Abstract:
A method for using a waveform segment in place of a missing portion of an audio waveform generated in response to a packet stream encoding portions of the audio waveform, the method comprising: phase matching a trailing portion of the waveform segment with a trailing portion of the audio waveform that follows the missing portion; and adding the phase matched waveform segment to the audio waveform.

Description:
FIELD 
       [0001]    The invention relates to audio transmission over packet switched networks. 
       BACKGROUND 
       [0002]    A packet switched network is a communication network that transmits data from a sender to a receiver packaged in packets, which are routed from the sender to the receiver over a network of switching nodes connected by “data links”. Each switching node receives packets via links that connect it to other switching nodes and switches packets that it receives to forward them over other data links that are suitable for bringing the packets to their destinations. Any two given packets may propagate over different routes, i e. different configurations of nodes and links, from a same sender to a same receiver. Examples of such packet switched networks are Arpanet, which was established more than thirty years ago and is the first packet switched network, and the Internet. The Internet is used today for all types of data communication and is commonly used to transmit multimedia data and for voice communication, conventionally known as Voice over Internet Protocol (VoIP). 
         [0003]    A packet comprises a header at the beginning of the packet, a payload in the middle of the packet, and a trailer at the end of the packet. The header generally includes information related to a destination address of the packet, routing information, a sequence number that identifies the packet&#39;s position in a transmitted sequence of packets, and information regarding a size of the packet The payload comprises data actually being communicated. The trailer typically includes error-checking data, which is used at the packet&#39;s destination to detect errors, which may have occurred in the packet on route. 
         [0004]    Since packets from a same sender to a same receiver may travel via different routes, packets, which are sequentially transmitted, may arrive at their common destination, i.e. receiver, in a different order than the order in which they were transmitted. As each packet is identified by a sequence number, its processing at the receiver will be done according to the sequence number regardless of the order in which it arrived at the receiver. 
         [0005]    In VoIP and other voice related packet switching applications, a sender&#39;s transmitter will generally digitize an analog voice stream and group the resultant digital data in sections. The transmitter packages each section in a payload portion of a packet and sends the packet to a receiver, or a plurality of receivers, via the Internet. The receiver decodes the data in the payloads of the packets it receives and orders the data according to the sequence numbers of the packets to regenerate the voice stream. In VoIP protocols, generally, packets are required to be received at the receiver within a delay time less than from about 250 msec to about 500 msec following their transmission in order to maintain voice continuity of a reconstructed voice stream. The network generally classifies packets that do not reach their destinations within this delay as “lost packets”, ceases attempts at routing them to their destinations and discards them. Packet losses may affect intelligibility of a received voice stream if sound encoded in lost packets has a generally continuous duration, hereinafter a “discontinuity duration”, between about 60 msec to about 100 msec. To make up for the lost packets, packet loss concealment (PLC) techniques are commonly used in VoIP and other voice related packet switching applications. PLC techniques are generally considered to be either sender based or receiver based. 
         [0006]    Sender based techniques may be classified as “active” or “passive”. Active techniques generally involve the receiver sending a message to the sender informing the sender which packets are lost, in response to which, the sender retransmits the lost packets. A drawback of this technique is that often a period, from a moment when a “lost packet” in a voice stream is first transmitted until a replacement packet is received at the receiver, exceeds the 250-500 msec delay time required to maintain voice continuity of the voice stream. 
         [0007]    There are generally considered to be two types of passive techniques: interleaving and forward error correction. In interleaving, the transmitter distributes bytes that encode temporally contiguous portions of an audio stream in different packets prior to transmission. As a result, loss of a single packet does not, in general, result in loss of audio data corresponding to a continuous period of time greater than that corresponding to audio data encoded in a single byte, which is generally less than the discontinuity duration. Forward error correction comprises sending additional data with each packet, often referred to as redundancy data, that is useable to reconstruct lost packets. Reed Solomon encoding/decoding is a well-known forward error correction technique. Passive methods usually require that all data in a given data stream be received prior to processing and reconstructing lost packets. As a result, these techniques may be time consuming and may requite large buffering capacity in the receiver. 
         [0008]    Receiver based techniques generally take advantage of a characteristic whereby variations in an audio waveform of a voice signal are relatively very small between adjacent packets. Numerous receiver-based techniques are known in the art, some of which are briefly discussed below.
   a. Silence Substitution—the method comprises replacing voice that is encoded in a lost packet with a period of silence.   b. Packet Repetition—the method comprises replacing a lost packet with a duplicate of a packet immediately preceding the lost packet.   c. Pitch Estimation—the method comprises determining a fundamental frequency of voice encoded in packets preceding a lost packet and duplicating the fundamental frequency during a period in which voice encoded in the missing packet would be made audible.   d. Linear Prediction—the method comprises determining waveform parameters from a portion of an audio waveform preceding a segment of the waveform encoded in a lost packet. The lost segment is synthesized responsive to the predicted parameters using linear interpolation techniques. Optionally, a portion of the audio waveform following the lost segment may also be used to perform linear prediction.   
 
         [0013]    For convenience of presentation, a portion of an audio waveform encoded in a packet immediately preceding a lost packet is referred to as a “leading portion”. A portion encoded in a packet immediately following the lost packet is referred to as a “trailing portion”. 
         [0014]    Typically, in replacing a missing portion of an audio waveform with a synthesized segment, the synthesized segment is matched to the leading portion of the audio waveform to provide a smooth transition between the leading portion and the synthesized segment. Generally, matching comprises overlapping and adding (OLA) a leading section of the synthesized segment with a trailing section of the leading portion so that the amplitude of the audio waveform is substantially preserved in a leading overlap region. In other matching techniques the trailing section of the leading portion is butted on to the leading section of the synthesized segment. Furthermore, several other matching techniques comprise phase matching, referred to as “synchronous overlap and add” (SOLA) techniques, wherein the leading section of a synthesized segment is overlapped with a trailing section of a leading portion of the waveform to preserve pitch as well as amplitude in the overlap region. 
         [0015]    PLC and techniques for synthesizing lost packets may be found in “Packet Loss Concealment for Voice Transmission over IP Networks”, Ejaz Mahfuz, Department of Electrical Engineering, McGill University, Montreal, Canada. September 2001, (www.tsp.ece.mcgill.ca/MMSP/Theses/2001/MaifuzT2001.pdf), “A Survey of Packet Loss Recovery Techniques for Streaming Audio”, C. Perkins, O. Hodson, V. Hardman, IEEE Network, September/October 1998, pp. 40-48, ANSI T1.521a-2000 (Annex B) “Standard for Packet Loss Concealment”, and ITU-T Recommendation G.711, Appendix I, “A High Quality Low-Complexity Algorithm for Packet Loss Concealment with G.711”, all of which are incorporated herein by reference. OLA and SOLA techniques are described in Chapter 2, “Sound modeling: signal based approaches” by Giovanni De Poli and Federico Avanzini (www.dei.unipd.it/˜musical/M06/Dispense06/2_signalmodels.pdf), incorporated herein by reference. 
       SUMMARY OF THE DISCLOSURE 
       [0016]    An aspect of some embodiments of the invention relates to providing a method and apparatus for synchronizing a synthesized waveform segment that is used in place of a missing portion of an audio waveform generated in response to a packet stream encoding portions of the audio waveform. 
         [0017]    According to an aspect of an embodiment of the invention, the synthesized waveform segment is synchronized with a leading portion of the audio waveform that precedes the missing portion and with a trailing portion of the audio waveform that follows the missing portion. 
         [0018]    In an embodiment of the invention, synchronizing the synthesized waveform segment with the trailing portion of the audio waveform comprises overlapping the trailing section of the synthesized segment with the leading section of the trailing portion and phase matching the synthesized segment with the trailing portion so that a fundamental frequency, i.e. “pitch”, as well as amplitude of the audio waveform, is substantially preserved in a trailing overlap region. Synchronizing the segment with the leading portion optionally comprises phase matching the synthesized segment with the leading portion of the audio waveform and optionally overlapping the leading section of the synthesized segment with the trailing section of the leading portion. 
         [0019]    Prior art techniques for replacing a lost segment with a synthesized segment generally provide for synchronous overlapping and addition of a leading section (SOLA) of the synthesized segment with a trailing section of the leading portion of an audio waveform. The rear section of the synthesized segment and leading section of the trailing portion of the audio waveform are weighted to provide relative continuity of amplitude. However, the synthesized segment and the trailing portion are not synchronized to provide continuity of pitch or phase. The rear section of the synthesized segment is allowed to “fall where it may”, presumably under an assumption that the rear section of the synthesized segment is properly synchronized to the trailing portion of the audio stream if the leading section of the segment is properly synchronized to the leading portion of the audio stream. The inventors have found however, that often in prior art replacement techniques, the rear section of a synthesized segment is not appropriately synchronized with a trailing portion of an audio waveform and that the lack of synchrony can cause noticeable degradation in quality of an audio stream generated responsive to the waveform. Synchronizing the rear section of the synthesized segment and the audio waveform, independent of synchronizing the leading section of the segment and waveform, in accordance with an embodiment of the invention, can result in noticeable improvement in the quality of the audio stream. 
         [0020]    In accordance with an embodiment of the invention, synchronizing the rear section of the synthesized segment with the trailing portion of the audio waveform comprises temporally displacing the trailing portion of the waveform relative to the segment after the segment is synchronized with the leading portion. Optionally, synchronizing the synthesized segment with the leading portion comprises temporally displacing the segment relative to the leading portion to provide a phase match with the leading portion. 
         [0021]    There is therefore provided, in accordance with an embodiment of the invention, a method for using a waveform segment in place of a missing portion of an audio waveform generated in response to a packet stream encoding portions of the audio waveform, the method comprising: phase matching a trailing portion of the waveform segment with a trailing portion of the audio waveform that follows the missing portion; and adding the phase matched waveform segment to the audio waveform. Optionally, phase matching the trailing portions comprises temporally displacing the trailing portion of the audio waveform. Additionally or alternatively, the method optionally provides for phase matching a leading portion of the waveform segment with a leading portion of the audio waveform that precedes the missing portion. 
         [0022]    Furthermore, in accordance with some embodiments of the invention, the method provides for overlapping the leading portions to generate a leading overlap waveform region. Optionally, the amplitudes of the overlapping leading portions are modulated so that the amplitude of the leading overlap waveform region is substantially the same as that of the leading portion of the audio waveform. 
         [0023]    In some embodiments of the invention, the method optionally further comprises overlapping the trailing portion to generate a trailing overlap waveform region. Optionally, the amplitudes of the overlapping trailing portions are modulated so that the amplitude of the trailing overlap waveform region is substantially the same as that of the leading portion of the audio waveform. 
         [0024]    There is further provided, in accordance with an embodiment of the invention, a receiver for receiving a packet stream encoding portions of an audio waveform, the receiver comprising: a generator that generates a waveform segment suitable for replacing a missing portion of the audio waveform; and circuitry adapted to phase match a trailing portion of the waveform segment with a trailing portion of the audio waveform that follows the missing portion. Optionally, the receiver includes circuitry comprising an overlap and add unit that overlaps and adds the trailing portion of the waveform segment with the trailing portion of the audio waveform. 
         [0025]    There is further provided in accordance with an embodiment of the invention, a computer readable medium containing a set of instructions for programming a processor to use a waveform segment to replace a missing portion of an audio waveform generated in response to a packet stream encoding portions of the audio waveform, the instructions comprising: a routine for phase matching a trailing portion of the waveform segment with a trailing portion of the audio waveform that follows the missing portion; and a routine for adding the phase matched waveform segment to the audio waveform. 
         [0026]    There is further provided in accordance with an embodiment of the invention, a signal set encoded with a set of instructions for programming a processor to use a waveform segment to replace a missing portion of an audio waveform generated in response to a packet stream encoding portions of the audio waveform, the instructions comprising: instructions for phase matching a trailing portion of the waveform segment with a trailing portion of the audio waveform that follows the missing portion; and instructions for adding the phase matched waveform segment to the audio waveform. 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0027]    Examples illustrative of embodiments of the invention are described below with reference to figures attached hereto. In the figures, identical structures, elements or parts that appear in more than one figure are generally labeled with a same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are generally chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below. 
           [0028]      FIG. 1  schematically shows an exemplary functional block diagram of a linear prediction (LP) based PLC module in accordance with prior art; 
           [0029]      FIG. 2  schematically illustrates synchronizing a waveform segment synthesized to replace a missing segment of an audio waveform with the audio waveform, in accordance with prior art. 
           [0030]      FIG. 3  schematically shows an exemplary functional block diagram of an improved PLC module in a receiver, in accordance with an embodiment of the invention; and 
           [0031]      FIG. 4  schematically illustrates synchronizing a waveform segment synthesized to replace a missing segment of an audio waveform with the audio waveform, in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    Reference is made to  FIG. 1 , which schematically shows an exemplary functional block diagram of a linear prediction (LP) based packet loss concealment (PLC) module  101  known in the art comprised in a receiver  100 . PLC module  101  uses a linear prediction technique to synthesize an audio waveform segment optionally based on a leading portion of the audio waveform. Incoming packets to the receiver are processed such that a last received packet is temporarily stored in a buffer for possible use in PLC applications should an immediately following packet not arrive. 
         [0033]    In a typical PLC application a leading portion  102  associated with the last received packet, or a plurality of last received packets, stored in a buffer  180 , is input to LP filter  120 . LP filter  120  comprises a finite impulse response (FIR) filter with frequency response characteristics determined by LP coefficients  118 , which are generated by a LP analysis circuitry  110 . Responsive to the LP coefficients LP filter  120  produces a residual signal  104  characterized by the fundamental frequency and amplitude of leading portion  102 . Generation of the LP coefficients in LP analysis circuitry  120  comprises windowing a section of the leading portion followed by computing an autocorrelation or alternatively, a covariance, of the windowed section. The LP coefficients are selected so that the energy level of residual signal  104  is substantially minimized. 
         [0034]    Residual signal  104  is fed into a Pitch Detector  130  and an Excitation Generator  140 . Pitch Detector  130  is adapted to estimate a pitch period of leading portion  102  by searching for peak locations, hereinafter referred to as “pitch peaks”, in the normalized autocorrelation function of residual signal  104 , or alternatively, in the normalized covariant function of the residual signal. Once the pitch period of leading portion  102  is estimated, Excitation Generator  140  may generate an excitation signal  108  responsive to the input of pitch period  106  from Pitch Detector  130  and the input of residual signal  104 . Excitation signal  108  comprises a portion of residual signal  104  a pitch period in length, replicated throughout substantially the entire length of the excitation signal. The entire length of excitation signal  108  is usually greater than that of the missing waveform. 
         [0035]    Inverse LP Filter  150  comprises an inverse FIR filter with frequency response characteristics determined by LP coefficients  118  and is adapted to add into Excitation signal  108  the frequency spectrum characteristics of the leading portion of the audio waveform. Inverse LP Filter  150  outputs a synthesized signal  112  comprising a synthesized segment of the audio waveform with a frequency spectrum and pitch period similar to leading portion  102 . Synthesized signal  112  is of a greater length than the missing portion of the audio waveform, the additional length used to optionally overlap-and-add with a trailing section of a leading portion of the audio waveform and to overlap and add with a leading section of a trailing portion of the audio waveform. 
         [0036]    An Overlap-and-Add (OLA) circuitry  160  is used to attach synthesized signal  112  onto the leading portion and the trailing portion. A window is used for phase matching the trailing section of the leading portion with a leading section of the synthesized signal. Optionally, in some embodiments of the invention the window is used for weighting and summing the trailing section of the leading portion with the leading section of the synthesized signal. OLA circuitry  160  comprises a buffer in which a rear section of synthesized signal  112  is stored. A window is also used for weighting and summing the rear section of synthesized signal  112  with the leading section of the trailing portion. The windowed section of synthesized signal  112  which comprises the missing portion in the audio waveform is referred to as a synthesized segment  114 . 
         [0037]    A scaling circuitry  170  is adapted to adjust the volume of synthesized segment  114  before being output as an output signal  116  to a loudspeaker (not shown). This is generally done to limit the effects of unwanted variations which may occur in the waveform of relatively long synthesized segments (usually exceeding  10  msec). As synthesized signal  114  passes through scaling circuitry  170  the amplitude of a section of the signal presently in the scaling circuitry is modified by a predefined “current” scaling value, which may vary up or down as a function of time. 
         [0038]    Reference is made to  FIG. 2 , which schematically illustrates waveform diagrams for an exemplary synthesizing process known in the art by a generic PLC module adapted to perform OLA. The generic PLC module may be the same or similar to PLC module  101  shown in  FIG. 1 . In the waveform diagrams the abscissa is graduated in sample numbers, the audio waveform samples represented by higher sample numbers are “played” or “vocalized” later than samples having lower sample numbers. 
         [0039]    An “original” signal  210  represents a section of an audio waveform prior to transmission through a packet switched network. Following routing through the network a packet, or several consecutive packets, is lost so that the signal at the receiving end is an exemplary corrupted signal  220 . Corrupted signal  220  is characterized by a leading portion  221 , which corresponds to the packet received immediately prior to the packet loss, a trailing portion  222  which corresponds to the packet received immediately following the packet loss, and a loss or missing portion  223  which corresponds to the lost packet and extends from sample  480  to  640 . 
         [0040]    In a synthesizing process by the generic PLC module ( FIG. 1 ) adapted to perform OLA, an exemplary synthesized segment  230  is synthesized to replace the lost packet. Synthesized segment  230  extends from sample  480  to approximately 680 and is longer than the loss portion  223 . Synthesized segment  230  is a copy of approximately 200 samples from the trailing section of leading portion  221  and comprises a leading edge  232 , four pitch peaks, such as that shown at pitch peak  231 , with the same fundamental frequency as in leading portion  221 . Leading edge  232  is adapted to match in phase with the trailing edge of leading portion  221  to which synthesized segment  230  will be attached. 
         [0041]    Application of OLA synthesis and the resulting audio waveform is shown by an exemplary reconstructed signal  240 . A leading section  242  of synthesized segment  230  is added to the trailing end of leading portion  221 . Possible discontinuity at the transition between leading portion  221  and synthesized segment  230  is minimized by phase matching at the edges. A rear section  241  of synthesized segment  230  is added to the leading section of trailing portion  222  using OLA windowing. A discontinuity in the transition between synthesized signal  230  and trailing portion  222  at rear section  241  is evidenced by the increase in the separation between two pitch peaks in the neighborhood of sample  640 . The increase in the separation represents a variation in the fundamental frequency of reconstructed signal  240  in that section of the audio waveform, resulting in degradation of quality of sound generated responsive to the waveform. 
         [0042]    Reference is made to  FIG. 3 , which schematically shows an exemplary functional block diagram of an improved PLC module  301  in a receiver  300 , in accordance with an embodiment of the invention, 
         [0043]    Improved PLC module  300  is adapted to synthesize an audio waveform segment, and to reconstruct an audio waveform in which synchronization is maintained in the transition between a leading portion of the audio waveform and the synthesized segment, and between the synthesized segment and a trailing portion of the audio waveform. The result is that the fundamental frequency of the audio waveform is substantially preserved preventing voice degradation. 
         [0044]    Improved PLC module  301  comprises a Generating Unit  310 , a Matching Unit  320 , an Overlap-Add Unit  330 , a Control Unit  340 , an Absorption Buffer  350 , and a Buffer  360 . In accordance with an embodiment of the invention, Generating Unit  310  is adapted to synthesize, using any method known in the art, an audio waveform segment  315 , also referred to as “synthesized signal”, using samples from a leading portion  305  of an audio waveform associated with a last packet, or a plurality of last received packets, arriving at a receiver  300 . Samples of leading portion  305  are continuously stored in a Buffer  360  irrespective of whether there is packet loss or not. The samples are stored in case the next packet does not arrive. If the packet arrives, the stored samples, or portion of stored samples, are replaced by samples from the newly arrived packet. In some embodiments of the invention, Generating Unit  310  may use samples stored in a buffer from leading portion  305  and a trailing portion  345  of the audio waveform, while in other embodiments of the invention, Generating Unit  310  may use samples stored in a buffer from trailing portion  345  of the audio waveform. Synthesized signal  315 , which may be similar or the same as synthesized signal  112  in  FIG. 1 , is generated by Generating Unit  310  only in response to a packet loss. In other embodiments of the invention, synthesized signal  315  may be generated continuously whether or not there is a packet loss. 
         [0045]    Matching Unit  320  is adapted to estimate a temporal shift in trailing portion  345  so that the pitch peaks in trailing portion  345  will be synchronized with the pitch peaks of synthesized signal  315 . Synchronization is performed by buffering and shifting forward or backward trailing portion  345  with respect to synthesized signal  315  until one or more of their pitch peaks are temporally matched. Shift estimation is performed optionally using cross-correlation techniques known in the art, such as, for example Maximum Correlation. When a packet, or several consecutive packets, is determined to be missing, Matching Unit  320 , in response to a control signal  355  from Control Unit  340 , outputs a delay signal  325 . Delay signal  325  is input to OLA Unit  330  and comprises information related to the estimated temporal shift, forward or backward, required in trailing portion  345  during the OLA windowing process so that the pitch peaks overlap. 
         [0046]    OLA Unit  330  is used to attach synthesized signal  315  onto trailing portion  345 . A window is used for phase matching a trailing section of leading portion  305  with a leading section of synthesized signal  315 . A resulting reconstructed signal  335  is then buffered in Absorption Buffer  350 . In accordance with some embodiments of the invention, OLA Unit  330  may be comprised in Generating Unit  310 . Leading portion  305  is continuously buffered also in Absorption Buffer  350 , irrespective of whether there is packet loss or not. Absorption Buffer  350  outputs an output signal  365  to a loudspeaker (not shown) comprising the leading portion and the reconstructed signal. If there is no packet loss the output signal comprises only the leading portion. Synchronization between the leading portion and the reconstructed signal is maintained by Control Unit  340 . Control Unit  340  also maintains synchronization in the absorption buffer between the leading portion and the reconstructed signal, relative to subsequently arriving trailing portions due to the temporal shifting, forward or backward, of the trailing portion. In some embodiments of the invention, Absorption Buffer  350  may comprise Buffer  360 . By temporally shifting forward (shifting forward in time) the trailing portion is output earlier in the audio stream than if there had there not been any packet loss. By temporally shifting backward the trailing portion it is output later in the audio stream than if there had not been any packet loss. 
         [0047]    Optionally, in some embodiments of the invention, the window is used for weighting and summing a trailing section of the leading portion with a leading section of the synthesized signal. A window is also used for weighting and summing a rear section of synthesized signal  315  with a leading section of trailing portion  345 . Reconstructed signal  335  is then also stored in Absorption Buffer  350  and subsequently output as part of output signal  365 . Control Unit  340  is adapted to manage the synchronization of the functions performed by Matching Unit  320 , OLA Unit  330 , and Absorption Buffer  350 . 
         [0048]    Reference is made to  FIG. 4  which schematically illustrates waveform diagrams for an exemplary synthesizing process by an improved PLC module in accordance with an embodiment of the invention. 
         [0049]    Improved PLC module may be similar or the same as improved PLC module  301  in  FIG. 3 . An original signal  410  represents a section of an exemplary audio waveform prior to transmission through a packet switched network. Following routing through the network, a packet, or several consecutive packets, is lost so that the signal at the receiving end is the exemplary corrupted signal  420 . Corrupted signal  420  is characterized by a leading portion  421  which corresponds to the packet received immediately prior to the packet loss, a trailing portion  422  which corresponds to the packet received immediately following the packet loss, and a loss or missing portion  423  which corresponds to the lost packet and extends from sample  480  to  640 . No information is available on that portion of original signal  410  due to the packet loss. 
         [0050]    In a synthesizing process by the improved PLC module, an exemplary synthesized segment  430  is synthesized to replace the lost packet. Synthesized segment  430  extends from sample  480  to approximately 680 and is longer than the loss portion  423 . Synthesized segment  430  is a copy of approximately 200 samples from the trailing section of leading portion  421  and comprises four pitch peaks, such as that shown at pitch peak  431 , with a same fundamental frequency as in leading portion  421 . In accordance with some embodiments of the invention, synthesized segment  430  may be longer and/or may comprise a greater number of pitch peaks, for example the synthesized segment may have a length of 250 samples and extend from sample  480  to  730  and comprise 5 pitch peaks. Furthermore, in some other embodiments of the invention, synthesized segment  430  may be shorter and/or may comprise a lesser number of pitch peaks, for example, the synthesized segment may have a length of 160 samples and extend from sample  480  to  640  and comprise 3 pitch peaks. 
         [0051]    Application of the matching process is shown for the audio waveform of exemplary corrupted signal  420 . Trailing portion  422  is shifted forward in time so that a first peak  445  is matched with the last pitch peak  432  of synthesized segment  430 , shifting forward by the same amount of time all other pitch peaks in trailing portion  422 , such as for example pitch peak  446 . 
         [0052]    Application of OLA synthesis and the resulting audio waveform is shown by an exemplary reconstructed signal  440 . A leading section  442  of synthesized segment  430  is added to the trailing section of leading portion  421  using phase matching, eliminating possible discontinuity at the transition between leading portion  421  and synthesized segment  430 . A rear section  441  of synthesized segment  430  is added to the leading section of trailing portion  422  using OLA windowing. A discontinuity in the transition between synthesized signal  430  and trailing portion  422  at rear section  441  is prevented by matching the last pitch peak  432  with pitch peak  445  and backward shifting of trailing portion  422 . Furthermore, the output audio quality is maintained as there in no substantial change in the fundamental frequency of reconstructed signal  440  compared to original signal  410 . 
         [0053]    In the description and claims of embodiments of the present invention, each of the words, “comprise” “include” and “have”, and forms thereof, are not necessarily limited to members in a list with which the words may be associated. 
         [0054]    The invention has been described using various detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. The described embodiments may comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the invention that are described and embodiments of the invention comprising different combinations of features noted in the described embodiments will occur to persons with skill in the art. The scope of the invention is limited only by the claims.