Method of processing a sequence of coded video frames

1. A method of modifying a digital data stream composed of a sequence of frames, where each frame represents an image and includes a data field specifying a presentation time stamp value for the frame, comprising:

a. initializing a first variable with a frame interval value based on a nominal frame rate;

b1. receiving a first frame and reading the presentation time stamp value of the first frame;

b2. assigning, to a second variable, the presentation time stamp value of the first frame;

b3. assigning, to a third variable for representing the first frame's clock time based on presentation time stamp, the value of the second variable;

b4. assigning, to a fourth variable for representing a frame's clock time based on presentation time stamp, the value of the second variable;

b5. assigning, to a fifth variable for representing a stream time stamp of interest, a value of zero;

c1. receiving a next frame and reading the presentation time stamp value of said next frame;

c2. assigning, to the second variable, the presentation time stamp value of said next frame;

c3. assigning, to a difference variable, the value of the second variable minus the fourth variable;

c4. assigning, to the fourth variable, the value of the second variable;

c5. assigning, to the fifth variable, a sum of the values of the fifth variable and the first variable; and

d. testing whether there is a discontinuity in presentation time stamp value between said next frame and the previous frame and, if so:

assigning, to a sixth variable for representing the value of the stream time stamp of interest at a point of discontinuity in presentation time stamp, the value of the fifth variable;

assigning, to the third variable, the value of the second variable; and

storing said next frame in a database using the value of the fifth variable as an index.

2. A method according to claim 1 , wherein the sequence of frames is composed of P frames, I frames and B frames, and

step a. further comprises initializing a seventh variable with a minimum frame interval value based on said nominal frame rate, and the method further comprises, if there is no discontinuity in presentation time stamp value between said next frame and said previous frame,

testing whether said next frame is either of a P frame or an I frame and, if so, and if the value of the difference variable is greater than the value of the seventh variable, assigning to the fifth variable the value of the fourth variable minus the third variable plus the sixth variable.

3. A method according to claim 1 , wherein the sequence of frames is composed of P frames, I frames and B frames, and the method further comprises, if there is no discontinuity in presentation time stamp value between said next frame and said previous frame, testing whether said next frame is a B frame.

4. A method according to claim 3 , further comprising, if said next frame is not a B frame, resetting a B frame flag false.

5. A method according to claim 3 , further comprising, if said next frame is a B frame, testing whether a B frame flag is set true.

6. A method according to claim 5 , further comprising, if said B frame flag is not set true, setting said B frame flag true.

7. A method according to claim 5 , wherein step a. further comprises assigning a value to a tolerance variable and step d. further comprises testing whether the absolute value of the difference variable exceeds the value of the tolerance variable, and the method further comprises, if said B frame flag is set true, assigning the value of the difference variable to the first variable and assigning the value of the first variable multiplied by three to the tolerance variable if the difference variable is greater than the seventh variable.

8. A non-transitory computer-readable storage medium comprising stored instructions which, when executed by one or more processors, cause the one or more processors to perform the method of claim 1 .