Patent Publication Number: US-2009220094-A1

Title: Processing circuit capable of modifying digital audio signals

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. application Ser. No. 10/707,858 filed Jan. 19, 2004, and incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an audio processing circuit, and more particularly, to an audio processing circuit capable of modifying digital audio signals that are appropriate for transmitting to other digital audio systems. 
     2. Description of the Prior Art 
       FIG. 1  illustrates the format of a stream according to the IEC 60958 standard. In the IEC 61937 standard, an interface format is defined for non-linear pulse-code modulation (PCM) encoded audio streams using the IEC 60958 standard. This IEC digital interface standard is also called S/PDIF (Sony/Philips Digital Interface). The IEC digital interface standard can be used for transmitting non-linear pulse-code modulation samples, and also can be used for transmitting data. Each encoded audio stream includes a plurality of S/PDIF frames. Each S/PDIF frame includes S/PDIF subframes such as a data burst section and a stuffing section having several stuffing bits. The length of the data burst section varies, and the stuffing section keeps the length of the S/PDIF frame constant. Each data burst section includes a preamble and a payload section. The preamble includes header information Pa, Pb, Pc, and Pd. Pa and Pb represent synchronization words of the S/PDIF standard. Pc represents the burst information. The payload section contains the information of an encoded audio frame of the encoded audio stream, and has several fields such as sync word, header, side information, audio samples, ancillary data, etc. 
       FIG. 2  illustrates a block diagram of an audio processing circuit  10  of an optical disk drive in the prior art. The audio processing circuit  10  includes a parser  12 , a stream buffer  14 , an audio processor  16 , a second buffer  18 , a digital to analog converter  20 , an IEC burst circuit  22 , and a digital interface  24 . Digital data recorded on the optical storage disk  26  is retrieved and preliminarily processed by a servo controller (which is not shown in  FIG. 2 ), is then sent to the parser  12 . The parser  12  parses the digital data, and passes the digital audio signals to the stream buffer  14  in a form of an audio stream. The audio stream includes a plurality of audio frames. The audio processor  16  decodes the audio frames stored in the stream buffer  14 . The decoded information is then stored in the second buffer  18 . Finally, the digital to analog converter  20  converts the decoded information stored in the second buffer  18  into an analog signal as an output signal of the optical disk drive. As the user probably desires using an external decoding/amplifying device (ex. the post-stage audio receiver  28  illustrated in  FIG. 2 ) for digital audio signal processing rather than using the internal audio processing circuit  10  incorporated inside the optical disk drive, the audio processing circuit  10  of the optical disk drive generally provides not only the above-mentioned decoding apparatus for reproducing the analog audio data which is digitally recorded on the optical storage disk  26  but also an digital interface  24  for connecting the optical disk drive to a post-stage audio receiver  28 . As mentioned, the digital data recorded on the optical storage disk  26  received by the parser  12  is sent to the stream buffer  14  in the form of the audio stream and stored in the stream buffer  14 . The audio frames of the audio stream stored in the stream buffer  14  can be decoded as mentioned or transferred into a S/PDIF stream, a stream of IEC 61937/IEC 60958 standard, and the S/PDIF stream is then sent from the digital interface  24  to the external post-stage audio receiver  28 . The IEC burst circuit  22  in  FIG. 2  retrieves the audio frames stored in the stream buffer  14  and partitions the audio frames into payload sections of proper sizes. As shown in  FIG. 1 , the corresponding preamble is added in front of each payload to form a data burst section, and the corresponding stuffing section is then added next to each data burst section. The transferred stream complying with the S/PDIF standard is then formed and sent to the post-stage audio receiver  28  through the digital interface  24 . 
     As mentioned above, the audio frames derived from the digital data on the optical storage disk  26  is stored in the stream buffer  14 , and the audio frames stored in the stream buffer  14  can be decoded by the audio processor  16 . The decoded information is then stored in the second buffer  18 , and the digital to analog converter  20  converts the decoded information into an analog signal as the output signal of the optical disk drive. In addition, the optical disk drive can be connected to the post-stage audio receiver  28  through the digital interface  24 , which is an interface for outputting the transferred signal generated by the IEC burst circuit  22  into the post-stage audio receiver  28 . However, in the prior art the IEC burst circuit  22  simply transfers the digital audio data of the audio frames stored in the bit stream buffer  14  without checking the correctness of the digital audio data. If the digital audio data extracted from the stream buffer  14  does not completely comply with a predetermined digital audio standard such as MPEG audio standard, the post-stage audio receiver  28  may fail to properly decode the received digital data. For example, some MPEG audio bit streams are encoded by improper audio signal encoding softwares or hardwares, and do not strictly follow the MPEG audio standard. In the prior art such audio bit streams would be output to the post-stage audio receiver  28  through the digital interface  24  without any error-check, and the post-stage audio receiver  28  may fail to decode them properly and thus unpleasant blast sound may occur. 
     Some technical background information is disclosed in several US patents, including U.S. Pat. Nos. 5,794,181, 5,884,048, 6,122,619, 6,128,579, and 6,272,153. 
     SUMMARY OF THE INVENTION 
     It is therefore an objective of the present invention to provide an apparatus and a method for modifying digital audio signals to solve the above-mentioned problem. 
     The present invention provides an audio processing circuit for receiving a first stream complying with a first standard and generating a second stream complying with a second standard which is a digital interface standard, the first stream includes a plurality of frames, each of the frames includes a plurality of fields, the audio processing circuit comprises a stream buffer, a stream recovering circuit, and a digital interface. The stream buffer stores the frames of the first stream. The stream recovering circuit electrically connected to the stream buffer receives the first stream having a mode field, the candidate values of the mode field in the first standard at least comprising a dual mono mode and a stereo mode, and for processing the first stream to generate the second stream so that the mode field of the second stream is the stereo mode value if the mode field of the first stream contains the dual mono mode value. The digital interface outputs the second stream to a post-stage audio receiver. 
     The present invention further provides a method for converting a first stream complying with a first standard into a second stream complying with a second standard which is a digital interface standard, the first stream includes a plurality of frames, each of the frames includes a plurality of fields, the method comprises the steps of: receiving the first stream having a mode field with a stream recovering circuit, the candidate values of the mode field in the first standard at least comprising a dual mono mode and a stereo mode; processing the first stream to generate the second stream with the stream recovering circuit so that the mode field of the second stream is the stereo mode value if the mode field of the first stream contains the dual mono mode value; and outputting the second stream with a digital interface to a post-stage audio receiver. 
     The present invention further provides an optical disk drive, comprising a parser, a stream buffer, a stream recovering circuit, and a digital interface. The parser parses a first stream from an optical disk. The stream buffer stores frames of the first stream. The stream recovering circuit electrically connected to the stream buffer receives the first stream having a mode field, the candidate values of the mode field in the first standard at least comprising a dual mono mode and a stereo mode, and for processing the first stream to generate a second stream so that the mode field of the second stream is the stereo mode value if the mode field of the first stream contains the dual mono mode value, the second stream being a digital interface standard. The digital interface outputs the second stream to a post-stage audio receiver. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a data format diagram of S/PDIF standard, which is the prior art. 
         FIG. 2  is a block diagram of an audio processing circuit for an optical disk drive according to the prior art. 
         FIG. 3  is a block diagram of an audio processing circuit for an optical disk drive according to one embodiment of the present invention. 
         FIG. 4  is a flowchart of detecting and modifying streams with the audio processing circuit of  FIG. 3 . 
         FIG. 5  is a flowchart of changing a field of a stream with the audio processing circuit of  FIG. 3 . 
         FIG. 6  is a flowchart of detecting and modifying errors in at least one field of a frame in a stream with the audio processing circuit of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  illustrates a block diagram of an audio processing circuit  30  of an optical storage device such as an optical disk drive according to one embodiment of the present invention. To easily compare the embodiment with the prior art, some of the elements in  FIG. 3  are labeled with the same numbers used in  FIG. 2 . An element in  FIG. 3  labeled with a previously used number in  FIG. 2  has the same functionality as that of the corresponding element in  FIG. 2 . The audio processing circuit  30  includes a parser  12 , a stream buffer  14 , an audio processor  32 , a second buffer  18 , a digital to analog converter  20 , a first buffer  38 , an IEC burst circuit  22 , and a digital interface  24 . The audio processor  32  includes a decoding circuit  34  and a stream recovering circuit  36 . That is, both the decoding circuit  34  and the stream recovering circuit  36  are integrated into the audio processor  32 . When an optical storage disk  26  is loaded into the optical disk drive, digital data previously recorded on the optical storage disk  26  is read and preliminarily processed by a servo controller (which is not shown in  FIG. 3 ). The digital data preliminarily processed is parsed by the parser  12  and then the audio part of the digital data is outputted in the form of a first stream complying with a first standard (for example, MPEG audio standard) and stored in the stream buffer  14 . The first stream includes a plurality of audio frames. Each of the audio frames includes a plurality of fields. The decoding circuit  34  of the audio processor  32  decodes the audio frames of the first stream stored in the stream buffer  14 , generates a PCM (pulse-code modulation) encoded stream and stores the PCM encoded stream in the second buffer  18 . The digital to analog converter  20  converts the PCM encoded stream stored in the second buffer  18  into an analog audio signal as an output signal of the optical disk drive. This embodiment provides the previously mentioned normal audio processing function as well, but the data conversion function using the digital interface  24  for connecting to an external post-stage audio receiver  28  is modified. First, the audio processing circuit  30  uses the stream recovering circuit  36  of the audio processor  32  to detect the audio frames of the first stream stored in the stream buffer  14  and to modify (to fix) the audio frames of the first stream according to a first standard (for example, an MPEG audio standard). And then, the frames detected or modified by the stream recovering circuit  36  are stored in the first buffer  38 . Finally, the IEC burst circuit  22  converts the modified frames into a second stream complying with a second standard (for example, an IEC digital interface standard, which is also called S/PDIF standard) and sends the second stream to the post-stage audio receiver  28  through the digital interface  24 . In more details, the IEC burst circuit  22  in  FIG. 3  retrieves the modified frames stored in the first buffer  38  and partitions the modified frames into payload sections of proper sizes. The corresponding preamble is then added in front of each payload to form a data burst section. The corresponding stuffing section (including several stuffing bits) is then added next to each data burst section. The second stream complying with the S/PDIF standard is thus formed and sent to the post-stage audio receiver  28  through the digital interface  24 . 
       FIG. 4  illustrates a flowchart for detecting and modifying streams with the audio processing circuit  30  of  FIG. 3 . The first stream (for example, an MPEG audio bit stream) includes a plurality of audio frames, and each audio frame includes a sync word at the beginning of the audio frame for data partitioning. The sync word of each audio frame has a unique pattern, for example, 0xfff in the MPEG audio bit stream. The expected position of the sync word is implied in the first stream. However, for some streams encoded by an improper audio encoding mechanism, the positions of the sync words may not reside in the expected position as implied in the streams. Instead, the actual positions of the sync words may be shifted to neighboring positions near the expected positions. A conventional audio processing circuit (such as those illustrated in  FIG. 2 ) does not check whether the actual positions of sync words match the expected positions (i.e. whether the positions of the sync words are shifted), and simply partitions the frames of the first stream as if the sync words do reside in the expected positions, and forming the second stream to the decoding/amplifying device  28  (the post-stage audio receiver  28 ) via the digital interface  24  (for example, an IEC digital interface). In such a circumstance, when the decoding/amplifying device  28  (the post-stage audio receiver  28 ) receives the second stream and tries to recover it back to the first stream, it may improperly (or even fail to) decode the second stream and/or the first stream because the partitioning of the first stream is incorrect, and a blast sound may occur. In order to prevent errors due to the above mentioned sync word shift, the audio processing circuit  30  of this embodiment uses the stream recovering circuit  36  of the audio processor  32  to detect the audio frames of the first stream stored in the stream buffer  14  and to modify (to recover) the audio frames of the first stream according to the predetermined first standard (for example, MPEG audio standard). The audio frames of the first stream are detected and modified by the stream recovering circuit  36 , and are then stored in the first buffer  38 . Finally, the IEC burst circuit  22  transfers the modified frames into a second stream complying with a second standard (for example, the IEC digital interface standard) and sends the second stream to the post-stage audio receiver  28  through the digital interface  24  (for example, the IEC digital interface). The steps for detection and modification are described as follows. 
     Step  110 : Retrieving an expected location indicating where the sync word should be in the bit stream buffer  14 , set the value of a pointer sft as zero, and then go to Step  120 ; 
     Step  120 : Is the sync word correct? If the value at the expected location matches a predetermined pattern (ex. 0xfff in this embodiment), go to Step  130 , if not, go to Step  140 ; 
     Step  130 : Copy the audio frame having its beginning pointed by the pointer sft from the stream buffer  14  to the first buffer  38  to complete the detection and the modification of the frame, and then go to Step  110  for further detection and modification of the next audio frame; 
     Step  140 : Set a new value of the pointer sft. The new value equals to the previous value of the pointer sft plus one. This step represents that the expected position is modified by one bit. Go to Step  150 ; and 
     Step  150 : The new value of the pointer sft indicates searching the sync word at a one-bit-shifted position. Now a bit at the leftmost end (i.e. MSB, Most Significant Bit) corresponding to the expected location is omitted, and a next bit of the first stream is added at the rightmost end (i.e. LSB, Least Significant Bit) corresponding to the expected location. Go back to Step  120 . 
     Although the shift direction in Step  150 , as a result of Step  140 , can be derived from the statement about the omitted bit at the leftmost end and the added bit at the rightmost end, this is not limiting. The shift direction is just an exemplary choice relating to the logical direction definition of the stream buffer  14 . As the original MSB mentioned in Step  150  can be omitted first and each bit can be replaced with the next bit, whether the shift direction is left or right does not hinder the implementation of this invention. Through the process of these steps (Step  110 ,  120 ,  130 ,  140 ,  150 ), the above-mentioned undesired shifted state of the data of the audio frames is corrected and the modified frames stored in the first buffer  38  are ready for partitioning into proper payload sections according to the S/PDIF standard. As previously mentioned, the IEC burst circuit  22  converts the modified frames stored in the first buffer  38  into a second stream complying with the second standard (for example, the S/PDIF standard) and sends the modified frames to the post-stage audio receiver  28  through the digital interface  24 . Therefore, the compatibility between the audio processing circuit  30  and the decoding/amplifying device  28  (the post-stage audio receiver  28 ) is enhanced. 
       FIG. 5  illustrates a flowchart for changing a specific field in a stream with the audio processing circuit  30  in  FIG. 3 . Under certain conditions, the decoding/amplifying device  28  (the post-stage audio receiver  28 ) cannot properly decode the audio bit stream because it does not recognize a specific field in the bit stream. By changing a specific field in the original audio bit stream (retrieved from the optical storage disk  26 ) with the audio processing circuit  30 , the problem can be solved and the decoding/amplifying device  28  (the post-stage audio receiver  28 ) can properly decode the audio bit stream. For example, in an MPEG audio signal, there is a two-bit “mode” field identifying a playback mode of the audio signal. The playback modes usually include a “mono” mode, a “dual mono” mode, and a “stereo” mode, where the “mono” mode represents reproducing a sound content with one audio channel, and the “dual mono” mode and the “stereo” mode represent reproducing different sound contents with two audio channels so there are stereo effects to the listeners. Some decoding/amplifying devices  28  (the post-stage audio receivers  28 ) do not recognize the “dual mono” mode. This type of decoding/amplifying devices  28  can correctly reproduce one audio channel at the “mono” mode and can also correctly reproduce two audio channels at the “stereo” mode, but will simply reproduce one audio channel at the “dual mono” mode. The listener would easily perceive the problem of the incompatibility between the decoding/amplifying device  28  (the post-stage audio receiver  28 ) and the optical disk drive. In this embodiment, the audio processing circuit  30  can use the stream recovering circuit  36  of the audio processor  32  to change a value of the “mode” field of the audio bit stream (retrieved from the optical storage disk  26 ) from an original value of “dual mono” mode to a new value of “stereo” mode, so the “stereo” mode decoding method of the decoding/amplifying device  28  (the post-stage audio receiver  28 ) is selected. Therefore, the decoding/amplifying device  28  can reproduce the “dual mono” mode data retrieved from the optical storage disk  26  at the “stereo” mode. As most of decoding/amplifying devices  28  (post-stage audio receivers  28 ) can recognize the “stereo” mode, the problem of the incompatibility between the decoding/amplifying devices  28  and the optical disk drive due to above mentioned problem is solved. The process of changing a field in the original stream, the first stream, retrieved from the optical storage disk  26  with the audio processing circuit  30  is described as follows. 
     Step  210 : Find the sync word in the stream buffer  14 ; 
     Step  220 : Get the data of the audio frame of the first stream from the stream buffer  14  until the “mode” field is found and store the data of the audio frame got from the stream buffer  14  in the first buffer  38 , where “Get” is a programming term representing an action of “retrieving” or “receiving”; 
     Step  230 : Parse the data of the “mode” field received from the stream buffer  14 ; 
     Step  240 : Change the “mode” field from the original mode value to a new mode value; 
     Step  250 : Get the stream until all the audio frames of the stream are detected and corrected; 
     Of concern, “Get”, the programming term representing an action of “retrieving” or “receiving” in the above steps (Step  220 ,  250 ), can be replaced by other terms while the implementation of the present invention is not hindered. In addition, although in this embodiment the field data to be changed is a single value, this is not limiting. For example, the data to be changed can be a plurality of values or even data of a plurality of fields. This leads to embodiments relating to copyright management. In some audio signals, there is a “copyright” field indicating the copyright management information of the audio signal. The copyright management information generally includes “no copy”, “copy always”, and “copy once”. When the “copyright” field of the stream retrieved from the optical storage disk  26  is recorded as “no copy”, the content (ex. video or audio data) recorded on the optical storage disk  26  is read-only and cannot be copied to any other digital storage devices (ex. other optical disks, mini disks, flash memory drives, hard drives, etc.). One embodiment is described as follows. When the “copyright” field of the stream retrieved from the optical storage disk  26  is recorded as “copy always”, the content (ex. video or audio data) recorded on the optical storage disk  26  can be copied as many times as desired without limitation. When the “copyright” field of the stream retrieved from the optical storage disk  26  is recorded as “copy once”, the stream recovering circuit  36  of the audio processor  32  in this embodiment will change the “copyright” field in the content recorded on the optical storage disk  26  from “copy once” to “no copy” after one copy process is done. 
       FIG. 6  illustrates a flowchart of detecting and modifying errors of a stream with the audio processing circuit  30  in  FIG. 3 . Another function of the stream recovering circuit  36  of the audio processor  32  is detecting the audio frames of the first stream received from the stream buffer  14  and modifying the content in at least one field of the audio frames as needed. The stream recovering circuit  36  can detect if there is any error in various fields of the audio frames in the first stream) and modify the first stream according to a predetermined digital audio standard if modification is required. After the audio processing circuit  30  uses the parser  12  to receive the first stream retrieved from the optical storage disk  26  and stores the first stream in the stream buffer  14 , the stream recovering circuit  36  checks each field of the first stream. As shown in  FIG. 6 , the stream recovering circuit  36  first finds the sync word of the first stream in the stream buffer  14 , and then checks the fields one by one, where the fields include the “sync word” field, “header” field, the “side information” field, the “scale factor” field, the “audio sample” field, and the “ancillary data” field. If the first stream is completely correct, the first stream is stored in the first buffer  38 . If the content of any field is detected to be in error by the stream recovering circuit  36 , the stream recovering circuit  36  will try to modify the field to recover a correct format of the stream according to a predetermined digital audio standard (for example, MPEG audio standard). If the stream recovering circuit  36  successfully corrects the fields, the modified fields are stored in the first buffer  38 . As shown in  FIG. 6 , the stream recovering circuit  36  will check the next field until each field is verified. If the stream recovering circuit  36  fails to correct the field during any iteration of the field data correction, the current frame is abandoned and the stream recovering circuit  36  detects the next frame and repeats the process shown in  FIG. 6 . That is to say, the stream recovering circuit  36  modifies the first stream received from the stream buffer  14  to conform with the predetermined digital audio standard (for example, MPEG audio standard) by correcting errors in the fields of the first stream. When the stream recovering circuit  36  is unable to correct some frames of the first stream received from the stream buffer  14 , the stream recovering circuit  36  abandons the uncorrectable frames which are not capable of being modified to conform with the predetermined standard. In this way, the stream recovering circuit  36  will not allow frames with uncorrectable errors to pass onwards. So there could probably be a short period of silence to the listeners when there are frames with uncorrectable errors in the first stream. However, considering the characteristic of the decoding/amplifying device  28  (the post-stage audio receiver  28 ) and the listeners&#39; comforts, no sound is better than blast sound because ordinary human ears could not perceive such a short period of silence. 
     As previously mentioned, the audio processing circuit  30  of the present invention provides the ordinary audio decoding function to reproduce the digital data retrieved from the optical storage disk  26  and further provides the stream recovering circuit  36  for processing the frames of the stream stored in the stream buffer  14 . The functions of the stream recovering circuit  36  include correcting the sync word shift, modifying the data contents of the stream, detecting (checking) the data contents of the stream, and trying to recover a correct format of the data content of the stream. The frames of the stream processed by the stream recovering circuit  36  of the audio processor  32  are stored in the first buffer  38 . The IEC burst circuit  22  then arranges the modified frames or verified frames stored in the first buffer  38  (for example, the arrangement includes adding the preambles and stuffing bits to form a second stream complying with the S/PDIF standard) and sends the second stream to the post-stage audio receiver  28  through the digital interface  24  (for example, S/PDIF interface). Therefore, the compatibility between the audio processing circuit  30  and the decoding/amplifying device  28  (the post-stage audio receiver  28 ) is enhanced. 
     In contrast to the prior art, the present invention method and device can use the stream recovering circuit  36  of the audio processor  32  to properly detect and modify the stream retrieved from the optical storage disk  26  and use the IEC burst circuit  22  to arrange the modified audio frames of the first stream stored in the first buffer  38  so that the compatibility between the audio processing circuit  30  and the external decoding/amplifying device  28  (the post-stage audio receiver  28 ) is enhanced. Adapting to the post-stage audio receiver  28  through the digital interface  24 , the audio processing circuit  10  of the prior art simply uses the IEC burst circuit  22  to transfer the audio frames of the first stream stored in the stream buffer  14  into the second stream without checking the content of the audio frames, so the audio frames which are not completely compliant with a predetermined digital audio standard will be output to the post-stage audio receiver  28  through the digital interface  24 . Hence when the post-stage audio receiver  28  receives the second stream derived from the audio frames of the first stream that is not completely compliant with the predetermined digital audio standard (for example, MPEG audio standard), the post-stage audio receiver  28  may improperly or even fail to decode the received second stream and/or the first stream, and a blast sound may occur. Adapting to the post-stage audio receiver  28  through the digital interface  24 , the audio processing circuit  30  of the embodiment uses the stream recovering circuit  36  of the audio processor  32  to process the audio frames of the first stream stored in the stream buffer  14  and stores the audio frames processed by the stream recovering circuit  36  in the first buffer  38 . The IEC burst circuit  22  then arranges the modified audio frames stored in the first buffer  38  to form a second stream complying with a second standard (for example, the S/PDIF standard) and sends the second stream to the post-stage audio receiver  28  through the digital interface  24 . Therefore, the audio processing circuit  30  can remove the frames with errors and/or can modify the frames which are not completely compliant with a predetermined digital audio standard (for example, the MPEG audio standard), so the decoding/amplifying device  28  (the post-stage audio receiver  28 ) can correctly decode the data of the digital audio signal and the compatibility is therefore enhanced. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. For example, in the above embodiments the first stream complies with the MPEG audio standard and the second stream complies with the S/PDIF standard (IEC digital interface standard). This is not limiting. Instead, the present invention should be construed as limited only by the metes and bounds of the appended claims.