from 4 to 5 seconds were made using the speech from this database For each

From 4 to 5 seconds were made using the speech from

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from 4 to 5 seconds, were made using the speech from this database. For each recording, there was a unique loudspeaker location, and the prerecorded speech was from a unique, male talker uttering a unique string of alpha-digits. The different source locations have been dubbed source 1 , source 2 and source 3 . The conference room set-up is illustrated by Figure 3.5. Source 1 was the farthest from the array and was positioned to simulate a person who was standing in front of a white-board, as if she were giving a presentation. Source 2 and source 3 were positioned to simulate talkers sitting at a 3.8m by 1.2m conference table, which was located approximately in the center of the room. Figure 3.5 Conference room layout. The microphone array was composed of fifteen omni-directional electret condenser microphones [30], which were randomly distributed on a plane within a 1.34-by-0.67- meter rectangle. This is illustrated by Figure 3.6. The microphones were attached to a rectangular sheet of acoustic foam, which was supported by an aluminum frame. This frame was mounted on a tripod that was placed 0.9 meters away from the wall. The acoustic foam damped some of the reflections from this wall and isolated the microphones from vibrations traveling along the mountings. The long dimension of the array was parallel to the y -axis of the room coordinate system, as shown in Figure 3.5. The center of the 25
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Figure 3.6 The planar, 15-element conference-room microphone array. array, corresponding to the origin of the local coordinate system shown in Figure 3.6, was located at (0.90, 2.15, 1.37) meters in the room coordinate system. During all three recordings, the loudspeaker was facing the microphones, and the volume had been adjusted to maximize the amplitude of the microphone signals without causing any clipping or noticeable distortion. This adjustment was made by adjusting the loudspeaker amplifier while watching the Megamike’s channel meters. The channel meters clearly indicated when the A/Ds reached their limit. Using this feature in conjunction with simply listening for audible distortion from the loudspeaker ensured that the recordings were made with the highest possible signal-to-noise ratios (SNRs). Immediately after each speech recording was made, another recording was made with the loudspeaker left undisturbed. During the second recording, a digitally generated, Gaussian noise signal was played through the loudspeaker. Again, the volume of the loudspeaker amplifier had been adjusted to maximize the amplitude of the microphone signals without causing any clipping or noticeable distortion. Finally, a single recording was made with no loudspeaker source. This source-free recording was used to measure the background noise present in the conference room. The sixteenth channel of the Megamike was connected to a high-quality microphone that was mounted approximately 7 centimeters in front of, and facing, the loudspeaker. The signal from this microphone was recorded simultaneously with the array signals, and it provided a true reference of the 26
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sound produced by the loudspeaker.
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