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MPEG monitoring

May 1, 2007 12:00 PM, BY JON HAMMARSTROM

Are PCR measurements critical?

A program clock reference (PCR) enables the MPEG decoder to synchronize to the encoder. The system time clock is locked to the stream PCRs. A 42-bit sample of the sine-wave of the system time clock at the encoder indicates to the demultiplexer what the clock's time should be at the decoder when each clock reference is received. Synchronization errors arise if the PCR value generated by the multiplexer is inaccurate, or if it is received late because of network delays, such as jitter.

The STC is used to create color burst and syncs. It is the reference for audio and video decoding and presentation time stamps. Jitter and inaccuracy errors can lead to decoder errors.

Figure 4. Service plan template.
Click image to enlarge.

PCR errors originate from faulty encoder PCR circuitry, faulty remultiplexer PCR circuitry or failure to seamlessly loop a transport file. PCR jitter can come from an unstable RF demodulator, unstable fiber demultiplexer, ATM network packet jitter or the way MPEG is packetized in IP networks. The buffers in set-top boxes should cope and smooth this, but there may be problems, particularly with large PCR spikes. Figure 3 shows evidence of a faulty encoder.

Modern H.264 systems (as in IPTV) may not strictly need PCR, but encoders allow it to be generated, as it is a good indicator of timing integrity (and jitter) of the IP pathway delivery system. Much test equipment has built-in alarms for limit violation to flag the errors, so operators are making sure that PCR is present in its output.

Ensuring that the contents of the transport stream are correct requires the monitoring equipment to have prior knowledge of what broadcasters plan to transmit. One method of achieving this is to enable the broadcaster to identify a small number of key parameters that can be used to verify the contents of the transport stream. These parameters form a service plan, or template, in which the operator enters the values that are expected to be present in the transport stream. The monitoring equipment extracts the actual values from the transport stream and compares them against the template, indicating when a discrepancy occurs.

Plans may vary according to the region and DTV standard. For example, service descriptor table is just one example for DVB, and there are other and similar service information tables for ATSC or Japanese ISDB services.

Figure 5. Error logs with highlight
Click image to enlarge.

Figure 4 shows an example template screen. Note that a single simple alarm template error on the summary screen will guide operators to this display, where it is possible to drill down and trace the fault in the transmission. In this case, an incorrect stream type is easily found in the service.

Monitoring equipment also should have the ability to change templates automatically at scheduled times — for example, when service plans change over on regional news insertion. Broadcasters may be asked to verify that local content has been inserted correctly and on time. This can be dealt with by templates as shown here. Legal service level agreements may involve tracking delivery of packets over time to indicate bandwidth delivered.

Most test equipment has error logs. These can make tracking of intermittent errors easier because broadcasters can detect time, date and nature of errors. Error filtering can help because broadcasters can restrict the type of errors logged to just the critical ones, or broadcasters may want to know all the errors, but just highlight and color code the critical ones they are worried about. (See Figure 5.)

Summary

There are basic critical tests that are common across most DTV systems. These are graded into three priorities by ETSI TR 101 290. Monitoring equipment and analyzers should support all three of levels of prioritization and prominently display results in a summary screen.

Figure 6. Multiple test view
Click image to enlarge.

There are additional tests that, for certain operators, may be most critical to their particular business. For example, cable operators may want to focus on modulation error ratio (MER), but once operators know it's within spec, they will exit the RF layer test menu. Thereafter, they may want to ensure their bit rates are maintained, and appropriate services are being delivered during a given time period. So the template and bit-rate alarms may be their prime concern.

Test equipment that provides alarms and a configurable user interface can contribute significantly to improved operational efficiency. (See Figure 6.) Polling can provide a cost-effective method to achieve broad MPEG, RF or IP measurements and deep MPEG protocol compliance testing across hundreds of channels.

Jon Hammarstrom is senior video marketing manager for Tektronix.