Method and apparatus for removing jitter and correcting timestamps in a packet stream
First Claim
1. In a system for transmitting packets of information from a transmission site to a reception site at a transmission bit rate, wherein timestamp values are inserted into selected packets prior to transmission that represent the value of a transmission site clock, and further wherein the packets experience jitter during transmission, a method of substantially removing jitter and adjusting the timestamp values prior to reception at the reception site, said method comprising the steps of:
- (a) receiving the packets at an intermediate site having a local clock that operates at a nominal frequency substantially equal to the nominal frequency of the transmission site clock;
(b) storing the received packets in a buffer at said intermediate site;
(c) outputting the packets from said buffer at an initial nominal rate for reception at the reception site;
(d) measuring an average transit time of packets through said buffer;
(e) adjusting said rate at which packets are output from the buffer to maintain a substantially constant average transit time of packets through the buffer; and
,(f) modifying the timestamps of said selected packets to reflect changes in temporal relationship between packets due to said adjusted rate of output thereof.
3 Assignments
0 Petitions
Accused Products
Abstract
In a system that transmits packets containing timestamps and information from a transmission site to a reception site at a transmission bit rate the packets may experience jitter, i.e. variable delay, during transmission. In such a case, the packets are temporally shifted relative to the timestamps and to other packets. Where the timestamp values are inserted into selected packets prior to transmission and that timestamp represent the value of the transmission site clock, the method and apparatus of the present invention substantially removes the jitter and adjusts the timestamp values prior to reception of the packets at the reception site. The present invention achieves this goal by receiving the packets at an intermediate site that has a local clock operates at a nominal frequency substantially equal to the nominal frequency of the transmission site clock and uses that clock as a jitter-free clock to correct the packets. The packets are stored in a buffer at the intermediate site and output from the buffer at a controlled rate to maintain a substantially constant average transit time of packets through the buffer. Before the packets are output for reception at the reception site the timestamps of selected packets are modified to reflect the new temporal relationship between packets due to the controlled rate of output from the intermediate site.
431 Citations
10 Claims
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1. In a system for transmitting packets of information from a transmission site to a reception site at a transmission bit rate, wherein timestamp values are inserted into selected packets prior to transmission that represent the value of a transmission site clock, and further wherein the packets experience jitter during transmission, a method of substantially removing jitter and adjusting the timestamp values prior to reception at the reception site, said method comprising the steps of:
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(a) receiving the packets at an intermediate site having a local clock that operates at a nominal frequency substantially equal to the nominal frequency of the transmission site clock; (b) storing the received packets in a buffer at said intermediate site; (c) outputting the packets from said buffer at an initial nominal rate for reception at the reception site; (d) measuring an average transit time of packets through said buffer; (e) adjusting said rate at which packets are output from the buffer to maintain a substantially constant average transit time of packets through the buffer; and
,(f) modifying the timestamps of said selected packets to reflect changes in temporal relationship between packets due to said adjusted rate of output thereof. - View Dependent Claims (2, 3, 4, 5, 6, 7)
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8. In a system for transmitting packets of information from a transmission site to a reception site at a transmission bit rate, wherein timestamp values are inserted into selected packets prior to transmission that represent the value of a transmission site clock, and further wherein the packet may experience jitter during transmission, an apparatus for substantially removing packet jitter and adjusting packet timestamp values to synchronize the reception site clock to the transmission site clock, said apparatus comprising:
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a local clock; means for receiving the packets; a buffer for storing the received packets; means for outputting the packets from said buffer at an initial transmission bit rate for reception at the reception site; means cooperating with said local clock for measuring average transit times of packets through said buffer; means cooperating with said means for measuring transit times for adjusting the rate at which packets are output from said buffer to maintain a substantially constant average transit time of packets through the buffer; and
,means for modifying the timestamps of said selected packets to reflect changes in temporal relationship between packets due to said adjusted rate of output thereof. - View Dependent Claims (9, 10)
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Specification