Abstract:
There is provided a voice activity detection method for indicating an active voice mode and an inactive voice mode. The method comprises receiving an input signal having a plurality of frames, determining whether each of the plurality of frames includes an active voice signal or an inactive voice signal, determining a second reflection coefficient for each frame determined to include the inactive voice signal, comparing the second reflection coefficient with a reflection threshold, and selecting the active voice mode if the second reflection coefficient is greater than the reflection threshold. The method may further comprise selecting the inactive voice mode if the second reflection coefficient is not greater than the reflection threshold. The method may also comprise analyzing the input signal to determine an energy level of the input signal, and selecting the active voice mode if the energy level is greater than an energy threshold.

Description:
RELATED APPLICATIONS 
   The present application is based on and claims priority to U.S. Provisional Application Ser. No. 60/665,110, filed Mar. 24, 2005, which is hereby incorporated by reference in its entirety. The present application also relates to U.S. application Ser. No. 11/342,104, filed contemporaneously with the present application, entitled “Adaptive Voice Mode Extension for a Voice Activity Detector,” and U.S. application Ser. No. 11/342,130, filed contemporaneously with the present application, entitled “Adaptive Noise State Update for a Voice Activity Detector,” which are hereby incorporated by reference in their entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to voice activity detection. More particularly, the present invention relates to a tone detection algorithm for a voice activity detector. 
   2. Related Art 
   In 1996, the Telecommunication Sector of the International Telecommunication Union (ITU-T) adopted a toll quality speech coding algorithm known as the G.729 Recommendation, entitled “Coding of Speech Signals at 8 kbit/s using Conjugate-Structure Algebraic-Code-Excited Linear-Prediction (CS-ACELP).” Shortly thereafter, the ITU-T also adopted a silence compression algorithm known as the ITU-T Recommendation G.729 Annex B, entitled “A Silence Compression Scheme for Use with G.729 Optimized for V.70 Digital Simultaneous Voice and Data Applications.” The ITU-T G.729 and G.729 Annex B specifications are hereby incorporated by reference into the present application in their entirety. 
   Although initially designed for DSVD (Digital Simultaneous Voice and Data) applications, the ITU-T Recommendation G.729 Annex B (G.729B) has been heavily used in VoIP (Voice over Internet Protocol) applications, and will continue to serve the industry in the future. To save bandwidth, G.729B allows G.729 (and its annexes) to operate in two transmission modes, voice and silence/background noise, which are classified using a Voice Activity Detector (VAD). 
   A considerable portion of normal speech is made up of silence/background noise, which may be up to an average of 60 percent of a two-way conversation. During silence, the speech input device, such as a microphone, picks up environmental noise. The noise level and characteristics can vary considerably, from a quiet room to a noisy street or a fast-moving car. However, most of the noise sources carry less information than the speech; hence, a higher compression ratio is achievable during inactive periods. As a result, many practical applications use silence detection and comfort noise injection for higher coding efficiency. 
   In G.729B, this concept of silence detection and comfort noise injection leads to a dual-mode speech coding technique, where the different modes of input signal, denoted as active voice for speech and inactive voice for silence or background noise, are determined by a VAD. The VAD can operate externally or internally to the speech encoder. The full-rate speech coder is operational during active voice speech, but a different coding scheme is employed for the inactive voice signal, using fewer bits and resulting in a higher overall average compression ratio. The output of the VAD may be called a voice activity decision. The voice activity decision is either 1 or 0 (on or off), indicating the presence or absence of voice activity, respectively. The VAD algorithm and the inactive voice coder, as well as the G.729 or G.729A speech coders, operate on frames of digitized speech. 
     FIG. 1  illustrates conventional speech coding system  100 , including encoder  101 , communication channel  125  and decoder  102 . As shown, encoder  101  includes VAD  120 , active voice encoder  115  and inactive voice encoder  110 . VAD  120  determines whether input signal  105  is a voice signal. If VAD  120  determines that input signal  105  is a voice signal, VAD output signal  122  causes input signal  105  to be routed to active voice encoder  115  and then routed to the output of active voice encoder  115  for transmission over communication channel  125 . On the other hand, If VAD  120  determines that input signal  105  is not a voice signal, VAD output signal  122  causes input signal  105  to be routed to inactive voice encoder  110  and then routed to the output of inactive voice encoder  110  for transmission over communication channel  125 . Further, VAD output signal  122  is also transmitted over communication channel  125  and received by decoder  102  as coding mode  127 , such that at the other end, coding mode  127  controls whether the coded signal should be decoded using inactive voice decoder  130  or active voice decoder  135  to produce output signal  140 . 
   When active voice encoder  115  is operational, an active voice bitstream is sent to active voice decoder  135  for each frame. However, during inactive periods, inactive voice encoder  110  can choose to send an information update called a silence insertion descriptor (SID) to the inactive decoder, or to send nothing. This technique is named discontinuous transmission (DTX). When an inactive voice is declared by VAD  120 , completely muting the output during inactive voice segments creates sudden drops of the signal energy level which are perceptually unpleasant. Therefore, in order to fill these inactive voice segments, a description of the background noise is sent from inactive voice encoder  110  to inactive voice decoder  130 . Such a description is known as a silence insertion description. Using the SID, inactive voice decoder  130  generates output signal  140 , which is perceptually equivalent to the background noise in the encoder. Such a signal is commonly called comfort noise, which is generated by a comfort noise generator (CNG) within inactive voice decoder  130 . 
   Due to an increase in deployment and use of VoIP applications, certain deficiencies of speech coding algorithms and, in particular, existing VAD algorithms have surfaced. For example, it has been experienced that the VAD erroneously may go off (indicative of inactive voice) at the tail end of a voice signal, although the voice signal is still present. As a result, the tail end of the voice signal is cut off by the VAD.  FIG. 2  is an illustration of this first problem, where VAD  120  goes off at point  210 , where voice signal still continues, and thus VAD  120  cuts off the tail end of voice signal  212 . In other words, the CNG matches the energy of the tail end of the voice signal (i.e. energy of the signal after VAD goes off) for generating the comfort noise. Because the matched energy is not that of a silence or background noise signal, but the matched energy is that of the tail end of a voice signal, the comfort noise that is generated by the CNG sounds like an annoying breathe-like noise. 
   In a further problem, it has been determined that existing VADs occasionally misinterpret a high-level tone signal as an inactive voice or background noise, which results in the CNG generating a comfort noise by matching the energy of the high-level tone signal. 
   Other VAD problems may also be caused due to untimely or improper initialization or update of the noise state during the VAD operation. It is known that the background noise can change considerably during a conversation, for example, by moving from a quiet room to a noisy street, a fast-moving car, etc. Therefore, the initial parameters indicative of the varying characteristics of background noise (or the noise state) must be updated for adaptation to the changing environment. However, when the background noise parameters are not timely or properly updated or initialized, various problems may occur, including (a) undesirable performance for input signals that start below a certain level, such as around 15 dB, (b) undesirable performance in noisy environments, (c) waste of bandwidth by excessive use of SID frames, and (d) incorrect initialization of noise characteristics when noise is missing at the beginning of the speech. As an example, when the incoming signal starts with silence followed by a sudden change in the level of noise signal, existing VADs do not initialize the noise state correctly, which can lead to the noise signal following the silence erroneously being considered as the active voice by the VAD. As a result of this improper initialization of the noise state, the VAD may go on during background noise periods causing an active voice mode selection, where the bandwidth is wasted for coding of the background noise. 
   Therefore, there is an intense need for a robust VAD algorithm that can overcome the existing problems and deficiencies in the art. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to system and method for voice activity detection. In one aspect of the present invention, there is provided a voice activity detection method for indicating an active voice mode and an inactive voice mode. In a separate aspect, the method comprises receiving an input signal having a plurality of frames, determining whether each of the plurality of frames includes an active voice signal or an inactive voice signal, determining a second reflection coefficient for each frame determined to include the inactive voice signal, comparing the second reflection coefficient with a reflection threshold, and selecting the active voice mode if the second reflection coefficient is greater than the reflection threshold. 
   In one aspect, the method further comprises selecting the inactive voice mode if the second reflection coefficient is not greater than the reflection threshold, where the comparing distinguishes between a noise signal and a tone signal, and where the reflection threshold is around 0.9. 
   In an additional aspect, the method may also comprise analyzing the input signal to determine an energy level of the input signal, selecting the active voice mode if the energy level is greater than an energy threshold, and selecting the inactive voice mode if the energy level is not greater than the energy threshold. 
   In another aspect, the method further comprises analyzing the input signal to determine a current tilt parameter of the input signal, analyzing the input signal to determine a previous tilt parameter of the input signal, selecting the active voice mode if a difference between the current tilt parameter and the previous tilt parameter is greater than a tilt threshold, and selecting the inactive voice mode if the difference between the current tilt parameter and the previous tilt parameter is not greater than a tilt threshold. 
   In other aspects, there is provided a voice activity detector comprising an input configured to receive an input signal having a plurality of frames, and an output configured to indicate an active voice mode or an inactive voice mode, where the voice activity detector operates according to the above-described methods of the present invention. 
   These and other aspects of the present invention will become apparent with further reference to the drawings and specification, which follow. It is intended that all such additional systems, features and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, wherein: 
       FIG. 1  illustrates a conventional speech coding system including a decoder, a communication channel and an encoder having a VAD; 
       FIG. 2  is an illustrative diagram of a problem in conventional VADs, where the VAD goes off at a point where voice signal still continues and the tail end of the voice signal is cuts off; 
       FIG. 3  illustrates the status of VAD mode selection versus time, where VAD voice mode is adaptively extended after detection of an inactive voice signal to remedy the problem of  FIG. 2 , according to one embodiment of the present invention; 
       FIG. 4A  illustrates a flow diagram for determining a voice mode status for adaptively extending VAD voice mode, according to one embodiment of the present invention; 
       FIG. 4B  illustrates a flow diagram for adaptively extending VAD voice mode using the voice mode status of  FIG. 4B , according to one embodiment of the present invention; 
       FIG. 5A  illustrates a tone signal having a sinusoidal shape in the time domain as stable as a background noise signal; 
       FIG. 5B  illustrates the tone signal of  FIG. 5A  in the spectrum domain having a sharp formant unlike a background noise signal; 
       FIG. 6  illustrates a flow diagram for use by a VAD of the present invention for distinguishing between tone signals and background noise signals, according to one embodiment of the present invention; 
       FIG. 7  illustrates a flow diagram for adaptively updating the noise state of a VAD, according to one embodiment of the present invention; and 
       FIG. 8  illustrates an input signal, where the noise level changes from a first noise level to a second noise level, and where a shifting window is used to measure the minimum energy is of the input signal. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Although the invention is described with respect to specific embodiments, the principles of the invention, as defined by the claims appended herein, can obviously be applied beyond the specifically described embodiments of the invention described herein. For example, although various embodiments of the present invention are described in conjunction with the VAD algorithm of the G.729B, the invention of the present application is not limited to a particular standard, but may be utilized in any VAD system or algorithm. Moreover, in the description of the present invention, certain details have been left out in order to not obscure the inventive aspects of the invention. The details left out are within the knowledge of a person of ordinary skill in the art. 
   The drawings in the present application and their accompanying detailed description are directed to merely example embodiments of the invention. To maintain brevity, other embodiments of the invention which use the principles of the present invention are not specifically described in the present application and are not specifically illustrated by the present drawings. It should be borne in mind that, unless noted otherwise, like or corresponding elements among the figures may be indicated by like or corresponding reference numerals. 
   As described above in conjunction with  FIG. 2 , in conventional VADs, while the voice signal is still being received, the VAD may improperly go off and, thus, cause the tail end of voice signal being cut off. The tail end is cut off because the CNG matches the energy of the tail end of the voice signal (i.e. energy of the signal after VAD goes off) for generating the comfort noise. To resolve this problem, the present application adaptively extends the active voice mode after VAD  120  goes off, as shown in  FIG. 3 .  FIG. 3  depicts the status of VAD mode selection versus time. For example, during time period  320 , VAD  120  indicates active voice. When VAD  120  goes off at the end of time period  320 , existing VADs indicate an inactive voice mode, which causes the tail end of voice signal (see  212 ) to be cut. However, as shown in  FIG. 3 , the present application extends time period  320  by adding VAD on-time extension period  322 , during which time period, VAD output remains high to indicate an active voice mode to avoid cutting off the tail end of the voice signal. According to one embodiment of the present invention, the period of time to extend the VAD on-time to indicate an active voice mode, after VAD determines that voice signal has ended, is selected adaptively, and not by adding a constant extension. For example, as shown in  FIG. 3 , VAD on-time extension period  322  is longer than VAD on-time extension period  332  or  334 . It should be noted that adding a constant VAD on-time extension period is undesirable, because communication bandwidth is wasted by coding the incoming signal as voice, where the incoming signal is not a voice signal. The present invention overcomes this drawback by adaptively adjusting the VAD on-time extension period. 
   In one embodiment of the present invention, the VAD on-time extension period is calculated based on the amount of time the preceding voice signal, e.g. voice signal  320 , is present, which can be referred to as the active voice length. The longer the preceding voice period before VAD goes off, the longer the VAD on-time extension period after VAD goes off. As shown in  FIG. 3 , voice period  320  is longer than voice periods  330  and  340 , and thus, VAD on-time extension period  322  is longer than VAD on-time extension periods  332  or  334 . 
   In another embodiment of the present invention, the VAD on-time extension period is calculated based on the energy of the signal about the time VAD goes off, e.g. immediately after VAD goes off. The higher the energy, the longer the VAD on-time extension period after VAD goes off. 
   In yet another embodiment, various conditions may be combined to calculate the VAD on-time extension period. For example, the VAD on-time extension period may be calculated based on both the amount of time the preceding voice signal is present before VAD goes off and the energy of the signal shortly after the VAD goes off. In some embodiments, the VAD on-time extension period may be adaptive on a continuous (or curve) format, or it may be determined based on a set of pre-determine thresholds and be adaptive on a step-by-step format. 
     FIG. 4A  illustrates a flow diagram for determining an adjustment factor for use to adaptively extend the voice mode of the VAD, according to one embodiment of the present invention. As shown, in step  402 , the VAD receives a frame of input signal  105 . Next, at step  404 , the VAD determines whether the frame includes active voice or inactive voice (i.e., background noise or silence.) If the frame is a voice frame, the process moves to step  406 , where the VAD initializes a noise counter to zero and increments a voice counter by one. At step  410 , it is decided whether the voice counter exceeds a predetermined number (N), e.g. N=8. If the voice counter exceeds the predetermined number (N), the process moves to step  416 , where a voice flag is set, where the voice flag is used to adaptively determine a VAD on-time extension period. However, if the voice counter does not exceed the predetermined number (N), the process moves to step  414 , where it is determined whether the signal energy, e.g. signal-to-noise ratio (SNR), exceeds a predetermined threshold, such as SNR&gt;1.4648 dB. If the signal energy is sufficiently high, the process moves to step  416  and the voice flag is set. 
   Turning back to step  404 , if the frame is a noise frame, the process moves to step  408 , where the VAD initializes the voice counter to zero and increments the noise counter by one. At step  412 , it is decided whether the noise counter exceeds a predetermined number (M), e.g. M=8. If the noise counter exceeds the predetermined number (M), the process moves to step  418 , where a voice flag is reset, where the voice flag is used to adaptively determine a VAD on-time extension period. 
     FIG. 4B  illustrates a flow diagram for adaptively extending the voice mode of the VAD, according to one embodiment of the present invention. At step  452 , it is determined if VAD output signal  122  is on, which is indicative of voice activity detection. If so, the process moves to step  454 , where it is determined if the present frame is a voice frame or a noise frame. If the present frame is the voice frame, the process moves back to step  452  and awaits the next frame. However, if the present frame is a noise frame, the process moves to step  456 . Unlike the conventional VADs, upon the detection of the noise frame, VAD output signal  122  is not turned off or a constant extension period is not added to maintain the on-time of VAD output signal  122 . Rather, according to the present invention, at step  456 , it is determined whether the voice flag is set. If so, the process moves to step  458  and the on-time for VAD output signal  122  is extended by a first period of time (X), such as an extension of time by five (5) frames, which is 50 ms for 10 ms frames. Otherwise, the process moves to step  460 , where the on-time for VAD output signal  122  is extended by a second period of time (Y), where X&gt;Y, such as an extension of time by two (2) frames, which is 20 ms for 10 ms frames. Furthermore, in one embodiment (not shown), at step  458 , the on-time for VAD output signal  122  may be extended by a third period of time (Z) rather than (X), where Z&gt;X, such as an extension of time by eight (8) frames, which is 80 ms for 10 ms frames, if the VAD determines that the signal energy is above a certain threshold, e.g. when the current absolute signal energy is more than 21.5 dB. The attached Appendix discloses one implementation of the present invention, according to  FIGS. 4A and 4B . 
   In another embodiment of the present application, a set of thresholds are utilized at step  404  (or  454 ) to determine whether the input frame is a voice frame or a noise frame. In one embodiment, these thresholds are also adaptive as a function of the voice flag. For example, when the voice flag is set, the threshold values are adjusted such that detection of voice frames are favored over detection of noise frames, and conversely, when the voice flag is reset, the threshold values are adjusted such that detection of noise frames are favored over detection of voice frames. 
   Turning to another problem, as discussed above, conventional VADs sometimes misinterpret a high-level tone signal as an inactive voice or background noise, which results in the CNG generating a comfort noise that matches the energy of the high-level tone signal. To overcome this problem, the present application provides solutions to distinguish tone signals from background noise signals. For example, in one embodiment, the present application utilizes the second reflection coefficient (or k 2 ) to distinguish between tone signals and background noise signals. Reflection coefficients are well known in the field of speech compression and linear predictive coding (LPC), where a typical frame of speech can be encoded in digital form using linear predictive coding with a specified allocation of binary digits to describe the gain, the pitch and each of ten reflection coefficients characterizing the lattice filter equivalent of the vocal tract in a speech synthesis system. A plurality of reflection coefficients may be calculated using a Leroux-Gueguen algorithm from autocorrelation coefficients, which may then be converted to the linear prediction coefficients, which may further be converted to the LSFs (Line Spectrum Frequencies), and which are then quantized and sent to the decoding system. 
   As shown in  FIG. 5A , a tone signal has a sinusoidal shape in the time domain as stable as a background noise signal. However, as shown in  FIG. 5B , the tone signal has a sharp formant in the spectrum domain, which distinguishes the tone signal from a background noise signal, because background noise signals do not represent such sharp formants in the spectrum domain. Accordingly, the VAD of the present application utilizes one or more parameters for distinguishing between tone signals and background noise signals to prevent the VAD from erroneously indicating the detection of background noise signals or inactive voice signal when tone signals are present. 
     FIG. 6  illustrates a flow diagram for use by a VAD of the present invention for distinguishing between tone signals and background noise signals. As shown, at step  602 , the VAD receives a frame of input signal. Next, at step  604 , the VAD determines whether the frame includes an active voice or an inactive voice (i.e., background noise or silence.) If the frame is determined to be a voice frame, the process moves back to step  602  and the VAD indicates an active voice mode. However, if the frame is determined to be an inactive voice frame, such as a noise frame, then the process moves to step  606 . Unlike conventional VADs, the VAD of the present invention does not indicate an inactive voice mode upon the detection of the inactive voice signal, but at step  606 , the second reflection coefficient (K 2 ) of the input signal or the frame is compared against a threshold (TH k ), e.g. 0.88 or 0.9155. If the VAD determines that the second reflection coefficient (K 2 ) is greater than TH k , the process moves to step  602  and the VAD indicates an active voice mode. Otherwise, in one embodiment (not shown), if the VAD determines that the second reflection coefficient (K 2 ) is not greater than TH k , the process moves to step  602  and the VAD indicates an inactive voice mode. 
   Yet, in another embodiment, background noise signals and tone signals may further be distinguished based on signal stability, since tone signals are more stable than noise signals. To this end, if the VAD determines that the second reflection coefficient (K 2 ) is not greater than TH k , the process moves to step  608  and the VAD compares the signal energy of the input signal or the frame against an energy threshold (TH e ), e.g. 105.96 dB. At step  608 , if the VAD determines that the signal energy is greater than TH e , the process moves to step  602  and the VAD indicates an active voice mode. Otherwise, in one embodiment, if the VAD determines that the signal energy is not greater than THE, the process moves to step  602  and the VAD indicates an inactive voice mode. 
   In another embodiment (not shown), if the VAD determines that the signal energy is not greater than TH e , signal stability may further be determined based on the tilt spectrum parameter (γ 1 ) or the first reflection coefficient of the input signal or the frame. In one embodiment, the tilt spectrum parameter (γ 1 ) is compared between the current frame and the previous frame for a number of frames, e.g. (|current γ 1 −previous γ 1 |) is determined for 10–20 frames, and a determination is made based on comparing with pre-determined thresholds, and the signal is classified as one of tone signals, background noise signals or active voice signals based on the signal stability. For example, if the result of (|current γ 1 −previous γ 1 |) for each frame of a plurality of frames is greater than a tone signal stability threshold, then the VAD will continue to indicate an active voice mode. Further, it should be noted that each of the second reflection coefficient (K 2 ), the signal energy and the tilt spectrum parameter (γ 1 ) can be used solely or in combination with one or both of the other parameters for distinguishing between tone signals and background noise signals. The attached Appendix discloses one implementation of the present invention, according to  FIG. 6 . 
   Now, turning to other VAD problems caused by untimely or improper update of the noise state, the present application provides an adaptive noise state update for resetting or reinitializing the noise state to avoid various problems. It should be noted that a constant noise state update rate can cause problems, e.g. every 100 ms, because the reset or re-initialization of the noise state may occur during active voice area and, thus, cause low level active voice to be cut off, as a result of an incorrect mode selection by the VAD. 
     FIG. 7  illustrates a flow diagram for adaptively updating the noise state of a VAD, according to one embodiment of the present invention. As shown, at step  702 , the amount of time elapsed since the last time the noise state was updated is determined. Next, at step  704 , it is determined whether the amount of time exceeds a predetermined period of time (Ti). For example, it is known that one speech sentence is spoken in about 2.5–3.5 seconds. Accordingly, in one embodiment, the pre-determined period of time after the last update is around 3.0 seconds. Therefore, at step  704 , it may be determined whether three (3) seconds has passed since the last time the noise state was updated. If so, the process moves to step  712 , where the noise state is updated. Otherwise, the process moves to step  706 , where the VAD determines the running mean of minimum energy (M 0 ) of the input signal, which is the average energy of the low energy of the input signal, and further determines current minimum energy (M 1 ) of the input signal. 
   Referring to  FIG. 8  of the present application, input signal  810  is shown, where the noise level changes from first noise level  815  to second noise level  820 . Further,  FIG. 8  shows a shifting window within which the minimum energy is measured. For example, the minimum energy within first window  805  is lower than the minimum energy within second window  807  due to the introduction of second noise level  820  in second window  807 . In one embodiment of the present invention, the shifting window shifts according to time and the minimum energy is measured as the shift occurs. The running mean of minimum energy (M 0 ) of the input signal is calculated based on the measurement of the minimum energy of a number of windows, and the current minimum energy (M 1 ) is the measurement of the minimum energy within the current window. 
   Turning back to  FIG. 7 , after step  706 , the process moves to step  708 , where the VAD determines whether the running mean of minimum energy (M 0 ) of the input signal is less than the current minimum energy (M 1 ), i.e. M 0 &lt;M 1 . Of course, without departing from the concept of the present invention, in some embodiments, a first predetermined value may be added to or subtracted from M 1  prior to the comparison, i.e. M 0 &lt;M 1 −0.015625 (dB). If the result of the comparison is true, e.g. M 0  is less than M 1 , then the process moves to step  712 , where the noise state is updated. Otherwise, the process moves to step  710 , where the VAD determines whether the running mean of minimum energy (M 0 ) of the input signal is greater than the current minimum energy (M 1 ) plus a second predetermined value, e.g. 0.48828 (dB), i.e. M 0 &gt;M 1 +0.48828 (dB). If so, then the process moves to step  712 , where the noise state is updated. Otherwise, the process returns to step  702 . 
   In one embodiment (not shown), at step  712 , prior to updating the noise state, the VAD considers the signal energy prior to updating the noise state to avoid updating the noise state during active voice signal, such that low level active voice can be cut off by the VAD. In other words, the VAD determines whether the signal energy exceeds an energy threshold, and if so, the VAD delays updating the noise state until the signal energy is below the energy threshold. The attached Appendix discloses one implementation of the present invention, according to  FIG. 7 . 
   From the above description of the invention it is manifest that various techniques can be used for implementing the concepts of the present invention without departing from its scope. Moreover, while the invention has been described with specific reference to certain embodiments, a person of ordinary skill in the art would recognize that changes can be made in form and detail without departing from the spirit and the scope of the invention. For example, it is contemplated that the circuitry disclosed herein can be implemented in software, or vice versa. The described embodiments are to be considered in all respects as illustrative and not restrictive. It should also be understood that the invention is not limited to the particular embodiments described herein, but is capable of many rearrangements, modifications, and substitutions without departing from the scope of the invention.