Engineering Academy Continuing Education Series

 Consistency Errors

Our first two issues of the Continuing Education series focused on digital audio and while there is enough information about that subject to last the entire year, we wanted to touch on a subject that came floating up in our “Alphabet Soup” of technical terms.

It’s called Consistency Errors….

Consistency errors can result in broken decoding, missing system components (such as closed captioning), and/or missing program guide information.

Broadcasters need to concern themselves with two kinds of metadata. Metadata defined by the ATSC is called “Program and System Information Protocol” (PSIP), and Metadata defined by MPEG is called “Program Specific Information” (PSI). Some of the functions of PSI are duplicated by PSIP. The duplication creates alternative means of locating system components; problems can arise if they conflict.

There are three categories of metadata consistency errors: external, internal, and collision errors.

External consistency errors occur when the PSIP and PSI information do not agree with one another. These errors typically occur when more than one device in the system is configured to generate metadata. If the PSIP is generated by one system device, and the PSI is generated by another system device, then it is possible for the two devices to get out of ‘sync’ with each other.

For example, the (Program Map Table) PMT is a PSI table, and the (Terrestrial Virtual Channel table) TVCT is a PSIP table. Both tables contain the program number field.

Suppose the PMT (generated on a multiplexer) lists one value for the program number, and the TVCT (generated in a PSIP generator) contains a different one. The receiver is forced to decide between the PSIP and the PSI, and cannot determine which one is correct. The response of a receiver to a specific external consistency error depends upon the receiver implementation. Two receivers from different vendors will frequently react differently to the same external table inconsistency.

Internal consistency errors occur when metadata contains references that are wrong but do not cross the boundary between PSI and PSIP. For example, the PMT is a PSI table. One of the data fields in the PMT is the PCR_PID. If the value of the PCR_PID field is incorrect, then receivers may have difficulty displaying video and audio. This is an internal constancy error because the PCR_PID value is found in a PMT (a PSI table) and not carried in any PSIP table.

Internal consistency errors are not limited to PSI tables; they can occur in PSIP tables too.

Consider the Master Guide Table (MGT) and the Channel Extended Text Table (Channel ETT), both of which are PSIP tables. Correctly formatted PSIP requires an MGT, but the Channel ETT is an optional PSIP table. If the MGT (required table) contains a reference to the Channel ETT (optional table), but the Channel ETT does not exist in the stream, then the resulting stream has a PSIP internal consistency error. Internal consistency errors can cause receivers to search for absent stream elements, or fail to find existing elements. Program guide information, closed captioning, and secondary audio channels are examples of program elements that may be affected by internal consistency errors.

Collision errors are the third kind of consistency error. They occur when similar metadata is generated on different devices in a system, and then multiplexed together into the same output.

For example, multiplexers and PSIP generators are frequently capable of generating PSI. Consider the case where PSI is generated on a multiplexer, and also generated by a PSIP generator. Unless steps are taken to consolidate the PSI from these two sources, the two different versions of PSI will collide at the output of the multiplexer. The output will contain alternating versions of PSI from each source (in this case the multiplexer and PSIP generator version). Many receiver implementations will simply fail to decode in the presence of a collision error.

We will continue our look into these errors and how to test for and correct them in the next issue.

We welcome comments and suggestions about this article plus other technical areas of interest that you would like to have covered. Previous issues of this series can be found here.

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