System and method for synchronizing audio-visual devices on a power line communications (PLC) network

• US 8,028,097 B2
• Filed: 03/10/2005
• Issued: 09/27/2011
• Est. Priority Date: 10/04/2004
• Status: Expired due to Fees

• Current Assignee: Sony Corporation (Sony Group Corp.), Sony Electronics Inc. (Sony Group Corp.)
• Original Assignee: Sony Corporation (Sony Group Corp.), Sony Electronics Inc. (Sony Group Corp.)
• Inventors: Iwamura, Ryuichi
• Primary Examiner(s): Hamza; Faruk
• Assistant Examiner(s): RECEK, JASON D
• Application Number:
  US11/078,251
• Publication Number:
  US 20060072695A1
• Time in Patent Office: 2,392 Days
• Field of Search: 709/248, 375/354, 375/356, 370/324, 370/507, 370509-510, 370/512
• US Class Current: 709/248
• CPC Class Codes: H04B 1/66 for reducing bandwidth of signals; for improving efficiency of transmission H04B1/68 takes precedence
• CPC Class Codes: H04B 2203/5408 using protocols
• CPC Class Codes: H04B 2203/5441 Wireless systems or telephone
• CPC Class Codes: H04B 2203/5445 Local network
• CPC Class Codes: H04B 2203/545 Audio/video application, e.g. interphone
• CPC Class Codes: H04B 2203/5483 using coupling circuits
• CPC Class Codes: H04B 3/542 the information being in digital form
• CPC Class Codes: H04J 3/0664 unidirectional timestamps
• CPC Class Codes: H04N 21/4147 PVR [Personal Video Recorder] H04N5/76 takes precedence ; arrangements for broadcast with accumulation-type receivers H04H20/40
• CPC Class Codes: H04N 21/42646 for reading from or writing on a non-volatile solid state storage medium, e.g. DVD, CD-ROM
• CPC Class Codes: H04N 21/4302 Content synchronisation processes, e.g. decoder synchronisation
• CPC Class Codes: H04N 21/4305 Synchronising client clock from received content stream, e.g. locking decoder clock with encoder clock, extraction of the PCR packets arrangements for synchronising receiver with transmitter by comparing receiver clock with transmitter clock H04L7/0012; arrangements for synchronising receiver with transmitter wherein the receiver takes measures against momentary loss of synchronisation H04L7/0083

Abstract

A method and apparatus for synchronizing streaming media devices within a PLC network. Output synchronization errors exceeding ˜30 ms become noticeable when multiple streaming media devices are outputting an audio stream. The present invention provides a system and method for isochronously sending periodic reference clocks from a master device to client devices coupled to the PLC network. The client devices set their clocks based on the reference clock. In addition the clients adjust their system clock time base in response to the average divergence of the system clock with the reference clock, or a count of the number of clocks between beacon frames. In this way the client clock is adjusted to closely track the server clock so that synchronization is maintained between each of the devices. Streaming audio shared between servers and client devices is thus output across the network with high fidelity due to the accurate synchronization.

Claims

1. An apparatus for synchronizing video and audio streams being communicated over a power line communications (PLC) network, comprising:

a first streaming media device configured for connecting to a power line for the receipt of operating power;

a power line communications circuit, within said first streaming media device, configured for communicating data over said power line operating as a PLC network, between said first streaming media device and at least a second streaming media device coupled to said power line;

wherein a periodic beacon signal is sent by said power line communications circuit to establish bus access timing for the devices on the PLC network whose video and audio streams are to be synchronized;

wherein said periodic beacon signal divides the power line channel into beacon cycles divided into multiple periods comprising a contention-free period and a non-contention-free period;

wherein streaming media devices communicate isochronous data within time slots of the contention-free period, and asynchronous data within time slots of the non-contention-free period, within said beacon cycles; and

means for isochronously communicating a reference clock, containing a multi-byte clock value, within a reserved time slot of said contention-free period from said first streaming media device over said PLC network to additional streaming media devices for maintaining synchronization of device output;

wherein said reference clock is available to the additional streaming media devices without a buffer delay;

said synchronization is maintained in response to adjusting a system clock in response to said reference clock, which controls timing of device output, within each of said additional streaming media devices to match the reference clock sent from said first streaming media device.

2. An apparatus as recited in claim 1, wherein said reference time clock is used to maintain a system time clock, in the additional streaming media devices, having an accuracy of approximately 5 milliseconds, or better, regardless of buffer size and packet arrival time.

3. An apparatus for synchronizing video and audio streams being communicated over a power line communications (PLC) network, comprising:

a first streaming media device configured for connecting to a power line for the receipt of operating power;

a power line communications circuit, within said first streaming media device, configured for communicating video and audio data over said power line operating as a PLC network, between said first streaming media device and at least a second streaming media device coupled to said power line;

means for generating a beacon signal for communicating boundaries of a beacon frame, which is divided into time slots within a contention-free portion for isochronous transmissions and a non-contention-free portion for asynchronous transmissions, to devices coupled to the PLC network; and

means for outputting a reference clock, as a multi-byte clock value, within said contention-free portion of said beacon frame for updating the system time clock, which controls timing of device output, on media devices coupled to the PLC network to maintain synchronization.

4. An apparatus as recited in claim 3:

wherein said reference clock is output by said first streaming media device operating in a server capacity; and

wherein said first streaming media device may comprise a separate media server, or media server capabilities integrated within a media device.

5. An apparatus as recited in claim 4, wherein said media device is selected from the group of media devices consisting essentially of set-top boxes, digital video recorders, media recorders/players, television sets, high-definition television sets, video monitors, and audio devices.

6. An apparatus as recited in claim 3, wherein said video and audio data is communicated over said power line communications (PLC) network in a digitally encoded format which is decoded at the receiving device.

7. An apparatus as recited in claim 6, wherein said encoding comprises encoding defined by the Motion Picture Experts Group (MPEG) standards.

8. An apparatus as recited in claim 3, wherein said means for outputting a reference time clock is configured to maintain the accuracy of the system time clock within an accuracy of approximately 5 milliseconds, or better, for said second streaming media device regardless of buffer size and packet arrival time.

9. An apparatus as recited in claim 8, wherein said reference time clock is output by said first streaming device to allow said second streaming device to update its system time clock value.

10. An apparatus as recited in claim 9, wherein said reference time clock is configured for receipt by said second streaming device which alters the frequency of its system time clock in response to a rate of divergence between the received reference clock value and the system time clock reference within said second streaming device.

11. An apparatus as recited in claim 10, wherein said divergence rate is determined over multiple periods to reduce the effects of signal jitter.

12. An apparatus as recited in claim 9, wherein said beacon framing signal is sent at a regular rate for receipt by said second streaming device which alters the frequency of its system time clock in response to a count of clock periods which occur during the beacon frame.

13. An apparatus as recited in claim 12, wherein said altering of the system time clock is determined over multiple beacon frames to reduce the effects of signal jitter.

14. An apparatus as recited in claim 3:

wherein said reference clock comprises a system time clock value of said first streaming media device which is combined with a time offset value; and

wherein said time offset value is determined in response to average network delay and buffering delay.

15. An apparatus as recited in claim 3:

wherein said means for outputting a reference clock is configured to output the reference clock within a time slot which is reserved within said contention-free portion of said beacon frame; and

wherein said reference clock is output approximately every five seconds or less.

16. A method of synchronizing the audio-video output from devices coupled to a power line communications (PLC) network, comprising:

generating a beacon framing signal on the PLC network;

communicating streaming media including audio, video, or a combination of audio and video between devices coupled to said PLC network;

reserving a portion of each beacon frame, bounded by said beacon framing signal, for use in communicating isochronous transmissions, said portion being referred to as a contention-free portion;

reserving a time slot within said contention-free period for communicating a reference time clock; and

generating a reference time clock, as a multi-byte clock value, within said reserved time slot to update the system time clock on devices coupled to the PLC network for maintaining synchronization of the streaming media being output;

wherein said synchronization is maintained on devices coupled to the power line communications (PLC) network in response to adjusting their system time clock, which controls timing of device output, to match the reference time clock.

17. A method as recited in claim 16, wherein a server, or a device acting in the capacity of a server, generates said beacon framing signal.

18. A method as recited in claim 16, wherein said reference clock signal is generated to update the system time clock values in the devices receiving the reference clock.

19. A method as recited in claim 16, wherein said beacon frames, or said reference clock, or a combination of beacon frames and reference clock is used for adjusting the frequency of the system time clocks within the devices coupled to the PLC network.

20. A method as recited in claim 19, wherein adjusting the frequency of said system time clock in the devices in performed over a number of said beacon frames, or reference clock, to reduce the effects of signal jitter.