Engineering Academy Continuing Education Series

I dare say that the majority of television viewers has ever heard of a “Transport Stream” but are quick to speak up when something is not right with it.

Problems with lip sync, missing captions or video description, incorrect or missing program guides are among the problems that transport stream errors can create.

The technical department should set up procedures to monitor the transport stream along with OTA and cable/satellite reception for any errors. The way that encoders and decoders work can sometimes remove and/or change important tables and files. It is important for all broadcast engineers (radio and television) to monitor and play close attention the quality of their signal plus listen to customers comments.

An article published by Triveni Digital stated that “the root cause of these types of errors can be cause by any of the MPEG devices in the chain”. They can cause dropped packets, metadata errors and inconsistencies, PCR jitter, video/audio under/overflow, and under provisioning.

Two major items that can create errors are continuity and PCR jitter.

Simply put the continuity counter is a 4 bit field in the header which increments by 1 each time a packet comes out on a specific PID. When a PID “skips” one value of the continuity Counter, we call it a ‘Continuity Error.’ This means one or more packets were lost.

Since a continuity error means some packets have been lost, frequent continuity errors should be one of the FIRST things you look for when conducting MPEG analysis. The Continuity Counter (CC) is carried in the header of every Transport Packet (TP) of the Transport Stream (TS).

These can occur due to initial setup and configuration errors but can also develop after the install by equipment drift/failures and loss of synchronization.

ATSC A78 recommends a practice of establishing a notion of error severity. Basically there are 5 levels of severity.

– TSID off air (TOA)
– Program off air (POA)
– Component missing (CM)
– Quality of service (QOS)
– Technically non-conform ant (TNC)

Probably the most important item in the transport stream is the Program Clock Reference (PCR). The entire ATSC system relies on correct timing of the creation and reception of all the elements in the transport stream. In a somewhat similar fashion we had the same thing in NTSC with Vertical and Horizontal sync.

The encoder has an internal clock operating at 27 MHz which the various elements are lock to. A sample of this clock, call the PCR is inserted in the stream every 100ms and is used to lock the clock located in the customers receiver. PCR is transmitted in the adaptation field of an MPEG-2 transport stream packet.

If the PCR time stamp does not occur when the receiver expects it “PCR jitter” can occur. This often will happen anytime you multiplex streams….and the more services you are multiplexing into a stream the harder it is to keep the spacing for PCR in the correct location. Not only can this happen in the station encoder but also during re-encoding at a cable head in. This can also lead to under- flow or over-flow in the receiver buffers.

This subject will be covered in more detail by John Bradford with Tektronix during the Television Engineering class March 23rd – 27th. We will also address it more in a later edition of the Continuing Education series.

CONTINUING EDUCATION SERIES ARCHIVE

We welcome comments and suggestions about this article plus other technical areas of interest that you would like to have covered.

Larry Wilkins CPBE®
ABA Engineering Academy Director
Alabama Broadcasters Association
334-303-2525