Its gain was adjusted independent of the array using a preamplifier and in line

Its gain was adjusted independent of the array using

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sound produced by the loudspeaker. Its gain was adjusted, independent of the array, using a preamplifier and in-line attenuators so that all sixteen channels were roughly at the same level (near maximum range). With its gain maximized, and its close proximity to the loudspeaker, this microphone essentially received a reverberation-free signal. Reference signals were included in all the array recordings made in the conference room. While the sampling rate of the Megamike is 20kHz, the recordings were re-sampled at 16kHz using an appropriate multi-rate filtering scheme [73]. Since the prerecorded speech signals that were played through the loudspeaker were originally sampled at 16kHz, none of the speech-content was lost during the down-sampling procedure. Furthermore, the down sampling reduced the amount of digital data, which in turn reduced the amount of storage and computation needed to process the data. It was also more convenient to have the same sampling rate for the prerecorded speech, the array recordings, and the sound card of the computer used to process the signals. Table 3.1 Source locations in the room coordinate system and DOAs relative to the array center. Location (Meters) DOA (Degrees) x y z θ φ Source 1 6.47 1.03 1.40 -11.09 0.33 Source 2 5.22 2.57 1.13 5.95 -3.11 Source 3 3.94 1.25 1.10 -15.92 -4.89 Table 3.1 lists the three source locations in relation to the room’s global coordinate system. Also listed in this table are the DOAs at the array’s origin, which correspond to the three source locations. These DOAs correspond to the azimuth angle, θ , and elevation angle, φ , as defined in Figure 2.3 for a planar array. According to this convention, sound waves traveling towards the array, on a path perpendicular to its axis, have azimuth and elevation that equal zero degrees. 27
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3.3 Signal-to-Noise Power The only obvious sources of noise in the conference room were the fans inside the Megamike and its host PC. In order to quantify this observation, the signal-to-noise ratios (SNRs) for each microphone channel were measured for the three Gaussian noise sources. The source-free recording was used to estimate the power of the background noise, which was presumed to be the same during all the recordings. Since the volumes of the source signals were set to overpower this noise, it is valid to assume that the background noise added a negligible component to the recordings of the Gaussian source. Hence, the powers of the microphone signals were used to compute the “signal” part of the SNRs. The microphone signals from the Megamike are band-limited, sampled and quantized versions of the true analog signals, . Let these discrete-time microphone signals be denoted . Denoting the ) ( ) ( 1 t x t x M K ] [ ] [ 1 n x n x M K m -th microphone signal from the l -th Gaussian noise recording by and from the source-free recording by , the SNR of the ] [ ) ( .
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