VoIP Acoustic Design - VLSI Solution

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VLSISolutionHHyVoIP Acoustic DesignVSMPG2. HOW TO COMBAT FEEDBACK2.3.4 Meas +30.5 dB: As Is0−10−20F=831.4209HzLevel=5.4292dBS/(N+THD)=−19.033dBS/N=−19.0415dBHD=26.9613dBAmplitude (dB)−30−40−50−60−700 500 1000 1500 2000 2500 3000 3500 4000Frequency (Hz)Figure 2.7: +30.5 dB: As IsFirst the unit was tested as is, without any modifications except for loading the passthroughsoftware. The spectrum is presented in Figure 2.7. As can be seen, frequenciesbetweem 200. . . 1400 Hz are greatly boosted.The overall signal power was a whopping 30.5 dB stronger than the reference so it is notwonder why echo cancellation wouldn’t perform properly.Rev. 2.11 2008-09-18 Page 10(22)
VLSISolutionHHyVoIP Acoustic DesignVSMPG2. HOW TO COMBAT FEEDBACK2.3.5 Meas +16.0 dB: No Chassis0−10−20F=1.3428HzLevel=−1.5676dBS/(N+THD)=−11.2087dBS/N=−11.1296dBHD=5.7821dBAmplitude (dB)−30−40−50−60−700 500 1000 1500 2000 2500 3000 3500 4000Frequency (Hz)Figure 2.8: +16.0 dB: No ChassisThe main chassis was removed in such a way that the speakers and microphone still wereat the same relative positions as before. This resulted in a much enhanced performanceas seen in Figure 2.8. All the peaks are gone and the resulting frequency response isrelatively straight.The signal’s power has decreased to +16.0 dB. Although it still is much greater thanwhat was with VLSI Solution’s Demonstration Unit, the difference is big enough thatVoIP functionality works with the official VoIP firmware.This solution was of course not practical in itself, because a product needs to have achassis. Nevertheless, now it was clear that the main chassis somehow acted as a soundconductor, because removing it decreased FES so significantly.The chassis was put together again for the next tests.Rev. 2.11 2008-09-18 Page 11(22)
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VoIP Acoustic Design - VLSI Solution

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