Abstract:
A process to synchronize information transmitted from an information provider to a plurality of network elements is provided. The process determines a temporal difference between related network elements and compensates for this difference by dilating an information signal provided to at least one of the network elements to resynchronize the data presented to the network elements. The process is repeated periodically in order to maintain a desired synchronization level.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 10/235,366 filed Sep. 5, 2002, now issued as U.S. Pat. No. 7,457,320, which claims priority to U.S. provisional application No. 60/317,274, filed Sep. 5, 2001, the contents of all of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     The use of packet-based communication in the home has increased dramatically in the last few years and is poised to explode in the near future. The medium for connecting devices in the home network is varied. It ranges from dedicated wired, to wireless, phone lines and recently also power lines connections. Currently, the main application of networks in the home are to connect computers and share Internet connections, but other applications loom on the horizon. One of the prominent candidates for home LAN applications is audio and video distribution. Synchronization of the different components is very important in making these applications viable. An advanced audio system, for example, may be composed of several speakers spatially separated from each other and interconnected with a LAN. To maintain a high fidelity, it is important that all the audio streams are synchronized to within about 10-20 milliseconds. Larger delays will be perceptible and will negatively impact the listening experience. 
     Another related problem that tends to occur when using today&#39;s digital music is the phenomenon of time drift. To illustrate the problem, assume that synchronization is achieved between the remote speakers at a certain point in time. Furthermore, assume that the long-term clock accuracy of the network element&#39;s crystal oscillator is on the order of 50*10 −6 . (Better accuracy is achievable, but at a price). The music on one device will run faster (or slower) than on the other devices. This time-drift from the point in time where the two devices are synchronized is readily calculated to yield (approximately):
 
 D=I*T   (Eq. 1)
 
where D is the time drift, T is the time lapsed from the point of synchronization, and I is the frequency inaccuracy.
 
     So, for example, after 5 minutes of play time (from synchronization) the drift between two speakers becomes:
 
 D= 100*10 −6 *300=30 Milliseconds  (Eq. 2)
 
     Clearly, this is not tolerable. Thus, it would be very beneficial if a practical solution to the above problem would be found. 
     SUMMARY OF INVENTION 
     The present invention is embodied in a method for synchronizing information transmitted from an information provider to a plurality of elements in a TCP/IP network. The method includes the step of transmitting at least a first control signal from the information provider to the plurality of elements using multicasting. 
     According to an aspect of the present invention further steps include receiving a signal from each of the network elements in response to the control signal; transmitting information to the network elements responsive to the signals received from the network elements; transmitting a further control signal from the information provider to the network elements using multicasting; receiving a state signal from each of the network elements in response to the further control signal; comparing the respective state signals with one another to determine a time drift between the plurality of elements; and transmitting a correction signal from the information provider to at least one of the plurality of elements based on the comparison. 
     According to one aspect of the invention, respective timers in each of the network elements are reset based on the initialization signal. 
     According to another aspect of the invention, the respective timer in each of the network elements is adjusted based on the correction signal. 
     According to yet another aspect of the present invention, a method for dilating a signal is provided. The method comprises the steps of duplicating at least a portion of the signal to generate a second signal; applying a first attenuation profile to the first signal and a second attenuation profile to the second signal to generate a first attenuated signal and a second attenuated signal; padding each of the first and second attenuated signals with a predetermined number of leading zeros and trailing zeros, respectively, to generate a first padded signal and a second padded signal; and summing the first padded signal and the second padded signal to generate a time dilated signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: 
         FIG. 1  is an illustration of a carrier wave (CW) section used to create two complementary signals according to the subject invention; 
         FIG. 2  is an illustration of a dilated signal and an original signal according to the subject invention; 
         FIG. 3  is an illustration of the section autocorrelation function, the allowed range and the peak defining the delay according to the subject invention; 
         FIG. 4  is an illustration of an example of the application of the present invention to a signal; 
         FIG. 5  is an illustration of sections used to form the signal shown in  FIG. 4 ; 
         FIG. 6  is a flow chart of an exemplary process incorporating the present invention; 
         FIG. 7  is a block diagram of a system incorporating the present invention; and 
         FIG. 8  is a flow chart of another exemplary process incorporating the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The subject invention is embodied in a process to synchronize information transmitted from an information provider to a plurality of network elements. 
     Overview 
     There are two main elements to the present invention: 
     1. Clock synchronization 
     2. Time drift compensation 
     Clock Synchronization 
     Clock synchronization, as defined herein, is the operation of setting up a unified time base in all network elements. The main problem is that the network presents unknown delays that make time synchronization difficult. In order to achieve accurate synchronization, the present invention uses multicasting in conjunction with server-client architecture. Table 1 represents an exemplary synchronization process between various remote network components  704  (in this case speakers) facilitated by a centralized server  702  (best shown in  FIG. 7 ). 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Exemplary process of establishing and maintaining 
               
               
                 synchronization between two 
               
               
                 remote network elements. 
               
             
          
           
               
                 Server 
                 Speaker-1 
                 Speaker-2 
                 Type 
                 When 
               
               
                   
               
               
                 Start-time, Δ 
                   
                   
                 Multicast 
                 At start-once 
               
               
                   
                 Ack 
                 Ack 
                 Unicast 
                 At Start-once 
               
               
                 Streaming 
                   
                   
                 Multicast/ 
                 Continuous 
               
               
                 information 
                   
                   
                 Unicast 
               
               
                 (audio/music) 
               
               
                 Request for 
                   
                   
                 Multicast 
                 Periodic 
               
               
                 timer state 
               
               
                   
                 Timer-state 
                 Timer-state 
                 Unicast 
                 Periodic- 
               
               
                   
                 (periodic) 
                 (periodic) 
                   
                 response 
               
               
                 Correction- 
                   
                   
                 Unicast 
                 As needed 
               
               
                 individual 
               
               
                   
                 Correction 
                 Correction 
                 Unicast 
                 On execution 
               
               
                   
                 Ack 
                 Ack 
               
               
                   
               
             
          
         
       
     
     The exemplary process steps are as follows: 
     1. Server multicasts a synchronization packet (referred to as “time of origin” packet) and start-time for streaming relative to the time-0 (usually a few seconds in the future). 
     2. Network elements acknowledge receipt of synchronization packet and start time. 
     3. Server streams information to network elements. In this example the network elements are speakers and the information is audio/music. 
     4. Periodically, server sends request for “timer state” from the network elements. 
     5. Network elements send their timer state in response to requests. 
     6. Server sends time correction needed to the different network elements to adjust for time-drifts. 
     7. Network elements correct their timers and act to adjust the time drift. 
     The details relating to these steps are described below. 
     Time Drift Compensation: 
     The present invention also discloses a method to adjust (compensate) for time drifts. According to an exemplary embodiment of the present invention, there are three parts to the time-dilation (gap filling) algorithm: 
     1. Selecting a digital audio section (typically a PCM signal) and deciding if the section is a candidate for time dilation according to its frequency response (i.e., most energy is above a set frequency (say 1000 Hz)). 
     2. Computing the section&#39;s autocorrelation function to determine the optimum time dilation (K) in a pre-specified range. 
     3. Creating two output sections from the digital audio section in the following way: The first output section composed of the digital audio section is padded with K zeros in front and the second output section with the digital audio section with K zeros at its end. The output sections are modulated in a complementary way such that they smoothly fade-in and fade-out into each other (see detailed description below). The two modulated sections are then summed to create a larger time dilated section without audible discontinuities. 
     Details of the Exemplary Algorithm 
     Although the exemplary embodiment is described herein with respect to speakers for simplicity, the present invention is not so limited. The present invention is equally suitable for synchronizing other types of remote network elements. 
     Clock Synchronization: 
     TCP/IP networks are extremely efficient in delivering data. The quality of service (QOS) of these networks, as defined by maximum latency allowed, is not appropriate in many cases, however, for delivering real-time information. When transmitting audio/music (or video) on such networks, there is no real need (in many cases) for real time processing. Conventionally, the variable delay between the server and the client is compensated by having a large buffer on the client side and by using forward error correcting codes (ECC) to reduce the need for retransmission of lost packets. When two different network components need to be synchronized, however, such methods do not help. In fact, buffering queues, usually put in the front end of a TCP/IP process, just add to the general difficulty in establishing a common timeframe in the different network elements on the same network. 
     One element of this part of the invention is the use of multicasting to improve the synchronization process. The improvement is primarily due to: (1) Multicast packets transmitted to multiple network elements leave the server as the same packet. This is in contrast to Unicasting where different packets are sent to different network elements. This eliminates any time differences in time-stamp transmittal in the server; (2) Instead of different packets traveling in the same route, it is the same packet traveling, again, eliminating any possible time-of-arrival differences on the same network. In the last “leg” of the trip, the same multicast packet is essentially duplicated and sent to the individual network element. 
     Another part of the present invention is the substantial reduction, and in some cases the total elimination, of the non-uniform delays (on the network elements) for processing of arriving packets. To achieve this result, a change in the conventional way of embodying TCP/IP stacks (in which buffering queues at the receiving end is introduced) is incorporated. In one embodiment, a latency of no more than 10 microseconds is achieved before a packet header is processed and its time-of-arrival stamp is established. In another embodiment, a latency of less than 5 milliseconds is achieved before a packet header is processed and its time-of-arrival stamp is established. 
     The computer synchronization process is shown in Table 1 (above) and described in detail below with reference to flow chart  600  shown in  FIG. 6  and the block diagram of system  700  shown in  FIG. 7 . 
     Line 1: A universal time base, T 0 , is established. At step  602 , the server  702  transmits a “time of origin” or synchronization signal, T 0 ,  706  using multicast. Once a client (network element)  704   a ,  704   b , . . .  704   n  receives the single packet establishing a time-base (or time of origin), at Step  604  the client  704   a ,  704   b , . . .  704   n  sets (resets) its respective timer (not shown) to zero. Note, that while the time it took the packet to reach client  704   a ,  704   b , . . .  704   n  may be large and unknown, the “time of origin” difference between two clients that are reasonably close (by network distance), such as  704   a  and  704   b , is very small. With the “time of origin” packet  706 , information regarding the “relative start-time” of playing audio or music is also included. This time is relative to the time of origin (5 seconds, for example). This “start time” is needed for two reasons: (1) to let the server  702  get the acknowledgements from clients  704   a ,  704   b , . . .  704   n  that they indeed received the “time of origin packet”  706  and (2) to allow the buffering of enough data to enable smooth client operation. The “time of origin” packet  706  is normally sent once in a session, and generally at the beginning of the session. In this context a session is defined as a connection that is established using the method described herein. It is important to note that, under certain conditions, Unicast is also possible and can be used where multicasting is not supported. 
     Line 2: Once the client  704   a ,  704   b , . . .  704   n  receives the “time of origin” packet. At step  606 , it transmits back a respective acknowledgement signal  708   a ,  708   b , . . .  708   n , indicating that it received synchronization signal  706  and initialized its respective timer. In the event that not all network elements acknowledged the receipt of the “time of origin” packet, a second “time of origin” packet is sent. Streaming of information (audio/music or video, for example) would not start until all network elements  704   a ,  704   b , . . .  704   n  acknowledge the receipt of the “time of origin” packet signal  706 . The “relative start time” takes into account the delays associated with the acknowledgements. The acknowledgement is accomplished using Unicast and is generally done once in a session. 
     Line 3: At step  608 , content  710  is transmitted to network elements  704   a ,  704   b , . . .  704   n  using Multicast or Unicast. 
     Line 4: If the clocks associated with the respective remote network elements were identical, further synchronization would not be necessary. This is true assuming that no packets are lost (and in case packets are lost they are retransmitted through the TCP protocol or by any other applicable packet loss recovery method). Unfortunately, in practical application the clocks on the different network elements are not locked with one another. Slight variations in otherwise identical crystals cause a time drift among the different network elements. Explained next is the approach for how to recover from such time drifts. Specifically, the present invention concentrates on detecting and quantifying the time drift. Time drifts accumulate as a function of time as shown in Eq. 1 above. Even very small clock differences can accumulate, given enough time, to induce noticeable, audible effects. In order to correct for such time drifts, at Step  610  server  702  sends a “request for timer state” signal  712  periodically, such as every minute for example, as measured by the server (the accuracy of the “period of time” is not important) to all network elements  704   a ,  704   b , . . .  704   n . This is preferably done using multicast to ensure that all network elements  704   a ,  704   b , . . .  704   n  receive the request signal  712  at the same time. Again, Unicast is also possible and can be used where multicasting is not supported. 
     Line 5: Once the network elements  704   a ,  704   b , . . .  704   n  receive the “request for timer state” signal  712 , at Step  612  they send back their respective timer state signal  714   a ,  714   b , . . .  714   n  at the time they received the request signal  712 . Note, that with the architecture modification, an arriving packet header is processed almost immediately and its time stamp is very accurately established. Timer state signal  714   a ,  714   b , . . .  714   n  is returned to server  702  using Unicast and always in response to a “request for timer state” signal  712 . 
     Line 6: Once the server  702  receives the “timer state” signal  712  from all network elements  704   a ,  704   b , . . .  704   n , it computes corrections (if needed) for each one of the elements. Note that the server  702  compares only the timer states of the elements  704   a ,  704   b , . . .  704   n  and does not consider its own timer in the calculation. The latest (most latent) timer state received from network elements  704   a ,  704   b , . . .  704   n , is the basis for defining the correction. In the present invention, this particular timer will not need any correction. At Step  614 , the latency difference between timer state signals  714   a ,  714   b , . . .  714   n  and the most latent timer state signal is determined. At Step  616 , any other timer that exceeds a pre-defined limit, based on the comparison of Step  612 , is sent an instruction  716   a ,  716   b , . . .  716   n  to time dilate its respective signal to compensate for the time drift. (This process will be detailed in the next section). This is done in Unicast as needed. 
     Line 7: At step  618 , clients  704   a ,  704   b , . . .  704   n  receive the respective time drift correction instruction  716   a ,  716   b , . . .  716   n  from server  702  as needed, execute the time dilation, readjust its respective clock to reflect the correction, and send an acknowledgement signal  718   a ,  718   b , . . .  718   n  to the server  702 . This is done in Unicast in response to a request for time drift correction. 
     Time Dilation: 
     To illustrate the exemplary “time dilation” method, performed in clients  704   a ,  704   b , . . .  704   n  as needed, reference is made to  FIGS. 1 ,  2  and the flow chart  800  illustrated in  FIG. 8 . 
     As shown in  FIGS. 1 and 8 , at Step  802 , a time section of a signal (best shown in the lower portion of  FIG. 2 ) is first duplicated. At Step  804 , a linear attenuation  104  is applied to the front part of the first section, such that signal  102  is subject to greater attenuation at the beginning  110  of the section. The attenuation  104  is then slowly and linearly reduced such that at a point before the section ends the signal is no longer subject to attenuation. Likewise, at Step  806 , the second section has a complementary attenuation profile  104 ′ applied for which the end  108  of the section  106  is completely attenuated and the attenuation is linearly decreased going backward in time. Next, at Step  808 , the two time series are offset, with the first section padded with leading zeros and the second section padded with trailing zeros, to create two sections that are identical in length. Next, at Step  810 , the two signals  102  and  106  are summed to create a time-dilated version of the input section. The exact profile of the attenuation and the number of zeros needed for padding are determined from the correlation function of the section. Using this method the two signals are summed in phase. The resultant signal  202  of the summation is shown in  FIG. 2  (top), while the original signal  204  without dilation is shown at the bottom of  FIG. 2 . 
     Time Drift Compensation: 
     A detailed description of the steps for time drift compensation according to the present invention is outlined below: 
     First, a time series is selected to which the time drift compensation will be applied. Preferably, the series size is composed of 2 N  points, where N is an integer. (This is because the present invention intends to use the FFT operation on the signal. Other sizes are also possible with appropriate padding). In the exemplary embodiment, either 256 or 512 points are used in the time series. The section is then tested for spectral distribution. In the present invention, it is important that signal energy be significantly existent in frequencies above a specified threshold. This is done because when the two sections are subsequently summed it is preferable that they be “in phase”. In order to be “in phase,” power above a set frequency has to exist. This will become clearer when the conditions needed for the spectral distribution are addressed in detail below. 
     Next, the autocorrelation function of the chosen section is computed and the peak is found at a time delay in a specified range away from the center. Referring now to  FIG. 3 , an example of the autocorrelation function of a carrier wave  302  is shown. The delay range 304 and the sought after peak  306  defining the delay are also shown. 
     As was explained above in the overview, a time section is taken and duplicated. Then a linear attenuation is applied to the front part of the first section such that the signal is greatly attenuated at the beginning of the section, and slowly, but linearly, the attenuation is reduced such that somewhere before the section ends the signal is no longer subject to attenuation (see, the attenuation profile  104  of  FIG. 1 ). Likewise, the second section has a complementary attenuation profile  104 ′ (shown in  FIG. 1 ) for which the end of the section is completely attenuated and the attenuation is linearly decreasing going backward in time. The two time series are then offset by the amount of delay determined above. This is done to provide an “in phase” summation of the two delayed sections. The first section is padded with leading zeros and the second section is padded with trailing zeros to create two sections that are identical in size. Next, the two signals are summed to create a time dilated version of the input section. For example, in the case where a tone is dilated, the result will be a perfect tone, but larger in size, as shown in  FIG. 2 . The exact profile of the attenuation (and the number of zeros needed for padding) is determined by the delay found above. The result of the summation  202  is shown in the top portion of  FIG. 2 , while the original signal  204  without dilation is shown in the lower portion of  FIG. 2 . Another example of a section of 512 points is provided with reference to  FIG. 4  for the WINDOWS “notify” sound. In  FIG. 4 , the original signal  404  is shown in the bottom portion of  FIG. 4  and the dilated signal  402  is shown in the upper portion of  FIG. 4 . In testing, the time dilation process did not produce any audible distinctions. Referring now to  FIG. 5 , the two sections  502 ,  504  used to create signal  402  are shown. 
     Smoothing Packet Loss Recovery: 
     The padding method described above can also be used in Voice over Internet Protocol (VoIP) transmission systems, for example. Often when packets are lost a previous packet is duplicated to “close” the time gap. In an exemplary embodiment of the present invention, a smoothing out of the transition between the identical packets and the one following the duplicated packet with the method just introduced is envisioned. 
     Shaping the Transition: 
     In the exemplary embodiment described above, a linear fading (attenuation) function was used to allow a slow transition between the two sections. The present invention, however, is not so limited. It is important to note that other functions with “low-pass” characteristics can be used as long as the sum of the fade-in and fade-out function is a unity. 
     While the invention has been described in terms of exemplary embodiments, it is contemplated that it may be practiced as described above with variations within the scope of the following claims.