Introduction

There are many instances within the broadcast chain where comparisons are beneficial between input and output operation. The most crucial situation occurs during the re-multiplexing of a broadcast stream. This operation is a common event at turn around facilities, such as Cable Head-Ends, that are being supplied digital programming by a national broadcaster and then manipulating program and system information before injecting it into their system. This application note will review the Cable Head-Ends and Re-Multiplexing and show how dual analysis can be used by broadcasters and integrators to verify proper operation.

The Broadcast Chain and Re-Multiplexing

The broadcast chain generally consists of two primary networks:

• The contribution network is sometimes referred to as a backhaul. This network is used by producers and broadcasters to distribute individual programs to a central location where multiple programs are combined to create a service (such as ABC, NBC, FOX, Comsat, Televisa, Globo, ZDF, etc.). These are generally small networks consisting of no more than 2000 subscribers/affiliates. It consists of program producers (including satellite news gathering), broadcaster service providers, data network service providers, and broadband network operators (Satellite, SDH, ATM).

• The distribution network is used by broadcasters and service operators to distribute programming to the end user. This network tends to consist of multiple tiers. The national broadcasters most commonly distribute their service to local affiliates who then distribute to the end user or a service operator. However, broadcasters can also broadcast directly to a service operator. These operators combine multiple programs with system information, conditional access, and other data to create a product which is then sold to the end user. These networks can be very large, consisting of millions of subscribers. The network includes of broadcast service providers, satellite, cable and terrestrial transmission network operators, conditional access management service providers and Internet service providers.

Broadcast Network

The successful launch of a digital service requires the involvement of numerous suppliers to the broadcaster. These include various system integrators, encoder manufacturers, multiplexer manufacturers, modulator manufacturers, set-top-box (STB) manufacturers, and chip-set manufacturers.

During the digital migration, many cable head-ends are subscribing to multi-program digital services and then customizing the information in the multiplex using a re-multiplexer before they inject it into the cable distribution system. Re-multiplexing is a method of reorganization such that the programming (audio and video) does not have to be converted to analog and then re-encoded (compressed) into a digital service.

Because a re-multiplexer creates the entire multiplex, only one is needed per multi-program digital channel (carrier frequency). Decoding a program to baseband and then compressing it an additional time would require an MPEG encoder for every re-transmitted program. As a result, the use of a re-multiplexer not only helps maintain program quality by avoiding another iteration of compression, it also reduces the cost of digital deployment and system complexity.

Multiple Channel Re-Multiplexer

The re-multiplexer may be used to perform several different tasks. It can operate on a single multiplex and manipulate network transmission parameters such as type (satellite, cable, terrestrial), position, frequency, etc. It can also be used to re-map the PIDs of program elements to avoid conflicts with other programming and insert new conditional access. Figure above shows how a re-multiplexer would be used to rearrange or combine different transport streams into a new multiplex. This is a valuable method of creating bouquets or service tiers.

In addition to translating between delivery systems (satellite to cable, terrestrial to cable, etc.), a re-multiplexer may also be converting from one standard to another (i.e. DVB to ATSC). In the first instance, the primary changes required involved description of the delivery system whereas the second instance requires a rebuilding of the DVB System Information (SI) into the ATSC Program and System Information Protocol (PSIP) - a much more involved process.

Dual Analysis

When using re-multiplexing within a broadcast system it is valuable compare the data going into the equipment versus the data coming out of the equipment. One method of comparing the I/O is to use a single MPEG2/ATSC/DVB transport stream analyzer. The unit would be connected to the input feed for analysis where the data of interest would then have to be noted. The analyzer would then be connected to the output of the re-multiplexer and then a comparison would be made between what was recorded and what is currently displayed. For systems not changing many of the parameters this may be sufficient. However, for systems where many of the parameters are changed, such as in the cable head end shown in the below, this method can prove impossible.

Satellite to Cable Broadcast System

Another method is to use two separate transport stream analyzers: one connected to the input of the re-multiplexer and one connected to its output. While this does provide a real-time display of the I/O analysis and eliminates the need to record information it has its drawbacks. Since there are two pieces of equipment it may prove to be very inconvenient to make comparisons between them because of the separation. This configuration is also both costly and cumbersome. Especially if you are a field service organization or integrator that has to transport the equipment around the world.

The ideal solution is having dual analysis capability with completely independent operation in the same unit (see figure below). The Acterna DTS-300 Digital Broadcast Test Platform can be configured with a Dual Analysis option. This arrangement is user friendly, cost efficient, and very transportable.

Acterna DTS-300 Dual Transport Stream Analysis

Using the DTS-300 with Dual Analysis, operators, integrators, and field service can easily analyze program and PID re-mapping, evaluate changes in transmission parameters, modifications to channel descriptors, conditional access data, timing information, etc. Below are some examples of the Acterna DTS-300 Dual Analysis capabilities. These captures were performed on a hypothetical system which consisted of several ATSC terrestrial channels being received by a cable headend and shown on channel 1 of the TSA. These channels were then manipulated in several ways before they were broadcast into the cable network and are being shown on channel 2 of the TSA.

Dual Analysis of Broadcast Parameter Changes

When translating from terrestrial to cable, several parameters must be changed in order to guarantee proper reception. These include changing from the mapping of information from Terrestrial Virtual Channel Tables (TVCT) to Cable Virtual Channel Tables (CVCT). As shown in the figure above, other changes may also be required which include carrier frequency and modulation. In this example, the type of modulation did not change, but in many instances the modulation used in a cable plant will be QAM (Quadrature Amplitude Modulation).

When going from one multi-channel source to another multi-channel source, some channels may be dropped and it is important to know if this has occurred properly. The figure below shows the system where two channels have been dropped from the multiplex: KUSI and TBS.

Dual Analysis of Channel Line Up Changes

When transports streams are translated and re-multiplexed from one system to another, they may also go through both a channel re-mapping and a PID. This is generally done to eliminate duplicate channels and PIDs numbers in the output stream. It may also be done at the operators discretion in order to customize their system in a certain way. It is important therefore to confirm that proper re-mapping has occurred. The previous figure and the figure below show how easily a comparison can be made between input and output channel and PID re-mapping using Dual Analysis. A comparison of errors between the input and output is also very valuable as to determining where a problem may exist. For instance if continuity count errors show up on the output there may be a problem with buffer overflow and the signal into the system may be dropping packets.

Dual Analysis PID Re-Mapping

Some of the other modifications broadcasters might perform deal with descriptors. As shown in the two figures below, the operator has changed both the title of the show and its description. Using Dual Analysis, the operator can verify that proper changes have been made and are being distributed into the system.

Dual Analysis of Program Title Changes

Any time modifications are made to a transport stream, there is the possibility timing errors can be introduced. It can be manifested either as undue PCR jitter or PCR spacing. As demonstrated in the example below, the input to the re-multiplexer (channel 1) has very consistent PCR spacing centered around 40 ms. The output of the re-multiplexer (channel 2) has tighter PCR spacing but there is much more variation. Maintaining proper timing characteristics is critical to proper reception and Dual Analysis can play an integral part of determining the source of the problem.

For more information or to receive a free evaluation software, please send your contact details to mpeg@jdsu.com.