Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04S—STEREOPHONIC SYSTEMS 
  • H04S7/00—Indicating arrangements; Control arrangements, e.g. balance control
  • H04S7/30—Control circuits for electronic adaptation of the sound field
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04S—STEREOPHONIC SYSTEMS 
  • H04S2420/00—Techniques used stereophonic systems covered by H04S but not provided for in its groups
  • H04S2420/11—Application of ambisonics in stereophonic audio systems
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04S—STEREOPHONIC SYSTEMS 
  • H04S3/00—Systems employing more than two channels, e.g. quadraphonic
  • H04S3/008—Systems employing more than two channels, e.g. quadraphonic in which the audio signals are in digital form, i.e. employing more than two discrete digital channels

Definitions

• the invention concerns a method for reproducing spatial impression of existing spaces in multichannel or binaural listening. It consists of following steps/phases:
• a human listener When listening to sound, a human listener always perceives some kind of a spatial impression.
• the listener can detect both the direction and the distance of a sound source with certain precision.
• the sound of the source evokes a sound field consisting of the sound emanating directly from the source, as well as of reflections and diffraction from the walls and other obstacles in the room. Based on this sound field, the human listener can make approximate deductions about several physical and acoustical properties of the room.
• One goal of sound technology is to reproduce these spatial attributes as they were in a recording space. Currently, the spatial impression cannot be recorded and reproduced without considerable degradation of quality.
• the mechanisms of human hearing are fairly well known.
• the physiology of the ear determines the frequency resolution of hearing.
• the wide-band signals arriving at the ears of a listener are analyzed using approximately 40 frequency bands.
• the perception of spatial impression is mainly based on the interaural time difference (ITD) and interaural level difference (ILD), that are also analyzed within the previously mentioned 40 frequency bands.
• ITD and ILD are also called localization cues. In order to reproduce the inherent spatial information of a certain acoustical environment, similar localization cues need to be created during the reproduction of sound.
• the problem is, how to record spatial sound to be reproduced with varying multichannel loudspeaker systems.
• the acoustics of the recording room have little effect on the recorded signals. In such a case, the spatial impression is added or created with reverberators while mixing the sound. If the sound is supposed to produce a perception as if it were recorded in a specific acoustical environment, the acoustics can be simulated by measuring a multichannel impulse response and convolving it with the source signal using a reverberator. This method produces loudspeaker signals that correspond to recording the sound source in the acoustical environment where the impulse responses were measured. The problem is then, how to create appropriate impulse responses for the reverberator.
• the invention is a general method for reproducing the acoustics of any room or acoustical environment using an arbitrary multichannel loudspeaker system. This method produces a sharper and more natural spatial impression than can be achieved with existing methods. The method also enables improvement of the acquired acoustics by modifying certain room acoustical parameters.
• the first principle utilizes one microphone per each loudspeaker in the reproduction system with intermicrophone distances of more than 10 cm.
• the second group of methods applies directional microphones positioned as close to each other as possible.
• Ambisonics technology is based on using such virtual microphones. Sound is recorded with a SoundField microphone or an equivalent system, and during reproduction, one virtual microphone is directed towards each loudspeaker. The signals of these virtual microphones are fed to the corresponding loudspeakers. Since first-order directivity patterns are broad, sound emanating from any distinct direction is always reproduced with almost all loudspeakers. Thus, there is plenty of cross-talk between the loudspeaker channels. Consequently, the listening area where the best spatial impression can be perceived is small, and the directions of the perceived auditory events are vague and their sound is colored.
• the purpose of the invention is to reproduce the spatial impression of an existing acoustical environment as precisely as possible using a multichannel loudspeaker system.
• responses continuous sound or impulse responses
• W omnidirectional microphone
• a common method is to apply three figure-of- eight microphones (X,Y,Z) aligned with the corresponding cartesian coordinate axes. The most practical way to do this is to use a SoundField or a Microflown system, which directly yield all the desired responses.
• the only sound signal fed to the loudspeakers is the omnidirectional response W. Additional responses are used as data to steer W to some or all loudspeakers depending on time.
• the acquired signals are divided into frequency bands, e.g., using a resolution of the human hearing or better. This can be realized, e.g., with a filterbank or by using short-time Fourier transform.
• the direction of arrival of the sound is determined as a function of time. Determination is based on some standard method, such as estimation of sound intensity, or some cross-correlation-based method [2].
• the omnidirectional response is positioned to the estimated direction. Positioning here denotes methods to place a monophonic sound to some direction regarding to the listener. Such methods are, e.g., pair- or triplet-wise amplitude panning [3], Ambisonics [4], Wave Field Synthesis [5] and binaural processing [6].
• the method is nevertheless not good enough. It assumes that the sound is always emanating from a distinct direction. This is not the case for example in diffuse reverberation.
• this is solved by estimating at each frequency band at each time instant also the diffuseness of sound, in addition to the direction of arrival. If the diffuseness is high, a different spatialization method is used to create a diffuse impression. If the direction of sound is estimated using sound intensity, the diffuseness can be derived from the ratio of the magnitude of the active intensity to the sound power. When the calculated coefficient is close to zero, the diffuseness is high. Correspondingly, when the coefficient is close to one, the sound has a clear direction of arrival. Diffuse spatialization can be realized by conveying the processed sound to more loudspeakers at a time, and possibly by altering the phase of sound in different loudspeakers.
• the method to compute sound direction is based on sound intensity measurement, and positioning is performed with pair- or triplet-wise amplitude panning. Steps 1-4 are referring to Figure 1 and steps 5-7 to Figure 2.
• the impulse response of an acoustical environment is measured or simulated, or continuous sound is recorded in an acoustical environment using one omnidirectional microphone (W) and a microphone system yielding the signals of three figure-of-eight microphones (X,Y,Z) aligned at the directions of the corresponding cartesian coordinate axes.
• W omnidirectional microphone
• X,Y,Z figure-of-eight microphones
• the acquired responses or sound are divided into frequency bands, e.g., according to the resolution of human hearing.
• the active intensity of sound is estimated as a function of time. 4
• the diffuseness of sound at each time instant is estimated based on the ratio of the magnitude of the active intensity and the sound power. Sound power is derived from the signal W.
• the signal of each frequency band is panned to the direction determined by the active intensity vector.
• the invention provides several advantages:
• the method When processing impulse responses, the method also provides means to alter the produced reverberation.
• Most existing room acoustical parameters describe the time-frequency properties of measured impulse responses. These parameters can be easily modified by time-frequency dependent weighting during the reconstruction of a multichannel impulse response. Additionally, the amount of sound energy emanating from different directions can be adjusted, and the orientation of the sound field can be changed. Furthermore, the time delay between the direct sound and the first reflection (in reverberation terms pre-delay) can be customized according to the needs of current application.
• a method according to the invention can also be applied to audio coding of multichannel sound. Instead of several audio channels, only one channel and some side information are transmitted.
• Christof Faller and Frank Baumgarte [7, 8] have proposed a less advanced coding method that is based on analyzing the localization cues from a multichannel signal.
• the processing method produces a somewhat reduced quality compared to the reverberation application, unless the directional accuracy is deliberately compromised. Nevertheless, especially in video and teleconferencing applications the method can be used to record and transmit spatial sound.
• Amplitude panning has for a long time been a standard method for positioning a non-reverberant sound source in a chosen point between loudspeakers.
• a method according to the invention improves the reproduction accuracy of a whole acoustical environment.
• the performance of the proposed system has been evaluated in formal listening tests using a 16-channel loudspeaker system including loudspeakers above the listener, as well as using a 5.1 setup. Compared to Ambisonics, the spatial impression is more precise and the sound is less colored. The spatial impression is close to the measured acoustical environment.
• Loudspeaker reproduction of the acoustics of a concert hall using the proposed method has also been compared to binaural headphone reproduction of recordings made with a dummy head in the same hall.
• Binaural recording is the best known method to reproduce the acoustics of an existing space.
• high quality reproduction of binaural recordings can only be realized with headphones. Based on comments of professional listeners, the spatial impression was in both cases nearly the same, but in the loudspeaker reproduction the sound was better externalized.
• Equipment standard PC; multichannel sound card, e.g. MOTU 818; measurement software, e.g. Cool Edit pro or WinMLS; microphone system, e.g. SoundField SPSS 422B.
• the loudspeaker system for reproduction is defined, for instance 5.1 standard without the middle loudspeaker.
• the middle loudspeaker is left out because the reverberation is reproduced with a four-channel reverberator.
• impulse responses are computed for all loudspeakers corresponding to each source- microphone combination.
• Desired source material is convolved with the impulse responses corresponding to one source-microphone combination and the resulting sound is assessed.
• the sound impression of different source- microphone combinations can be compared in order to choose the one most suitable for current application.
• different source material can be positioned at different locations in the sound field.
• Equipment can consist of a standard PC or of a convolving reverberator, e.g. Hyundai SREV1 ; in this case additionally four loudspeakers.

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• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Acoustics & Sound (AREA)
• Signal Processing (AREA)
• Stereophonic System (AREA)

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• 2003
  • 2003-02-26 FI FI20030294A patent/FI118247B/fi active IP Right Grant
• 2004
  • 2004-02-25 US US10/547,151 patent/US7787638B2/en not_active Expired - Lifetime
  • 2004-02-25 JP JP2006502072A patent/JP4921161B2/ja not_active Expired - Lifetime
  • 2004-02-25 WO PCT/FI2004/000093 patent/WO2004077884A1/en not_active Ceased
• 2010
  • 2010-06-01 JP JP2010125832A patent/JP5431249B2/ja not_active Expired - Lifetime
  • 2010-07-20 US US12/839,543 patent/US8391508B2/en not_active Expired - Lifetime

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