Title of Invention

SYSTEM FOR AUDIO EXTERNALIZATION IN STEREOPHONIC HEADPHONES FOR PRODUCING NATURAL SOUND EFFECTS

Abstract

A system and method to produce natural sound e.ffects in a stereophonic headphone system through audio externalization. It uses a time domain approach, and a novel algorithm to achieve its goals. The system comprises of three modules -Difference circuit, Cross- feed circuit and the Early Reflector circuit. The difference circuit makes the side components of the Land R signals more prominent. The cross-feed circuit simulates the loudspeaker environment over headphones using inter-channel time difference (ITD) and inter-channels level difference (ILD). The early reflector circuit extracts one dominant early reflection and adds it back to the original signal with different delay lengths for both the channels. The resultant audio signal will be closer to natural sound effects

Full Text

FIELD OF THE INVENTION The present invention, in general, relates to the field of audio technology and particularly to of Audio Enhancement in Multimedia Signal processing. More particularly, it relates to a method and system to produce natural sound effects in a stereophonic headphone system subjected to audio externalization. DESCRIPTION OF RELATED ART The use of stereophonic headphones is very common now a days.. Stereophonic system is an audio device producing two channels of audio output over headphones/external speakers. It comprises of an audio device with two channels of information over headphone and hence produces natural sound effects. However, in stereophonic reproduction, the listener perceives sound images along a line between the headphones. Further, it creates a spatial impression of 60 degrees. When listening to audio over headphones, the sound can become localized inside the head (eg. bass guitar) causing the sound field to appear flat and lacking dimensional qualities. It could be awkward to listen to such unnatural sound for a long time and exposure to such sound effect for a prolonged duration of time may lead to in-head localization - a sense of sound appearing from with-in the head - and in turn listening fatigue. This 'in-head' localization, referred to as lateralization, is caused by the lack of normal spatial cues that the human brain uses to place the sound source in the listening environment. Another interesting aspect is that this is a phenomenon that is unique to recorded music. Therefore it has become necessary for an implementation of a system, which perceives sound expands to a region outside the listener's head. As it is observed, it is the low frequency components in the music - like the voice of bass guitar - that always causes in-head localization. This 'in-head' localization, referred to as lateralization, is caused by the lack of normal spatial cues that the human brain uses to place the sound source in the listening environment. Several solutions have been proposed by researchers to externalize the sound - remove in-head localization - with the use of binaural cues and other techniques. For example, the US patent titled "Monaural signal to artificial stereo signals converting and processing circuit for headphones" (publication number: US4359605), details a system and method for producing stereo sound effect by mixing localization information and indirect sound signal to the original audio signal. In this invention a circuit localizing information of imaginary sources are added to the already obtained artificial stereo signals. However in this system the addition of the indirect sounds, the sound expansion felt by a listener listening to a headphone expands to regions outside the listener's head. Another patent, titled "Stereo headphone sound source localization system" (publication number: US5371799), describes a method for source localization in a stereo headphone system. The method involves introduction of early-reflection component in the direct sound signal to produce source localization outside the listener's head. The early reflection signal is obtained by introducing delay and attenuation in the direct sound signal. Here the apparent source of the audio signal is located outside of the head of the headphone user. The system comprises a range processor, an early reflection portion and a reverberations portion. The range processor processes the input signal as if it where made of direct wave portion, which is processed in filters whose filter coefficients are chosen based upon the desired azimuth of the virtual sound source location. The early reflection portion is passed through a bank of filters connected in parallel whose coefficients are chosen based on each reflection azimuth. The outputs of these filters are passed through scalars so as to adjust the amplitude and to simulate a desired range of the virtual sound source. The reverberation portion is processed without any sound source location information, using a random number generator, and the output is attenuated in an exponential attenuator to be faded out. The outputs of the scalars and attenuators are then all summed to produce left and right headphone signals for playback over the respective headphone transducers. Thus the stereo sound effect is produced by introduction of indirect sound component to direct sound signals. Even though the state of the art depicts several alternatives for externalization of stereophonic headphone none of these functions effectively, none of them talks about a method for producing stereo effect by the combination of decorrelation, cross-feed, and indirect sound addition, as being proposed by the current invention. In other words the state of art provides the use of a combination of decorrelator and crossfeed network to explore binaural cues (binaural cues are the parameters that measure the extent of the 'spaciousness' of the sound) for natural sound effect, while they fail to bring out a combination of the two. The existing literature speaks about systems, which either involves a localization of information through a cross-feed circuit or producing stereo effect by introducing indirect sound component to direct sound component signals. In view of the above drawbacks in the existing systems, the present invention proposes a method and system to produce natural sound effects in a stereophonic headphone system through audio externalization. This is achieved by a method of decorrelation, cross-feeding indirect sound component to direct sound signals for natural sound effect. It uses a time domain approach, and a novel algorithm to achieve its goals. SUMMARY OF INVENTION It is therefore the primary object of the invention to provide a system and method thereof to produce natural sound effects in a stereophonic headphone by implementation of audio externalization. It is another object of the invention is to provide a method for time domain approach for externalization in headphone. It is another object of the invention is to provide a method for reducing in- head localization and hence externalization the stereo widening image. It is yet another objective of the invention is to provide a method for better a better audio externalization by boosting of side, signals to central signal (i.e., L-R, difference signal) and thereby reducing the correlation between the two channels. It is yet another objective of the invention to provide a method to boost the side signal in order to make the side components of the L and R signals more prominent. It is a further object of the invention to provide a method to extract one dominant early reflection and adds it back to the original signal with different delay lengths for both the channels It is a further object of the present invention to provide a system that simulates the loudspeaker environment over headphones using inter-channel time difference (ITD) and inter-channels level difference (ILD). It is yet another objective of the invention to provide a system, which highly minimizes laterization and thereby maximizes externalization through effective implementation of inter-channel level difference (IID), inter-channel time difference (ITD) and inter-channel coherence (ICC). Accordingly the invention provides a system for audio externalization over stereophonic headphone on the basis of embedded program fitted inside the headphone as an audio post-processing tool in order to produce natural sound effects in the said headphone, wherein the system comprises a) Difference Circuit boosting the side signal in order to make the side components of the L and R signals more prominent than a Cross- feed circuit in to which the out put of difference circuit is fed; b) The cross-feed circuit simulating the loudspeaker environment over headphones using inter-channel time difference (ITD) and inter- channels level difference (ILD); c) An early reflector circuit being fed with the output of the cross-feed circuit, wherein the early reflector circuit is capable of extracting one dominant early reflection and adding it back to the original signal with different delay lengths for both the channels, thereby lowering the inter channel coherence (ICC). d) Associated circuitry to enable the production of natural sound effects. The invention also provides a method for audio externalization over stereophonic headphone in order to produce natural sound effects in the said headphone, the method comprises of a) Boosting the side signal in order to make the side components of the L and R signals more prominent than a cross- feed circuit in to which the out put of difference circuit is fed; b) Simulating the loudspeaker environment over headphones using inter- channel time difference (ITD) and inter-channels level difference (ILD); and c) Extracting one dominant early reflection of the simulated output using a reflector circuit and adding it back to the original signal with different delay lengths for both the channels, thereby lowering the inter channel coherence (ICC). Other objects, features, and advantages of the present invention will become more apparent from the ensuing detailed description of the invention, taken in conjunction with the accompanying drawings, which depicts the flowchart of the various stages of the methods involved, eventually leading to audio externalization. BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS Figure 1 depicts Stereo Widening System. Figure 2 depicts Block Diagram of Difference Circuit. Figure 3 depicts Block Diagram of Cross-feed Circuit. Figure 4 depicts Block Diagram of Early Reflector Circuit. Figure 5 depicts Results of subjective listening test. Figure 6 depicts Histogram of difference signal (conventional stereo (top) and widened stereo (bottom)). DETAILED DESCRIPTION OF THE INVENTION The preferred embodiments of the present invention will now be explained with reference to the accompanying drawings. However, it should be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore the details disclosed herein are not to be interpreted as limiting but merely as the basis for the claims and as a basis for teaching one skilled in the art how to make or use the invention. Before delving into the details of the present invention, let us briefly take a look at the scenario for which solution the present invention has been proposed. The stereophonic system is an audio device producing two channels of audio output over headphones/external speakers. However, in stereophonic reproduction, the listener perceives sound images along a line between the headphones. Further, it creates a spatial impression of 60 degrees. Exposure to such unnatural sound effect for a prolonged duration of time may lead to in-head localization - a sense of sound appearing from with-in the head - and fatigue in the listener. As it is observed, it is the low frequency components in the music - like the voice of bass guitar - that always causes in-head localization. Another interesting aspect is that the in-head localization phenomenon is unique to recorded music only. The present invention tries to overcome these handicaps through audio externalization and puts the manufacturers in an advantageous situation to utilize particular features present in the invention. An attraction of the current invention is that the system reduces the in-head localization and produces a wide stereo image towards the L-R ears to obtain the natural sound effect. This is achieved through an algorithm, which is fitted inside the headphone as an audio post-processing tool. The algorithm is based on center signal to side signal ratio. It also uses suitable feedback delay lines and tunable gain parameters along with binaural cues. Even though it be used independently, the preferred embodiment of the invention encourages implementation in real-time as it involves less computational frequency. According to the preferred embodiment of the invention, it proposes a stereo widening scheme, which involves boosting of side signal (i.e., L-R, difference signal) as well as reducing the correlation between the two channels. A notable advantage of this invention is that the systematic evaluation has been done for the proposed model on the basis of the subjective listening tests and the objective testing. The objective test incorporates the statistical evaluation of the stereo widening effect with the help of implementation software MATLAB. The subjective tests perceive the extent of pleasing widening effect by the listener. The system according to the preferred embodiment comprises of three modules, Difference circuit, Cross- feed circuit, and the Early Reflector circuit. Difference circuit boosts the side signal in order to make the side components of the L and R signals more prominent. The cross-feed circuit simulates the loudspeaker environment over headphones using inter-channel time difference (ITD) and inter- channels level difference (ILD). The early reflector circuit extracts one dominant early reflection and adds it back to the original signal with different delay lengths for both the channels. This helps to lower the inter-channel coherence (ICC) by making the signals out of phase. Fig. 1 depicts the Stereo Widening System, which demarcates how the input signal gets a widened output by passing through the three modules. Now, let us see each module in detail. Difference Circuit Fig 2 portraits the functional Block diagram of the Difference Circuit. As mentioned already, the purpose of the Difference Circuit is to boost the side signal such that the side components of the L and R signal becomes more prominent. As shown in figure (2), the signals - Lin and Rin - are passed through a decorrelator, consisting of two all-pass filters of linear phase characteristics that introduce complementary phase shifts in the left and right audio channel. Decorrelation reduces the timbral coloration when applied to individual channels. Further, by this method, the cross correlation coefficient of left and right channels is highly reduced as shown in Figure (2). The widening is achieved by enhancing the directional information in each channel. The side signal is usually delayed in the range of 10 ms to 25 ms. Empirically, the delay of 15 ms is selected as optimum as any delay exceeding 25 ms may incorporate echo artifacts and more timbral coloration. The delayed difference signal is then passed through the band pass filter (BPF) for frequency selective spatial enhancement. Default is the band pass filter with cutoff frequencies 250Hz and 12 KHz for lower bound and upper bound respectively. The output of the BPF is then attenuated by a widening depth parameter and added to the left channel and the 180 degrees phase shifted signal is added to the right channel, eventually increasing the side to centre signal ratio, thereby enhancing the spatial fidelity of the audio signal. Cross- feed circuit Cross talk is quite common in loudspeaker set-up owing to the reason that the left ear hears a little of the right channel and vice versa. In order to create similar listening effect on headphones a crossfeed network can be used to recreate inter-aural cross talk. Fig 3 depicts a block diagram of cross-feed networks and explains how does it work in accordance with the inter-aural cross talk. The low frequency sounds gets diffracted as they go behind the head. The high frequency sounds above 1.5 KHz are blocked, as the sound wavelength is short compared to the head dimension, also termed as head shadow effect. ITD and ILD cues are prominent cues below 1.5 kHz and head shadowing becomes prominent above 1.5 kHz. The cross-feed circuit as is shown in figure (3) simulates the inter- channel cross talk. The delay4d) is chosen as 15 samples (We assume that it takes 0.3 ms for sound to reach from loudspeaker to listener at 44.1 KHz sampling frequency). The delay introduced in cross talk patfoxis? (Ad+itd) where itd is the inter-aural time difference which is approximately 9 samples (0.2 ms). The direct path signal is passed through an all pass filter (Hd). Cross path signal is passed through a cross talk filter (Hx) which is a low pass filter of cutoff frequency 2 KHz. Delayed cross talk filtered output from the left channel is multiplied by cross-feed gain parameter 'P' and added to right channel and vice versa. The cross-feed gain parameter controls ILD between the cross path and direct path. The gain parameter is chosen in the range of 0.3 to 0.7. Early Reflector circuit Now referring to Fig.4, which portraits an early reflector circuit it could be possible to explain how the out of head localization is achieved? Figure shows how it simulates the addition of a single, dominant early-reflection by using a feed¬forward delay network. The early reflections provide most of the spatial information, recognizable directionalities as well as distinct arrival times of an environment. A low pass filter is inserted in the delay loop so that the walls tend to absorb more high frequencies than low frequencies in a room environment. While preserving the quality of the signal, the implementation of the present invention reduces the inter-channel coherence by adding each channel signal to a delayed attenuated version of the channel itself. The two slightly different delays are used in both the channels to minimize the Inter channel coherence. Empirically the chosen gain parameter, y as 0.5 and the Ap' and 'Aq' delays can li e in the range of 5 ms to 10 ms and the difference between two delays to be kept as 3 ms for good results. Thus, the lower ICC in audio channels improves the spatial legibility and immersiveness. Finally, in order to evaluate the invention more systematically, subjective listening tests and objective tests were performed on the model. In subjective listening tests, the task was to perceive the extent of pleasing widening effect as being experienced by the listener. Fig 5 depicts the results of subjective listening test. The purpose of the subjective listening test is to perceive the extent of pleasing widening effect by the listener. Subjective evaluation of the group of audience has been done with the help of a dataset containing variety of music. The listeners evaluated how pleasing they feel the widened audio sounds in comparison with the hidden reference widened sound. They were asked to rate the quality by giving a 1 - 5 score. From Fig.5 shows the average rating for each dataset corresponding to various music genres. In the objective test, the complete design of stereo widening system was implemented in MATLAB, and the extent of widening was statistically measured by plotting the histogram of the difference signals, as shown in the figure (6). Most part of the difference signal of the conventional stereo is concentrated at the centre while the difference signal of the widened audio is more spread towards left and right ears. It is quite obvious from the histogram that the centre signal is almost reduced to one third when compared to the original stereo. All of the above requires different logic on the functioning of externalization over stereophonic headphone .In the present invention it has been understood that externalization has been occurred as a combination of decorrelation, crosstalk and indirect sound component to direct sound signals for natural sound effect. Therefore it will be obvious to those skilled in the art that other control methods and apparatuses can be derived from the combinations of the various methods and apparatuses of the present invention as taught by the description and the accompanying drawings and these shall also be considered within the scope of the present invention. Further, description of such combinations and variations is therefore omitted above. The biggest advantage of the present system, according to the invention, is that the proposed model reduces the in-head localization and externalizes the stereo phantom image towards the left and right ears. This algorithm is expected to be useful as an audio post-processing tool and shall be fit inside the headphones or can be used independently for a rich widened stereo listening experience. Further, the model encourages for implementation in real¬time as it involves less computational complexity. Although, the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are possible and are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart there from. GLOSSARY OF TERMS AND DEFINITIONS THEREOF Filter: A circuit which alters the level of a limited range of frequencies. Lateralization: It is a phenomenon in which sound appears to be coming from inside the head. Localization: It is a phenomenon in which sound appears to be coming from one particular location. Externalization: It is a phenomenon in which sound appears to be coming from outside the head. Stereo widening: It is a process to make the stereo sound coming from wider angle more than 60 degrees. Binaural Cues: Binaural cues are the parameters that measure the extent of the 'spaciousness' of the sound. We Claim 1) A system for audio externalization over stereophonic headphone on the basis of embedded program fitted inside the headphone as an audio post¬processing tool in order to produce natural sound effects in the said headphone wherein the system comprises a) Difference Circuit boosting the side signal in order to make the side components of the L and R signals more prominent than a Cross- feed circuit in to which the out put of difference circuit is fed; b) The cross-feed circuit simulating the loudspeaker environment over headphones using inter-channel time difference (ITD) and inter- channels level difference (ILD); c) An early reflector circuit being fed with the output of the cross-feed circuit, wherein the early reflector circuit is capable of extracting one dominant early reflection and adding it back to the original signal with different delay lengths for both the channels, thereby lowering the inter channel coherence (ICC); and d) Associated circuitry to enable the production of natural sound effects. 2) The system as claimed in claim 1, wherein the difference circuit comprises a decorrelator, band pass filter, attenuator and the associated circuitry for enabling effective decorrelation of towards L - R Signals. 3) The system as claimed in claim 1, wherein the cross-feed circuit comprises an all pass filter (Hd) forming the direct path, a low pass filter forming (Hx) the cross talk path and a circuit to multiply the cross-feed gain parameter 'P', which controls ILD between the cross path and direct path. 4) The system as claimed in claim 1, wherein the early reflector circuit comprises, the required circuitry to add a delayed attenuated version to each channel. 5) The system as claimed in Claim 2, wherein the decorrelator consists of two all- pass filters that introduce complementary phase shifts in the left and right audio channel before sending the output to the band pass filters having The bad pass filters have cutoff frequencies 250Hz and 12 KHz for lower band and upper band respectively. 7) The system as claimed in claim 2, wherein the band pass filter induces a frequency selective spatial enhancement in the decorrelator output signal before passing it over to the attenuator circuit. 8) The system as claimed in claim 2, wherein the attenuator produces an output signal attenuated by a widening depth parameter and feeds to the left channel and the 180 degrees phase shifted signal to the right channel respectively. 9) The system as claimed in claim 2, wherein the side signal is delayed in the range of 10 ms to 25 ms. 10) The system according to claim 3, wherein the gain parameter falls in the range 0.3 - 0.7. 11) The system as claimed in claim 4, where in each of the channel signals having slight delay, is input to a delayed attenuated version of the channel itself in order to reduce the inter-channel coherence. 12) The circuit as claimed in claim11, wherein the differences between the delays in the two channels are kept as 3 ms. 13) A method for audio externalization over stereophonic headphone in order to produce natural sourid effects in the said headphone, the method comprises of a) Boosting the side signal in order to make the side components of the L a) and R signals more prominent than a cross- feed circuit in to which the out put of difference circuit is fed; b) Simulating the loudspeaker environment over headphones using inter- channel time difference (ITD) and inter-channels level difference (ILD); and c) Extracting one dominant early reflection of the simulated output using a reflector circuit and adding it back to the original signal with different delay lengths for both the channels, thereby lowering the inter channel coherence (ICC). 14) The method as claimed in claim 13, wherein decorrelation of the input signal reduces the timbral coloration when applied to individual channel. 15) The method as claimed in claim13, wherein widening is achieved by the enhancing the directional information in each channel. 16) The method as claimed in claim13, wherein the side to center signal ratio is increased for effecting the spatial enhancement of the signal in the difference circuit 17) The method as clamed in claim 13, wherein the direct path signal is allowed to pass through an all pass filter (Hd) and the Cross path signal is allowed to pass through a cross talk filter (Hx) which is a low pass filter of cutoff frequency 2 KHz. in order to simulate the interaural crosstalk. 18) The method as claimed in claim 13, wherein the a low pass filter is inserted in the delay loop of the early reflector so that the walls tend to absorb more high frequencies than low frequencies in a room environment. 19) A system for audio externalization over stereophonic headphone on the basis of embedded program fitted inside the headphone as an audio post¬processing tool in order to produce natural sound effects in the said headphone, substantially as herein described with reference to the accompanying drawings. 20) A method for audio externalization over stereophonic headphone in order to produce natural sound effects in the said headphone, substantially as herein described with reference to the accompanying drawings. Dated this the 27th day of August 2007 SANTOSH VIKRAM SINGH Patent Agent Agent for the Applicant

Documents:

1549-CHE-2006 AMENDED PAGES OF SPECIFICATION 01-04-2013.pdf

1549-CHE-2006 AMENDED CLAIMS 01-04-2013.pdf

1549-CHE-2006 AMENDED CLAIMS 20-05-2013.pdf

1549-CHE-2006 EXAMINATION REPORT REPLY RECEIVED 20-05-2013.pdf

1549-CHE-2006 FORM-1 01-04-2013.pdf

1549-CHE-2006 FORM-13 01-04-2013.pdf

1549-CHE-2006 FORM-5 01-04-2013.pdf

1549-CHE-2006 OTHER PATENT DOUCMENT 20-05-2013.pdf

1549-CHE-2006 POWER OF ATTORNEY 01-04-2013.pdf

1549-CHE-2006 POWER OF ATTORNEY 20-05-2013.pdf

1549-CHE-2006 ABSTRACT.pdf

1549-CHE-2006 CLAIMS.pdf

1549-CHE-2006 CORRESPONDENCE OTHERS.pdf

1549-CHE-2006 DESCRIPTION (COMPLETE).pdf

1549-CHE-2006 DRAWINGS.pdf

1549-CHE-2006 EXAMINATION REPORT REPLY RECEIVED 01-04-2013.pdf

1549-CHE-2006 FORM-1.pdf

1549-CHE-2006 FORM-18.pdf

1549-CHE-2006 FORM-5.pdf

1549-che-2006-correspondnece-others.pdf

1549-che-2006-description(provisional).pdf

1549-che-2006-drawings.pdf

1549-che-2006-form 1.pdf

1549-che-2006-form 26.pdf