are speaking, without at least partial loss of comprehension.
Participants in a teleconference is now given the same level of freedom of interaction that they would sitting around a table. The solution lies in applying echo cancellation technology that dynamically filters away the sound from the speaker. This sound is of course picked up by the microphone and a full duplex system needs to filter away all of this signal. This is a challenging task as the microphone receives the sound both as direct waves and as reflections from walls and ceiling with varying time delays. The reflections - or echo - changes continuously as persons moving around in the room affects the way sound is reflected back to the microphone. Echo cancellation thus necessitates a much more sophisticated technology than what is used in standard half duplex speakerphones. One of the earlier solutions to minimise echo, was to crate specially constructed conference rooms with a high degree of acoustic absorption. The rooms were equipped with directional microphones as well as carefully positioned loudspeakers to allow for sound waves to cancel each other out. Such rooms and their system involved a considerable expense as well as restricting conferencing to a single location.
In recent years, more and more powerful signal processors have become available making it possible to apply sophisticated real time software algorithms to achieve full duplex. However, sound reflections from walls and ceiling still makes this a very complex task as the echo cancelling unit must update its filter to constantly changing echo pattern. Though the human ear cannot perceive sound reflections of less than 35 ms (35/1000 seconds), such echo still distorts the overall sound picture and makes the speech less recognisable. Even the most sophisticated software can not yet solve all the challenges introduced by echo. One way to compensate for this, is to bring the sound source closer to each participants, thus lowering the sound level and consequently reducing echo and increasing clarity of the speach. This is the philosophy behind the Eclipse systems as it expands with satellites units which are placed relatively near each person. For sound distribution to and from the satellites, Eclipse applies a unique infrared (IR) transmission technology.
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Sound Transmission by Infrared Light (IR) Infrared light(IR) is electromagnetic waves with a frequency of approximately 300 THz. This compares to a wavelength of approximately 1 micrometer (um) or 1/1000 millimetre. Compared to visible light with a wavelength range of 0.4 um (blue) to 0,8 um (red), Eclipse uses IR wavelengths between 0.8 um and 1.0 um.
Eclipse conference system uses IR light to transmit audio to and from its satellite units. The system converts the audio to IR waves and floods the room with IR light. As with the visible light, IR waves travels in straight lines and is spread throughout the room via reflections off walls and ceiling. It is therefore not required to have free visibility between the master unit and its satellites. Eclipse system works well off the reflections and persons may move freely around the room without disturbing the transmission.
The IR transmitter and receiver are hidden behind the specially design Eclipse housing. Even though the Eclipse house is completely black, it is still nearly completely transparent to infrared light. Eclipse supports two satellite units with microphone/speaker and an unlimited number of satellites with a speaker but no microphones. Three IR channels are applied - one to broadcast audio to the satellites and one for each of the two satellites equipped with microphones.
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In a conversation across the table, we may speak and listen at the same time. On the telephone we can do the same when using the handset of a regular phone.
The illustration shows the principal operation of an ordinary speakerphone. The voice travels in just one direction at a time (half duplex) - either the microphone is blocked or the loudspeaker is blocked. The system switches voice direction by whom is talking the loudest.
