CA2220795A1 - Method and apparatus for pcr jitter measurement in an mpeg-2 transport stream using sliding window - Google Patents

Abstract

A method and a test instrument are provided for measuring program clock reference (PCR) jitter in a data stream, such as an MPEG-2 transport stream. The test instrument includes a line interface circuit for receiving the data stream, a digital computer for processing the data stream and a local clock for generating timestamps. The digital computer extracts from the data stream a PCR sample value which represents a count of a system time clock used to generate the data stream. The timestamp is obtained from the local clock and is associated with the PCR sample value. The digital computer determines a reconstructed system time clock from a set of values including the PCR sample value and the timestamp, and previously-received PCR sample values and associated timestamps within a window representative of a predetermined time interval. The window is shifted as new PCR sample values are received. The digital computer determines a PCR jitter value for the PCR sample value by determining a deviation of the PCR sample value from the reconstructed system time clock. The PCR jitter value may be displayed on a display unit, preferably in a histogram.

Description

CA 0222079~ l997-ll-l2 METHOD AND APPARATUS FOR PCR Jl l l ~;~ MEASUREMENT

Cross Reference to Related Application s This application is a continll~tion-in-part of application Serial No. 08/632,036, filed April 12, 1996.

Field of the Invention This invention relates to analysis of data streams transmitted through a data 0 communication channel and, more particularly, to test instruments and methods for measuring program clock reference (PCR) jitter in an MPEG-2 transport stream.

Ba~k~round of the Invention In the field of data communications, one highly dem~ntling application is the s tr~n.smission of full motion video, such as may be required for video-on-demand applications.
Various protocols which involve colllpres~ion of digitized video information have been proposed. One such protocol is the MPEG-2 transport stream, as defined in standard ISO/IEC
13818-1: 1996 promulgated by the ISO/IEC. The MPEG-2 transport stream involves tr~n~mi~sion of video and audio information in transport stream packets of 188 bytes in 20 length. Each packet includes a header, which contains control information, and a payload, which contains video or audio information. The MPEG-2 transport stream may carry multiple different programs simultaneously. Each packet in the transport stream is associated with a program by a packet identifier (PID) contained in the header. The header is of variable length, depending on whether it collLaills an adaptation field. The adaptation field contains control 2s information that is not necessarily present in every transport stream packet.One of the fields that may be contained in the adaptation field is a program clock reference (PCR). The PCR is re~l~se~ ion of a system time clock that was used at the source to encode the data in the transport stream packet. The system time clock typically has a frequency of 27 MHz. Each PCR field contains a PCR sample value which represents a 30 count of the system time clock at the time when the transport stream packet was encoded for tr~n.cmission. PCR sample values for a particular program are received with different 120965.1 CA 0222079~ l997-ll-l2 transport stream packets having the same PID. A series of PCR samples can be used to reconstruct the system time clock at the decoder end of the communication channel.
The MPEG-2 transport stream may, for example, be transmitted through asynchronous transfer mode (ATM) networks developed for high speed, packetized digital tr~n.cmi.~sion of s data, audio and video. One of the parameters that is used in characterizing an MPEG-2 transport stream is PCR jitter. In an ideal communication channel, all transport stream packets, and therefore all PCR samples, are received a fixed time after tr~n~mi~sion.
However, in a real communication channel, variable delays may be introduced by different channel elements. For example, different transport stream packets may follow dirrelell~
0 network paths in reaching the final clestin~tion. Variations in arrival times produce PCR jitter.
PCR jitter is described in standard ISO/~EC 13818-9:1996.

Summary of the Invention According to the present invention, methods and appaldlus for measuring program 5 clock reference (PCR) jitter in a data stream cont~ining digitally encoded information and PCR samples are provided. The data stream typically comprises an MPEG-2 transport stream.
The method of the invention comprises the steps of providing a test instrument including a line interface circuit for receiving a data stream, a digital computer connected to the line interface circuit for processing the data stream, and a local clock for generating timestamps.
20 The digital computer extracts from the data stream a PCR sample value which represents a count of a system time clock used to generate the data stream. A timestamp, which represents a time of arrival of the PCR sample value, is obtained from the local clock and is associated with the PCR sample value. The digital computer determines a reconstructed system time clock from a set of values comprising the PCR sample value and the timestamp, and 2s previously-received PCR sample values and associated timestamps within a window representative of a predetermined time interval that terminates with the PCR sample value and the timestamp. The digital computer determines a PCR jitter value for the PCR sample value by determining a deviation of the PCR sample value from the reconstructed system time clock. The PCR jitter value represents a variation in the arrival time of the PCR sample value 30 with respect to the reconstructed system time clock.
The reconstructed system time clock is preferably determined by determining a line of ~20965.1 CA 0222079~ l997-ll-l2 best fit to the set of values within the window. The slope and the offset of the line of best fit are calculated. The line of best fit represents the reconstructed time clock.
The above procedure is preferably repeated for a plurality of PCR sample values extracted from the data stream to provide a plurality of PCR jitter values. The PCR jitter s values are determined in real time as the PCR sample values are received, without waiting for all PCR sample values to be received. The window is shifted relative to the set of PCR
sample values and timestamps for each new PCR sample value received, so that a predetermined number of the most recently received sample values is used to determine the line of best fit. Thus, a sliding window is used to determine PCR jitter values.o The step of determining the line of best fit preferably includes a recursive linear regression calculation on the set of PCR sample values and timestamps within the window.
The variance of the PCR jitter values may also be determined using a recursive calculation.
The invention provides an accurate measurement of PCR jitter for each PCR samplevalue received. The PCR jitter measurement is available in real time as the PCR samples are received. The accuracy of the PCR jitter measurement is independent of the number of PCR
sample values received. Furthermore the PCR jitter measurement quickly adjusts to a change in parameters, such as a change in network delay or clock slew.

Brief Description of the Dr~
For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:
FIG. 1 is a block diagram of an example of a test instrument suitable for implementing PCR jitter measurement in accordance with the invention;
FIG. 2 is a flow diagram of a first embodiment of a routine for PCR jitter 25 measurement;
FIG. 3 is a graph of norrnalized PCR values as a function of normalized PCR arrival times;
FIG. 4 is a block diagram that illustrates hardware and software components involved in an implementation of the invention;
FIG. 5 is an example of a histograrn showing the frequency of different PCR jitter values;

120965.1 CA 0222079~ l997-ll-l2 FIG. 6 is a graph of norm~li7e(1 PCR values as a function of norm~lized PCR arrival times, illustrating the change in slope of the line of best fit that may result from a change in network delay;
FIG. 7 is a graph of norm~li7e~1 PCR values as a function of norm~lizPcl PCR arrival s times, illustrating di~lenl lines of best fit at different window times; and FIG. 8 is a flow diagrarn of a second embodiment of a routine for PCR jitter measurement.

DetailedDcsl ;~tion A block diagram of a test instrument for analyzing a MPEG-2 transport stream is shown in FIG. 1. Line interface hardware 10 receives an MPEG-2 transport stream, which may be transmitted from a remote location. The MPEG-2 transport stream is typically received on ATM and possibly a physical layer, such as SONET. The MPEG-2 transport stream data is supplied to a protocol processor l 2. The protocol processor 12 receives each 5 packet in the MPEG-2 transport stream and identifies information in the header in accordance with the MPEG-2 protocol. The protocol processor 12 also includes a decoder for decoding the payload information. Selected information can be extracted from the MPEG-2 transport stream by the protocol processor 12. Suitable line interface hardware and protocol processors are known in the art and are commercially available. For example, the line 20 interface hardware may be a Model E1697A, and the protocol processor may be a Model E4209B, both sold by Hewlett-Packard Company.
Selected information from the protocol processor 12 is supplied to a host controller 14, which analyzes the selected information and provides results to a display unit 20. The display unit 20 may, for exarnple, be a video monitor. The host controller 14 may be a 2s general purpose computer, such as, for example, a type E 1499, and may utilize the Unix operating system.
The MPEG-2 transport stream analyzer shown in FIG. 1 may perform functions such as real time analysis of program clock reference statistics, packet identifier statistics, program specific information table monitoring, transport stream and global error indicators, header 30 statistics and header field matching. The test instrument may also provide triggering and pattern matching functions, and decoding and display of the video contained in the transport 120965.1 CA 0222079~ l997-ll-l2 stream.
As indicated above, the MPEG-2 transport stream is 188 bytes long and includes aheader and a payload. The following header information is relevant to the present invention.
The header includes a two-bit adaptation control field, which indicates whether the transport stream packet includes an adaptation field. The header also includes a 1 3-bit packet identifier (PID) field, which identifies the program with which the packet is associated. The MPEG-2 transport stream may carry multiple programs simultaneously, with each program identified by the PID. The adaptation field includes a one-bit PCR flag, which indicates whether the adaptation field contains a PCR field. The program clock reference (PCR) field is a 42-bit 0 field coded in two parts, a 33-bit program clock reference base and a 9-bit program clock reference extension. The PCR base and the PCR extension are defined by the MPEG-2 protocol. The PCR indicates the intended time of arrival of the byte cont~ining the last bit of the PCR base at the input of the system target decoder. The PCR represents the time at which the packet was transmitted in units of a system time clock count. PCR samples are usually not s included in every transport stream packet.
The present invention provides a method and appaldlus for measuring PCR jitter in the MPEG-2 transport stream. The PCR jitter is defined as the difference between the actual arrival time of a PCR sample value and the expected arrival time based on system time clock (STC) at the source of the transport stream. Before the PCR jitter can be estim~te~l> the 20 system time clock must be recovered from the PCR sample values in the transport stream.
The PCR jitter can then be measured as deviations from the system time clock. The system time clock is subject to frequency variations, but such frequency variations can be ignored for short segments of the transport stream. In determining PCR jitter, the system time clock frequency is assumed to be constant.
2s A flow chart of a first embodiment of a routine for determining PCR jitter is shown in FIG. 2. PCR jitter is determined by a software routine in the host controller 14 without requiring special dedicated hardware. The PCR jitter measurement routine may, for example, be implemented using the C progr~mming language. In step 40, a PCR sample value is extracted from the transport stream. As indicated above, several programs may be carried by 30 the MPEG-2 transport stream simultaneously. One or more programs may be selected for analysis of PCR jitter. These programs are identified by the program identifier, PID, in the 120965.1 CA 0222079~ l997-ll-l2 header of the transport stream packet. Although the instrument may analyze more than one program simultaneously, the analysis of one program is described. The routine is repeated for each program of interest. When a transport stream packet with the selected PID and cont~inin~ a PCR sample value is received, the PCR sample value is extracted from the 5 transport stream and is assigned a timestamp from a local clock. The timestamp establishes the arrival time of the PCR sample value. The received PCR sample value and the timestamp from the local clock are norm~1i7.ed in step 42. The received PCR sample value is normalized as follows:

o PCR n=((PCR ba5enx 300) + PCR extn)/27 (1) where PCRb"se represents the 33-bit PCR base field, PCR ~x. represents the 9-bit PCR
extension field and PCR n represents a norm~1i7eA PCR sample value, given in units of microseconds. The timestamp ticks provided by the local clock are counts of tens of nanoseconds. Norm~li7~od timestamp values TL n are obtained by dividing the raw local clock count by 100. Thus, the data associated with each PCR sample value includes a norm~1i7ed PCR sample value, PCRn, and an associated normalized timestamp, TL n~ both in microseconds.
In order to determine PCR jitter, the system time clock is recovered from the PCR
sample values, and the PCR jitter for each PCR sample value is measured as a deviation of the PCR sample value from the recovered system time clock. The system time clock may be recovered using a linear regression model based on the local timestamps of the transport stream packets cont~inin~ the PCR sample values. The timestamp is based on the arrival time 2s of the last byte in the transport stream packet cont~ining the PCR.
After two PCR sample values are received, a line connecting those sample values may be drawn. The line represents the first estim~tion of the system time clock. As further PCR
samples are received, the least squares linear regression model may be used to make successive estim~tions of the line of best fit, which represents the reconstructed system time clock. A graph of norm~1i7~.~1 PCR sample value, PCR n7 as a function of norm~1i7ed timestamp, TL n~ should approach unity slope when there is no frequency variation in the 120965.1 CA 0222079~ 1997-11-12 encoder system time clock or the local clock.
A plot of PCR in microseconds as a function of TL in microseconds after several PCR
sample values is shown in FIG. 3. Nonn~ d PCR samples values, PCR n and corresponding norrn~li7ed timestamps, TLn are plotted as points 60, 62, 64, etc. A line of best fit 70 can be s determined for any set of two or more sample values. The line of best fit 70 represents the recovered system time clock from the source of the transport stream. The line of best fit 70 is a graph of the time at which the packet was transmitted (norm~ d PCR sample value) as a function of PCR arrival time (norn~1i7~cl timestamp). The line of best fit 70 should have a slope of one or approxill~ately one. The PCR jitter, PJ n of each PCR sample value is the lo hofizolllal distance parallel to the TL axis between each sample point and the line of best fit 70. The PCR jitter represents a time deviation of the PCR sample value from the recovered system time clock.
In order to ~letennine PCR jitter, the line of best fit 70 may be first determined using a linear regression model. The line of best fit is given by:
PcR=mnxTL+cn (2) where m n represents the slope of the line of best fit, and c n represents the offset (the PCR axis hllelcel)l) of the line of best fit after n samples. The values of slope m n and offset c n are 20 determined in accordance with the linear regression model by the following equations:

n ~, TLj PCRj ~ , TL,.~ , PCR,¦ (3) m = i=~ i=o i=o n n n 2 n ~ TL,2 - ~ TLj n n ~,PCRj - m ~ TLj Cn = i O j=o n 120965.1 CA 0222079~ l997-ll-l2 On each successive sample (TL n, PCR n ), the slope m n and the offset c n of the line of best fit are recalculated in accordance with Equations (3) and (4). However, it will be understood that it is not necessary to repeat each calculation in its entirety. Instead, the sums in the 5 expressions for slope and offset are saved after each calculation. When each new sample (TL
n~ PCR n )~ is received, the new values are added to the applopl;ate sums in the expressions for slope and offset to provide new values of slope and offset. Thus, a new line of best fit is determined for each new PCR sample value with a minimum of calculation. As discussed below, by calculating the line of best fit in real time, the PCR jitter and PCR jitter variance 0 can also be calculated in real time, rather than waiting until all PCR sample values are received. If it is assumed that there is little or no slew in the system time clock and the local time~llpillg clock, the successive estim~tions of slope and offset become stable after an initial training period and the mean jitter approaches zero for a large number of PCR samples.
Referring again to FIG. 2, when a sufficient number of norm~li7~1 PCR n values have 5 been received, as detçrmined in step 46, the slope m n and offset cn of the line of best fit are calculated in step 50, as described above. When a sufficient number of PCR n values has not been received, the process simply waits for more PCR sample values to be received in the MPEG-2 transport stream. Preferably, the process waits until the slope and offset are stable within defined limits before using those values in the jitter calculation. In an alternative 20 approach, a line of best fit can be determined after two PCR sample values have been received. The line of best fit is then updated for each new PCR sample value received until a sufficient number of PCR sample values have been received to provide a desired accuracy in the PCR jitter measurement. In step 52, the PCR jitter, PJ n ~ is determined from the norm~ ecl PCR sample value, PCR n~ norm~li7~-~l timestamp, TL n and from 2s the slope m n and offset c n of the line of best fit in accordance with the following equation:

PCR - c PJn = TLn - mn (5) 120965.1 CA 0222079~ l997-ll-l2 The PCR jitter variance, VPJ n may optionally be determined in step 54 in accordance with the following equation:

VPJ ~ ([pJj])2 (6) The values of PCR jitter and PCR jitter variance are measurements of the physical timing characteristics of a selected program in the MPEG-2 transport stream. The PCR jitter measurement provides an individual PCR jitter value for each PCR sample value received and is available in real time shortly after the PCR sample value is received. The PCR jitter o variance is a statistical representation of the PCR jitter values and is recomputed for each new PCR sample value. It will be understood that the calculation of PCR jitter variance is optional. The measured values of PCR jitter and PCR jitter variance may be displayed and/or stored in any desired manner. For example, the measured values may be displayed on the video display unit 20, may be printed and/or may be stored for subsequent analysis. In a s preferred embodiment, PCR jitter values are presented on the video display unit 20 in a histogram, as shown in FIG. 5. In the histogram, the horizontal axis is the PCR jitter, PJ n~
and the vertical axis is frequency of occurrence. Routines for generating a histogram display using a set of input values are well known to those skilled in the art. As discussed above, PCR jitter values are determined in real time. The histogram may be updated for each new 20 PCR jitter value determined. It will be understood that different display formats can be utilized for presenting the PCR jitter measurements.
A plerelled implementation of the present invention is described with reference to FIG. 4, which shows portions of the protocol processor 12 (FIG. 1 ) and the host controller 14.
The host controller 14 provides a PID list to the protocol processor 12. The PID list specifies 2s one or more programs in the MPEG-2 transport stream that are to be analyzed. As indicated above, the packet identifier (PID) associates each transport stream packet with a program.
The MPEG-2 transport stream is input to a capture RAM 102 in the protocol processor 12 120965.1 CA 0222079~ l997-ll-l2 through a PID filter 106. The PID filter 106 selects transport stream packets specified by the PID list and inputs filtered transport stream packets to capture RAM 102. A local clock 104 provides timestarnps that are associated with the transport stream packets stored in the capture RAM 102. A microprocessor 108 provides raw PCR and timestamp values from captureRAM 102 to a shared RAM 110. The raw PCR data is accessed by the host controller 14 for computation of the PCR jitter and PCR jitter variance values by software routines in block 1 12. The computations are performed as described above in connection with Equations ( 1 ) to (6). The measured values may be provided to computer display 20 as a histogram and are preferably stored by the host controller 14 for subsequent analysis.
o A histogram may be generated for each program, or PID, being monitored. Each bin in the histogram is defined by a range of PCR jitter. The user may specify the histogram range. The bin boundaries are calculated from the range. Respective bin values are incremented for each matching value of PCR jitter, PJ n. The PCR jitter variance, VPJ n ~ and the number PCR samples may be shown as numerical values on the display.
The technique for determining PCR jitter described above is generally satisfactory, but has certain drawbacks. In particular, the summation of product terms and product of sllmm~tion terms in Equations (3) and (4) are cumulative over all PCR sample values received. Since the values of these terms increase over time, precision may be lost due to floating point limitations of the computer. Specifically, the computer has a fixed number of 20 bits, such as 32 bits, for representing the summation terms. As the summation terms increase, the least significant bits are dropped, and precision is lost. The loss of precision reduces the accuracy of the calculated slope and offset of the line of best fit, which in turn compromises the accuracy of the PCR jitter measurement.
Calculating the slope and offset of the line of best fit over all PCR sample values 25 received may compromise the sensitivity of the PCR jitter measurement to changes in parameters associated with the PCR jitter measurement. Examples include changes in network delay and/or system clock slew. The reduction in sensitivity is greatest when such changes occur after a large number of PCR sample values has been received by the test instrument.
The situation is illustrated in FIG. 6, which is a graph of normalized PCR values (PCR) as a function of normalized PCR arrival times (TL). A line of best fit 200 is 120965.1 CA 0222079~ l997-ll-l2 determined using Equations (3) and (4) for a large number of received PCR sample values.
Assume that a change in network delay occurs following PCR sample value 202. Since the parameters of the line of best fit 200 have been calculated over a large number of PCR sample values, the sensitivity to the change in network delay is reduced. Due to the change in 5 network delay, a better line of best fit would be the dashed line 206 shown in FIG. 6.
However, due to the weighting of the previous PCR sample values, the calculated line of best fit is likely to be very close to line of best fit 200. As a result, the PCR jitter calculation would be less accurate following the change in network delay.
In order to avoid these drawbacks, a sliding window may be used for calculating the o slope and offset of the line of best fit. Instead of using all the available PCR sample values in calculating the slope and offset of the line of best fit, the sliding window uses only the PCR
sample values within a window l~,e;,elll~Li~e of a predetermined time interval. The phrase "sliding window" may be defined as follows. The window includes a predetermined number of recently received sample values and slides in the sense that it is shifted to include each 5 newly-received sample value. Sliding, or shifting, may be implemented by adding the newly-received sample value to the sample values in the window and discarding the oldest sample value. Thus, only recently-received PCR sample values are used m calculating the slope and offset. The window termin~tes with the current PCR sample value and timestamp.
The size of the window may be varied within the scope of the present invention. A
20 preferred window size includes about 500 PCR sample values and associated timestarnps. By limiting the number of sample values used in the calculation, there is minim~1 loss in precision due to the floating point limitation of the computer, and the PCR jitter measurement is more accurate.
In addition, the sensitivity of the PCR jitter measurement to changes, such as changes 25 in network delay and/or clock slew, is improved. A graph of normalized PCR values as a function of normalized PCR arrival times is shown in FIG. 7. The sliding window technique is illustrated as a window 230 at time tl, a window 232 at time t2 and a window 234 at time t3. It is assumed that a change in network delay occurs during time tl. Lines of best fit 240, 242 and 244 are calculated at times tl, t2 and t3, respectively. As illustrated, the line of best 30 fit changes in response to the change in network delay. The PCR jitter measurement using the sliding window approach converges to the ideal line of best fit faster, since the ratio of the 120965.1 CA 0222079~ l997-ll-l2 number of sample values affected by the change in network delay to tne nurnber of sample values prior to the change increases much more quickly. As the window slides, it reaches a point where the line of best fit is determined solely by the sarnple values after the change in network delay.
s The sliding window approach to PCR jitter measurement may be understood by considering sample values (TLI, PCR~), (TL2, PCR2), ... (TLn, PCRn), (TLn+l, PCRn+l). For a window having a size n, the slope and offset of the line of best fit are first calculated using the sample values 1 to n. When the sample value n + 1 is received, the sample value (TL" PCR~) is discarded, and the slope and offset of the line of best fit are calculated using sample values o 2 to n+1. This process is repeated for each new sample value received, so that the window slides with respect to the set of received sample values. The window slides one sample value at a time, such that the new sample value falls within the window and the oldest sample value is discarded.
Through the use of recursive formulas, the computational complexity and overhead15 associated with the calculation of the slope and offset of the line of best fit, and also the PCR
jitter variance, may be reduced. This is achieved by deriving the m~them~tical equations which express the latest value of each of these quantities as a function of the latest sarnple value and the previously calculated value of the corresponding quantity.
Since these equations are lengthy, it is convenient to change the notation used above.
20 Henceforth, TL; and PCR; shall be represented by xj and y;, respectively. Also, PCR jitter shall be represented by j. Using this notation, Equation (3) for calculating the slope of the line of best fit can be rewritten as follows.

A n ~; XIY~ xl)( yl) (7) n Bn n ~ xl--( ~ Xl) 1~1 1.1 For a window size of n sarnples, the slope of the line of best fit upon analysis of a new PCR
30 sample value can be calculated using the following recursive equation.

120965.1 CA 02220795 l997-ll-l2 A ~n~ (Y~ ~'n+~ n+~)~;y~(n+l)(~+p~+~ ~P~ (8) n + I; 8n t ~ Bn + ~ I + (~ ~ ~n + ~)( 2 ~ + I ) (~ + ~

s where n represents the number of sample values in the window, k is a positive integer that represents the number of the window as it slides and i is an index. In Equation (8), A n+k-l and B n+k-l represent the numerator and denominator, respectively, from the previous calculation and in the case where k = 1, they represent the numerator and denominator, respectively, of Equation (7). Similarly, the variance Vn+l (j) of the PCR jitter upon analysis of the next 0 sample value, given the previously calculated variance Vn (j), is given by the following recursive equation.

V (j) (n--l) V"(j) (intl~Jn) (9) The PCR jitter variance is calculated in accordance with Equation (9) over all sample values, not just the sample values within the window.
A flow chart of a second embodiment of a routine for determining PCR jitter is shown 20 in FIG. 8. In step 300, a PCR sample value is extracted from the transport stream and is assigned a timestamp from the local clock. The received PCR sample value and thetimestamp from the local clock are normalized in step 302. Steps 300 and 302 may be performed as described above in connection with steps 40 and 42, respectively, of FIG. 2. In step 304, the new normalized PCR sample value and timestamp are written in a list in 2s memory. The list is an array of sample pairs, including the norm~li7e~ PCR sample values and timestamps. A maximum number of sample pairs in the list is determined by the window size. In step 306, a determination is made whether a sufficient number of PCR sample values has been received to determine the line of best fit within a given accuracy. When a sufficient number of PCR sample values has not been received, the process waits for more PCR sample 30 values to be received. When a sufficient number of PCR sample values has been received, the slope and offset of the line of best fit are determined in step 310. The slope is determined in 120965.1 CA 0222079~ l997-ll-l2 -l4-laccordance with Equation (8), and the offset is cletennined in accordance with Equation (4), where the summations are performed over the sample values that fall within the window. The PCR jitter is determined in step 312 in accordance with Equation (5). Then the PCR j itter variance may optionally be d~te~ ed in step 314 in accordance with Equation (9). A
s display of jitter and jitter variance may be generated in step 320, as described above in connection with step 56 of FIG. 2. In step 324, a determination is made as to whether the number sample values received is equal to or greater than the number of sample values in the window. When the number of sample values received is less than the window size, the process r eturns to step 300 to wait for another PCR sample value. When the number of lo sample values received is equal to the window si~, the oldest PCR sample value and timestamp are removed from the list in step 330. The process then returns to step 300 to wait for another PCR sample value.
While there have been shown and described what are at present considered the prerellec~ embodiments of the present invention, it will be obvious to those skilled in the art s that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

120965.1

Claims (20)

1. A method for measuring program clock reference (PCR) jitter in a data stream containing digitally encoded information and PCR samples, comprising the steps of:
a) providing a test instrument including a line interface circuit for receiving a data stream, a digital computer connected to the line interface circuit for processing the data stream and a local clock for generating timestamps;
b) said digital computer extracting a PCR sample value from the data stream, said PCR sample value representing a count of a system time clock used to generate said data stream;
c) said digital computer obtaining a timestamp from said local clock and associating the timestamp with said PCR sample value, said timestamp representing a time of arrival of said PCR sample value;
d) said digital computer determining a reconstructed system time clock from a set of values comprising said PCR sample value and said timestamp, and previously-received PCR sample values and associated timestamps within a window representative of a predetermined time interval that terminates with said PCR sample value and said timestamp; and e) said digital computer determining a PCR jitter value for said PCR sample value by determining a deviation of said PCR sample value from said reconstructed system time clock, said PCR jitter value representing a variation in arrival time of said PCR sample value with respect to said reconstructed system time clock.

2. A method as defined in claim 1 wherein the step of determining a reconstructed system time clock includes determining a line of best fit to said set of values within said window, said line of best fit representing said reconstructed system time clock.

3. A method as defined in claim 1 further including repeating steps b) - e) for a plurality of PCR sample values from said data stream to provide a plurality of PCR jitter values in real time.

4. A method as defined in claim 3 wherein the step of determining a reconstructed system time clock for each new PCR sample value includes shifting said window to include said new PCR sample value.

5. A method as defined in claim 1 wherein the step of determining a reconstructed system time clock includes performing a recursive linear regression calculation on said set of values within said window.

6. A method as defined in claim 2 wherein the step of determining a line of best fit includes determining a slope and an offset of said line of best fit.

7. A method as defined in claim 3 further including the step of determining the variance of said PCR jitter.

8. A method as defined in claim 1 wherein said data stream comprises an MPEG-2 transport stream.

9. A method as defined in claim 3 further including the steps of providing said test instrument with a display unit and displaying said plurality of PCR jitter values on said display unit.

10. A method as defined in claim 3 further including the steps of providing said test instrument with a display unit and displaying said plurality of PCR jitter values in a histogram on said display unit.

11. A test instrument for measuring program clock reference (PCR) jitter in a data stream containing digitally encoded information and PCR samples, comprising:
a line interface circuit for receiving a data stream;
a local clock for generating timestamps;

a digital computer connected to the line interface circuit for processing the data stream, said digital computer comprising:
means for extracting a PCR sample value from the data stream, said PCR sample value representing a count of a system time clock used to generate said data stream;
means for obtaining a timestamp from said local clock and associating the timestamp with said PCR sample value, said timestamp representing a time of arrival of said PCR sample value;
means for determining a reconstructed system time clock from a set of values comprising said PCR sample value and said timestamp, and previously-received PCR sample values and associated timestamps within a window representative of a predetermined time interval that terminates with said PCR sample value and said timestamp; and means for determining a PCR jitter value for said PCR sample value by determining a deviation of said PCR sample value from said reconstructed time clock.

12. A test instrument as defined in claim 11 wherein said means for determining a reconstructed system time clock includes means for determining a line of best fit to said set of values within said window, said line of best fit representing said reconstructed system time clock.

13. A test instrument as defined in claim 11 wherein said data stream comprises an MPEG-2 transport stream.

14. A test instrument as defined in claim 11 wherein said digital computer includes means for updating said reconstructed system time clock and means for determining a PCR jitter value for each of a plurality of new PCR sample values received in said data stream to provide a plurality of PCR jitter values.

15. A test instrument as defined in claim 14 wherein said means for updating said reconstructed system time clock includes means for shifting said window to include each of said new PCR sample values.

16. A test instrument as defined in claim 11 wherein said means for determining a reconstructed system time clock includes means for performing a recursive linear regression calculation on said set of values within said window.

17. A test instrument as defined in claim 12 wherein said means for determining a line of best fit includes means for determining a slope and an offset of said line of best fit.

18. A test instrument as defined in claim 14 wherein said digital computer further includes means for determining the variance of said PCR jitter.

19. A test instrument as defined in claim 14 wherein said digital computer further includes a display unit and means for displaying said plurality of PCR jitter values on said display unit.

20. A test instrument as defined in claim 14 wherein said digital computer further includes a display unit and means for displaying said plurality of PCR jitter values in a histogram on said display unit.