Transport stream monitoring
Sep 8, 2008 8:00 AM
Priority 2
Second-priority errors are those that could affect individual programs, but the TS is still intact. The types of problems these errors can cause are frozen frames and loss of lip sync. Tests for these faults on the stream analyzer include:
- Transport error: This flag is set in the TS header by the demodulator if it can't correct errors in the stream.
- CRC error: This indicates that a CRC error (data corruption) occurred in any of the following tables — CAT, PAT, PMT, NIT, EIT, BAT, SDT or TOT.
- PCR error: This flag is raised if the primary clock reference (PCR) is not seen for more than 100ms. The time interval between two consecutive PCR values should be no more than 40ms. This type of error can cause the decoder to lose lock on the 27MHz clock.
- PCR repetition error: This error occurs when the time interval between two consecutive PCR values is more than 40ms and can cause the 27MHz clock to drift or lose lock.
- PCR discontinuity indicator error: If the difference between two consecutive PCR values is outside the range of 100ms, this error can occur.
- PCR accuracy error: This error can occur when the PCR accuracy of the selected program is outside the range of ±500ns.
- PTS error: This occurs when the presentation time stamp (PTS) repetition is more than 700ms. The PTS is contained in the MPEG-2 program stream and is used to aid the decoder in presenting the program on time, at the correct speed and synchronized. The PTS is compared to the PCR.
- CAT error: This is used for conditional access programs (paid programming).
Priority 3
Figure 4
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Third-priority errors are application dependent and are concerned mostly with errors in the network information tables (NIT), buffer over- and underruns for the transport, multiplexer and elementary streams.
The most likely error under priority three is “unreferenced PID,” which is a packet with a PID that is not listed in the PMT. This is not critical, but it indicates that packets are being disassociated from the PMT, possibly through remultiplexing.
PCR monitoring
A very important parameter to monitor is the PCR and its relative advancement or delay in the transport stream. This can be shown as a graph that displays this timing variation in a variety of ways. The point is to know how close the PCR is to arriving at the correct time and if it comes close to causing an error. Each program or channel must be selected and monitored separately because each has its own PCR. (See Figures 4 and 5.)
ATSC recommendations
Figure 5
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The Advanced Television Systems Committee (ATSC) has its own publication entitled “ATSC Recommended Practice: Transport Stream Verification” (document A/78a) that provides a common method for describing TS parameters that should be verified for it to be considered decodable. While A/78a does overlap with TR 101 290, the ATSC recommendation specifies five groups of priority tests, each with a range of deviation. The five groups are:
- TOA (transport stream off-air): In this case, the station is technically off-air, because the TS errors are so severe. Receivers will not be able to tune and decode anything within this broadcast — e.g., repeated absence or complete absence of sync bytes.
- POA (program off-air): Here, a major service is in error to the point where decoders will not be able to decode the program — e.g., the absence of an entry in the virtual channel table (VCT) for a service.
- CM (component missing): Typically, in this case, one of the audio or video elementary streams cannot be found, like a mismatch between the video PID signaled in the service location descriptor (SLD) and the actual PID used for the video elementary stream.
- QOS (quality of service): In this error, parameters are out of specification by such a margin that a significant fraction of the receivers can be expected to produce flawed outputs — e.g., the master guide table (MGT) cycle time being somewhat longer than the specification, which would cause slower than normal tuning.
- TNC (technically nonconformant): In this case, the error violates the letter of the standard, but in practice will have little effect on the viewing experience.
Monitoring equipment
Today’s stream monitoring devices come in a variety of configurations and price ranges. Some devices will continually monitor the stream and alert an operator to any error or potential error, while other devices must be monitored manually by an observer with a screen. Most, if not all, of these devices will list the TR 101 290 parameters for reporting, because they are used in virtually all transport streams, while some others will have the ATSC A/78a parameters as well.
For more critical tests involving the PCR, look for the following in a stream analyzer: PCR accuracy, which looks for accuracy in the originating 27MHz clock at the encoder; PCR drift rate, which looks for slow changes in the PCR frequency; and PCR jitter, which covers high-frequency changes in the 27MHz clock. These last two measurements cover errors introduced at the encoder/multiplexer and in the transmission path. The PCR accuracy test is only meant for testing of the encoder/multiplexer, however all these tests require highly-accurate local 27MHz clocks, which will be found in only the more expensive test equipment.
Some lower-cost devices connect to a computer through a USB connection and use the power of the computer to do the hard work of decoding the stream. Some are also capable of recording and playing back a transport stream, which is useful for later analyses of a suspect transport stream. If the device will also play back, then it can be used to feed the transmitter with a prerecorded transport stream when maintenance needs to be performed on the STL or data link from the studio. It also can be used in case the studio link is lost. In this case, a hard disk at the transmitter with evergreen programming can be kept and used in an emergency.
References
Tektronix has a thorough booklet entitled “MPEG Fundamentals and Protocol Analysis.” It covers a variety of MPEG formats as well as transport stream fundamentals. It can be downloaded at http://www.tek.com/Measurement/programs/mpeg_fundamentals/.
The ATSC has all of the specifications for DTV broadcasting available for download at www.atsc.org/.
The Digital Video Broadcasting (DVB) Project is the repository for the specifications for MPEG TS as well as satellite and cable transmissions. Many of the specifications can be downloaded at www.dvb.org/.
Next time
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