Methods and circuits for synchronizing streaming data and systems using the same

• US 6,356,871 B1
• Filed: 06/14/1999
• Issued: 03/12/2002
• Est. Priority Date: 06/14/1999
• Status: Expired due to Term

• Current Assignee: Cirrus Logic Incorporated
• Original Assignee: Cirrus Logic Incorporated
• Inventors: Hemkumar, Nariankadu Datatreya, Rao, Raghunath Krishna, Dokic, Miroslav
• Primary Examiner(s): Korzuch, William
• Assistant Examiner(s): ABEBE, DANIEL DEMELASH
• Application Number:
  US09/332,804
• Time in Patent Office: 1,002 Days
• Field of Search: 375/372, 375/354, 375/355, 704/500, 704/502
• US Class Current: 704/500
• 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 21/44 Processing of video elementary streams, e.g. splicing a video clip retrieved from local storage with an incoming video stream or rendering scenes according to encoded video stream scene graphs

Abstract

A method for synchronizing stream processing with a locally generated system time clock is disclosed. A program clock reference is recovered from a transport layer and used to vary the frequency of the locally generated system time clock. A presentation time stamp is recovered from a packetized elementary stream derived from the transport layer. The relationship between a reference sample associated with the presentation time stamp from the packetized elementary stream and a current sample being streamed is determined relative to the system time clock. Data samples are added or subtracted from a stream of data samples being streamed in response to the step of determining to establish a relationship between the stream of samples and the presentation time stamp relative to the system time clock.

Claims

1. A method synchronizing data streaming in a system including a buffer in memory and a first-in-first-out memory comprising the steps of:

receiving a reference sample along with a corresponding time stamp, the time stamp indicating a presentation time for the reference sample in system time clock units;

storing the reference sample in the buffer, the buffer storing previously received samples;

loading a selected number of samples from the buffer into the first-in-first-out-memory when the first-in-first-out memory empties to a predetermined level; and

determining a time difference in system time clock units between the presentation time and an output time of the reference sample from the first-in-first-out memory comprising the substeps of;

determining a first value representing a difference in system time clock units between the presentation time and a time of executing said step of loading;

determining a second value representing a delay in system time clock units required to transfer the reference sample through the buffer to an output of the first-in-first-out memory; and

taking the difference in system clock units between the first and second values.

2. The method of claim 1 wherein data is output from the first-in-first-out memory at a sampling frequency and said step of determining the second value comprises the substep of multiplying a number of samples in the memory buffer and the first-in-first-out memory between the reference sample and the output by a ratio of a frequency of the system time clock and the sampling frequency.

3. The method of claim 1 wherein said step of loading is performed when the first-in-first-out memory is half empty.

4. The method of claim 1 wherein the streaming data comprises a packetized elementary stream of data.

5. The method of claim 4 wherein the packetized stream comprises audio data.

6. The method of claim 1 and further comprising the step of adding samples to a stream of samples output from the first-in-first-out memory when the time difference between the presentation time and the output time of the reference sample is a positive value and dropping samples when the time difference between presentation time and the time of output of the reference sample is a negative value.

7. The method of claim 1 and further comprising the step of determining a time difference prior to step of decompressing said prior data.

8. A method synchronizing data streaming in a system including a buffer in memory and a first-in-first-out memory comprising the steps of:

receiving a reference sample along with a corresponding time stamp, the time stamp indicating a presentation time for the reference sample in system time clock units;

storing the reference sample in the buffer, the buffer storing previously received samples;

loading a selected number of samples from the buffer into the first-in-first-out memory when the first-in-first-out memory empties to a predetermined level; and

determining a difference in number of samples being output between the presentation time and the output time of the reference sample from the first-in-first-out memory comprising the substeps of;

determining a first value representing a difference in the number of samples between the current sample being output and the reference sample at the time of performing said step of loading;

subtracting a number of previously stored samples in the buffer and first-in-first-out memory from the first value.

9. The method of claim 8 wherein samples are output from the first-in-first-out memory at a sampling rate and said step of determining the first value comprises the substeps of:

determining a difference in number of time clock units between the presentation time and the time of performing said step of loading; and

multiplying the difference in number of time clock units by a ratio of the sampling frequency and a frequency of the system time clock.

10. The method of claim 8 and further comprising the step of adding samples to a stream of samples output from the first-in-first-out memory when the difference in samples being output between the presentation time and the output time of the reference sample is a positive value and subtracting samples from the stream when the difference in samples is a negative value.

11. The method of claim 8 wherein said step of loading is performed when the first-in-first-out memory is half empty.

12. The method of claim 8 wherein the streaming data comprises a packetized elementary stream of data.

13. The method of claim 9 wherein the packetized stream comprises audio data.

14. The method of claim 8 and further comprising the step of determining a difference in number of samples prior to step of decompressing said prior data.

15. An audio decoder comprising:

an input port for receiving a packetized elementary stream of audio data, presentation time stamps periodically inserted in said stream;

a data buffer for storing audio samples retrieved from said stream;

an output first-in-first-out memory for sourcing decoded audio data to an external device at a selected sampling rate, said output first-in-first-out memory loaded from the data buffer when said first-in-first-out memory reaches a partially empty level; and

a digital signal processor for determining a time advantage between a time indicated by a said presentation time stamp for output of a reference sample from the first-in-first-out memory and an actual time of output of said reference sample in accordance with the formula;

TA=PTS−

STC_lhe−

{(DIPSTICK+FIFO_SIZE )*(STCfreq/Fs)}

 

wherein;

TA is the time advantage in system time clock units;

PTS is the presentation time stamp in units of the system time clock;

STC_lhe is the time of reload of the first-in-first-out memory;

DIPSTICK is the number of samples in the buffer at time of reload;

Fs is the frequency at which samples are output from the first-in-first-out memory;

STCfreq is the frequency of the system time clock; and

FIFO_SIZE is the number of samples in the first-in-first out memory.

16. The audio decoder of claim 15 wherein said digital signal processor comprises a selected one of a plurality of digital signal processors fabricated on a single chip.

17. The audio decoder of claim 15 wherein the buffer, first-in-first-out memory and digital signal processor are fabricated on a single chip.

18. The audio decoder of claim 15 wherein said audio stream comprises a stream of compressed audio data and said digital signal processor is operable to decompress said data stream prior to storage of said samples in said buffer.

19. The audio decoder of claim 15 wherein said system time clock is derived from a local oscillator and said decoder further comprises circuitry for varying an output frequency of said oscillator in response to a received program clock reference.

20. An audio decoder comprising:

an input port for receiving a packetized elementary stream of audio data, presentation time stamps periodically inserted in said stream;

a data buffer for storing audio samples retrieved from said stream;

an output first-in-first-out memory for sourcing decoded audio data to an external device at a selected sampling rate, said output first-in-first-out memory loaded from the data buffer when said first-in-first-out memory reaches a partially empty level; and

a digital signal processor for determining a sample advantage representing a difference in a number of samples being output between a presentation time indicated by a said presentation time stamp for output of a reference sample and an actual output time of the reference sample from the first-in-first-out memory in accordance with the formula;

SA={(PTS−

STC_lhe)*(F_s/STCfreq)}−

(DIPSTICK+FIFO_SIZE)

 

wherein;

TA is the time advantage in system time clock units;

PTS is the presentation time stamp in units of the system time clock;

STC_lhe is the time of reload of the first-in-first-out memory;

DIPSTICK is the number of samples in the buffer at time of reload;

F₂ is the frequency at which samples are output from the first-in-first-out memory;

STCfreq is the frequency of the system time clock; and

FIFO_SIZE is the number of samples in the first-in-first-out memory.

21. The audio decoder of claim 20 wherein said digital signal processor comprises a selected one of a plurality of digital signal processors fabricated on a single chip.

22. The audio decoder of claim 20 wherein the buffer, first-in-first-out memory and digital signal processor are fabricated on a single chip.

23. The audio decoder of claim 20 wherein said audio stream comprises a stream of compressed audio data and said digital signal processor is operable to decompress said data stream prior to storage of said samples in said buffer.

24. The audio decoder of claim 20 wherein said system time clock is derived from a local oscillator and said decoder further comprises circuitry for varying an output frequency of said oscillator in response to a received program clock reference.

25. A method for synchronizing stream processing with a locally generated system time clock comprising the steps of:

recovering a program clock reference from a transport layer;

varying the frequency of the locally generated system time clock using the program clock reference;

recovering a presentation time stamp from a packetized elementary stream derived from the transport layer;

determining the relationship between a reference sample associated with the presentation time stamp from the packetized elementary stream and a current sample being streamed relative to the system time clock, comprising the substeps of;

storing the reference sample in a buffer holding previously received samples;

loading a selected number of samples from the buffer into a first-in-first-out-memory when the first-in-first-out memory empties to a predetermined level; and

determining a time difference in units of the system time clock between the presentation time and an output time of the reference sample from the first-in-first-out memory comprising the substeps of;

determining a first value representing a difference in system time clock units between a time indicated by the presentation time stamp and a time of executing said step of loading;

determining a second value representing a delay in system time clock units required to transfer the reference sample through the buffer to an output of the first-in-first-out memory; and

taking the difference in system clock units between the first and second values; and

adding and subtracting data samples from a stream of data samples being output in response to said step of determining to establish a relationship between the presentation time stamp and the stream of samples relative to the system time clock.

26. A method for synchronizing stream processing with a locally generated system time clock comprising the steps of:

recovering a program clock reference from a transport layer;

varying the frequency of the locally generated system time clock using the program clock reference;

recovering a presentation time stamp from a packetized elementary stream derived from the transport layer;

determining the relationship between a reference sample associated with the presentation time stamp from the packetized elementary stream and a current sample being streamed relative to the system time clock, comprising the substeps of;

loading a selected number of samples from a buffer into a first-in-first-out-memory when the first-in-first-out memory empties to a predetermined level; and

determining a difference in number of samples being output between the presentation time and the output time of the reference sample from the first-in-first-out memory comprising the substeps of;

determining a first value representing a difference in a number of samples between the current sample being output from the first-in-first-out memory and the reference sample at the time of performing said step of loading; and

subtracting a number of previously stored samples in the buffer and first-in-first-out memory from the first value; and

adding and subtracting data samples from a stream of data samples being output in response to said step of determining to establish a relationship between the presentation time stamp and the stream of samples relative to the system time clock.