Clock recovery for video encoding/transcoding applications
View Patent ↗Approaches for clock synchronization in digital video environments. In an embodiment, an encoder/transcoder calculates a ratio between a system clock and a source clock. The source clock is used by a source device to encode or transcode digital video. The system clock is used by the encoder/transcoder. After the encoder/transcoder receives the digital video from the source device, the encoder/transcoder uses the calculated ratio to create a recovered clock. The recovered clock is locked to a frequency of the source clock but not to the phase of the source clock. The encoder/transcoder uses the recovered clock to encode or transfer the digital video received from the source device. The encoder/transcoder ensures that the frequency of the recovered clock does not change faster than a certain rate, e.g., 0.075 Hz/second.
1. A non-transitory computer-readable storage medium storing one or more sequences of instructions for synchronizing a system clock to a source clock, which when executed by one or more processors, cause:
a digital video encoder/transcoder calculating a ratio between the system clock and the source clock, wherein the system clock is used by the digital video encoder/transcoder; and
the digital video encoder/transcoder using the calculated ratio to create a recovered clock that is locked to a frequency of the source clock but not to the phase of the source clock by:
maintaining a circular queue of linear conversion functions, and
selecting a particular linear conversion function, from said circular queue, to create said recovered clock for a fixed duration of time; and
the digital video encoder/transcoder using said recovered clock for said fixed duration of time to encode or transcode digital video received from a source that uses said source clock.
2. The non-transitory computer-readable storage medium of claim 1 , wherein the digital video encoder/transcoder using the calculated ratio to create the recovered clock comprises:
the digital video encoder/transcoder ensuring that the frequency of the recovered clock does not change more than 0.075 Hz/second.
3. The non-transitory computer-readable storage medium of claim 1 , wherein the digital video encoder/transcoder using the calculated ratio to create a recovered clock comprises:
the digital video encoder/transcoder, after expiration of a period of time, recalculate the calculated ratio to create a new calculated ratio; and
the digital video encoder/transcoder using the new calculated ratio to create a new recovered clock.
4. The non-transitory computer-readable storage medium of claim 1 , wherein during said fixed duration of time, a ratio of said recovered clock to said system clock is constant.
5. The non-transitory computer-readable storage medium of claim 1 , wherein the digital video encoder/transcoder using the calculated ratio to create the recovered clock comprises:
after expiration of said fixed duration of time, selecting a next linear conversion function, after said particular linear conversion function in said circular queue, to create said recovered clock for a new fixed duration of time.
6. The non-transitory computer-readable storage medium of claim 1 , wherein values of a scale factor (K) used in said particular linear conversion function, a change in the slope of a line equation used to convert said system clock to said recovered clock (Δm), and a duration of time in which said slope of said line equation is fixed (Δs) are values which are the power of 2.
7. The non-transitory computer-readable storage medium of claim 3 , wherein said period of time is between 4-6 seconds.
8. The non-transitory computer-readable storage medium of claim 6 , wherein K has a value greater than 1/Δm.
9. An apparatus for synchronizing a system clock to a source clock, comprising:
one or more processors; and
one or more non-transitory computer-readable storage mediums storing one or more sequences of instructions, which when executed, cause:
a digital video encoder/transcoder calculating a ratio between the system clock and the source clock, wherein the system clock is used by the digital video encoder/transcoder; and
the digital video encoder/transcoder using the calculated ratio to create a recovered clock that is locked to a frequency of the source clock but not to the phase of the source clock by:
maintaining a circular queue of linear conversion functions, and
selecting a particular linear conversion function, from said circular queue, to create said recovered clock for a fixed duration of time; and
the digital video encoder/transcoder using said recovered clock for said fixed duration of time to encode or transcode digital video received from a source that uses said source clock.
10. The apparatus of claim 9 , wherein the digital video encoder/transcoder using the calculated ratio to create the recovered clock comprises:
the digital video encoder/transcoder ensuring that the frequency of the recovered clock does not change more than 0.075 Hz/second.
11. The apparatus of claim 9 , wherein the digital video encoder/transcoder using the calculated ratio to create a recovered clock comprises:
the digital video encoder/transcoder, after expiration of a period of time, recalculate the calculated ratio to create a new calculated ratio; and
the digital video encoder/transcoder using the new calculated ratio to create a new recovered clock.
12. The apparatus of claim 9 , wherein during said fixed duration of time, a ratio of said recovered clock to said system clock is constant.
13. The apparatus of claim 11 , wherein said period of time is between 4-6 seconds.
14. A method for synchronizing a system clock to a source clock, comprising:
a digital video encoder/transcoder calculating a ratio between the system clock and the source clock, wherein the system clock is used by the digital video encoder/transcoder; and
the digital video encoder/transcoder using the calculated ratio to create a recovered clock that is locked to a frequency of the source clock but not to the phase of the source clock by:
maintaining a circular queue of linear conversion functions, and
selecting a particular linear conversion function, from said circular queue, to create said recovered clock for a fixed duration of time; and
the digital video encoder/transcoder using said recovered clock for said fixed duration of time to encode or transcode digital video received from a source that uses said source clock.
15. The method of claim 14 , wherein the digital video encoder/transcoder using the calculated ratio to create the recovered clock comprises:
the digital video encoder/transcoder ensuring that the frequency of the recovered clock does not change more than 0.075 Hz/second.
16. The method of claim 14 , wherein the digital video encoder/transcoder using the calculated ratio to create a recovered clock comprises:
the digital video encoder/transcoder, after expiration of a period of time, recalculate the calculated ratio to create a new calculated ratio; and
the digital video encoder/transcoder using the new calculated ratio to create a new recovered clock.
17. The method of claim 16 , wherein said period of time is between 4-6 seconds.