Audio and video clock synchronization in a wireless network

• US 20050259754A1
• Filed: 05/18/2004
• Published: 11/24/2005
• Est. Priority Date: 05/18/2004
• Status: Active Grant

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Ho, Jin-Meng, Baker, Richard T., Hicks, Allison Winifred
• Granted Patent: US 7,668,243 B2
• Time in Patent Office: Days
• US Class Current: 375/240.280
• 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
• CPC Class Codes: H04N 21/8547 involving timestamps for synchronizing content
• CPC Class Codes: H04W 28/14 using intermediate storage

Abstract

System and method for synchronizing clocks and maintaining packet timing relationships in a wireless communications system. A preferred embodiment further comprises periodically synchronizing local clocks at a transmitter and a receiver to a clock reference, adding a timestamp to each application packet at a transmitter of a wireless network, setting the timestamp to a value of a local time at the transmitter plus a link delay, buffering a received packet at a receiver, and releasing the buffered packet to an application level when a value of a local time at the receiver equals the timestamp value in the packet. This can help to ensure that the timing relationships between data packets present at a transmitter is maintained at a receiver, regardless of transport delays (waiting, transmission and processing) incurred by the data packets.

Claims

1. A method for preserving packet timing relationships at a source and a destination, the method comprising:

synchronizing local clocks at the source and the destination to a reference clock at periodic intervals;

appending a timestamp to each packet of a plurality of packets at the source;

transmitting each packet to the destination;

adjusting the timestamp of each packet at the destination; and

releasing each packet to an application level when the local clock of the destination is essentially equal to the adjusted timestamp of the packet.

2. The method of claim 1, wherein the packets being appended with the timestamp are data packets.

3. The method of claim 1, wherein the synchronizing comprises:

receiving a message, wherein the message is one of a periodic sequence of messages;

calculating a start time for the message;

adjusting the start time for the message;

computing a clock error value; and

correcting a local clock value with the clock error value.

4. The method of claim 1, wherein the timestamp specifies a time of the local clock at the source when a packet enters a link level of the source.

5. The method of claim 4, wherein the timestamp further includes an anticipated link delay.

6. The method of claim 5, wherein the link delay includes delays incurred by transmission, reception, processing, buffering, and retransmissions in case of transmission failures.

7. The method of claim 1, wherein the packets are MPEG-2 transport stream packets, and the timestamp specifies a time to release the MPEG-2 transport stream packets to an MPEG-2 application player for MPEG-2 packet decoding and presentation.

8. The method of claim 1, wherein the adjusting comprises adding a link delay to the timestamp.

9. The method of claim 8, wherein the link delay includes delays incurred by transmission, reception, processing, buffering, and retransmissions in case of transmission failures.

10. The method of claim 1 further comprising prior to the releasing, buffering the data.

11. The method of claim 10, wherein the buffering occurs at a receive buffer of the destination.

12. The method of claim 1, wherein the source and the destination are wireless connected.

13. The method of claim 1, wherein the source contains a transmitter and the destination contains a receiver.

14. A method for correcting a clock in a wireless network, the method comprising:

receiving a frame, wherein the frame is one of a periodic sequence of frames;

calculating a start time for the frame;

adjusting the start time for the frame;

computing a clock error value; and

correcting a local clock value with the clock error value.

15. The method of claim 14, wherein the frames are uniquely numbered and a start time for the frame can be determined from the frame'"'"'s unique number.

16. The method of claim 15, wherein the start time is equal to the unique number times the period of the periodic sequence.

17. The method of claim 16, where the period of the periodic sequence of frames is contained in each frame.

18. The method of claim 14, where each frame contains a value corresponding to its start time.

19. The method of claim 14, wherein the adjusting comprises adding an update delay to the start time of the frame.

20. The method of claim 19, wherein the update delay corresponds to a transmission and processing delay for each of the periodic frames in the wireless network.

21. The method of claim 20, wherein the update delay is consistent for all frames.

22. The method of claim 14, wherein the computing comprises subtracting a local clock value from the adjusted start time for the frame.

23. The method of claim 14, wherein a voltage-controlled oscillator is used to generate a signal to control the local clock, and wherein the correcting comprises changing a frequency of the signal with the clock error value.

24. The method of claim 23, wherein the change to the frequency of the signal is performed instantaneously.

25. The method of claim 23, wherein the change to the frequency of the signal is performed gradually.

26. The method of claim 23, wherein a voltage controlled oscillator is used as the oscillator.

27. The method of claim 14 further comprising filtering the clock error value prior to using it as an input to the oscillator.

28. The method of claim 27, wherein the filter performs a low pass filtering.

29. A circuit comprising:

an adder, the adder to combine an update delay with a beacon start time;

a subtractor coupled to the adder, the subtractor to subtract a local time from an output of the adder;

a voltage controlled oscillator coupled to the subtractor, the voltage controlled oscillator configured to generate a signal at a certain frequency dependant upon an output of the subtractor; and

a local timer coupled to the voltage controlled oscillator, the local timer configured to keep track of elapsed time based on the frequency of the signal.

30. The circuit of claim 29, wherein the local time is provided by the local timer.

31. The circuit of claim 30, wherein the local timer maintains a frame counter and a frame offset.

32. The circuit of claim 31, wherein the frame counter is equal to a beacon number contained in a beacon.

33. The circuit of claim 31, wherein the beacon start time is equal to a beacon number times a beacon interval.

34. The circuit of claim 33, wherein the beacon start time is equal to a timestamp included in the beacon.

35. The circuit of claim 29 further comprising a filter having an input coupled to the subtractor and an output coupled to the voltage controlled oscillator, the filter to smoothen the output of the subtractor.

36. The circuit of claim 35, wherein the filter is a low pass filter.

37. The circuit of claim 29, wherein the circuit is used in a wireless receiver.

38. The circuit of claim 37, wherein the wireless receiver is used in an IEEE 802.11 compliant communications system.

39. The circuit of claim 37, wherein the wireless receiver is used in a wireless personal area network.