Abstract:
A method is provided for synchronizing the playback of a digital audio broadcast on a plurality of network output devices using a microphone near a source, embedded control codes, and the audio patterns from the network output devices. An optional, additional manual adjustment method relies on a graphical user interface for adjustment and audible pulses from the devices which are to be synchronized. Synchronization of the audio is accomplished with clock synchronization of the network output devices. The digital audio broadcast from multiple receivers does not present to a listener any audible delay or echo effect.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention generally relates to audio playback for multiple devices, and specifically, to synchronizing the audio playback.  
         BACKGROUND OF THE INVENTION  
         [0002]    When two or more audio devices play at the same time, there may be an echo effect that reduces the enjoyment of the sound by the listener. These audio devices may be cabled to a controller device which provides the audio data to them. A bus may be used to supply this data. Alternatively, the different devices may communicate with each other through wireless communication, such as through an RF or infrared port.  
           [0003]    In a system in which one computer or device broadcasts a single digital audio stream that is then simultaneously received by more than one receiving device, the different receiving devices will often play their audio slightly out of sync with each other, due to differing latencies in receiving and processing the digital audio stream. This produces an echo or delay effect which causes a listener to receive the same audio at slightly different times from the multiple devices. An additional echo effect may arise from a difference in distance from the listener to the various receiving devices. The listener&#39;s enjoyment is impaired.  
           [0004]    Prior devices have used clock synchronized receiving devices. In one prior device, a transmitting device inserted control track pulses into the digital audio stream at known intervals. Each receiving device obtained the time of the received track pulse and then delayed the playback of the audio stream until the received pulse was aligned with the next pulse at the known interval. In another prior device, the transmitting device transmitted to each receiving device a waveform sample, the playback time of the sample, and the latency value of the receiving device. Each receiving device then delayed the playback of the audio stream until the waveform sample was aligned with the received playback time of the sample.  
           [0005]    None of the prior devices used audio patterns and a microphone for obtaining feedback as part of the synchronization method. Furthermore, these prior devices required clock synchronization of the transmitting device and/or the playback devices. Clock synchronization tends to increase circuit complexity, and thereby increase costs.  
           [0006]    A method and apparatus for synchronizing the playback of audio from two or more devices is needed which does not require clock synchronization.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a method and apparatus for synchronizing the playback of the audio from two or more audio receivers so that there is no audible delay or echo effect between them when listened to simultaneously without using clock synchronization.  
           [0008]    In the closed-loop feedback system for synchronizing playback of multicast audio streams on a local network of the present invention, control codes are embedded in the audio stream to cause the playback devices to emit audio patterns that are detected by a microphone connected to a source of the audio stream. The source determines the latency from sending to playback, and then sends packets to each playback device to introduce additional latency into the audio stream to achieve synchronized playback.  
           [0009]    In a first aspect of the present invention, a method of audio synchronization on a home network is disclosed which includes sending a control code to a specific one of two or more receiver devices, outputting audible signals from two or more receiver devices on the home network, recording a first time difference between the time of sending a control code to the specific one of the two or more receiver devices and the time the audible signal is received from the specific one of the two or more receiver devices, recording a second time difference between the time of sending a control code to an other specific one of the two or more receiver devices and the time the audible signal is received from the other one of the two or more receiver devices, and automatically adjusting the relative delay of audio information signals to one of the specific ones of the two or more receiver devices.  
           [0010]    In a second aspect of the present invention, a system for synchronizing audio playback of multiple receiving devices is disclosed which includes a transmitting device and two or more receiving devices, wherein the receiving devices are synchronized through control codes sent from the transmitting device to each of the two or more receiving devices and audio patterns transmitted from each of the two or more receiving devices to the transmitting device.  
           [0011]    In a third aspect of the invention, a system for synchronizing the audio playback of two or more receiving devices is disclosed which includes means for transmitting control codes embedded in an audio stream, means for receiving the control codes embedded in the audio stream, means for transmitting audio patterns generated in response to the control codes, and means for receiving the audio patterns.  
           [0012]    It is to be understood that both the forgoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The numerous advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying figures in which:  
         [0014]    [0014]FIG. 1 illustrates a system having a plurality of audio playback receiver devices;  
         [0015]    [0015]FIG. 2 illustrates exemplary responses from three receiver devices;  
         [0016]    [0016]FIG. 3 illustrates a flow chart of method steps of the present invention;  
         [0017]    [0017]FIG. 4 illustrates an exemplary circuit for deriving the time differences or delay in response from the receiver devices;  
         [0018]    [0018]FIG. 5 illustrates a flow chart for an exemplary embodiment of the process for manually synchronizing the audio playback of two or more devices;  
         [0019]    [0019]FIG. 6 illustrates horizontal sliding bars for a graphical user interface for manual synchronization; and  
         [0020]    [0020]FIG. 7 illustrates vertical bars for a graphical user interface for manual synchronization. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    Reference will now be made in detail to the presently preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings.  
         [0022]    Referring generally now to FIGS. 1 through 7, exemplary embodiments of the present invention are shown.  
         [0023]    The present invention describes a method of enabling simultaneous output of audio signals on multiple output devices by the use of control codes embedded in multicast multimedia stream to cause specific receiving devices to emit easily detectable audio patterns which can then be detected by a microphone connected to the source of the multimedia stream. These audio patterns may or may not be audible to the human ear. The source may then accurately determine the total latency from sending to playback. The source may then send command packets to each playback device to determine how much additional latency it needs to introduce into the playback stream to achieve synchronization.  
         [0024]    Optionally, this invention also allows manual fine tuning the synchronization of audio output on multiple networked output devices. The method comprises the audible output of a audio signal from two or more receiving devices and allows the user to increase or decrease the timing delay in reference to the audible sound. Once the signals sound simultaneous to the user, the delay is recorded for the devices and added to subsequent audio output to reproduce the delay identified by the user to make the two devices sound synchronized.  
         [0025]    The control codes embedded in the multicast multimedia stream are only a small part of the audio stream. The transmitting source device might or might not be playing audio, and could be remote from the receiving devices. If the transmitting device is local and playing audio, it may be made to participate in the same synchronized audio playback method as described below. This method requires no clock synchronization of the audio transmitting device, only modification of the digital audio stream to include the control codes. The source device keeps a record of the latency value for each receiver.  
         [0026]    All devices may buffer the audio stream. Some amount of buffering of the audio stream is occurring, to allow the receiving devices to search forward and backward in the audio data, and to allow them to delay or shift audio playback. Signal transmission may be in analog or digital format.  
         [0027]    [0027]FIG. 1 illustrates a system of several devices. Transmitting device XMTR  5  sends the audio stream, sync information, time and other data to receiving devices PCn, such as PCA, PCB and PCn. Transmitting device XMTR  5  controls audio playback for the system. The devices may be interconnected by a bus cabling or may communicate with one another through wireless communication such as radio frequency or infrared. A microphone  30  or other audio or sound pickup device is attached or proximate to the transmitting device XMTR  5 .  
         [0028]    Several discrete sub-processes are used in the present invention. These include a latency detector and manual fine tune control.  
         [0029]    An average latency detector detects the average latency between transmitting a signal to a device, and the device receiving the signal. “Symmetrical latency” is not assumed between two computers because the time for the sound waves generated in the testing and synchronization of the receiver devices to travel may be considerably longer than the combined time for the control code to travel between the source and the receiver device and the processing of the control code to generate an audio pattern. This is because in air, between 0 and 100 degrees Fahrenheit, sound travels at slightly more than 1 foot per millisecond and the electromagnetic waves, used to transmit data between the source and the receiver device and processing within the receiver device, travel at nearly a million times faster. Even the propagation delays and computer instruction cycle times, at most a low number of microseconds, found in the receiver device are insignificant compared to the sound wave travel time.  
         [0030]    All receiver devices may account for latency of their own audio playback subsystem. There are three different methods for this, each of which would occur after the other processes described herein have been used to synchronize the clocks on all audio playback devices: 1) shifting the playback by a predetermined value, such value determined through empirical testing of the actual playback device; 2) shifting the playback by a predetermined value, such value determined at run time, by the customer, using the manual audio synchronization fine-tune control, to determine the actual latency of the device&#39;s internal audio subsystem; and 3) fine-tuning the clock synchronization at run time, by the customer, using the manual audio synchronization fine-tune control.  
         [0031]    [0031]FIG. 2 illustrates an example of an n receiver device system, where n is a natural number. The value of n may be two, three, ten, or some other positive integer. Here, relative times are displayed from the time a control code or command packet is sent by the transmitter  100 ,  130 ,  160  to the receipt  110 ,  140 ,  170  of the audio pattern by the source or audio pattern receiving device coupled to the source. As shown, travel time differences  120 ,  150 ,  180  may be appreciable between the receiver devices. Because of the high speed of electromagnetic wave signals in transmitting the control codes and the high speed of electronic processing circuitry, the great bulk of the travel time is due to the travel of the sound waves from the receiver devices to the acoustic pick up device, such as a microphone. Roughly, for every millisecond of travel time, the acoustic wave travels slightly over one foot. At 0 degrees Fahrenheit, sound travels at approximately 1051 feet per second. At 100 degrees Fahrenheit, sound travels at approximately 1162 feet per second.  
         [0032]    [0032]FIG. 3 illustrates a flow chart of the method of the present invention. The method may be implemented as hardware, software, firmware, or a combination on the transmitter or source device. The test, step  200 , may be conducted with a periodicity or as needed. In step  210 , the receiver device address is initialized. The transmitter sends a command packet to PC number n. This command packet may consist of control codes. It may be placed directly in the audio stream. The designated receiver device PC number n then receives the device and generates an audio pattern. The audio pattern may or may not be audible. In step  230 , the audio pattern is received by the pickup device near or attached to the transmitter or source device. The difference in time between sending out the command packet and receipt of the audio pattern is calculated in step  240  and stored for further processing. In step  250 , there is a determination made as to whether this is the last one of the receiver devices to be tested. If it is not, the next receiver device is selected in step  260  and processing repeats for that unit. The time differences of the various receiver devices are compared to one another. The one which has the longest travel time may be taken as a reference. The delay values for the other receiver devices may be derived by subtracting the travel time of each of the other receiver devices from the longest travel time. This information is then sent to the respective receiver devices which, accordingly, postpone their playback times. Other variations of the method may be employed. In fact, if buffering of the audio stream is used, the playback of the audio by the receiving devices may be arranged in various ways, including advancing the playback of the audio to the time of the audio playback of the fastest response audio playback receiver device.  
         [0033]    [0033]FIG. 4 illustrates an exemplary hardware embodiment of calculating the travel times of the receiver devices PCn, PC number n, where n, as an identifier of the PC, varies from 1 to a positive integer greater than one. An internal clock  335  continually increments a time counter  330 . The time count generated is always presented to the input of the send time latch  340  and the receive time latch  350 . The resolution of the counter  330  may be 8 bit, 16 bit, or another number of bits. For example, 18 bit resolution provides an resolution greater than one in one hundred thousand. Upon the receipt of transmission signal of a command packet representing embedded control codes in an audio stream, a transmission of command packet clock pulse is generated from a device  320  such as a monostable multivibrator. This command packet transmission clock pulse causes the time to be latched by the send time latch  340  and the PC number of the receiver device to be updated. The designated receiver device generates an audio pattern which is received by the microphone  300  in response to the command packet. This triggers another clock pulse device  310  which may be from a monostable multivibrator. This audio pattern received clock causes latching of the time of receipt of the audio pattern. After a small period of time, several nanoseconds, the input signals to the time difference arithmetic logic unit  370  are sufficiently stable. The time difference is then presented to the time difference latch  380 . On the next time difference latch clock pulse the data is latched and presented to the memory  390 . The address of the time difference is linked to the device number of the receiver device PCn. Further processing, such as adding an address offset to the PC number may be used. A clock for the memory  390  causes the data to be entered into the memory. This information may later be picked up for further processing to determine which receiver device should be used as a reference.  
         [0034]    A manual audio synchronization fine-tune control allows the user to “fine tune” the end results of automated synchronization. This means the user may want either to add relative delays in the audio for certain effects or to eliminate echo. It also allows the user to manually determine the internal latency of a device audio playback subsystem, by comparing the amount and direction of playback latency error between itself and a reference system with a known internal audio subsystem latency value. The system requires at least two devices playing audio, one that is the reference (PCA), and one that is adjusted by the user (PCB). In one embodiment, the method assumes that the reference player PCA always buffers and delays its own audio playback, so that PCB is able to move its own playback either forward or backward in time, relative to PCA. PCA synchronizes its clock with PCB. PCA emits an audible high-pitched pulse every n seconds, on even n second boundaries. PCB emits an audible pulse every n seconds, on even n second boundaries. PCB displays a graphic slider control to the user, defaulted to “centered” position. As the user slides the control left or right, PCB increments/decrements a correction value, and simultaneously shifts the audio click forward or backward in time. The user adjusts the slider until the two clicks converge and sound to the user as a single click. The resultant correction value may be added or subtracted from PCA&#39;s known internal latency value, to determine PCB&#39;s internal latency value.  
         [0035]    [0035]FIG. 5 illustrates a flow chart for an exemplary embodiment of the process for manually synchronizing the audio playback of two or more devices. Initially, a graphical user interface (GUI) may be set up to display several display options, including the identification of all or part of a set of audio playback devices, steps  500 ,  510 ,  520 . An offset value for the latency value is zeroed, step  530 . The processor checks to see if a user is moving the slider bar, step  540 . If the user is not moving the scroll bar, the offset is calculated by multiplying the change value by the unit time (for instance, the base clock period), step  550 . This value is added or subtracted from the latency value, step  560 , and stored, steps  570  and  580 . If the user is scrolling, then the direction of change is determined, step  585 . If the scrolling has been consistent and of a sufficient length of time, scrolling speed is increased, step  600 . The change value is increased or decreased as a result of the scrolling direction and speed, step  605 .  
         [0036]    [0036]FIG. 6 illustrates horizontal sliding bars for a graphical user interface for manual synchronization. Two slider bars  700  and  720  are placed one above another. The sliding bar for subordinate device PCn has a movable marker  730  to allow the user to manually set the latency value offset for that device. The marker for the sliding bar  700  for control device PCA may or may not be movable.  
         [0037]    [0037]FIG. 7 illustrates vertical bars for a graphical user interface for manual synchronization. Two or more subordinate devices, PCB and PCC, may be represented on a GUI. The number and names of the devices displayed may be chosen by the user. It may be preferable to select a small number of subordinate devices since it may be easier to determine which, if any, of those devices need to be adjusted. Here, control device PCA is shown as a vertical bar  800  with a separate scroll bar  810  having up and down arrows  820  and  830 . Likewise, subordinate device PCB is shown as a vertical bar  840  with a separate scroll bar  850  having up and down arrows  860  and  870  and subordinate device is shown as a vertical bar  880  with a separate scroll bar  890  having up and down arrows  900  and  910 . The vertical bars  800 ,  840 , and  880  may grow, shrink, or remain the same size during manual adjustment. A screen may appear to allow the user to select which devices should appear on the GUI.  
         [0038]    Data entry on the GUI may be accomplished by manual entry on a keyboard, through a touch screen, or by a hand held device such as a mouse or track ball, or other means.  
         [0039]    It is believed that the method to synchronize playback of multicast audio streams on a local network of the present invention and many of its attendant advantages will be understood by the forgoing description. It is also believed that it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof. It is the intention of the following claims to encompass and include such changes.