Method for decoding an audio/video stream and corresponding device
After a stream transition event such as a channel change, a drift between audio and video presentation time stamps in an audio/video stream is determined. The thus determined drift serves to compute a duration of a slow video motion start sequence with a given slow video rate, so that video can be shown early without yet being synchronized with audio. The slow video motion start enables the audio to catch up with the video and allows to ensure a synchronization of audio with video after the duration of the slow video motion start sequence, where after video and audio are synchronized and evolve normally. The present principles use a single decoder clock that is fed to the audio and video decoders, the decoder clock fed to the video decoder being offset with a decreasing value during the period of slow video motion.
1. A method for decoding an audio/video stream, wherein the method is implemented by an audio/video stream receiver and comprises:
receiving a stream transition event;
receiving an audio/video stream further to the stream transition event;
computing a drift between a video presentation time stamp comprised in said audio/video stream and an audio presentation time stamp comprised in said audio/video stream;
obtaining a duration of a slow video motion start sequence;
initializing a decoder clock with a value of a program clock reference from said audio/video stream;
providing said decoder clock to an audio decoder in said audio/video stream receiver;
providing said decoder clock to a video decoder in said audio/video stream receiver, said decoder clock provided to said video decoder being offset with an initial non-zero offset value and with a decreasing offset value until said offset reaches a zero offset value at the end of said duration of said slow video motion start sequence; and
synchronizing offsetting of said decoder clock provided to said video decoder with a frame rate of video comprised in said audio/video stream.
2. The method according to claim 1 , wherein said offset value is decreased during said duration according to a linear function.
3. The method according to claim 1 , wherein said offset value is decreased during said duration according to an exponential function.
4. The method according to claim 1 , wherein the decreasing of the offset value is reevaluated as a function of variations of said drift.
5. The method according to claim 1 , wherein said initial non-zero offset value being a function of said computed drift and of a frequency of said decoder clock, said decreasing offset value being a function of said initial non-zero offset value and of a number of clock ticks of said decoder clock during said duration of said slow video motion start sequence.
6. A device for decoding an audio/video stream, wherein said device comprises:
at least one reception interface configured to:
receive a stream transition event; and
receive an audio/video stream further to the stream transition event;
a processor configured to:
compute a drift between a video presentation time stamp and an audio presentation time stamp comprised in said audio/video stream;
obtain a duration of a slow video motion start sequence;
initialize a decoder clock with a value of a program clock reference from said audio/video stream;
provide said decoder clock to an audio decoder comprised in said device;
provide said decoder clock to a video decoder comprised in said device, said decoder clock provided to said video decoder being offset with an initial non-zero offset value and with a decreasing offset value until said offset reaches a zero offset value at the end of said duration of said slow video motion start sequence; and
synchronize the offsetting of said decoder clock provided to said video decoder with a frame rate of video comprised in said audio/video stream.
7. The device according to claim 6 , wherein said processor is further configured to decrease said offset value during said duration according to a linear function.
8. The device according to claim 6 , wherein said processor is further configured to decrease said offset value during said duration according to an exponential function.
9. The device according to claim 6 , wherein said processor is further configured to reevaluate decreasing of the offset value as a function of variations of said drift.
10. The device according to claim 6 , wherein said processor is further configured to compute said initial non-zero offset value as a function of said drift and of a frequency of said decoder clock, said decreasing offset value being a function of said initial non-zero offset value and of a number of clock ticks of said decoder clock during said duration of slow video motion start sequence.