Television – Synchronization – Locking of video or audio to reference timebase
Reexamination Certificate
1998-10-14
2001-09-04
Hsia, Sherrie (Department: 2614)
Television
Synchronization
Locking of video or audio to reference timebase
C348S515000, C348S513000
Reexamination Certificate
active
06285405
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to communications systems and more particularly to a method and apparatus for synchronizing audio and video data in video conferencing systems.
2. Description of the Related Art
A video conference environment typically includes a plurality of conference sites, commonly referred to as endpoints, which are geographically separated but electronically linked together to enhance collaboration between and among individuals at the various conference sites. A video conference system attempts to replicate the interpersonal communication and information sharing which would occur if all the participants were together in the same room at the same time.
Two or more endpoints participating in a video conference are coupled together via digital communication lines. For example, in a point-to-point conference, two endpoints are coupled in a video conference. One endpoint dials the other endpoint directly to initiate the video conference. In a multi-point video conference involving more than two endpoints, each endpoint dials a multiple point control unit (MCU) which couples the endpoints in the same video conference.
Each endpoint typically transmits and receives conference information (e.g., audio and video information) to and from the other endpoint(s). Each conference site includes at least one source of conference information. For example, each endpoint has one or more audio and video sources from which to select for transmission to the other endpoint(s). When audio and video signals are transmitted, synchronizing presentation of the signals at the other endpoints is referred to as “lip synching” and is an important element in user satisfaction with video conferencing. For example, when a close-up image of an individual speaking at one endpoint is displayed on monitors at the other endpoints, it is desirable for the speaker's voice to match movement of the speaker's mouth.
Communication standards such as H.320, H.324, and H.323 for video conferencing systems use separate data streams for audio data and video data. Digital video images require more information to represent each frame of data compared to digital audio messages, and accordingly, more time is required to process video images compared to audio data streams.
A common technique for achieving lip sync in video conferencing systems is to use a table of static values representing the delay between the audio and video messages at various line rates. Currently, a user selects an audio delay setting from a property page on a system configuration property sheet. The property page allows the user to set delay values for both transmitted and received audio for each line rate, or to reset the list of values to default values. Controlling delay using a constant, static value is inadequate, however, because total delay is affected by line rate and video bit rate. Video bit rate is correspondingly affected by factors such as data rates, communication protocol, audio algorithm selection, and the number of data channels transmitted. These factors often change throughout the course of a video conference and it is therefore desirable for a video conferencing system to be capable of automatically adjusting the delays to keep the audio and video data streams synchronized.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention, a method and an apparatus for adaptively synchronizing data signals are provided which substantially eliminate or reduce disadvantages and problems associated with the prior art. The present invention is applicable to systems where it is desirable to synchronize two or more data signals that require different amounts of time to encode, transmit, and decode, such as audio and video data signals in video conferencing systems.
In particular, a system for synchronizing transmission of encoded frames of data is provided which includes a first encoder coupled to a first interface to receive a first frame of data. The first encoder associates a first time tag with the first frame of data wherein the first time tag is based on the time the first frame of data was received from the first interface. The first encoder encodes the first frame of data to generate a first encoded frame of data, and associates a second time tag with the first encoded frame of data. The second time tag is based on the time the first frame of data was substantially encoded. The first encoder determines an encoding delay based on the first time tag and the second time tag, and transmits the encoding delay to a data processor. The system also includes a second encoder coupled to a second interface to receive a second frame of data. The second encoder is coupled to the data processor. The data processor is operable to transmit data to the second encoder and receive data from the second encoder. Further, the data processor generates a target delay value based on the encoding delay and an offset value, and transmits the target delay value to the second encoder. The second encoder receives the target delay value from the data processor, encodes the second frame of data, and transmits the second encoded frame of data at a time based on the target delay value.
The present invention also provides a system for synchronizing presentation of decoded frames of data which includes a data processor coupled to a first decoder and a second decoder. The first decoder associates a first time tag with a first frame of data based on the time the first frame of data was received from the data processor. The first decoder generates a first decoded frame of data, and associates a second time tag with the first decoded frame of data based on the time the first frame of data was substantially decoded. The first decoder determines a decoding delay based on the first time tag and the second time tag, and transmits the decoding delay to the data processor. The data processor generates a target delay value based on the decoding delay and an offset value, and transmits the target delay value to the second decoder. The second decoder receives the target delay value from the data processor, decodes the second frame of data to generate a second decoded frame of data, and transmits the second decoded frame of data at a time based on the target delay value.
A method and apparatus for measuring the amount of time required to encode a frame of video data is also provided which may be used to synchronize an audio data signal with a video data signal. The method includes receiving a frame of video data, associating a first time tag with the frame of video data based on the time the video data was received, encoding the frame of video data, associating a second time tag with the encoded video data based on the time the video data was substantially encoded, and determining the video encoding delay based on the first time tag and the second time tag.
A method and apparatus for measuring the amount of time required to decode a frame of encoded video data is also provided which may be used to synchronize an audio data signal with a video data signal. The method includes parsing encoded video data for a picture start code and a video encoding temporal reference, sampling a system timer to generate a first timer value, decoding the frame of encoded video data, sampling the system timer to generate a second timer value after decoding the frame of encoded video data, and determining the video decoding delay based on the first timer value and the second timer value.
A method and apparatus for introducing a target time delay value in an audio codec is also provided. The target time delay value is used to synchronize an audio data signal with a video data signal. The method includes determining a video time delay value, generating the target time delay value based on an offset value and the video time delay value, and transmitting the target time delay value to the audio codec.
In another embodiment, a plurality of video time delay values are determined, and an average value of the video time de
Binford, Jr. Donald D.
Jackman William J.
Korte Matthew W.
Bertani Mary Jo
Hsia Sherrie
Skyjerven Morrill MacPherson LLP
Vtel Corporation
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