Method and apparatus for reducing delay jitter
View Patent ↗Elastic buffering takes place within a head end terminal ( 200 ). An elastic buffer ( 206 ) has the effect of shifting time at its output frame-of-reference backward by a fixed amount to compensate for unknown time differences between time at its input frame-of-reference and time at the frame-of-reference of a remote node ( 150 ). The amount of time that the elastic buffer ( 206 ) shifts data is determined by a time-stamping technique, where data packets are time stamped at the remote node ( 150 ) and the time stamp is analyzed at the head end terminal ( 200 ) to determine the time shift.
1. A method comprising the steps of:
during an initial startup procedure, performing the steps of:
receiving a first plurality of packets from a remote source, i, wherein each packet within the first plurality of packets is time stamped;
determining a time of reception for each packet within the first plurality of packets;
comparing the time stamp with the time of reception for each packet within the first plurality of packets to produce a plurality of comparisons Δ i ;
determining a smallest Δ i from the comparisons;
after the initial startup procedure, performing the steps of:
receiving a second plurality of packets from the remote source, i, wherein each packet within the second plurality of packets is time stamped;
converting each time stamp on the second plurality of packets to the frame of reference of the receiver by adding the smallest Δ i that was determined during the startup procedure to the time stamp;
determining a play out time from an elastic buffer based on the converted time stamps in order to reduce delay jitter.
2. The method of claim 1 wherein the step of receiving the first plurality of packets from the remote source comprises the step of receiving the first plurality of packets from a node within a subscriber access network.
3. The method of claim 2 wherein the step of receiving the first plurality of packets from the node comprises the step of receiving the first plurality of packets from the node, wherein the first plurality of packets are time stamped by the node.
4. The method of claim 1 wherein the step of determining the time of reception comprises the step of determining the time of reception utilizing an internal time base that is not referenced to any external time base.
5. The method of claim 1 wherein the step of comparing the time stamp with the time of reception comprises the step of determining a plurality of differences (Δ i ) between the an internal time base and the time stamp for each packet within the first plurality of packets.
6. The method of claim 1 wherein the step of determining a play out time comprises the steps of:
adding the smallest Δ i that was determined during the startup procedure to each timestamp to produce a modified time stamp; and
adding the modified time stamp to a constant jitter buffering time, yielding a play out start time.
7. An apparatus comprising:
means for receiving a first plurality of packets during an initial startup procedure from a remote source, wherein each packet within the first plurality of packets is time stamped;
means for determining a time of reception for each packet within the first plurality of packets;
means for comparing the time stamp with the time of reception for each packet within the first plurality of packets to produce a plurality of comparisons Δ i ;
means for determining a smallest Δ i from the comparisons;
means for receiving a second plurality of packets after the startup procedure from the remote source, wherein each packet within the second plurality of packets is time stamped;
means for converting each time stamp on the second plurality of packets to the frame of reference of the receiver by adding the smallest Δ i determined during the startup procedure to the time stamp;
means for determining a play out time from an elastic buffer based on the converted time stamps in order to reduce delay jitter.
8. The apparatus of claim 7 wherein each packet within the first plurality of packets is received from a node within a subscriber access network.
9. The apparatus of claim 8 wherein the first plurality of packets are time stamped by the node.
10. The apparatus of claim 7 wherein the time of reception is determined by utilizing an internal time base that is not referenced to any external time base.
11. The apparatus of claim 7 wherein the plurality of comparisons (Δ i ) comprise a plurality of differences (Δ i ) between an internal time base and the time stamp for each packet within the first plurality of packets.