Method and Apparatus for PCR Jitter Measurement in an MPEG-2 Transport Stream

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

In the field of data communications, one highly demanding application is the transmission of full motion video, such as may be required for video-on-demand applications. Various protocols which involve compression of digitized video information have been proposed. One such protocol is the MPEG-2 transport stream, as defined in standard ISO/IEC 13818-1: 1994(E) promulgated by the ISO/IEC. The MPEG-2 transport stream involves transmission of video and audio information in transport stream packets of 188 bytes in 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 contains an adaptation field. The adaptation field contains control 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 representation 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 count of the system time clock at the time when the transport stream packet was encoded for transmission. PCR sample values for a particular program are received with different 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 transmission of 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 transmission. However, in a real communication channel, variable delays may be introduced by different channel elements. For example, different transport stream packets may follow different network paths in reaching the final destination. Variations in arrival times produce PCR jitter. PCR jitter is described in standard ISO/IEC 13818-9 DIS 1995.

According to the present invention, methods and apparatus for measuring program clock reference (PCR) jitter in a data stream containing 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, a local clock for generating timestamps and a display unit. 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 line of best fit to a set of values comprising the PCR sample value and the timestamp, and previously received PCR sample values and associated timestamps. The line of best fit represents a reconstructed system time clock derived from the system time clock. At least two PCR samples values are required to estimate a line of best fit. The digital computer determines a PCR jitter value for the PCR sample by determining a distance between the line of best fit and the PCR sample value. The PCR jitter value represents a variation in the arrival time of the PCR sample value with respect to the reconstructed system time clock. The PCR jitter value may be displayed on the display unit, preferably in a histogram.

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 values are determined in real time as the PCR sample values are received, without waiting for all PCR sample values to be received. An accurate estimate of PCR jitter is obtained after only a few PCR sample values are received.

The step of determining the line of best fit preferably includes performing a linear regression calculation on the set of PCR sample values and timestamps. In particular, the linear regression calculation includes determining the slope and the offset of the line of best fit using a least squares calculation. The line of best fit is recalculated for each new PCR sample value by updating the previously calculated values of slope and offset with the new PCR sample value and the associated timestamp. The variance of the PCR jitter values may also be determined.

The invention provides an accurate measurement of PCR jitter for each PCR sample value received. The PCR jitter measurement is available in real time as the PCR samples are received. Thus, the user is not required to wait until all samples have been received before a PCR jitter measurement is available. An accurate measurement of PCR jitter has been shown to be available after about 5 to 10 PCR samples have been received in a typical transport stream.

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 an embodiment of the routine for PCR jitter measurement in accordance with the invention;

FIG. 3 is a graph of normalized 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; and

FIG. 5 is an example of a histogram showing the frequency of different PCR jitter values.

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 12. The protocol processor 12 receives each 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 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 example, be a video monitor. The host controller 14 may be a general purpose computer, such as, for example, a type E1499, 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 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 stream.

As indicated above, the MPEG-2 transport stream is 188 bytes long and includes a header 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 13-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 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 containing 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 included in every transport stream packet.

The present invention provides a method and apparatus 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 estimated, the 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.

A flow chart of a preferred routine for determining PCR jitter in accordance with the invention is shown in FIG. 2. In accordance with the invention, 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 programming language.

In step 40, a PCR sample value is extracted from the transport stream. As indicated above, several programs may be carried by 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 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 containing a PCR sample value is received, the PCR sample value is extracted from the 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 normalized in step 42. The received PCR sample value is normalized as follows:

where PCR .sub.base represents the 33-bit PCR base field, PCR .sub.ext represents the 9-bit PCR extension field and PCR .sub.n represents a normalized PCR sample value, given in units of microseconds. The timestamp ticks provided by the local clock are counts of tens of nanoseconds. Normalized timestamp values TL .sub.n are obtained by dividing the raw local clock count by 100. Thus, the data associated with each PCR sample value includes a normalized PCR sample value, PCR .sub.n, and an associated normalized timestamp, TL .sub.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 is recovered using a linear regression model based on the local timestamps of the transport stream packets containing the PCR sample values. The timestamp is based on the arrival time of the last byte in the transport stream packet containing the PCR.

After two PCR sample values are received, a line connecting those sample values may be drawn. The line represents the first estimation of the system time clock. As further PCR samples are received, the least squares linear regression model is used to make successive estimations of the line of best fit, which represents the reconstructed system time clock. A graph of normalized PCR sample value, PCR .sub.n, as a function of normalized timestamp, TL .sub.n, should approach unity slope when there is no frequency variation in the 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. Normalized PCR samples values, PCR .sub.n and corresponding normalized timestamps, TL.sub.n, are plotted as points 60, 62, 64, etc. A line of best fit 70 can be 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 (normalized PCR sample value) as a function of PCR arrival time (normalized timestamp). The line of best fit 70 should have a slope of one or approximately one. The PCR jitter, PJ .sub.n of each PCR sample value is the horizontal 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 determine PCR jitter, the line of best fit 70 is first determined using a linear regression model in accordance with the invention. The line of best fit is given by:

where m .sub.n represents the slope of the line of best fit, and c .sub.n represents the offset (the PCR axis intercept) of the line of best fit after n samples. The values of slope m .sub.n and offset c .sub.n are determined in accordance with the linear regression model by the following equations: ##EQU1## On each successive sample (TL .sub.n, PCR .sub.n), the slope m .sub.n and the offset c .sub.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 expressions for slope and offset are saved after each calculation. When each new sample (TL .sub.n, PCR .sub.n), is received, the new values are added to the appropriate 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 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 timestamping clock, the successive estimations 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 normalized PCR .sub.n values have been received, as determined in step 46, the slope m .sub.n and offset c.sub.n of the line of best fit are calculated in step 50, as described above. When a sufficient number of PCR .sub.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 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 .sub.n, is determined from the normalized PCR sample value, PCR .sub.n, normalized timestamp, TL .sub.n, and from the slope m .sub.n and offset c .sub.n of the line of best fit in accordance with the following equation: ##EQU2## The PCR jitter variance, VPJ .sub.n may optionally be determined in step 54 in accordance with the following equation: ##EQU3## 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 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 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 .sub.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 is updated for each new PCR jitter value determined in accordance with the present invention. It will be understood that different display formats can be utilized for presenting the PCR jitter measurements.

A preferred 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 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 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 timestamps that are associated with the transport stream packets stored in the capture RAM 102. A microprocessor 108 provides raw PCR and timestamp values from capture RAM 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 112. 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.

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 .sub.n. The PCR jitter variance, VPJ .sub.n, and the number PCR samples may be shown as numerical values on the display.

While there have been shown and described what are at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.