Abstract:
A method and apparatus for processing data samples utilizes a channel map populated by device descriptor, or by an application program interface. Packet processing code loops through all of the samples contained in a packet while incrementing through a channel map and steering table without having to look up a table to determine in what audio buffer the sample is to be stored or read. Additionally, the present invention utilizes a stride map, so the audio subsystem knows how many samples to skip in order to reach the next sample frame. The present invention can be used for handling received packets as well as forming packets to send over a bus.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates broadly to digital audio transmitted between devices on a network. Specifically, the present invention relates to storing audio data in audio buffers in locations as determined by the contents of steering registers and channel maps.  
       BACKGROUND OF THE INVENTION  
       [0002]     A data bus can be utilized for interconnecting electronic devices such as computers, audio systems, television receivers, display devices, video recorders, and home control devices such as security system or appliance control systems. Communication using a data bus occurs in accordance with a bus protocol recognized by devices attached to the bus. Examples of bus protocols include the IEEE 1394 High Performance Serial Bus and the Universal Serial Bus (USB). A bus protocol typically provides for communicating both control information and data. On an IEEE 1394 serial bus, control information is generally passed using the asynchronous services of the serial bus. Control information for a particular application can be defined using, for example, Common Application Language (CAL) or Audio-Video/Control (AV/C).  
         [0003]     Like video processing, in audio processing applications, audio samples can be packed in the order the samples enter an audio processing engine. The order could be by stereo pairs, mono channels, interleaved channels, or whatever order the audio hardware chooses to packetize the audio data. This places a significant processing burden on the packetizer or depacketizer to determine which audio channel buffer is associated with each audio sample in the packet. The complexity is further compounded when multiple devices transmit audio data to the audio processing engine, as the different devices do not conform to a single standard or sample ordering. Rather, existing audio devices order the audio samples within the packet as efficiently as possible for themselves, and this efficiency does not necessarily apply to the target device that receives the audio packets.  
         [0004]     While device descriptors are commonly used on the communication bus, current device descriptors can only describe the channel ordering used to insert audio samples in the packet, this only works for in-order processing. With multiple devices, there are multiple channel orderings, so there is a question as to how to handle all possible sample orderings and interleaved as well as non-interleaved buffers. As there is no current solution to this problem, the burden on the audio processing engine remains, and device performance suffers.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention solves the problems described above and provides a method and apparatus for processing audio samples that utilizes a channel map that is populated by a modified device descriptor, or by an application program interface. In accordance with the present invention, low-level packet processing code loops through all of the samples contained in a packet while incrementing through a channel map and steering table without having to look up a table to determine in what audio buffer the sample is to be stored or read. This method and apparatus functions regardless of whether or not the audio buffer is interleaved or not, and regardless of how many channels are accommodated by an interleaved audio buffer. Additionally, the present invention utilizes a stride map, so the audio subsystem knows how many samples to skip in order to reach the next sample frame. The present invention can be used for handling received packets as well as forming packets to send over a bus.  
         [0006]     In a preferred embodiment, the present invention encompasses software commands executed by a processor to perform the methods of the present invention. In another preferred embodiment, hardware is configured to execute the methods of the present invention.  
         [0007]     Many other features and advantages of the present invention will become apparent from reading the following detailed description, when considered in conjunction with the accompanying drawings, in which: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  illustrates in block diagram form functional components used in embodiments of the present invention;  
         [0009]      FIG. 2  illustrates the format of a CIP packet used in embodiments of the present invention;  
         [0010]      FIG. 3  illustrates a steering table used in accordance with the present invention;  
         [0011]      FIGS. 4 and 5  illustrate sample buffers having different stride sizes;  
         [0012]      FIG. 6  illustrates the organization of a stride map array used in accordance with the present invention;  
         [0013]      FIG. 7  illustrates the organization of a channel map used in accordance with the present invention;  
         [0014]      FIG. 8  illustrates the organization of audio channel buffers used in accordance with the present invention;  
         [0015]      FIG. 9  illustrates in flow diagram form a sequence of acts performed in accordance with the present invention; 
     
    
     DETAILED DESCRIPTION  
       [0016]     Directing attention to  FIG. 1 , there is shown a contemplated audio subsystem  100 . Device  102  is in communication with driver  104  and sends driver  104  audio packets as well as control information. Driver  104  typically includes a packetizing/depacketizing engine that functions to either form packets in the case of packetization, or process received packets into audio streams in the case of depacketization. Driver  104  consults steering table  106 , channel map  108  and stride map  110  to select the appropriate buffer from audio channel buffers  112 . Audio buffers  112  can feed output to an operating system of the host of audio subsystem  100 , or audio buffers  112  can be fed to hardware devices. Steering table  106  is a lookup table that returns an address of a sample buffer based on the channel index of the audio sample in the packet. Channel map  108  is a pointer array that contains starting addresses of individual sample buffers associated with samples in the packet. Stride map  110  is an array of step sizes for incrementing audio pointers to the next sample in the packet.  
         [0017]     Channel map  108  can be allocated to the size of the audio packets&#39; sampleframe width. Channel map  108  can also accommodate mismatched sample size or packet size with respect to application stream size. For example, if an audio application is only recording a stereo stream but device  102  is sending data over 24 audio channels, channel map  108  can be configured to publish only a stereo stream while still extracting a large sample frame from the packet. This increases efficiency because driver  104  is only processing samples belonging to the stereo stream that will be recorded by the application.  
         [0018]     The present invention thus also provides efficient hardware play-through support. By steering the input channel map to an output channel map on a channel-by-channel-basis, any channel can be played through directly in the driver. Custom channel-steering can also be performed by the present invention; a single channel can be sent to all available channels by duplicating the same starting sample buffer pointer in channel map  108 .  
         [0019]     A Common Isochronous Packet (CIP packet) having an AM  824  format is contemplated for use with an embodiment of the present invention utilizing an IEEE 1394 High Performance Serial Bus, but other packet formats can be used as well. Directing attention to  FIG. 2 , the organization of exemplary CIP packet  200  is shown. CIP packet  200  has two headers, header  202  and  204 . Following headers  202 ,  204  are various audio samples, packed in numerical order from left to right, top to bottom. As illustrated in  FIG. 2 , CIP packet  200  is divided into sample frames. Sample frame  206  comprises samples  1  through  6 . Next is sample frame  208 , comprising audio samples  7 - 12 . This 6-sample frame size continues for additional sample frames until the end of CIP packet  200 . Typically, there are eight sample frames in one CIP packet. However, other packet configurations can be used in embodiments of the present invention.  
         [0020]     Directing attention to  FIG. 3 , steering table  106  describes an indexed channel order of 1, 3, 5, 2, 4, 6 for each 6-channel sample frame in CIP packet  200 . This means that the first sample is processed in the sample frame, followed by the third sample, the fifth sample, the second sample, the fourth sample, and the sixth sample. This indexed order can be provided in a device descriptor passed from device  102  to system  100 . In an alternative embodiment, the indexed order can be supplied during execution to audio system  100  by an application program interface (API) or other software executing on audio subsystem  100 . This is especially useful when different devices are connected to audio subsystem  100 , and packet processing shifts from one device to another.  
         [0021]      FIGS. 4 and 5  illustrate sample buffer  150  having different stride sizes as used in accordance with stride map  110 . In each case, the address offset can be obtained from the pointer to sample buffer  150 .  FIG. 4  shows sample buffer  150  as a two-channel sample buffer used for a stereo audio stream, having a left channel and a right channel. In this example, the stride size is two, as the samples alternate between left and right, so one sample is skipped for example, when reading or writing the left channel or the right channel. Similarly,  FIG. 5  illustrates sample buffer  150  as four-channel sample buffer, where the stride size is four, as the samples repeat a four-element sequence. In this case, to read channel  1 , three samples are skipped after each time the sample for channel  1  is read. Thus, the stride size indicates the number of samples to skip in a sample buffer. In  FIG. 4 , where the stride size is two, two samples are skipped to reach the desired channel, and, in  FIG. 5 , where the stride size is four, four samples are skipped.  
         [0022]      FIG. 6  illustrates stride map  110 , which is an array of stride sizes for sample buffers used in embodiments of the present invention. As shown, the first exemplary element indicates a stride size of two, and corresponds to a sample buffer as shown in  FIG. 4 . The next two elements each have a stride size of four, followed by a stride size of two and additional elements having stride sizes of four.  
         [0023]      FIG. 7  illustrates channel map  108 , which is an array of pointers that constitute the starting addresses of the individual sample buffers in audio channel buffers  112 . As shown, the first element of channel map  108  has a pointer having the value 100, which indicates a starting address of 100. The next element is a pointer having the value 200. The third pointer has a value 208, followed by a pointer having the value 104, a pointer having the value 204 and another pointer having the value 212.  
         [0024]     Audio subsystem  100  initializes by obtaining the channel order and number of channels to process. As described earlier, this information can be obtained by driver  104  from device  102  in the form of a device descriptor passed from device  102  to driver  104 . Also as described above, this information can be passed to driver  104  from a process executing on audio subsystem  100 , such as an API or other process. In an embodiment, such information could be passed to driver  104  when a user manipulates a device interface, such as an interface that allows the user to select from a plurality of devices connected to audio subsystem  100 . Such information also could be passed to driver  104  when a user selects a particular operating mode of a connected audio device, such as an operating mode that requires a change in the amount of data processed by audio subsystem  100 . For example, if a user wishes to change from four-channel audio to stereo audio, channel information sent to driver  104  would also change to reflect the change in operating mode.  
         [0025]     At act  302 , driver  104  uses the information received in act  300  to construct steering table  106 , channel map  108 , and stride map  110 . Channel order information is reflected in steering table  106 , number of channels is reflected in stride map  110 , and assignment of audio sample streams is made to audio channel buffers  112  in channel map  108 .  
         [0026]     Operation of the present invention in a depacketizing embodiment is illustrated in flow diagram form as shown in  FIG. 8 . Beginning at act  300 , initialization having already been performed on driver  104 , a packet is received at audio subsystem  100 . Driver  104  obtains the starting address from channel map  108  based on the sample frame index supplied by steering table  106  (which channel in the frame is being processed) in act  302 . At act  304 , driver  104  reads the sample buffer index, indicating which sample in main sample buffer  150 . At act  306 , device driver  104  reads the stride size from stride map  110 . At act  308 , an address within audio channel buffers  112  is calculated as the starting address of the channel read in act  302  plus the product of the offset value read in act  304  multiplied by the value read from stride map  110  in act  306  multiplied by the size of the sample.  
         [0027]     At act  310 , the audio data read from the packet is then written in audio channel buffers  112  at the address generated in act  308 . If the end of a frame has not been reached (decision act  312 ), control proceeds to act  313 , where the sample frame index is incremented. If the end of a frame has been reached, control transitions to act  314 , where the sample frame index is reset and the sample buffer index is incremented (act  316 ). At decision act  318 , if the end of a packet being processed has been reached, control transitions back to act  300  where processing of a new packet begins. If the end of the packet has not been reached, then control transitions back to act  302 , where processing advances on the current packet. This sequence of acts repeats until there are no more additional packets to be processed.  
         [0028]      FIG. 9  illustrates a sequence of acts performed when driver  104  is applied as a packetizing function on data stored in audio channel buffers. Rather than receiving packets, as shown in  FIG. 8 , packets are being constructed and transmitted to device  102 . Directing attention to  FIG. 9 , initialization having already been performed on device driver  104 , in preparation for sending data stored in audio channel buffers  112 , obtains the starting address from channel map  108  based on the sample frame index supplied by steering table  106  (which channel in the frame is being processed) in act  402 . At act  404 , driver  104  reads the sample buffer index, indicating which sample in main sample buffer  150 . At act  406 , driver  104  reads the stride size from stride map  110 . At act  408 , an address within audio channel buffers  112  is calculated as the starting address read in act  402  plus the product of the offset value read in act  404  multiplied by the value read from stride map  110  in act  306 .  
         [0029]     At act  410 , audio data is read from audio channels buffer  112  at the address generated in act  408 . This data is then written to a packet formed by driver  104  in act  412 . If the end of a frame has not been reached (decision act  414 ), control proceeds to act  415 , where the sample frame index is incremented. If the end of a frame has been reached, control transitions to act  416 , where the sample frame index is reset and the sample buffer index is incremented (act  418 ). At decision act  420 , if the end of a packet being processed has been reached, control transitions to act  422 , where a new packet is begun and control loops back to act  402  where processing of the new packet begins. If the end of the packet has not been reached, then control still transitions back to act  402 , where processing advances on the current packet. This sequence of acts repeats until there are no more additional packets to be processed. The constructed packets are then sent to device  102 .  
         [0030]     Combining the functionality of  FIGS. 8 and 9  into a single driver  104  allows audio subsystem  100  to operate in play-through mode. Referring back to  FIG. 1 , in this embodiment, there are two channel maps  108 - 1  and  108 - 2 . Channel map  108 - 1  is designated for input as described with respect to  FIG. 8  and channel map  108 - 2  is designated for output as described with respect to  FIG. 9 . By steering channel map  108 - 1  to channel map  108 - 2  on a channel-by-channel basis, the audio data received from device  102  can be played through any channel in driver  104  to device  103 .  
         [0031]     While the present invention has been described and illustrated in detail, it is to be understood that many changes and modifications can be made to the various embodiments of the invention without departing from the spirit thereof.