Abstract:
This invention relates to methods, a computer program product and apparatuses in the context of frame buffering. A buffering time for one or more frames received by a frame buffer is determined based on a specific buffering time associated with a specific frame and on information representative of a specific amount of data stored in the frame buffer.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to buffering of frames, and in particular to determining a buffering time for one or more frames received by a frame buffer. 
       BACKGROUND OF THE INVENTION 
       [0002]    Frame buffers are inter alia deployed in the context of video and audio processing, such as for instance in Voice over IP (VoIP) systems. 
         [0003]    In general, network jitter and packet loss conditions can cause degradation in quality for example in conversational speech services in packet switched networks, such as the Internet. The nature of the packet switched communications can introduce variations in transmission of times of the packets (containing frames), known as jitter, which can be seen by the receiver as packets arriving at irregular intervals. However, an audio playback device requires constant input to maintain good audio quality, and no interruptions can be allowed. Thus, if some packets/frames arrive at the receiver after they are required for playback, the decoder may have to consider those frames as lost and perform error concealment. 
         [0004]    Typically, a fixed buffer to manage network jitter can be utilised to store incoming frames for a predetermined amount of time (specified e.g. upon reception of the first packet of a stream) to hide the irregular arrival times and provide constant input to the decoder and playback components. 
         [0005]    This approach is illustrated in  FIG. 1 , which shows timelines of a transmitter TX, a receiver RX and a decoder DEC, respectively. On the uppermost timeline, the regularly spaced transmission points of time of frames are illustrated by vertical lines. Similarly, in the middle timeline, the reception points of time of the transmitted frames are illustrated by vertical lines, wherein the reception points of time, for instance due to network jitter, are irregularly spaced. Finally, the lowermost timeline shows the decoding points of time of the frames, which are, due to the fact that the decoder DEC needs to provide constant input to the playback device, regularly spaced. Therein, the relationship between transmission, reception and decoding points of time of the same frame is illustrated by means of dashed arrows. As can be seen by comparing the reception point of time of the first received frame in the middle timeline with its associated decoding point of time in the lowermost timeline, the jitter buffer introduces an additional delay increasing the end-to-end delay since the received frames are stored in the jitter buffer before the decoding process. 
         [0006]    A shortcoming of this basic approach is that a jitter buffer management scheme requiring fixed playback timing is inevitably a compromise between low enough buffering delay and low enough number of delayed frames, and finding an optimal trade-off can be a difficult task. For example, in the situation shown in  FIG. 1 , the frame sent at 100 ms arrives at the receiver RX after it is needed for further processing by decoder DEC, and therefore it needs to be replaced with error concealment (i.e. a “late loss” occurs, shown as a dashed vertical line). 
         [0007]    There may exist special environments and applications where the amount of expected jitter can be estimated to remain between predetermined limits. In general, however, the network delay associated with jitter can vary from a scintilla of time to hundreds of milliseconds within the same session. Using a jitter buffer management scheme requiring fixed playback timing with an initial buffering delay set to a large enough value to cover the jitter, according to an expected worst case scenario, would keep the number of delayed packets in control. However, at the same time there may arise a risk of introducing an end-to-end delay which can be too long to enable a natural conversation. In this case applying a fixed jitter buffer management scheme may not be a practical choice in most audio transmission applications operating over a packet switched network, e.g. in VoIP over the 3GPP IP Multimedia Subsystem (IMS). 
         [0008]    In contrast to a fixed jitter buffer management scheme, an adaptive jitter buffer management scheme can be used to dynamically control the balance between short enough delay and low enough number of delayed frames. In this approach, an entity controlling the jitter buffer constantly monitors the incoming packet stream and adjusts the buffering delay (or buffering time, these terms are used interchangeably) according to observed changes in the network delay behaviour. If the transmission delay seems to increase or the jitter becomes worse, the buffering delay may need to be increased to meet the network conditions. In the opposite situation, where the transmission delay seems to decrease, the buffering delay can be reduced, and hence, the overall end-to-end delay can be minimised. 
         [0009]    One of the challenges in adaptive jitter buffer management is reliable estimation—or actually prediction—of the transmission characteristics. Although adaptation based on the reception statistics of most recent packets usually gives a reasonable estimate on the short-term network behaviour, it may be impossible to avoid the fact that some frames arrive after their scheduled decoding time—i.e. too late for normal decoding, especially when applying relatively strict buffering delay requirement. 
         [0010]    Jitter buffer adaptation during active speech requires additional processing to shorten or extend the speech signal (i.e. time scaling, also known as time warping) to maintain good voice quality and intelligibility. For example, suitable methods are disclosed in documents WO 03/021830 and U.S. 2006/0056383. To avoid complex time scaling, a commonly used method for jitter buffer management is to perform the adaptation during comfort noise signal periods typically at beginning of a new talk spurt (i.e. at a speech onset). This approach can be expected to provide low complexity adaptation functionality with high quality, since the comfort noise signal does not carry information that is important for intelligibility or actual voice quality. The minor drawback of the onset adaptive jitter buffer management is that even though the network analyser detects changes in the delay characteristics, the jitter buffer adaptation needs to wait for the next speech onset to take place. However, jitter buffer management solutions apply the onset adaptive approach as part of the adaptation functionality. Where the basic approach is to re-estimate the required buffering time and perform adaptation at each speech onset, while only urgent adaptation steps are taken during active speech. 
         [0011]    A basic (adaptive) jitter buffer management approach uses the statistics on the current number of frames in the buffer as an indication of the buffer status. If, for example, the number of frames in the buffer falls below a predetermined (or adaptively determined) threshold, an adaptation step to increase the buffering time can be initiated to take place at the next speech onset in order to decrease the risk of subsequent frames arriving too late. If, however, the number of frames in the buffer grows above another predetermined (or adaptively determined) threshold, an adaptation step to decrease the buffering time can be initiated to reduce delay for improved interactivity. 
         [0012]    An alternative approach may use statistics computed over several frames instead of considering only single frames. For instance, the number of instances when the number of frames in the buffer falls below or above predetermined limits over an analysis window consisting of several frames (or time corresponding to several frames&#39; duration) may be counted. Equally well, the average number of frames in the buffer may be considered over an analysis window as the indication of buffer status which is used for controlling the jitter buffer management operation. 
         [0013]    When considering the buffer status indication based on the number of frames, one possible way to apply adaptation is to estimate the target buffer level as a number of frames, and then to wait for the selected number of frames to arrive and accumulate in the buffer before decoding (or playing back) the first frame of a talk spurt at the point a speech onset is detected. 
         [0014]    This is illustrated in  FIG. 2 , wherein the structure of  FIG. 2 . In this example, the speech onset frame is the one transmitted at 20 ms (and received approximately at 36 ms), and the decoding &amp; playback is started after three frames (the first one to be decoded plus two other ones providing assumed 40 ms of jitter protection) have arrived at the buffer. In this example this approach would result in approximately 52 ms buffering delay for the first frame of the talk spurt. 
         [0015]    A considerably different jitter buffer management approach is to indicate the buffer status based on the buffering time. The buffering time may for instance be an observed buffering time recorded when a frame is passed to the decoder (or playback device), or a predicted buffering time computed when a frame is received. Methods for predicting a buffering time are for instance disclosed in document WO 2006/044696. As in the aforementioned approaches, which are based on the number of frames stored in the buffer, the buffer adaptation approach based on the buffering time may also consider statistics computed over several frames instead of keeping track of single frames only. 
         [0016]    Jitter buffer management based on the buffering time may for instance be performed by estimating the required buffering time (e.g. in milliseconds), and applying this buffering time for the first frame of a talk spurt when a speech onset is encountered. 
         [0017]    The example in  FIG. 3  illustrates this approach. In this case, the estimated buffering delay of 40 ms is applied for the first frame of the talk spurt simply by directly delaying its decoding &amp; playback by 40 ms. 
         [0018]    The previously described jitter buffer management approaches for speech onset handling can be expected to provide, on average, approximately equal performance (both in terms of delay and late-loss). However, there can be special cases where these approaches may fail, either by introducing unnecessarily high buffering delay until the next adaptation step, or by providing inadequate jitter protection, leading to an unnecessarily high rate of late-loss frames. 
         [0019]    For instance, problematic cases for jitter buffer management based on the number of frames in the buffer are scenarios where the frame triggering the start of the decoding &amp; playback is an “outlier”—i.e. it arrives too early or too late (with respect to the subsequent frames). In the former case, which is illustrated in  FIG. 4 , buffer adaptation may provide too short buffering time causing high late-loss rate, while in the latter case, which is illustrated in  FIG. 5 , the buffering time would be unnecessarily high (and there may be also a risk of buffer overflow) until the next adaptation point. 
         [0020]    If the “trigger frame”, denoting the frame triggering the start of the decoding and playback, is lost in the transmission path, the jitter buffer manager will wait until the next frame is successfully received before starting the decoding &amp; playback. In case of a long lost burst this may obviously increase buffering time significantly. 
         [0021]    On the other hand, for jitter buffer management based on the buffering time, the critical frame is the actual speech onset frame: if it arrives “early”, the buffering time may be too short (see  FIG. 6 ), causing increased late-loss rate. If it arrives “late”, the buffering time may become unnecessarily long (see  FIG. 7 ). 
       SUMMARY 
       [0022]    A method is disclosed, comprising determining a buffering time for one or more frames received by a frame buffer based on a specific buffering time associated with a specific frame and on information representative of a specific amount of data stored in the frame buffer. 
         [0023]    Furthermore, a method is disclosed, comprising determining a buffering time for one or more speech frames received by a jitter buffer based on a specific buffering time associated with a specific speech frame and on information representative of a specific amount of data stored in the jitter buffer. 
         [0024]    Furthermore, a computer-readable medium having a computer program stored thereon is disclosed. The computer program comprises instructions operable to cause a processor to determine a buffering time for one or more frames received by a frame buffer based on a specific buffering time associated with a specific frame and on information representative of a specific amount of data stored in the frame buffer. Therein, the computer-readable medium may be any medium that is capable of storing digital data in electric, magnetic, electro-magnetic or optic form. The medium may be a fixedly installed or removable medium. 
         [0025]    Furthermore, a computer program is disclosed, comprising instructions operable to cause a processor to determine a buffering time for one or more frames received by a frame buffer based on a specific buffering time associated with a specific frame and on information representative of a specific amount of data stored in the frame buffer. 
         [0026]    Furthermore, an apparatus is disclosed, comprising a determining unit configured to determine a buffering time for one or more frames received by a frame buffer based on a specific buffering time associated with a specific frame and on information representative of a specific amount of data stored in the frame buffer. 
         [0027]    Furthermore, an apparatus is disclosed, comprising an adaptation control unit configured to determine a buffering time for one or more speech frames received by a jitter buffer based on a specific buffering time associated with a specific speech frame and on information representative of a specific amount of data stored in the jitter buffer. 
         [0028]    Finally, an apparatus is disclosed, comprising means for determining a buffering time for one or more frames received by a frame buffer based on a specific buffering time associated with a specific frame and on information representative of a specific amount of data stored in the frame buffer. 
         [0029]    The above-disclosed apparatuses may be embodied as a module, respectively. This module may for instance be connected to a device that contains the frame buffer, in order to determine the buffering time for one or more frames received by the frame buffer. The apparatuses may equally well comprise the frame buffer itself. The apparatuses may for instance be receivers that receive, buffer and further process the frames. The apparatuses may for instance be Multimedia Telephony Service for Internet Protocol Multimedia Subsystem (MTSI) speech receivers or parts of such receivers. 
         [0030]    According to the methods, computer program product, computer programs and apparatuses of the present invention, a buffering time for one or more frames received by a frame buffer is determined. The frames may for instance be received by the buffer in the form of packets, and the packets may be unpacked by the buffer to obtain the frames. The buffering time to be determined may for instance be defined as the time between the reception of a frame in the frame buffer and the output of this frame out of the frame buffer. The buffering time may for instance be determined for a single frame only, e.g. for the specific frame, or for several frames. The frames received by the frame buffer may have the same length or have different lengths. The determined buffering time may for instance be used to trigger reading of frames out of the frame buffer by a downstream processing unit, such as for instance a decoder. 
         [0031]    The buffering time is determined based on a specific buffering time associated with a specific frame, and on information representative of a specific amount of data stored in the frame buffer. The specific frame may for instance be a reference frame, e.g. a frame marking or representing a start, an end or a change. The specific frame may for instance be an onset speech frame, which marks the beginning of a talk spurt. The buffer then buffers speech frames, for instance to compensate for jitter, and the buffering time for the one or more frames is then determined, inter alia, based on a specific buffering time of this onset speech frame. 
         [0032]    The specific buffering time may for instance be an estimated required buffering time for the specific frame, or may define the time lapsed since the reception of the specific frame in the frame buffer until a current point of time. 
         [0033]    The information representative of a specific amount of data stored in the frame buffer may refer to all data stored in the frame buffer, or to only a part thereof. The information representative of a specific amount of data stored in the frame buffer may for instance comprise a number of frames contained in the frame buffer, and/or a time span covered by a number of frames contained in the frame buffer to name but a few exemplary possibilities. The number of frames contained in the frame buffer may for instance relate to all frames currently contained in the frame buffer. This definition of the number of frames contained in the frame buffer may for instance be applied in scenarios where frames that have already read from the frame buffer are also removed from the frame buffer. Equally well, the number of frames may relate only to those frames in the frame buffer that are subsequent (e.g. in processing order, for instance in decoding order) with respect to the specific frame (e.g. an onset frame), including or excluding the specific frame. Moreover, if the frame buffer is configured to buffer frames of different frame sequences, the number of frames may relate to only one of these frame sequences. 
         [0034]    Considering the time span covered by a number of frames contained in the frame buffer may give more reliable information on the buffer status (than considering the number of frames contained in the frame buffer) in case some of the frames are lost in the transmission path. Consider an example: frames with frame numbers n, n+1 and n+3 are stored in the buffer. Considering a number of frames contained in the frame buffer as frame information may indicate the “buffer depth” to be three frames, whereas considering the time span covered by a number of frames contained in the frame buffer may take into account the fact that the frame with frame number n+2 is missing from the sequence, and that the frames in the buffer actually cover a time span of four frames. 
         [0035]    According to the methods, computer program product, computer program and apparatuses of the present invention, thus a buffering time for one or more frames is determined under consideration of both i) specific buffering time information and ii) information representative of a specific amount of data stored in the frame buffer This approach may particularly contribute to improve the buffering performance in terms of lower late loss rate and/or shorter buffering delay, especially in challenging transmission conditions introducing large jitter and/or high numbers of lost packets. 
         [0036]    According to an exemplary embodiment of the present invention, the information comprises a time span covered by a number of frames contained in the frame buffer. 
         [0037]    In this exemplary embodiment of the present invention, the determining may comprise calculating an output point of time for the specific frame as the sum of a point of time at which the specific frame is received and the specific buffering time; and checking, under consideration of the time span covered by a number of frames contained in the frame buffer and the specific buffering time, if the specific frame shall be output at the output point of time or at another point of time. The buffering time is thus determined for the specific frame as the difference between the point of time at which the specific frame is received and the point of time at which the specific frame is output. 
         [0038]    In this exemplary embodiment of the present invention, the specific buffering time may be a required buffering time of the specific frame. The required buffering time may for instance be estimated. 
         [0039]    In this exemplary embodiment of the present invention, the checking may comprise checking, if the output point of time has elapsed; checking, in case the output point of time has elapsed, if the time span is smaller than a first value that at least partially depends on the specific buffering time, increasing the output point of time and returning to the checking if the output point of time has elapsed in case the time span is smaller than the first value, and determining that the specific frame shall be output at a current point of time in case the time span is not smaller than the second value; and checking, in case the output point of time has not yet elapsed, if the time span is larger than a second value that at least partially depends on the specific buffering time, determining that the specific frame shall be output at a current point of time in case the time span is larger than the second value, and returning to the checking if the output point of time has elapsed in case the time span is not larger than the second value. Therein, the output point of time may for instance be increased by a constant or an adaptively determined value. This value may for instance be decreased at or after each time when an increasing of the output point of time is performed. 
         [0040]    Therein, the first value may be the specific buffering time minus a first margin value. The second value may be a specific buffering time plus a second margin value. The first and/or second margin values may for instance be constant or be adaptively determined. 
         [0041]    According to a further exemplary embodiment of the present invention, the information comprises a number of frames contained in the frame buffer. 
         [0042]    In this exemplary embodiment of the present invention, the determining may comprise determining a target number of frames; and checking, under consideration of a time duration derived from the number of frames contained in the frame buffer and the specific buffering time, if the specific frame shall be output at a point of time at which a number of frames in the frame buffer has reached the target number of frames or at another point of time. 
         [0043]    In this exemplary embodiment of the present invention, the specific buffering time may be a time elapsed since the specific frame has been received. 
         [0044]    In this exemplary embodiment of the present invention, the checking may comprise checking, if a number of frames in the frame buffer has reached the target number of frames; checking, in case the number of frames in the frame buffer has reached the target number of frames, if a time duration derived from the number of frames contained in the frame buffer is larger than a third value that at least partially depends on the specific buffering time, increasing the target number of frames and returning to the checking if a number of frames in the frame buffer has reached the target number of frames in case the time duration is larger than the third value, and determining that the specific frame shall be output at a current point of time in case the time duration is not larger than the third value; and checking, in case the number of frames in the frame buffer has not yet reached the target number of frames, if the time duration is smaller than a fourth value that at least partially depends on the specific buffering time, determining that the specific frame shall be output at a current point of time in case the time duration is smaller than the fourth value, and returning to the checking if a number of frames in the frame buffer has reached the target number of frames in case the time duration is not smaller than the fourth value. 
         [0045]    Therein, the third value may be the specific buffering time plus a third margin value. The fourth value may be the specific buffering time minus a fourth margin value. The third and/or fourth margin values may for instance be constant or be adaptively determined. 
         [0046]    According to a further exemplary embodiment of the present invention, the determining of the buffering time for one or more frames is performed in a multimedia telephony service for internet protocol multimedia subsystem speech receiver. Equally well, the determining may be performed in any other type of speech receiver or in any other type of receiver that deploys a buffer. 
         [0047]    These and other aspects of the invention will be apparent from and elucidated with reference to the detailed description presented hereinafter. The features of the present invention and of its exemplary embodiments as presented above are understood to be disclosed also in all possible combinations with each other. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0048]    In the figures show: 
           [0049]      FIG. 1 : An exemplary illustration of frame buffering according to a fixed jitter buffer management scheme; 
           [0050]      FIG. 2 : an exemplary illustration of frame buffering according to a jitter buffer management scheme based on the number of frames in the buffer; 
           [0051]      FIG. 3 : an exemplary illustration of frame buffering according to a jitter buffer management scheme based on a buffering time; 
           [0052]      FIG. 4 : an exemplary illustration of frame buffering according to a jitter buffer management scheme based on the number of frames in the buffer in case of an early arriving trigger frame; 
           [0053]      FIG. 5 : an exemplary illustration of frame buffering according to a jitter buffer management scheme based on the number of frames in the buffer in case of a late arriving trigger frame; 
           [0054]      FIG. 6 : an exemplary illustration of frame buffering according to a jitter buffer management scheme based on a buffering time in case of an early arriving onset frame; 
           [0055]      FIG. 7 : an exemplary illustration of frame buffering according to a jitter buffer management scheme based on a buffering time in case of a late arriving onset frame; 
           [0056]      FIG. 8 : a schematic block diagram of an exemplary embodiment of an apparatus according to the present invention; 
           [0057]      FIG. 9 : a flowchart of a first exemplary embodiment of jitter buffer management according to the present invention; 
           [0058]      FIG. 10 : an exemplary illustration of frame buffering according to the first exemplary embodiment of jitter buffer management according to the present invention in case of an early arriving onset frame; 
           [0059]      FIG. 11 : a flowchart of a second exemplary embodiment of jitter buffer management according to the present invention; 
           [0060]      FIG. 12 : an exemplary illustration of frame buffering according to the second exemplary embodiment of jitter buffer management according to the present invention in case of an early arriving trigger frame; and 
           [0061]      FIG. 13 : an exemplary illustration of frame buffering according to the second exemplary embodiment of jitter buffer management according to the present invention in case of a late arriving trigger frame. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0062]    In the following detailed description of the present invention, exemplary embodiments of the present invention will be described in the context of a speech coding system. It should be noted that the present invention is however not restricted to speech coding systems, but is equally well applicable in all other types of systems where frames are buffered. 
         [0063]      FIG. 8  is a schematic block diagram of an exemplary embodiment of an apparatus  8  according to the present invention. Therein, transfer of content between the blocks is illustrated by means of solid lines, whereas the transfer of control information is illustrated in dashed lines. Apparatus  8  is exemplarily considered to be a speech receiver, for instance in the context of the Multimedia Telephony Service for Internet Protocol Multimedia Subsystem (MTSI). 
         [0064]    Apparatus  8  comprises a jitter buffer  800 , a speech decoder  801 , a network analyzer  802 , an adaptation control logic  803  and an adaptation unit  804 . 
         [0065]    Jitter buffer  800  is configured to at least temporarily store one or more speech frames, which are received via a (wired or wireless) network for instance in the form of packets  806 . These packets  806  may for instance be Real-time Transport Protocol (RTP) packets, which are unpacked by buffer  800  to obtain the speech frames. Buffer  800  is linked to speech decoder  801  to output speech frames when they are requested for decoding. 
         [0066]    Buffer status information  808 , such as for instance information on a number of frames contained in buffer  800 , or information on a time span covered by a number of frames contained in the buffer, or a buffering time of a specific frame (such as an onset frame), is transferred between buffer  800  and adaptation control logic  803 . 
         [0067]    Network analyzer  802  monitors the incoming packets  806 , for instance to collect reception statistics (e.g. jitter, packet loss). Corresponding network analyzer information  807  is passed from network analyzer  802  to adaptation control logic  803 . 
         [0068]    Adaptation control logic  803 , inter alia, controls buffer  800 . This control comprises determining buffering times for one or more frames received by buffer  800 , and is performed based on network analyzer information  807  and/or buffer status information  808 . The steps performed by adaptation control logic  803  when determining buffering times will be explained in more detail with reference to the flowcharts of  FIGS. 9 and 11  below. The buffering delay of buffer  800  may for instance be controlled during comfort noise periods, during active speech periods or in-between. For instance, a buffering time of an onset speech frame may be determined by adaptation control logic  803 , and speech decoder  801  may (for instance via adaptation unit  804 , signals  809  and  810 ) then be triggered to extract this onset speech frame from buffer  800  when this determined buffering time has elapsed. 
         [0069]    Adaptation unit  804 , if necessary, shortens or extends the output speech signal  812  according to requests given by adaptation control logic  803  to enable buffer delay adjustment in a transparent manner. 
         [0070]    Speech decoder  801  extracts frames from buffer  800  for decoding and outputs the decoded speech  811  to adaptation unit  804 . Speech decoder  801  may for instance be an Adaptive Multirate (AMR) or Adaptive Multirate Wideband (AMR-WB) speech decoder. 
         [0071]      FIG. 9  is a flowchart  9  of a first exemplary embodiment of jitter buffer management according to the present invention. The steps of this flowchart may for instance be performed by adaptation control logic  803  of  FIG. 8  to determine a buffering time for an onset frame. 
         [0072]    In a first step  900 , it is checked if an onset frame has arrived at buffer  800 . This may for instance be signaled to adaptation logic  803  in the form of buffer status information  808  (see  FIG. 8 ). If an onset frame has arrived, a required buffering time Tb for the onset frame is estimated in a step  901 . This may for instance be performed by adaptation control logic  803  based on network analyzer information  807  obtained from network analyzer  802  (see  FIG. 8 ). 
         [0073]    In a step  902 , an output point of time Td is computed as the sum of the current point of time T and the estimated required buffering time Tb. 
         [0074]    In a step  903 , it is checked if the output point of time Td has elapsed. If this is the case, in a step  904 , it is checked if the time span Tr covered by all frames contained in buffer  800  is smaller than the estimated required buffering time Tb minus a first margin value Tm 1 , which may be either constant or adaptive. Therein, it is assumed that frame buffer  800  only contains “new” frames, i.e. frames that are subsequent in decoding order with respect to the onset frame. This may imply that “old” frames that have already been decoded have been removed from frame buffer  800 . If “old” frames are at least temporarily kept in frame buffer  800 , the time span may for instance be considered to be related only to the onset frame and the frames that are subsequent in playback order with respect to the onset frame. The information on the time span or information required for computing the time span may for instance be signaled to the adaptation control logic in the form of buffer status information  808  (see  FIG. 8 ). If the check in step  904  is positive, in a step  905 , the output point of time Td is increased by a value Ta, which may either be constant or adaptively determined, and the flowchart jumps back to step  903 . Therein, the value of Tm 1  could for instance be adapted to a smaller value each time Td is increased. This may help avoiding excessive buffering delay in case of a long loss burst (if one keeps pushing Td forward in time, but no new frames come in to increase the value of Tr, one might end up adding more buffering delay until the first frame after a lost burst arrives). 
         [0075]    If the check in step  904  is negative, the immediate output of the onset frame for decoding by speech decoder  801  (see  FIG. 8 ) is triggered in a step  906 . By triggering the output of the onset frame, also the buffering time of the onset frame is determined. 
         [0076]    If the check if the output point of time Td has elapsed in step  903  is negative, it is checked in step  907  if the time span Tr covered by the frames contained in the buffer is larger than the estimated required buffering time Tb plus a second margin value Tm 2 , which may be either constant or adaptively determined. If the check in step  907  is negative, the flowchart jumps back to step  903 . If the check in step  907  is positive, the immediate output of the onset frame for decoding by speech decoder  801  (see  FIG. 8 ) is triggered in a step  906 . 
         [0077]    As can be seen from flowchart  9  of  FIG. 9 , according to this first exemplary embodiment of the present invention, the buffering time of the onset frame is determined under consideration of an estimated required buffering time Tb for the onset frame, and on the time span Tr covered by a number of frames contained in buffer  800  of  FIG. 8 . Instead of immediately triggering the output of the onset frame when the check in step  903  is positive, the flowchart  9  performs further checks that consider knowledge of the time span Tr. 
         [0078]    The processing of the adaptation control logic  803  then takes the following shape: At the start of a talkspurt region, buffer  800  is populated with incoming packets/frames of speech data. When the decoder timeline reaches the determined output point of time Td (step  903 ), the jitter buffer management system notes how many frames are stored in buffer  800  and determines the time span covered by these speech frames. If the network has experienced delays, then buffer  800  will not be as full as it should be. In this case the time span covered by these speech frames may be less than the estimated required buffering time Tb minus some margin value Tm 1  (step  904 ). The time line is pushed back by a value Ta (step  905 ), which effectively delays the decoding of the onset speech frame by this value. This then gives the opportunity for buffer  800  to fill up with further packets/frames from the network. This “loop” is continued until there are enough packets/frames of speech data that cover a time span that is large enough to satisfy the estimated required buffering time Tb. Using this technique may allow better control of underflows, which results in an improvement in speech quality for packet based systems. 
         [0079]    Alternatively, in network conditions where frames arrive in bursts, the time span Tr occupied by the frames in buffer  800  may be greater than the estimated required buffering time Tb plus some margin Tm 2  (step  907 ). The decoding will commence. This effectively pushes the timeline of the decoder forward to a point in time before the output point of time Td has been reached. Using this technique results in keeping the delay of the decoder time line to a minimum, thereby allowing the end to end speech communication delay to be as small a possible. Furthermore, the risk of buffer overflow is reduced. 
         [0080]      FIG. 10  illustrates the enhancement that is achieved by the first exemplary embodiment of the present invention in case of an early arriving onset frame (compare to  FIG. 6 ). As a result, instead of using the estimated required buffering time Tb for the onset speech frame, an increased buffering time is used, so that, compared to the jitter buffer management approach of  FIG. 6 , which only considers the estimated required buffering time Tb and not additionally the time span Tr, all frame losses can be avoided. 
         [0081]      FIG. 11  is a flowchart  11  of a second exemplary embodiment of jitter buffer management according to the present invention. The steps of this flowchart  11  may for instance be performed by adaptation control logic  803  of  FIG. 8  to determine a buffering time for an onset frame. 
         [0082]    In a first step  1100 , it is checked if an onset speech frame has arrived at buffer  800  (see  FIG. 8 ). If this is not the case, the flowchart jumps back to step  1100 ; otherwise, a target number of frames Nr is determined in a step  1101 , for instance based on network analyzer information  802  received from network analyzer  802  (see  FIG. 8 ). The target number of frames Nr implies Tr milliseconds of jitter protection (i.e. Tr=Nr*frame duration in milliseconds). 
         [0083]    In a step  1102 , it is then checked if the number of all frames contained in buffer  800  has reached the target number of frames Nr. Therein, it is assumed that frame buffer  800  only contains “new” frames, i.e. frames that are subsequent in decoding order with respect to the onset frame. This may imply that “old” frames that have already been decoded have been removed from frame buffer  800 . If “old” frames are at least temporarily kept in frame buffer  800 , the time span may for instance be considered to be related only to the onset frame and the frames that are subsequent in playback order with respect to the onset frame. 
         [0084]    If the check in step  1102  is positive, it is checked in a step  1103 , if Tr is larger than a value Tb plus a third margin value Tm 3 , which may either be constant or adaptively determined. Therein, value Tb is the time elapsed since the reception of the onset frame in buffer  800 . It should be noted that value Tb represents a buffering time, but, with respect to the value Tb used in the first exemplary embodiment has a different meaning. 
         [0085]    If the check in step  1103  is positive, the target number of frames Nr is increased in a step  1104 , for instance by one frame, and the flowchart jumps back to step  1102 . Otherwise, the immediate output of the onset frame for decoding by speech decoder  801  (see  FIG. 8 ) is triggered in a step  1105 . By triggering the output of the onset frame, also the buffering time of the onset frame is determined. 
         [0086]    If the check in step  1102  is negative, it is checked in a step  1106  if the value Tr is smaller than the value Tb minus a fourth margin value Tm 4 . If this is not the case, the flowchart jumps back to step  1102 . Otherwise, the immediate output of the onset frame for decoding by speech decoder  801  (see  FIG. 8 ) is triggered in a step  1105 . 
         [0087]    As can be seen from flowchart  11  of  FIG. 11 , according to this second exemplary embodiment of the present invention, the buffering time of the onset frame is determined under consideration of information on a number of frames in buffer  800  and information on the buffering time Tb of the onset frame. Instead of immediately triggering the output of the onset frame when the check in step  1102  is positive, the flowchart  11  performs further checks that consider knowledge of the buffering time Tb of the onset frame. 
         [0088]    The processing of the adaptation control logic  803  then takes the following shape: jitter buffer management sets out from a target number of frames Nr. This number has an equivalent time value Tr, which is the estimated buffer delay in time (based on the number of frames). 
         [0089]    The value Tr is used to delay the decoder timeline. So in an ideal world where the speech frames arrive at regular intervals (e.g. 20 ms) from the network, the filling up rate of buffer  800  would be equal to the time which the decoder timeline has to be delayed, therefore in this case Tb=Tr. 
         [0090]    However, if a burst of speech frames is received in buffer  800  at the start of talk spurt region, then buffer  800  will fill up at a faster rate than the required delay value Tr in the decoder time line. So in this case the buffering time needs to be increased to accommodate the extra frames arriving before the target delay Tr has been reached in the decoder timeline. The buffer size will increase by one frame. Note that the actual value of Tr is not updated when this test and calculation loop is performed. This methodology effectively allows the jitter buffer management to handle a burst in the number of packets delivered by the network. 
         [0091]    Alternatively, the network may be delivering packets/frames at a slower rate at the beginning of a talk spurt region. In this case buffer  800  will be filling up (to the required buffer depth) at a slower rate than which the decoder timeline is advancing. In this case in order to avoid excessive jitter buffer delay, the decoder playback will commence as soon as Tb&gt;Tr+Tm 4 . 
         [0092]      FIG. 12  illustrates the enhancement that is achieved by the second exemplary embodiment of the present invention in case of an early arriving trigger frame (compare to  FIG. 4 ). As a result, the buffering time is increased when jitter buffer management jitter buffer management waits for the desired buffering time to elapse. Compared to  FIG. 4 , thus due to the additional consideration of the buffering time Tb and its comparison to Tr, the buffering time is increased and all frame losses can be avoided. 
         [0093]      FIG. 13 , on the other hand, presents the enhancement that is achieved by the second exemplary embodiment of the present invention in case of a late arriving trigger frame (compare to  FIG. 5 ). Here, the buffering delay can be significantly reduced compared to  FIG. 5  by additionally considering the buffering time Tb and comparing it to Tr. 
         [0094]    The proposed embodiments of the present invention provide computationally simple mechanisms to improve the buffering performance (in terms of lower loss rate and/or shorter buffering delay), especially in challenging transmission conditions introducing large jitter and/or high numbers of lost packets. 
         [0095]    The invention has been described above by means of exemplary embodiments. It should be noted that there are alternative ways and variations which are obvious to a skilled person in the art and can be implemented without deviating from the scope and spirit of the appended claims. 
         [0096]    Furthermore, it is readily clear for a skilled person that the logical blocks in the schematic block diagrams as well as the flowchart and algorithm steps presented in the above description may at least partially be implemented in electronic hardware and/or computer software, wherein it depends on the functionality of the logical block, flowchart step and algorithm step and on design constraints imposed on the respective devices to which degree a logical block, a flowchart step or algorithm step is implemented in hardware or software. The presented logical blocks, flowchart steps and algorithm steps may for instance be implemented in one or more digital signal processors, application specific integrated circuits, field programmable gate arrays or other programmable devices. The computer software may be stored in a variety of storage media of electric, magnetic, electro-magnetic or optic type and may be read and executed by a processor, such as for instance a microprocessor. To this end, the processor and the storage medium may be coupled to interchange information, or the storage medium may be included in the processor.