Microphone Array and BeamformingBy Pattarapong Rojanasthien ECE 5525 Dr. Kepuska December 8th, 2008OverviewIntroductionBackground Microphone Arrays Beamforming Signal to Noise RatioSolution and ApproachResultsConclusionIntroductionThe goals for the project are the following Learn the basic concept of microphone array Become familiar and learn how to use a program that perform Beamforming Show that Beamforming will enhance the signal quality based on SNRMicrophone ArrayMultiple microphones set up in certain formation depend on the application. Line Circle RectangleEach microphone will capture the sound from the source at the different time due to distance from the source.Therefore, there is a time delay in each signal.Microphone ArrayMulti-Channel Signal ComparisonTime DelayNote that the distance between another microphone (not the reference) to the source, which denote as g, can be compute with trigonometry cos = d/g = d = g cos. The distance from source to Ref Mic: d = 30 in.The distance from source to Mic x: g = 34.73 in.The distance between mic: N = 17.5 in.Sampling Frequency: fs = 22050 samples/sec.Speed of sound: c = 345 m/sec.Find d = 34.73 30 = 4.73.Find Sample/meter = 22050/345 = 63.913 samples/m.Turn into inches = 63.913 x 0.0254 = 1.62To find time delay, we need distance d x = 4.73 x 1.62 = 7.7 samples, round up to be 8 samples.Example of Microphone ArrayBeamformingA technique that rearrange the mixture signals from the microphone array, so that the signals from the source that we want are lined up before combine them all up into one signalSince signals from the specified source are on the same phase, they will add each other up.The noise signals may either add each other up or cancel each other out. The general form of the beamforming output based on delay and sumBeamforming PatternSignal to Noise RatioThe measurement of the signal qualityBasically, it is the magnitude of One way to calculate it is to find noise from voice activity detection 1.Measure the mean of the energy during the last t1 seconds = E1 2.Measure the mean of the energy during the last t2 seconds = E2 3.Calculate the speech threshold T2 = E2 + Ex 4.If E1 Ts, speech is detected. (speech onset) 5.Freeze E2 and calculate the noise threshold Tn = E2 + En 6.Measure the mean of the energy of recent t1 seconds = E1 7.IF E1 Tn, noise is detected. (speech offset)Voice Activity DetectionUse the average energy compute from two windows size. One windows is smaller than another another.ExperimentUse the microphone array data recorded by Tom Sullivan at Carnegie Mellon University.The data will be converted into wavefiles for each channel using MATLAB.Put the wavefiles into BeamformIt to perform beamforming. The output is in form of NIST .sph file. Convert it back to .wav using the program sox.Take the result back into MATLAB and compare the SNR with the signals from microphone array.Results an102-mtms-arr4A.adc Source of SignalSNRMic 121.00937389 Mic 222.48577961 Mic 322.80745788 Mic 425.49945084 Mic 523.06647821 Mic 626.87451164 Mic 724.8231068 Mic 821.54103218 Mic 920.9528169 Mic 1023.69206855 Mic 1124.28739145 Mic 1223.09365172 Mic 1320.78050523 Mic 1420.96611645 Mic 1520.77844073 Result from Beamforming46.79534715Results (contd)The average SNR of 15 microphone is about 22.84 dB, which the beamforming result is roughly twice higher than that.Also look at the spectrogram of the the raw data and beamforming data respectively.Note that the noise energy is reduced.ConclusionBy beamforming the signals from the microphone array, we can enhance the quality of the signal that we want by rearrange the microphone array signals together and combine up into one signal.The most difficult part of this project is rather on the technicality (convert and compile the programs) rather than understanding the general concepts behind it.Special ThanksDr. Kepuska and Za Hniang Za for guidance on the concepts and MATLABReferencesAcoustic Beamforming for Signal Enhancement, Localization, and Separation. Kung Yao. DARPA Air-Coupled Acoustic Sensors Workshop Aug 24. 1999. http:/www.darpa.mil/mto/archives/workshops/sono/presentations/ucla_yao.pdfAudio and Speech Processing. Geert Van Meerbergen, et al. http:/homes.esat.kuleuven.be/gvanmeer/s&a/oefenzittingen/opgave2/node3.htmlBeamformit (Fast and Robust Acoustic Beamformer). Xavier Anguera. Nov 11 2008. http:/www.icsi.berkeley.edu/xanguera/beamformit/LOUD: A 1020-Node Microphone Array and Acoustic Beamformer. Eugene Weinstein. http:/www.cs.nyu.edu/eugenew/publications/loud-slides.pdfMicrophone Data by Tom Sullivan at Carnegie Mellon University.Microphone Array Project in Microsoft Research. http:/research.microsoft.com/users/ivantash/MicrophoneArrayProject.aspxTrinov SRP: Surround Microphone Array. http:/www.trinnov Questions