Packet loss concealment for a sub-band predictive coder based on extrapolation of excitation waveform

Systems and methods are described for performing packet loss concealment using an extrapolation of an excitation waveform in a sub-band predictive speech coder, such as an ITU-T Recommendation G.722 wideband speech coder. The systems and methods are useful for concealing the quality-degrading effects of packet loss in a sub-band predictive coder and address some sub-band architectural issues when applying excitation extrapolation techniques to such sub-band predictive coders.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to systems and methods for concealing the quality-degrading effects of packet loss in a speech or audio coder.

2. Background Art

In digital transmission of voice or audio signals through packet networks, the encoded voice/audio signals are typically divided into frames and then packaged into packets, where each packet may contain one or more frames of encoded voice/audio data. The packets are then transmitted over the packet networks. Sometimes some packets are lost, and sometimes some packets arrive too late to be useful, and therefore are deemed lost. Such packet loss will cause significant degradation of audio quality unless special techniques are used to conceal the effects of packet loss. There exist prior-art packet loss concealment methods for full-band predictive coders based on an extrapolation of the excitation signal, which is sometimes also referred to as the prediction residual signal. For example, see U.S. Pat. No. 5,615,298 to Chen, entitled “Excitation Signal Synthesis during Frame Erasure or Packet Loss.” However, issues arise when such techniques are applied to sub-band predictive coders such as the ITU-T Recommendation G.722 wideband speech coder due at least in part to the architecture of those coders. A sub-band predictive coder first splits an input signal into different frequency bands using an analysis filter bank and then applies predictive coding to each of the sub-band signals. At the decoder side, the decoded sub-band signals are recombined in a synthesis filter bank into a full-band output signal.

SUMMARY OF THE INVENTION

Embodiments of the present invention may be used to conceal the quality-degrading effects of packet loss (or frame erasure) in a sub-band predictive coder. Embodiments of the present invention address sub-band architectural issues when applying excitation extrapolation techniques to such sub-band predictive coders.

In particular, a system for replacing a portion of an audio signal that is deemed lost in a sub-band predictive coder is described herein. The system includes a first excitation extrapolator, a second excitation extrapolator, a first synthesis filter, a second synthesis filter, and a synthesis filter bank. The first excitation extrapolator is configured to generate a first sub-band extrapolated excitation signal based on a first sub-band excitation signal associated with one or more previously-received portions of the audio signal. The second excitation extrapolator is configured to generate a second sub-band extrapolated excitation signal based on a second sub-band excitation signal associated with one or more previously-received portions of the audio signal. The first synthesis filter is configured to filter the first sub-band extrapolated excitation signal to generate a synthesized first sub-band audio signal. The second synthesis filter is configured to filter the second sub-band extrapolated excitation signal to generate a synthesized second sub-band audio signal. The synthesis filter bank is configured to combine at least the synthesized first sub-band audio signal and the synthesized second sub-band audio signal to generate a full-band output audio signal corresponding to the portion of the audio signal that is deemed lost.

The foregoing system may further include a first decoder and a second decoder. The first decoder is configured to decode a first sub-band bit-stream associated with a portion of the audio signal that is not deemed lost and the second decoder is configured to decode a second sub-band bit-stream associated with the portion of the audio signal that is not deemed lost. The first decoder may be a low-band adaptive pulse code modulation (ADPCM) decoder and the second decoder may be a high-band ADPCM decoder. The first synthesis filter may be a low-band ADPCM decoder synthesis filter and the second synthesis filter may be a high-band ADPCM decoder synthesis filter.

A method for replacing a portion of an audio signal that is deemed lost in a sub-band predictive coder is also described herein. In accordance with the method, a first sub-band extrapolated excitation signal is generated based on a first sub-band excitation signal associated with one or more previously-received portions of the audio signal. A second sub-band extrapolated excitation signal is generated based on a second sub-band excitation signal associated with one or more previously-received portions of the audio signal. The first sub-band extrapolated excitation signal is filtered in a first synthesis filter to generate a synthesized first sub-band audio signal. The second sub-band extrapolated excitation signal is filtered in a second synthesis filter to generate a synthesized second sub-band audio signal. At least the synthesized first sub-band audio signal and the synthesized second sub-band audio signal are combined to generate a full-band output audio signal corresponding to the portion of the audio signal that is deemed lost.

The foregoing method may further include decoding a first sub-band bit-stream associated with a portion of the audio signal that is not deemed lost in a first decoder and decoding a second sub-band bit-stream associated with the portion of the audio signal that is not deemed lost in a second decoder. The first decoder may be a low-band ADPCM decoder and the second decoder may be a high-band ADPCM decoder. The first synthesis filter may be a low-band ADPCM decoder synthesis filter and the second synthesis filter may be a high-band ADPCM decoder synthesis filter.

An alternative system for replacing a portion of an audio signal that is deemed lost in a sub-band predictive coder is also described herein. The system includes a first synthesis filter bank, a full-band excitation extrapolator, an analysis filter bank, a first synthesis filter, a second synthesis filter, and a second synthesis filter bank. The first synthesis filter bank is configured to combine at least a first sub-band excitation signal associated with one or more previously-received portions of the audio signal and a second sub-band excitation signal associated with one or more previously-received portions of the audio signal to generate a full-band excitation signal. The full-band excitation extrapolator is configured to receive the full-band excitation signal and generate a full-band extrapolated excitation signal therefrom. The analysis filter bank is configured to split the full-band extrapolated excitation signal into at least a first sub-band extrapolated excitation signal and a second sub-band extrapolated excitation signal. The first synthesis filter is configured to filter the first sub-band extrapolated excitation signal to generate a synthesized first sub-band audio signal. The second synthesis filter is configured to filter the second sub-band extrapolated excitation signal to generate a synthesized second sub-band audio signal. The second synthesis filter bank is configured to combine at least the synthesized first sub-band audio signal and the synthesized second sub-band audio signal to generate a full-band output audio signal corresponding to the portion of the audio signal that is deemed lost.

The foregoing system may further include a first decoder and a second decoder. The first decoder is configured to decode a first sub-band bit-stream associated with a portion of the audio signal that is not deemed lost and the second decoder is configured to decode a second sub-band bit-stream associated with the portion of the audio signal that is not deemed lost. The first decoder may be a low-band ADPCM decoder and the second decoder may be a high-band ADPCM decoder. The first synthesis filter may be a low-band ADPCM decoder synthesis filter and the second synthesis filter may be a high-band ADPCM decoder synthesis filter.

An alternative method for replacing a portion of an audio signal that is deemed lost in a sub-band predictive coder is also described herein. In accordance with this alternative method, at least a first sub-band excitation signal associated with one or more previously-received portions of the audio signal and a second sub-band excitation signal associated with one or more previously-received portions of the audio signal are combined to generate a full-band excitation signal. A full-band extrapolated excitation signal is then generated based on the full-band excitation signal. The full-band extrapolated excitation signal is then split into at least a first sub-band extrapolated excitation signal and a second sub-band extrapolated excitation signal. The first sub-band extrapolated excitation signal is filtered in a first synthesis filter to generate a synthesized first sub-band audio signal. The second sub-band extrapolated excitation signal is filtered in a second synthesis filter to generate a synthesized second sub-band audio signal. At least the synthesized first sub-band audio signal and the synthesized second sub-band audio signal are then combined to generate a full-band output audio signal corresponding to the portion of the audio signal that is deemed lost.

The foregoing method may further include decoding a first sub-band bit-stream associated with a portion of the audio signal that is not deemed lost in a first decoder and decoding a second sub-band bit-stream associated with the portion of the audio signal that is not deemed lost in a second decoder. The first decoder may be a low-band ADPCM decoder and the second decoder may be a high-band ADPCM decoder. The first synthesis filter may be a low-band ADPCM decoder synthesis filter and the second synthesis filter may be a high-band ADPCM decoder synthesis filter.

Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the art based on the teachings contained herein.

The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.

DETAILED DESCRIPTION OF INVENTION

The following detailed description of the present invention refers to the accompanying drawings that illustrate exemplary embodiments consistent with this invention. Other embodiments are possible, and modifications may be made to the illustrated embodiments within the spirit and scope of the present invention. Therefore, the following detailed description is not meant to limit the invention. Rather, the scope of the invention is defined by the appended claims.

It will be apparent to persons skilled in the art that the present invention, as described below, may be implemented in many different embodiments of hardware, software, firmware, and/or the entities illustrated in the drawings. Any actual software code with specialized control hardware to implement the present invention is not limiting of the present invention. Thus, the operation and behavior of the present invention will be described with the understanding that modifications and variations of the embodiments are possible, given the level of detail presented herein.

It should be understood that while the detailed description of the invention set forth herein may refer to the processing of speech signals, the invention may be also be used in relation to the processing of other types of audio signals as well. Therefore, the terms “speech” and “speech signal” are used herein purely for convenience of description and are not limiting. Persons skilled in the relevant art(s) will appreciate that such terms can be replaced with the more general terms “audio” and “audio signal.” Furthermore, although speech and audio signals are described herein as being partitioned into frames, persons skilled in the relevant art(s) will appreciate that such signals may be partitioned into other discrete segments as well, including but not limited to sub-frames. Thus, descriptions herein of operations performed on frames are also intended to encompass like operations performed on other segments of a speech or audio signal, such as sub-frames.

Additionally, although the following description discusses the loss of frames of an audio signal transmitted over packet networks (termed “packet loss”), the present invention is not limited to packet loss concealment (PLC). For example, in wireless networks, frames of an audio signal may also be lost or erased due to channel impairments. This condition is termed “frame erasure.” When this condition occurs, to avoid substantial degradation in output speech quality, the decoder in the wireless system needs to perform “frame erasure concealment” (FEC) to try to conceal the quality-degrading effects of the lost frames. For a PLC or FEC algorithm, the packet loss and frame erasure amount to the same thing: certain transmitted frames are not available for decoding, so the PLC or FEC algorithm needs to generate a waveform to fill up the waveform gap corresponding to the lost frames and thus conceal the otherwise degrading effects of the frame loss. Because the terms FLC and PLC generally refer to the same kind of technique, they can be used interchangeably. Thus, for the sake of convenience, the term “packet loss concealment,” or PLC, is used herein to refer to both.

B. Review of Sub-Band Predictive Coding

In order to facilitate a better understanding of the various embodiments of the present invention described in later Sections, the basic principles of sub-band predictive coding are first reviewed here. In general, a sub-band predictive coder may split an input audio signal into N sub-bands where N≧2. Without loss of generality, the two-band predictive coding system of the ITU-T G.722 coder will be described here as an example. Persons skilled in the relevant art(s) will readily be able to generalize this description to any N-band sub-band predictive coder.

FIG. 1shows a simplified encoder structure100of a G.722 sub-band predictive coder. Encoder structure100includes an analysis filter bank110, a low-band adaptive differential pulse code modulation (ADPCM) encoder120, a high-band ADPCM encoder130and a bit-stream multiplexer140. Analysis filter bank110splits an input audio signal into a low-band audio signal and a high-band audio signal. The low-band audio signal is encoded by low-band ADPCM encoder120into a low-band bit-stream. The high-band audio signal is encoded by high-band ADPCM encoder130into a high-band bit-stream. Bit-stream multiplexer140multiplexes the low-band bit-stream and the high-band bit-stream into a single output bit-stream. In the packet transmission applications discussed herein, this output bit-stream is packaged into packets and then transmitted to a sub-band predictive decoder200, which is shown inFIG. 2.

As shown inFIG. 2, decoder200includes a bit-stream de-multiplexer210, a low-band ADPCM decoder220, a high-band ADPCM decoder230, and a synthesis filter bank240. Bit-stream de-multiplexer210separates the input bit-stream into the low-band bit-stream and the high-band bit-stream. Low-band ADPCM decoder220decodes the low-band bit-stream into a decoded low-band audio signal. High-band ADPCM decoder230decodes the high-band bit-stream into a decoded high-band audio signal. Synthesis filter bank240then combines the decoded low-band audio signal and the decoded high-band audio signal into the full-band output audio signal.

C. First Example Embodiment for Performing Packet Loss Concealment in a Sub-Band Predictive Coder Based on Extrapolation of an Excitation Waveform

FIG. 3is a block diagram of a system300in accordance with a first example embodiment of the present invention. For convenience, system300is described herein as part of an ITU-T G.722 coder, but persons skilled in the relevant art(s) will readily appreciate that the inventive ideas described herein may be generally applied to any N-band sub-band predictive coding system.

As shown inFIG. 3, system300includes a bit-stream de-multiplexer310, a low-band ADPCM decoder320, a low-band excitation extrapolator322, a low-band ADPCM decoder synthesis filter324, a first switch326, a high-band ADPCM decoder330, a high-band excitation extrapolator332, a high-band ADPCM decoder synthesis filter334, a second switch336, and a synthesis filter bank340. Bit-stream de-multiplexer310operates in essentially the same manner as bit-stream de-multiplexer210ofFIG. 2, and synthesis filter bank340operates in essentially the same manner as synthesis filter bank240ofFIG. 2.

The input bit-stream received by system300is partitioned into a series of frames. A frame received by system200may either be deemed “good,” in which case it is suitable for normal decoding, or “bad,” in which case it must be replaced. As described above, a “bad” frame may result from a packet loss.

If the frame that is received by system300is good, then low-band ADPCM decoder320decodes the low-band bit-stream normally into a decoded low-band audio signal. In this case, first switch326is connected to the upper position marked “good frame,” thus connecting the decoded low-band audio signal to synthesis filter bank340. Similarly, high-band ADPCM decoder330decodes the high-band bit-stream normally into a decoded high-band audio signal. In this case, second switch336is connected to the upper position marked “good frame,” thus connecting the decoded high-band audio signal to synthesis filter bank340. Hence, during good frames the system inFIG. 3operates in an essentially equivalent manner to system200ofFIG. 2with one exception—the low-band excitation signals of the signal are stored in low-band excitation extrapolator322for possible use in a future bad frame, and likewise the high-band excitation signals of the signal are stored in high-band excitation extrapolator332for possible use in a future bad frame.

If the frame that is received by system300is bad, then the excitation signal of each sub-band is individually extrapolated from the previous good frames to fill up the gap in the current bad frame. This function is performed by low-band excitation extrapolator322and high-band excitation extrapolator332. There are many excitation extrapolation methods that are well-known in the art. U.S. Pat. No. 5,615,298 provides an example of one such method and is incorporated by reference herein. In general, for voiced frames where the speech waveform is nearly periodic, the excitation waveform also tends to be somewhat periodic and therefore can be extrapolated in a periodic manner to maintain the periodic nature. For unvoiced frames where the speech waveform appears more like noise, the excitation signal also tends to be noise-like, and in this case the excitation waveform can be obtained using a random noise generator with proper scaling. In a transition region of speech, a mixture of periodic extrapolation and noise generator output can be used.

The extrapolated excitation signal of each sub-band is passed through the synthesis filter of the predictive decoder of that sub-band to obtain the reconstructed audio signal for that sub-band. Specifically, the extrapolated low-band excitation signal at the output of low-band excitation extrapolator322is passed through low-band ADPCM decoder synthesis filter324to obtain a synthesized low-band audio signal. Similarly, the extrapolated high-band excitation signal at the output of high-band excitation extrapolator332is passed through high-band ADPCM decoder synthesis filter334to obtain a synthesized high-band audio signal.

During processing of a bad frame, first switch326and second switch336are both at the lower position marked “bad frame.” Thus, they will connect the synthesized low-band audio signal and the synthesized high-band audio signal to synthesis filter bank340, which combines them into a synthesized output audio signal for the current bad frame.

Before the system inFIG. 3completes the processing for a bad frame, it needs to perform at least one more task: updating the internal states of low-band ADPCM decoder320and high-band ADPCM decoder330. Such internal states include filter coefficients, filter memory, and a quantizer step size. This operation of updating the internal states of each sub-band ADPCM decoder is shown inFIG. 3as dotted arrows from low-band ADPCM decoder synthesis filter324to low-band ADPCM decoder320and from high-band ADPCM decoder synthesis filter334to high-band ADPCM decoder330. There are many possible methods for performing this task as will be understood by persons skilled in the art.

A first exemplary technique for updating the internal states of sub-band ADPCM decoders320and330is to pass the reconstructed sub-band signal through the corresponding ADPCM encoder of that sub-band (blocks120and130inFIG. 1, respectively). Since each sub-band ADPCM encoder has the same internal states as the corresponding sub-band ADPCM decoder, after encoding the entire current reconstructed frame of the synthesized sub-band signal (the output of either low-band ADPCM decoder synthesis filter324or high-band ADPCM decoder synthesis filter334), the filter coefficients, filter memory, and quantizer step size left at the end of encoding the entire reconstructed frame of synthesized sub-band signal is used to update the corresponding internal states of the ADPCM decoder of that sub-band.

Alternatively, in a second exemplary technique, the extrapolated excitation signal of each sub-band can go through the normal quantization procedure and the normal decoder filtering and decoder filter coefficients updates in order to update the internal states of the ADPCM decoder of that sub-band. In this case, rather than performing an update of such internal states in a separate step, a more efficient approach is to quantize the extrapolated sub-band excitation signal and use the quantized extrapolated excitation signal to drive the sub-band decoder synthesis filter (low-band ADPCM decoder synthesis filter324or high-band ADPCM decoder synthesis filter334) while at the same time updating the filter coefficients following the same coefficient update method used in low-band ADPCM decoder320and high-band ADPCM decoder330. This way, the updating of the internal states will be performed as a by-product of performing the task of low-band ADPCM decoder synthesis filter324and high-band ADPCM decoder synthesis filter334.

There are other methods for updating the internal states. For example, for certain situations or signal segments it may be better to use an averaged version of previous states in previous good frames to update the internal states at the end of the current bad frame, and in some other situations (for example, in a packet loss with very long duration), it may be better to reset all internal states of each sub-band ADPCM decoder to their initial states.

After the internal states of sub-band predictive decoders320and330are properly updated at the end of a bad frame, the system is then ready to begin processing of the next frame, regardless of whether it is a good frame or a bad frame.

To further illustrate this first example embodiment,FIG. 4illustrates a flowchart400of a method by which system300operates to process a single frame of an input bit-stream. As shown inFIG. 4, the method of flowchart400begins at step402, in which system300receives a frame of the input bit-stream. At decision step404, system300determines whether the frame is good or bad. If the frame is good, then a number of steps are performed starting with step406. If the frame is bad, then a number of steps are performed starting with step416.

The series of steps that are performed starting with step406in response to receiving a good frame will now be described. At step406, bit-stream de-multiplexer310de-multiplexes a bit-stream associated with the good frame into a low-band bit-stream and a high-band bit-stream. At step408, low-band ADPCM decoder320normally decodes the low-band bit-stream to generate a decoded low-band audio signal. At step410, high-band ADPCM decoder330normally decodes the high-band bit-stream to generate a decoded high-band audio signal. At step412, synthesis filter bank340combines the decoded low-band audio signal and the decoded high-band audio signal to generate a full-band output audio signal. At step414, low-band excitation signals associated with the current frame are stored in low-band excitation extrapolator322for possible use in a future bad frame and high-band excitation signals associated with current frame are stored in high-band excitation extrapolator332for possible use in a future bad frame. After step414, processing associated with the good frame ends, as shown at step428.

The series of steps that are performed starting with step416in response to receiving a bad frame will now be described. At step416, low-band excitation extrapolator322extrapolates a low-band excitation signal based on low-band excitation signal(s) associated with one or more previous frames processed by system300. At step418, high-band excitation extrapolator332extrapolates a high-band excitation signal based on high-band excitation signal(s) associated with one or more previous frames processed by system300. At step420, the low-band extrapolated excitation signal is passed through low-band ADPCM decoder synthesis filter324to obtain a synthesized low-band audio signal. At step422, the high-band extrapolated excitation signal is passed through high-band ADPCM decoder synthesis filter334to obtain a synthesized high-band audio signal. At step424, synthesizer filter bank340combines the synthesized low-band audio signal and the synthesized high-band audio signal to generate a full-band output audio signal. At step426, the internal states of low-band ADPCM decoder320and high-band ADPCM decoder330are updated. After step426, processing associated with the bad frame ends, as shown at step428.

D. Second Example Embodiment for Performing Packet Loss Concealment in a Sub-Band Predictive Coder Based on Extrapolation of an Excitation Waveform

In a second example embodiment, sub-band excitation signals associated with one or more previously-received good frames (which are stored in buffers) are first passed through a synthesis filter bank to obtain a full-band excitation signal for the previously-received good frame(s), and then extrapolation is performed on this full-band excitation signal to fill the gap associated with a current bad frame. This full-band extrapolated excitation signal is then passed through an analysis filter bank to split it into sub-band extrapolated excitation signals, which are then passed through sub-band decoder synthesis filters and eventually a synthesis filter bank to produce an output audio signal. The rest of the steps for updating the internal states of the predictive decoder of each sub-band may be performed in a like manner to that described in reference to the first example embodiment above.

A block diagram of this second example embodiment of the present invention is shown inFIG. 5. In the system500shown inFIG. 5, like-numbered blocks perform the same functions as inFIG. 3. For example, blocks520and530perform the same functions as block320and330, respectively. Again,FIG. 5shows only an exemplary system according to a second example embodiment of the present invention. Those skilled in the art will appreciate that the sub-band predictive coding system can be an N-band system rather than the two-band system shown inFIG. 5, where N can be an integer greater than 2. Similarly, the predictive coder for each sub-band does not have to be an ADPCM coder as shown inFIG. 5, but can be any general predictive coder, and can be either forward-adaptive or backward-adaptive.

Refer now toFIG. 5. When system500is processing a good frame, switches526and536are both in the upper position labeled “good frame,” and a bit-stream de-multiplexer510, a low-band ADPCM decoder520, a high-band ADPCM decoder530, and a synthesis filter bank540operate in essentially the same manner as bit-stream de-multiplexer310, low-band ADPCM decoder320, high-band ADPCM decoder330, and synthesis filter bank540, respectively, to decode the input bit-stream normally. In addition, a low-band excitation signal produced in low-band ADPCM decoder520during good frames is stored in a low-band excitation buffer540. Likewise, a high-band excitation signal produced in the high-band ADPCM decoder530during good frames is stored in a high-band excitation buffer550.

When system500is processing a bad frame, switches526and536are both in the lower position labeled “bad frame.” In this case, a synthesis filter bank560receives a low-band excitation signal from low-band excitation buffer540and a high-band excitation signal from high-band excitation buffer550, and combines the two sub-band excitation signals into a full-band excitation signal. A full-band excitation extrapolator570then receives this full-band excitation signal and extrapolates it to fill up the gap associated with the current bad frame. In an embodiment, full-band excitation extrapolator570extrapolates the signal beyond the end of the current bad frame in order to compensate for inherent filtering delays in synthesis filter bank560and an analysis filter bank580. Analysis filter bank580then splits this full-band extrapolated excitation signal into a low-band extrapolated excitation signal and a high-band extrapolated excitation signal, in the same way the analysis filter bank110ofFIG. 1performs its band-splitting function.

A low-band ADPCM decoder synthesis filter524then filters the low-band extrapolated excitation signal to produce a synthesized low-band audio signal, and high-band ADPCM decoder synthesis filter534then filters the high-band extrapolated excitation signal to produce a high-band synthesized audio signal. These two sub-band audio signals pass through switches526and536to reach the synthesis filter bank440, which then combines these two sub-band audio signals into a full-band output audio signal.

Like system300ofFIG. 3, in system500ofFIG. 5the internal states of low-band ADPCM decoder520and high-band ADPCM decoder530need to be updated to proper values before the normal decoding of the next good frame starts, otherwise significant distortion may result. The update of the internal states of low-band ADPCM decoder520and high-band ADPCM decoder530can be performed using one of the methods outlines in the description of the first example embodiment above.

To further illustrate this second example embodiment,FIG. 6illustrates a flowchart600of a method by which system500operates to process a single frame of an input bit-stream. As shown inFIG. 6, the method of flowchart600begins at step602, in which system500receives a frame of the input bit-stream. At decision step604, system500determines whether the frame is good or bad. If the frame is good, then a number of steps are performed starting with step606. If the frame is bad, then a number of steps are performed starting with step616.

The series of steps that are performed starting with step606in response to receiving a good frame will now be described. At step606, bit-stream de-multiplexer510de-multiplexes a bit-stream associated with the good frame into a low-band bit-stream and a high-band bit-stream. At step608, low-band ADPCM decoder520normally decodes the low-band bit-stream to generate a decoded low-band audio signal. At step610, high-band ADPCM decoder530normally decodes the high-band bit-stream to generate a decoded high-band audio signal. At step612, synthesis filter bank540combines the decoded low-band audio signal and the decoded high-band audio signal to generate a full-band output audio signal. At step614, a low-band excitation signal associated with the current frame is stored in low-band excitation buffer540for possible use in a future bad frame and a high-band excitation signal associated with current frame is stored in high-band excitation buffer550for possible use in a future bad frame. After step614, processing associated with the good frame ends, as shown at step630.

The series of steps that are performed starting with step616in response to receiving a bad frame will now be described. At step616, synthesis filter bank560receives a low-band excitation signal from low-band excitation buffer540and a high-band excitation signal from high-band excitation buffer550, and combines the two sub-band excitation signals into a full-band excitation signal. At step618, full-band excitation extrapolator570receives this full-band excitation signal and extrapolates it to generate a full-band extrapolated excitation signal. At step620, analysis filter bank580splits the extrapolated full-band excitation signal into a low-band extrapolated excitation signal and a high-band extrapolated excitation signal. At step622, low-band ADPCM decoder synthesis filter524filters the low-band extrapolated excitation signal to produce a synthesized low-band audio signal, and at step624, high-band ADPCM decoder synthesis filter534filters the high-band extrapolated excitation signal to produce a high-band synthesized audio signal. At step626, synthesis filter bank640combines the two synthesized sub-band audio signals into a full-band output audio signal. At step628, the internal states of low-band ADPCM decoder520and high-band ADPCM decoder530are updated. After step628, processing associated with the bad frame ends, as shown at step630.

The main differences between the embodiments ofFIG. 5andFIG. 3are the addition of synthesis filter bank560and analysis filter bank580, and the fact that the excitation signal is now extrapolated in the full-band domain rather than the sub-band domain. The addition of synthesis filter bank560and analysis filter bank580can potentially add significant computational complexity. However, extrapolating the excitation signal in the full-band domain provides an advantage. This is explained below.

When system300ofFIG. 3extrapolates the high-band excitation signal, there are some potential issues. First, if it does not perform periodic extrapolation for the high-band excitation signal, then the output audio signal will not preserve the periodic nature of the high-band audio signal that can be present in some highly periodic voiced signals. On the other hand, if it performs periodic extrapolation for the high-band excitation signal, even if it uses the same pitch period as used in the extrapolation of the low-band excitation signal to save computation and to ensure that the two sub-band excitation signals are using the same pitch period for extrapolation, there is still another problem. When the high-band excitation signal is extrapolated periodically, the extrapolated high-band excitation signal will be periodic and will have a harmonic structure in its spectrum. In other words, the frequencies of the spectral peaks in the spectrum of the high-band excitation signal will be related by integer multiples. After this high-band excitation signal is passed through high-band ADPCM decoder synthesis filter334, the spectral peaks of the resulting high-band audio signal will still be harmonically related. However, once this high-band audio signal is re-combined with the low-band audio signal by the synthesis filter bank340, the spectrum of the high-band audio signal will be “translated” or shifted to the higher frequency, possibly even with mirror imaging taking place. Thus, after such mirror imaging and frequency shifting, there is no guarantee that the spectral peaks in the high band portion of the full-band output audio signal will have frequencies that are still integer multiples of the pitch frequency in the low-band signal. This can potentially cause degradation in the output audio quality of highly periodic voiced signals. In contrast, system500inFIG. 5will not have this problem. Since system500performs the excitation signal extrapolation in the full-band domain, the frequencies of the harmonic peaks in the high band is guaranteed to be an integer multiple of the pitch frequency.

In summary, the advantage of this second example embodiment is that for voiced signals the extrapolated full-band excitation signal and the final full-band output audio signal will preserve the harmonic structure of spectral peaks. On the other hand, the first example embodiment has the advantage of lower complexity, but it may not preserve such harmonic structure in the higher sub-bands.

E. Hardware and Software Implementations

The following description of a general purpose computer system is provided for the sake of completeness. The present invention can be implemented in hardware, or as a combination of software and hardware. Consequently, the invention may be implemented in the environment of a computer system or other processing system. An example of such a computer system700is shown inFIG. 7. In the present invention, all of the steps ofFIGS. 4 and 6, for example, can execute on one or more distinct computer systems700, to implement the various methods of the present invention.

Computer system700includes one or more processors, such as processor704. Processor704can be a special purpose or a general purpose digital signal processor. The processor704is connected to a communication infrastructure702(for example, a bus or network). Various software implementations are described in terms of this exemplary computer system. After reading this description, it will become apparent to a person skilled in the relevant art(s) how to implement the invention using other computer systems and/or computer architectures.

Computer system700also includes a main memory706, preferably random access memory (RAM), and may also include a secondary memory720. The secondary memory720may include, for example, a hard disk drive722and/or a removable storage drive724, representing a floppy disk drive, a magnetic tape drive, an optical disk drive, or the like. The removable storage drive724reads from and/or writes to a removable storage unit728in a well known manner. Removable storage unit728represents a floppy disk, magnetic tape, optical disk, or the like, which is read by and written to by removable storage drive724. As will be appreciated, the removable storage unit728includes a computer usable storage medium having stored therein computer software and/or data.

In alternative implementations, secondary memory720may include other similar means for allowing computer programs or other instructions to be loaded into computer system700. Such means may include, for example, a removable storage unit730and an interface726. Examples of such means may include a program cartridge and cartridge interface (such as that found in video game devices), a removable memory chip (such as an EPROM, or PROM) and associated socket, and other removable storage units730and interfaces726which allow software and data to be transferred from the removable storage unit730to computer system700.

Computer system700may also include a communications interface740. Communications interface740allows software and data to be transferred between computer system700and external devices. Examples of communications interface740may include a modem, a network interface (such as an Ethernet card), a communications port, a PCMCIA slot and card, etc. Software and data transferred via communications interface740are in the form of signals which may be electronic, electromagnetic, optical, or other signals capable of being received by communications interface740. These signals are provided to communications interface740via a communications path742. Communications path742carries signals and may be implemented using wire or cable, fiber optics, a phone line, a cellular phone link, an RF link and other communications channels.

As used herein, the terms “computer program medium” and “computer usable medium” are used to generally refer to media such as removable storage units728and730, a hard disk installed in hard disk drive722, and signals received by communications interface740. These computer program products are means for providing software to computer system700.

Computer programs (also called computer control logic) are stored in main memory706and/or secondary memory720. Computer programs may also be received via communications interface740. Such computer programs, when executed, enable the computer system700to implement the present invention as discussed herein. In particular, the computer programs, when executed, enable the processor700to implement the processes of the present invention, such as any of the methods described herein. Accordingly, such computer programs represent controllers of the computer system700. Where the invention is implemented using software, the software may be stored in a computer program product and loaded into computer system700using removable storage drive724, interface726, or communications interface740.

In another embodiment, features of the invention are implemented primarily in hardware using, for example, hardware components such as application-specific integrated circuits (ASICs) and gate arrays. Implementation of a hardware state machine so as to perform the functions described herein will also be apparent to persons skilled in the relevant art(s).