Patent Publication Number: US-8989884-B2

Title: Automatic audio configuration based on an audio output device

Description:
RELATED CASE 
     This application is related to and claims the benefit of priority from provisional Patent Application No. 61/431,806 filed Jan. 11, 2011, entitled “AUTOMATIC AUDIO CONFIGURATION BASED ON AN AUDIO OUTPUT DEVICE”; the entire content of which is incorporated by this reference for all purposes as if fully disclosed herein. 
     This application is related to U.S. application Ser. No. 11/824,320, entitled, “Data-Driven Media Management Within An Electronic Device,” filed Jun. 28, 2007, the entire contents of which is hereby incorporated by reference as if fully set forth herein for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to audio processing and, more specifically, determining whether an audio processing device should produce high quality or low quality audio output depending on the current conditions associated with the audio processing device. 
     BACKGROUND 
     Computing devices can often handle playback of multiple types of media. The media that may be played back by computing devices often includes numerous forms/formats of video, and numerous forms/formats of audio. Within such computing devices, one or more applications may play back media. Non-limiting examples of types of applications that may attempt to play back media within a handheld computing device include a telephone application, a web browser, an e-mail application, multimedia message service (MMS), a music player, and video player. 
     One factor that determines the perceived quality of audio is bit depth. In digital audio, bit depth indicates the number of bits recorded for each audio sample. Bit depth directly corresponds to the resolution of each audio sample in a set of digital audio data. The larger the bit depth, the more bits are allocated for each audio sample and, thus, more information is available to produce audio with higher fidelity. Common examples of bit depth include CD quality audio, which is recorded at 16 bits, and DVD audio, which can support up to 24-bit audio. Thus, “16-bit audio” refers to a bit depth of 16, “24-bit audio” refers to a bit depth of 24, etc. 
     For some desktop or laptop computers, a user is able to, via an audio setup application, configure an output device by selecting 16-bit or 24-bit as the bit depth. However, smaller audio processing devices, such as handheld electronic devices (e.g., mobile phones and tablet devices), typically only produce 16-bit audio, regardless of the output device (e.g., integrated or built-in speakers, headphones, USB audio devices) that is connected to the handheld device. One reason for only producing 16-bit audio is that handheld devices are power-constrained devices and producing 24-bit audio requires additional processing relative to producing 16-bit audio. Many handheld devices include one or more fixed-point decoders (e.g., one decoder for each audio format, such as AAC). Each decoder (either hardware or software) includes simple multiply and add units, each of which operate on integer numbers and produce 16-bit audio. Thus, even though a USB audio device that is connected to a handheld device may be able to output an analog signal based on 24-bit audio, the handheld device to which the USB audio device is connected only produces 16-bit audio for the USB audio device. 
     Other handheld devices have greater power and more sophisticated circuitry that can operate on floating-point numbers instead of integer numbers and can produce floating-point audio samples. Thus, the same decoders that traditionally exist for laptop and desktop computer may be used for these handheld devices. If the source audio content was in 24-bit, then operating in the floating point domain allows the dynamic range of the source audio content to be maintained and a high fidelity 24-bit audio can be produced. 
     However, an audio processing device always producing 16-bit audio or always producing 24-bit audio may have some disadvantages, depending on the connected output device and other factors that correspond to the state of the audio processing device. For example, some output devices can produce noticeably better-sounding audio based on 24-bit audio rather than 16-bit audio. Thus, always decoding source audio content to 16-bit audio will not realize the benefits available when such output devices are connected to the audio processing device. As another example, some output devices do not produce noticeably better-sounding audio based on 24-bit audio compared to 16-bit audio. Thus, always decoding source audio content to 24-bit may not be worth the extra processing required to decode to and/or operate on 24-bit audio. 
     The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a block diagram that depicts components of a handheld device upon which an embodiment of the invention may be implemented; 
         FIG. 2  is a block diagram that depicts additional components of device  100 , according to an embodiment of the invention; 
         FIG. 3  is a flow diagram that depicts a process for determining a decode format for encoded audio content, according to an embodiment of the invention; and 
         FIG. 4  is a block diagram that illustrates a computer system upon which an embodiment of the invention may be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention. 
     General Overview 
     An audio processing device determines, from among a plurality of possible decode formats, a particular decode format to which source audio content is to be decoded. One or more factors are considered in making the determination of which decode format to select. One factor may include the format associated with output device (i.e., “hardware format”) to which the audio output is destined. Examples of hardware formats includes 16-bit audio and 24-bit audio. However, embodiments of the invention are not limited to these particular bit depths. Other possible bit depths include, for example, 8, 32, and other bit depths that are not on a byte boundary (e.g., 20). Non-limiting examples of output devices include built-in speakers, headphones, and USB audio devices. Another factor in making the determination of which decode format to select may include whether certain operations are to be performed on the decoded audio before the decoded audio is converted to an analog signal. 
     In some situations, the perceived quality of high quality audio output (e.g., corresponding to 24-bit audio) may be the same as the perceived quality of lower quality audio output (e.g., corresponding to 16-bit audio) due to various factors. One such factor may be the limitations of the output device or the quality of the digital-to-analog converter (DAC) associated with the output device. Thus, the extra processing to produce high quality audio output may not be warranted, especially since producing low quality audio output will conserve resources (e.g., processing time and power consumption), of the audio processing device, that would otherwise be used to produce 24-bit audio. For example, analog audio output to headphones may have the same perceived quality regardless of whether the corresponding digital audio was in 16-bit or 24-bit format. 
     Example Handheld Device 
     The techniques described herein may be applied to any type of electronic device for which one or more applications play back media. For example, the electronic device may be an iOS™ device, such as an iPhone™, iPod Touch™, iPad™, or Apple TV™, or any other type of electronic device. Handheld electronic devices, such as personal digital assistants (PDAs) and mobile phones, are examples of electronic devices to which the techniques may be applied. For the purpose of explanation, examples of the techniques will be given with reference to handheld devices, but it should be noted that the techniques are not limited to such devices. 
       FIG. 1  depicts an example handheld device  100  upon which an embodiment of the invention may be implemented. Handheld device  100  comprises at least a display  102  (which may or may not be touch-sensitive), a main speaker  104 , a telephone receiver/speaker  108 , a jack  112  for either headphones (listen only) or headset (listen and talk), a wireless transmitter/receiver  116  such as but not limited to Bluetooth®, a line out port  120  (suitable for a docking station used by, e.g., a larger home-audio system), and also a user interface (UI) speaker  124  (e.g. for keypad click-tones). Other types of speakers and audio ports may also be included. 
     The touch-sensitive version of display  102  may contain a keypad which in turn can generate sounds when touched. 
     Telephone receiver/speaker  108  may be similar to a receiver used within a conventional telephone. 
     Handheld device  100  further comprises a mute switch  128 , and a vibration means  132 . A mechanism for enabling/disabling the vibration means  132  may be available through display  102  by accessing a software application loaded on device  100 . 
     Handheld device  100  may execute one or more software applications (not shown), non-limiting examples of which include applications for e-mail, telephone, voice-mail, web-browser, short messaging service (SMS), entertainment player either for music or video, camera functions, and slideshow presentation (with music accompaniment). 
     UI speaker  124  may be used for playing keypad sounds (e.g., clicks) and notifying a user (e.g., via alert sounds) that certain steps and key-actions may not be permitted. 
     Audio Processing Components 
       FIG. 2  is a block diagram that depicts audio processing components of an audio processing device  200 , according to an embodiment of the invention. In an embodiment, audio processing device  200  is handheld device  100 . In  FIG. 2 , audio processing device  200  includes an audio storage  210 , a decoder  220 , a mixer  230 , a digital-to-analog converter (DAC)  240 , and an output device  250 . Each of decoder  220 , mixer  230 , and DAC  240  may be implemented in hardware, software, or any combination of hardware and software. 
     Audio storage  210  contains one or more audio files. Non-limiting example of audio files that are contained in audio storage  210  include music files, audio files corresponding to system-generated sounds, and multimedia files that include video data in addition to audio data. The audio data in the audio files contained in audio storage  210  may be encoded in different formats. Non-limiting examples of different formats include Advanced Audio Coding (AAC), MPEG-1 or MPEG-2 Audio Layer 3 (MP3), 16-bit pulse-code modulation (PCM), 24-bit PCM, Apple Lossless, Waveform Audio File Format (WAV), and Audio Interchange File Format (AIFF). 
     As depicted in  FIG. 2 , an audio file  212  is input to decoder  220 . Decoder  220  decodes the encoded audio data of audio file  212  to generate decoded audio  222 . Non-limiting examples of decode formats of decoded audio data at this stage include 32-bit floating point numbers, 8.24 fixed-point integers, 24-bit integers, and 16-bit integers. In an embodiment, 16-bit integers are considered low quality while 32-bit floating point numbers, 8.24 fixed-point integers, and 24-bit integers are considered high quality. 
     If audio processing device  200  supports the storage and playback of audio data encoded in multiple formats, then audio processing device  200  may include a decoder for each encoded format. For example, audio processing device  200  may include an AAC decoder and an MP3 decoder. 
     Decoded audio  222  is input to mixer  230  which combines multiple audio streams into a single audio stream. In the example depicted in  FIG. 2 , audio  224  is also input to mixer  230 . An example of audio file  212  is a music file. While the audio data in audio file  212  is being decoded, an email may be received at audio processing device  200 . The reception of the email may trigger the generation of a system sound, which is represented as audio  224  before the corresponding audio is played by output device  250 . Audio  224  may be decoded audio data (and, thus, does not require decoder  220 ) or may be encoded audio data (e.g., from another music file) that first requires decoding by decoder  220  (or another decoder not shown). 
     Mixer  230  generates mixed audio  232  based on decoded audio  222  and audio  224 . Audio  232  may be of the same format as decoded audio  222  described above, namely 32-bit floating point numbers, 8.24 fixed-point integer, and 16-bit integers. If multiple audio streams do not need to be mixed, then decoded audio  222  may be input to DAC  240  without passing through mixer  230 . 
     Mixed audio  232  is input to DAC  240 , which converts digital audio data into an analog signal that is transmitted to output device  250 . Prior to being input to DAC  240 , mixed audio  232  may need to be converted to a different format that DAC  240  may recognize. For example, if mixed audio  232  is based on 32-bit floating point numbers, then mixed audio  232  may be converted to a 24-bit format or a 16-bit format, depending on the hardware format associated with each output device  250 . Non-limiting examples of hardware formats include a 16-bit format and a 24-bit format. 
     In  FIG. 2 , DAC  240  is a component of audio processing device  200 . However, audio processing device  200  may include zero DACs or multiple DACs. If audio processing device  200  does not include any DACs, then any output devices connected to audio processing device  200  must have, or be connected to, a DAC. 
     In  FIG. 2 , output device  250  is a component of audio processing device  200 . Thus, output device  250  may be built-in speakers, such as speaker  108  of handheld device  100 . In a related embodiment, output device  250  is external to device  200 . In that scenario, output device  250  may be, for example, headphones, USB audio devices, a High-Definition Multimedia Interface (HDMI) device, line out speakers, a stereo Bluetooth (i.e., A2DP) accessory, ad DisplayPort device, or an AirPlay receiver. The type and quality of audio output devices vary widely. As noted herein, some output devices are built into an audio processing device, such as a laptop computer or handheld device. Other output devices are external to the audio processing device. An external output device may be connected to an audio processing device via multiple types of ports, such as a USB port, a Firewire port, a serial port, a parallel port, an earphone jack, a wireless port (e.g., Bluetooth), etc. 
       FIG. 2  does not indicate all the possible operations that may be performed on digital audio data other than decoding and mixing. Thus, one or more operations may be performed on decoded audio  222  (i.e., before mixer  230  processes decoded audio  222 ) and/or on mixed audio  232  (i.e., after mixer  230  processes decoded audio  222 ). Non-limiting examples of such operations include scaling, equalization, time-pitch adjustment (e.g., speeding up or slowing down audio), sample rate conversion, and dynamic range compression. Equalization is the process of adjusting (e.g., boost or cut/attenuate) the strength of certain frequencies within a signal. 
     Determining a Decode Format 
       FIG. 3  is a flow diagram that depicts a process  300  for determining a decode format to which certain audio content is to be decoded, according to an embodiment of the invention. Process  300  may be implemented in hardware, software, or a combination of hardware and software. For example, process  300  may be hardware implemented using a device (e.g. a programmable logic array) having a plurality of elements, including logical elements, wherein the elements are programmed/configured to implement the method described herein. As an alternative, process  300  may be hardware implemented by way of another device such as an application specific integrated circuit (ASIC) having elements, including logical elements, that are constructed/configured to implement the method described herein. As a further alternative, process  300  may be software implemented such that the method described herein is set forth in a set of instructions that are stored in a machine readable storage medium and executed by one or more computing devices (such as the sample computer system depicted in  FIG. 4 ) to carry out the method described herein. These and other implementations are possible. All possible implementations are within the scope of the present invention. 
     At step  310 , an audio processing device (e.g., handheld device  100 ) determines the hardware format (e.g., 16-bit or 24-bit) associated with an output device. The hardware formats are dictated by the capabilities of the output device. One or more of the hardware formats may be communicated to the audio processing device, such as in the case of HDMI. Additionally or alternatively, one or more hardware formats are stored on the audio processing device based on prior knowledge of the supported hardware formats of a particular output device. For example, the DAC chip built into the audio processing device will support a particular set of hardware formats. 
     The association between hardware formats and output devices may be reflected in a plurality of mappings that are stored on the audio processing device. Each mapping maps a type of output device (or specified port) to a hardware format. Thus, one mapping may associate an HDMI device with a 24-bit hardware format (e.g., after the HDMI device communicates that hardware format to the audio processing device). Another mapping may associate a line out port with a 16-bit hardware format. 
     Certain output devices and/or ports may be considered high quality (HQ) or low quality (LQ) even though each output device requires a 16-bit hardware format. For example, a line out port and built-in speakers are considered LQ while USB audio devices, AirPlay receivers, and a DisplayPort port may be considered HQ. Thus, in an embodiment, an output device or port may be associated with data that indicates whether it is HQ or LQ. This association may be stored within, or separate from, the mappings described previously. 
     In an embodiment, even though an output device (e.g., headphones) is capable of processing 24-bit audio (or rather, the DAC associated with the output device is capable of processing 24-bit audio), the output device (e.g., via a mapping) may be associated with a different hardware format (16-bit). This may be due to the fact that the output device is not associated with a DAC of sufficient quality. It may be determined that the perceived quality of 16-bit audio played by the output device is the same as the perceived quality of 24-bit audio played by the same output device. Therefore, the benefits of not performing the extra processing required to generate 24-bit audio may outweigh any perceived increase in quality from the 24-bit audio. 
     At step  320 , the audio processing device determines the encoding of the source audio content. This step may be optional if the audio processing device stores and processes audio data that is encoded in only a single format (e.g., AAC). However, if the audio processing device stores one set of audio data that is encoded in one format and another set of audio data that is encoded in another format, then the audio processing device determines the encoding format of the source audio content. 
     At step  330 , the audio processing device determines, based on the hardware format and the encoding format, a processing mode decode format to which the audio content is to be decoded. Table 100 provides examples of different encoding formats, different hardware formats, and corresponding decode formats. Embodiments of the invention are not limited to the hardware formats, encoding formats, or decode formats listed in Table 100. 
                             TABLE 100               ENCODING               FORMAT   LQ HW FORMAT   HQ HW FORMAT                  16-BIT PCM   16-bit integer   8.24 fixed-point integer               (w/dithering)       24-BIT PCM   16-bit integer   32-bit floating point number       AAC   16-bit integer   32-bit floating point number       MP3   16-bit integer   8.24 fixed-point integer               (w/dithering)                    
Thus, according to Table 100, the fidelity of the output device is a significant factor in determining the decode format to which source audio content is to be decoded.
 
     For example, if the source audio content is encoded in the 16-bit PCM format and the output device is associated with a low quality hardware format (e.g., 16 bits), then the decode format is determined to be a 16-bit integer format. Alternatively, if the output device is considered high quality, then the decode format is determined to be a 8.24 fixed-point integer format. Later, the audio content that is in the 8.24 fixed-point integer format is dithered and quantized down to a 16-bit format and the 16-bit audio is transmitted to a DAC. 
     As another example, if the source audio content is encoded in the AAC format and the output device is considered low quality, then the decode format is determined to a 16-bit integer format. Alternatively, if the output device is considered high quality, then the decode format is determined to be a 32-bit floating point number format. 
     In an embodiment, even if the audio processing device determines that the hardware format is high quality and that dithering to a 16-bit integer format is required, the audio processing device still decodes the source audio content to a 16-bit integer format without decoding the source audio content to a 8.24 fixed-point integer format. Thus, no dithering is necessary. This step may be warranted in the scenario where (1) there are no other audio streams that are to be mixed with the decoded source audio content and (2) certain operations are not performed on the decoded source audio content. Such operations may include applying volume, equalization, time-pitch adjustment, sample rate conversion, and dynamic range compression. 
     “Applying volume” refers to automatically (either in hardware or software) boosting or attenuating the audio samples. An example of applying volume is the “sound check” operation in iTunes™. If sound check is performed, then a music library is analyzed to determine a volume adjustment to apply to each music track so that each music track is played at the same perceived volume level. Then, for a particular song, if sound check is applied and data indicates that a certain amount of volume needs to be applied to the particular song in order to be perceived as loud as another song, then that amount of volume will be applied to the particular song. 
     Therefore, in this embodiment, at step  340 , the audio processing device determines whether dithering is considered necessary (e.g., according to Table 100). If not, then the process proceeds to step  350 . If so, then the process proceeds to step 360. 
     At step  350 , the audio processing device decodes the source audio content to the determined decode format. 
     At step  360 , the audio processing device determines whether one or more criteria are satisfied. The one or more criteria may comprise whether (a) the source audio content is uncompressed or losslessly compressed 16-bit audio content and (b) certain operations are not to be performed on the to-be-decoded source audio content. Such operations may include scaling, mixing, volume adjustment, equalization, and mixing other audio streams with the to-be-decoded source audio content. 
     If (a) and (b) are satisfied, then the process proceeds to step  370  where the source audio content is decoded directly to a 16-bit integer format without any dithering of the decoded source audio content. If at least one of the (1) and (2) is not satisfied, then the process proceeds to step  380 . 
     At step  380 , the source audio content is decoded to the determined decode format (e.g., one of 24-bit integer, 32-bit floating point number, or 8.24 fixed-point integer formats) and later “dithered down” to a 16-bit integer format. Step  380  may include additional operations (such as scaling, mixing, volume adjustment, equalization, etc.) that are performed on the decoded audio, i.e., prior to the dithering step. 
     Changing the Hardware Format 
     While audio content is being decoded, the destination of processed audio data may be changed. For example, while audio data produced by decoding particular audio content is streamed to built-in speakers of an audio processing device, a user of the device may plug a USB cord (connected to a USB audio device) into a USB port of the device. Prior to the USB cord being plugged in, the audio processing device may have made a determination to decode audio to a 16-bit integer format based on the fact that the built-in speakers are considered to be a low quality output device. However, the USB audio device may be considered to be a high quality output device. 
     In an embodiment, the decode format is changed “mid-stream.” In other words, a portion of a particular audio source is decoded to, for example, a 16-bit integer format. Then, before the remainder of the particular audio source is decoded, the decoding process changes such that the remainder of the particular audio source is decoded to, for example, a 32-bit floating point number format. The decode format is changed mid-stream only if no synchronization issues will arise from the change. A synchronization issue may arise if another data source (whether audio or video) is being streamed or if a user interface is displaying the precise playback location in the audio stream. If so, then both data streams, for example, might no longer be synchronized with each other. 
     In another embodiment, the decode format is only changed if the audio stream (i.e., generated from the source audio content) is complete or when playback is stopped and restarted. Stopping and restarting playback of source audio content may occur when the user moves (via a user interface) the playback location to another position in the audio stream. 
     Hardware Overview 
     According to one embodiment, the techniques described herein are implemented by one or more special-purpose computing devices. The special-purpose computing devices may comprise a plurality of elements, including logic elements, that are hard-wired to perform the techniques, or may include digital electronic devices such as one or more application-specific integrated circuits (ASICs) or field programmable gate arrays (FPGAs) that are persistently programmed to perform the techniques, or may include one or more general purpose hardware processors programmed to perform the techniques pursuant to program instructions in firmware, memory, other storage, or a combination. Such special-purpose computing devices may also combine custom hard-wired logic, ASICs, or FPGAs with custom programming to accomplish the techniques. The special-purpose computing devices may be desktop computer systems, portable computer systems, handheld devices, networking devices or any other device that incorporates hard-wired and/or program logic to implement the techniques. 
     For example,  FIG. 4  is a block diagram that illustrates a computer system  400  upon which an embodiment of the invention may be implemented. Computer system  400  includes a bus  402  or other communication mechanism for communicating information, and a hardware processor  404  coupled with bus  402  for processing information. Hardware processor  404  may be, for example, a general purpose microprocessor. 
     Computer system  400  also includes a main memory  406 , such as a random access memory (RAM) or other dynamic storage device, coupled to bus  402  for storing information and instructions to be executed by processor  404 . Main memory  406  also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor  404 . Such instructions, when stored in non-transitory storage media accessible to processor  404 , render computer system  400  into a special-purpose machine that is customized to perform the operations specified in the instructions. 
     Computer system  400  further includes a read only memory (ROM)  408  or other static storage device coupled to bus  402  for storing static information and instructions for processor  404 . A storage device  410 , such as a magnetic disk or optical disk, is provided and coupled to bus  402  for storing information and instructions. 
     Computer system  400  may be coupled via bus  402  to a display  412 , such as a cathode ray tube (CRT), for displaying information to a computer user. An input device  414 , including alphanumeric and other keys, is coupled to bus  402  for communicating information and command selections to processor  404 . Another type of user input device is cursor control  416 , such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor  404  and for controlling cursor movement on display  412 . This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane. 
     Computer system  400  may implement the techniques described herein using customized hard-wired logic, one or more ASICs or FPGAs, firmware and/or program logic which in combination with the computer system causes or programs computer system  400  to be a special-purpose machine. According to one embodiment, the techniques herein are performed by computer system  400  in response to processor  404  executing one or more sequences of one or more instructions contained in main memory  406 . Such instructions may be read into main memory  406  from another storage medium, such as storage device  410 . Execution of the sequences of instructions contained in main memory  406  causes processor  404  to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions. 
     The term “storage media” as used herein refers to any non-transitory media that store data and/or instructions that cause a machine to operation in a specific fashion. Such storage media may comprise non-volatile media and/or volatile media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device  410 . Volatile media includes dynamic memory, such as main memory  406 . Common forms of storage media include, for example, a floppy disk, a flexible disk, hard disk, solid state drive, magnetic tape, or any other magnetic data storage medium, a CD-ROM, any other optical data storage medium, any physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, NVRAM, any other memory chip or cartridge. 
     Storage media is distinct from but may be used in conjunction with transmission media. Transmission media participates in transferring information between storage media. For example, transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus  402 . Transmission media can also take the form of acoustic or light waves, such as those generated during radio-wave and infra-red data communications. 
     Various forms of media may be involved in carrying one or more sequences of one or more instructions to processor  404  for execution. For example, the instructions may initially be carried on a magnetic disk or solid state drive of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system  400  can receive the data on the telephone line and use an infra-red transmitter to convert the data to an infra-red signal. An infra-red detector can receive the data carried in the infra-red signal and appropriate circuitry can place the data on bus  402 . Bus  402  carries the data to main memory  406 , from which processor  404  retrieves and executes the instructions. The instructions received by main memory  406  may optionally be stored on storage device  410  either before or after execution by processor  404 . 
     Computer system  400  also includes a communication interface  418  coupled to bus  402 . Communication interface  418  provides a two-way data communication coupling to a network link  420  that is connected to a local network  422 . For example, communication interface  418  may be an integrated services digital network (ISDN) card, cable modem, satellite modem, or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface  418  may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface  418  sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. 
     Network link  420  typically provides data communication through one or more networks to other data devices. For example, network link  420  may provide a connection through local network  422  to a host computer  424  or to data equipment operated by an Internet Service Provider (ISP)  426 . ISP  426  in turn provides data communication services through the world wide packet data communication network now commonly referred to as the “Internet”  428 . Local network  422  and Internet  428  both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link  420  and through communication interface  418 , which carry the digital data to and from computer system  400 , are example forms of transmission media. 
     Computer system  400  can send messages and receive data, including program code, through the network(s), network link  420  and communication interface  418 . In the Internet example, a server  430  might transmit a requested code for an application program through Internet  428 , ISP  426 , local network  422  and communication interface  418 . 
     The received code may be executed by processor  404  as it is received, and/or stored in storage device  410 , or other non-volatile storage for later execution. 
     In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. The sole and exclusive indicator of the scope of the invention, and what is intended by the applicants to be the scope of the invention, is the literal and equivalent scope of the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction.