System and method for clock skew compensation between encoder and decoder clocks by calculating drift metric, and using it to modify time-stamps of data packets

• US 6,598,172 B1
• Filed: 10/29/1999
• Issued: 07/22/2003
• Est. Priority Date: 10/29/1999
• Status: Expired due to Term

• Current Assignee: Intel Corporation
• Original Assignee: Intel Corporation
• Inventors: Marshall, Robert A., VanDeusen, Mark P.
• Primary Examiner(s): Lee, Thomas
• Assistant Examiner(s): PATEL, NITIN C
• Application Number:
  US09/431,297
• Time in Patent Office: 1,362 Days
• Field of Search: 370/912, 370/516, 370/389, 714/744, 713/503
• US Class Current: 713/503
• CPC Class Codes: H04J 3/0632 Synchronisation of packets and cells, e.g. transmission of voice via a packet network, circuit emulation service [CES] queuing arrangements in packet switching elements H04L49/90; synchronising systems for the synchronous transmission of a pulse code modulated video signal with one or more other pulse code modulated signals H04N7/56
• CPC Class Codes: H04N 21/2365 Multiplexing of several video streams
• 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 21/4341 Demultiplexing of audio and video streams
• CPC Class Codes: H04N 21/4347 Demultiplexing of several video streams
• CPC Class Codes: Y10S 370/912 Packet communications

Abstract

In a coordinated computer system for encoding, transmitting, and decoding a series of data packets such as audio and/or video data, there may be a skew between the clock used by an encoder and the clock used by a decoder. In a method and device for compensating for this clock skew, the decoder calculates a drift metric representing the clock skew and modifies the time stamps of the data packets based on the drift metric. The decoder also performs a sample rate conversion on the digital data, in order to compensate for the clock skew between the encoder and decoder.

Claims

1. A method for compensating for clock skew in a coordinated computer system adapted to transmit a series of digital data packets, each digital data packet including a digital data sample, from an encoder to a decoder, comprising:

calculating a drift metric, based on a plurality of received digital data packets, to represent the clock skew between an encoder clock and a decoder clock;

modifying a time stamp of a digital data packet based on the drift metric; and

performing a sample rate conversion to adjust the playback rate of digital data.

2. The method of claim 1, the calculating of a drift metric including monitoring a queue of digital data packets.

3. The method of claim 1, further comprising:

recovering the drift metric used to modify the time stamp by calculating a difference between a time stamp of a first digital data packet and a time stamp of a second digital data packet, and dividing said difference by a playback time of the first digital data packet.

4. The method of claim 1, the performing of a sample rate conversion to adjust the playback rate of the digital data including

selectively replicating a digital data sample and adding the replicated digital data sample to the digital data, if the drift metric indicates that the encoder clock is slower than the decoder clock.

5. The method of claim 1, the performing of a sample rate conversion to adjust the playback rate of the digital data including

selectively deleting a digital data sample from the digital data, if the drift metric indicates that the encoder clock is faster than the decoder clock.

6. The method of claim 1, the series of digital data packets including digital audio data samples.

7. The method of claim 6, the series of digital data packets including digital video data samples.

8. The method of claim 1, the digital data transmission system adapted to comply with the MPEG Specification.

9. A computer system adapted to receive and decode a series of digital data packets from an encoder having an encoder clock, comprising:

a decoder clock;

a queue monitor, adapted to calculate a drift metric based on a plurality of received digital data packets;

a time stamp modification module, adapted to modify a time stamp of a digital data packet based on the drift metric;

an adjustment module, adapted to perform a sample rate conversion on a series of digital data samples, based on the drift metric.

10. The computer system of claim 9, the adjustment module adapted to perform the sample rate conversion by selectively replicating a digital data sample and adding the replicated digital data sample to the series of digital data samples, if the encoder clock is slower than the decoder clock.

11. The computer system of claim 9, the adjustment module adapted to perform the sample rate conversion by selectively deleting a digital data sample from the series of digital data samples, if the drift metric indicates that the encoder clock is faster than the decoder clock.

12. The computer system of claim 9, the series of digital data packets including digital audio data samples.

13. The computer system of claim 9, the series of digital data packets including digital video data samples.

14. The computer system of claim 9, the digital data transmission system adapted to comply with the MPEG Specification.

15. The computer system of claim 9, further comprising:

an audio decode module adapted to recover the drift metric used to modify the time stamp by calculating a difference between a time stamp of a first digital data packet and a time stamp of a second digital data packet, and dividing said difference by a playback time of the first digital data packet.

16. An article comprising a storage medium including a set of instructions, said set of instructions capable of being executed by a processor to implement a method for compensating for clock skew in a coordinated computer system adapted to transmit a series of digital data packets from an encoder to a decoder, the method comprising:

calculating a drift metric, based on a plurality of received digital data packets, to represent the clock skew between an encoder clock and a decoder clock;

modifying a time stamp of a digital data packet, based on the drift metric; and

performing a sample rate conversion to adjust the playback rate of digital data.

17. The article of claim 16, the method further including:

selectively replicating a digital data sample and adding the replicated digital data sample to the digital data, if the drift metric indicates that the encoder clock is slower than the decoder clock.

18. The article of claim 16, the method further including:

selectively deleting a digital data sample from the digital data, if the drift metric indicates that the encoder clock is faster than the decoder clock.

19. The article of claim 16, the series of digital data packets including digital audio data.

20. The article of claim 16, the series of digital data packets including digital video data.

21. The article of claim 16, the digital of transmission system adapted to comply with the MPEG Specification.