Headend Digital Video Processing Page 5.1 DigiPoints Volume 2 Module 5 Headend Digital Video Processing Summary In this module, students learn engineering theory and operational information about Headend Digital Video Processing. Module Objectives Upon successful completion of this module, the student will be able to: Describe how digital video signals are processed at the headend. Identify key components in the distribution of digital video. Describe the functions of each headend component. Recognize terminology used by the MPEG-2 standard for digital video. Describe how program and system information is integrated within the MPEG-2 digital data stream. Describe the typical architecture of a practical digital headend. State key operation and maintenance requirements for digital video equipment. Describe the role of the technician in supporting digital video equipment installation and maintenance at the headend. Prerequisites Read, Chapter 5. As you complete this module, you should be filling in answers and definitions for the items found on the following pages.
Headend Digital Video Processing Page 5.2 Module 5 Headend Digital Video Processing Take notes on the following pages as your instructor covers the topics in this session. For some topics, key points are identified. You should add to them with supplementary information from the class discussions. For other topics, you will need to fill in key points as requested in the workbook. Constructing meaningful notes in this way helps you to understand the subject and can serve as a valuable future reference.
Headend Digital Video Processing Page 5.3 What is Headend Digital Video? Digital signals can enter a headend system via satellite feed, various land-links, or local origination. Each signal arriving at the headend is combined at the multiplexer, processed and remodulated prior to cable distribution. Figure 1 Digital Headend Digital Monitor IRD IRT Out of Band Modulator Control Info RF Out Multiplexer Modulator Local Programming Combiner Based on the diagram in Figure 1 and class discussion, describe briefly the function of the following headend equipment. IRT IRD Out-of-Band Modulator
Headend Digital Video Processing Page 5.4 Multiplexer Modulator Combiner Identify the method of signal encryption in the following. Line Shuffler VideoCipher DigiCipher In digital encryption, MPEG-2 is the transport mechanism of the encrypted signal. The system section of the MPEG-2 standard specifies transport, which facilitates signal multiplexing. To minimize errors in data transmission, FEC (Forward Error Correction) is applied to the MPEG-2 signal before transmission and removed at the receiver.
Headend Digital Video Processing Page 5.5 Workbook Exercise # 1 Complete Workbook Exercise #1 by labeling the processes applied to the digital signal as your instructor identifies them. Figure 2 Digital Signal Processing T Transmitter MPEG-2 Transport MPEG Framing FEC Encoder QAM Modulator Channel Receiver
Headend Digital Video Processing Page 5.6 QAM Demodulator FEC Decoder MPEG-2 Framing MPEG-2 Transport
Headend Digital Video Processing Page 5.7 Overview of MPEG-2 Transport Layer Multiplexing MPEG-2 data streams contain all the information needed to identify each program within the stream. Pointers are contained in a hierarchical set of tables, which are sent as part of the transport stream in packetized form. Figure 9 MPEG-2 Transport Layer Multiplexing The Program Association Table (PAT) is contained in the packet with packet identifier PID0 Program 0 Program 1 Program 3 17 22 33 Program n 55 The Conditional Access Table (CAT) is contained in the packet with packet identifier PID1 Stream 1 Stream 2 Stream 3 Stream n Video Audio Audio Data 54 48 49 55 A Program Map Table (PMT) is contained in a Service Control Channel (SCC) Stream 1 Stream 2 Stream 3 Stream n Video Audio Audio Data 16 81 82 88 Conditional Access Data See Figure 8 for details of conditional access Program Map Table for Program 1 Program Map Table for Program 3 PAT Prog 1 Map Prog 3 Map EMM Prog 1 Audio 2 Prog 3 Audio 3 Prog 3 Video 1 Prog 3 Video 1 Prog 1 Video 1 Prog 3 Audio 2 Prog 3 Video 1 0 22 33 7 48 82 16 16 54 81 16 PID (Packet Identifier) numbers are contained in the packet headers Source: Tektronix and DigiPoints
Headend Digital Video Processing Page 5.8 Each packet or message contains a fixed packet identifier (PID) as part of its header. The Program Association Table (PAT) provides the associations between the packets and lists all the services (programs) in the transport multiplex. Provide the definition of these acronyms. SCC PMT ECM
Headend Digital Video Processing Page 5.9 Integrated Program and System Information (IPSI) Protocol IPSI protocol is a collection of hierarchically associated tables, each of which describe particular elements of typical digital TV services. Figure 3 Main Structure for the IPSI Tables B-PID STT RRT MGT VCT for channel x: source_id PID-K PID-L PID-M EIT-0 EIT-1 EIT-2 source_id source_id source_id for channel y: source_id source_id source_id source_id List the four base tables and provide a short functional description of each.
Headend Digital Video Processing Page 5.10 Event Information Tables (EIT) are a second tier of tables. Describe their function. Extended Text Tables (ETT) are a third tier of tables. Describe their function. Figure 4 Extended Text Tables in the IPSI Hierarchy MGT PID-V PID-X PID-Y PID-Z ETT-V text messages for VCT ETT-0 ETT-1 ETT-2 text messages for EIT-0 text messages for EIT-1 text messages for EIT-2
Headend Digital Video Processing Page 5.11 For an ATSC terrestrial digital transport stream, list the minimum amount of information required. For each table, the sections are appended one after the other, and then segmented into 184-byte packets. After adding the 4-byte MPEG-2 transport stream header, the packets are mult iplexed at constant bit rates with others carrying audio, video, data, etc. Figure 5 Packetization and Transport of the Program Guide MGT STT RRT VCT EIT-0 with instances ETT-0 with messages PID 0x1FFB PID K PID X MPEG-2 Transport Stream Packets audio data other Multiplex Output to Modulator
Headend Digital Video Processing Page 5.12 Generic Equipment Functions Integrated Receiver/Decoder (IRD) IRD is tuned to a desired carrier frequency within a range of transponder frequencies and demodulates QPSK formatted carriers arriving from a satellite downlink. Is dedicated to one program channel Provides composite video and baseband audio components Identify additional functions of the IRD. Integrated Receiver/Transcoder (IRT) Identify the main functional differences between the IRD and the IRT.
Headend Digital Video Processing Page 5.13 Multiplexer Multiplexers provide the ability to share a high-speed output line between a number of lower speed input signals. Add drop multiplexing operates on an MPEG-2 signal stream, remultiplexing the received signal by selecting services for output, dropping unwanted services, and multiplexing the selected services into a new data stream. Figure 6 Add-Drop Multiplexing Incoming Program Stream Select Programming Added Programming Create New Multiplex Output Program Stream Dropped Programming
Headend Digital Video Processing Page 5.14 To accomplish add-drop, the ADM reads the PID0 packet stream to obtain the Program Association Table (PAT) The PAT provides information to determine the packet streams with the program maps contained in the Service Control Channels (SCCs) for all services. SCCs contain the Program Map Tables (PMTs), containing details of the packet streams making up the services in the multiplex. Operates on each of the packet streams, adding and dropping streams as specified, and then adjusting the PIDs, related tables, and timing to correspond to a new MPEG output multiplex. Bandwidth management is a key responsibility of the headend technician. Identify the parameters that determine the output bandwidth.
Headend Digital Video Processing Page 5.15 Modulator A modulator accepts baseband video, audio and data in digital or analog formats and provides a RF modulated output. Control signals for both forward and reverse path are also modulated on carriers in digital systems. There are a few standard modulation schemes within the broadcast, satellite and CATV industries. QAM is common in CATV. In noisy environments QPSK is used, such as satellite and cable return paths. In digital broadcast, Vestigial Sideband (VSB) is the format employed.
Headend Digital Video Processing Page 5.16 Workbook Exercise # 2 Write a short description of the following Digital Video Processing keywords. Bootp ECM FEC HSTM IRT PID PMT
Headend Digital Video Processing Page 5.17 The Motorola Solution: An Example of How Digital Signals are Processed. The digital headend system below is a typical example of headend digital signal processing. Describe the process of signal acquisition.
Headend Digital Video Processing Page 5.18 Decryption The key points in the IRT decryption process of satellite transmitted programming are detailed below. Six decryption processors are employed (TSODAs), connected in series. The decryption key is derived from either the input data stream or data generated locally from a headend key list server. Authorization to decrypt the data is obtained from CAT messages. CAT messages contain PID numbers for the Entitlement Management Message (EMM) streams in the multiplex and a provider identification number associated with each EMM stream. Incoming multiplex is searched for: An EMM stream that matches the PID numbers.
Headend Digital Video Processing Page 5.19 An EMM provider identification value that matches the provider identification configured into the IRT processor. Once a match is found, the EMM is delivered to the appropriate decryption processor (TSODA). Enables decryption of the assigned service using the delivered access control key The TSODA extracts the key from the EMM, searches the incoming multiplex for the PIDs that comprise the service, and uses the decryption key to decrypt all components of the system.
Headend Digital Video Processing Page 5.20 PAT Figure 8 MPEG Signal Decryption PID0 identifies the container for the Program Association Table (PAT) PID1 identifies the container for the Conditional Access Table (CAT) CAT PID0 PID1 Program n Program n is one entry in the PAT EMM The Entitlement Management Message (EMM is an entry in the CAT, and contains the encryption key Program Map Program Messages Decrypted Program
Headend Digital Video Processing Page 5.21 Clearing Packets from the Transport Multiplex Tables and identifiers are contained in the transport multiplex. It is necessary to clear the processed data to make room for new PID1 and EMM packet streams, used for encrypting information destined for the subscriber. The system access controller informs the IRT to null the contents of the PID1 and EMM packet values.
Headend Digital Video Processing Page 5.22 Inserting Messages into the Transport Multiplex IRT then adds new messages that are required to process the signal at the subscriber s terminal. Examples of such messages are: PAT messages are reinserted from the original service map that came from the satellite transport multiplex stream or a local system access controller. CAT messages define access control for the set-top terminal and are generated from a local system access controller. For data encryption, both PAT and CAT messages must be inserted Program-Specific Information (PSI) messages deliver special features to the set-top, such as violence rating, generated from a local system access controller.
Headend Digital Video Processing Page 5.23 Encrypting the transport multiplex Processor receives an encryption EMM from the transport stream. Extracts the access control key. Map of services matches to PID numbers of service streams. Access control key applied to said service streams. Results in an encrypted MPEG-2 signal.
Headend Digital Video Processing Page 5.24 Upconverting the Bit Rate The IRT input signal may be a split multiplex. Two separate signals, I and Q, whose data rate is 23.6 Mbps. The appropriate signal must be forwarded to access control processing. Up Rate Converter circuit inserts null packets into the multiplex stream to increase the information rate to 26.97 Mbps.
Headend Digital Video Processing Page 5.25 Generating the Output Signal FEC is applied to the multiplex stream. What input data rate is required by the FEC encoder? FEC process involves Reed-Solomon encoding, interleaving and convolutional encoding. The resultant data rate is 5.056 Megasymbols per second. The signal is then upconverted to a center frequency of 44 MHz within an IF bandwidth of 41-47 MHz.
Headend Digital Video Processing Page 5.26 Parameters That a Technician Sets The headend technician may be required to set certain parameters during installation and system updates. Table 1 lists the Field Settable Parameters. Category Parameter Setting Location Comments Satellite Receiver Input Port Front Panel Required setting Transponder Front Panel Required setting number L-Band frequency Front Panel Required setting I or Q Multiplex Front Panel Required when split multiplex is selected Network Addresses IP address of the IRT Front Panel Required if the IRT is connected to a headend Local Area Network for Operations and Access Control Remote Access Control Local Control or Control Embedded in the Signal Access Controller IP address Embedded Control Messaging Stream Acquisition PID Embedded control messaging stream program clock reference (PCR) PID Embedded control messaging EMM provider ID RPC Commands and Bootp software RPC Commands and Bootp software RPC and Bootp RPC RPC and Bootp Maintenance Required Required if local control is used and extracted messages are forwarded to the controller Required if an embedded control messaging stream is used Required if an embedded control messaging stream is used Required if an embedded control messaging stream is used
Headend Digital Video Processing Page 5.27 Decryption Embedded control messaging multicast address Embedded control messaging UDP port number Embedded control messaging isochronous configuration Default EMM provider ID Number of EMM provider IDs List of EMM provider ID/service pairs RPC RPC and Bootp RPC RPC and Bootp RPC RPC Required if an embedded control messaging stream is used Required if an embedded control messaging stream is used, and the stream is transmitted to other IRT 2000s over Ethernet Required if an embedded control messaging stream is used Required Required if more than one EMM provider ID is used for encrypted services in satellite signal Required if more than one EMM provider ID is used for encrypted services in satellite signal Expansion interface Port configuration RPC Required if the expansion port is used
Headend Digital Video Processing Page 5.28 Modular Processing System Newer Motorola unit for processing digital signals Multicard chassis with configurable backplane Field upgradeable Includes IRT functions
Headend Digital Video Processing Page 5.29 Digital Modulators In the typical headend there are two types of standalone equipment used to add or remove digital information from an analog carrier. Detail the main operating features of: Out-of-Band Modulator Return Path Demodulator
Headend Digital Video Processing Page 5.30 Practical Digital In general, once a system has been installed and booted, it remains functional unless there is a power outage, channel reconfiguration, or some physical disaster that causes damage to the headend. The technician s main responsibilities will occur during device installation and configuration. List these responsibilities.
Headend Digital Video Processing Page 5.31 Workbook Exercise # 3 Testing Your Knowledge Your instructor has the answers to these questions in the Appendix to the DigiPoints, Volume 2, Module 5 Leader Guide. 1. What are the differences between an IRD and an IRT? As part of your answer, explain the advantages of an IRT over an IRD, and why it is common to use an IRD to monitor digital signal input to the headend, even when IRTs are used. 2. What major processes must be applied to a modulated digital signal, from the point at which it is received from the satellite downlink to the point at which it is placed on the distribution network? 3. How does MPEG-2 multiplexing differ from digital multiplexing in the North American Digital Signal hierarchy? How does it differ from SONET? 4. What are possible signal sources for a digital headend?
Headend Digital Video Processing Page 5.32 5. Where does the system get the information it needs to decrypt signals? Where are the encrypted signals received? 6. What are the two ways that Motorola digital headend equipment can be configured? 7. How does MPEG keep track of program streams?
Headend Digital Video Processing Page 5.33 Laboratory Exercise #1 Scope This laboratory exercise is intended to allow the student to participate in and better understand digital video processing and in particular the installation and operational issues of a key component in the process: The Integrated Receiver/Transcoder (IRT). Overview The Integrated Receiver/Transcoder (IRT) transcodes the digital signal format from a QPSK input to a QAM output. This process can be summarized and simplified in the following stages. Signal Acquisition: Demodulates the QPSK modulated L-Band carrier, and decodes the FEC encoding. The resultant information is an MPEG-2 multiplexed data stream called the High-Speed Transport Multiplex (HSTM). Decryption: Decrypts the data stream using decryption keys derived from the input data stream or locally generated. Clearing Packets from the Transport Multiplex: Removes information that will not be used in downstream transmission. Inserting Messages into the Transport Multiplex: Inserts data that will be needed for downstream processing. Encrypting the Transport Multiplex: Re-encrypts the stream by applying a new access control key. Upconverting the Bit Rate: After encryption, an Up Rate Converter inserts null packets (when necessary) into the multiplex stream to increase the information rate to 26.97 Mbps, as required by the FEC encoder. Generating the Output Signal: Forward Error Correction is applied, the resultant data stream is then QAM modulated, ready for cable distribution.
Headend Digital Video Processing Page 5.34 1. IRT-1OOO Input Level Measurement Application: Digital Video Processing at the headend Outline: The IRT is designed to receive RF input in the L-Band (950-1450 MHz). The L-band signal source could be derived from a satellite dish or generated by a local source with a L-Band Upconverter. Determine the IRT input signal meets specification of between -65 and -25 dbm per carrier. Follow local procedures and configure the system such that the L-Band signal under test is connected to the spectrum analyzer (SA) input. Note: Consult measurement note for details on SA configuration. 2. IRT input configuration Application: Digital video processing at the headend Outline: Using the front panel four arrow keys and enter key, configure the IRT parameters as per system (test) requirements. Port number: Select 1 for port 1 (top input port) or 2 for port 2 (bottom-input port). Symbol rate/trellis Coding: Select CI9_3/4 as the default setting for HITS. Front Panel Display Symbol Rate Trellis Coding Signal bandwidth C19_3/4 19.5l048 Msps 3/4 24 MHz C29_1/2 29.26572 Msps 1/2 36 MHz Transponder number and L-band frequency: The front panel controls can be used to tune either the transponder number or the L-band frequency directly. As the transponder number is changed, the corresponding frequency is automatically updated.
Headend Digital Video Processing Page 5.35 Transponder Number L-Band Frequency (MHz) Transponder Number L-Band Frequency (MHz) 1 1430 2 1410 3 1390 4 1370 5 1350 6 1330 7 1310 8 1290 9 1270 10 1250 11 1230 12 1210 13 1190 14 1170 15 1150 16 1130 17 1110 18 1090 19 1070 20 1050 21 1030 22 1010 23 990 24 970 3. IRT operational status Application: Digital video processing at the headend Outline: The IRT has several status LEDs that indicate IRT mode of operation: Authorized LED: LED conducts when authorized. DigiCipher LED: LED conducts when the IRT has locked onto a valid input signal. Manual LED: LED conducts when power is applied to the IRT. Port 1 LED: LED conducts when the L-band input has been applied to port 1, the top input port of the IRT. Port 2 LED: LED conducts when the L-band input has been applied to port 2, the bottom input port of the IRT. If the IRT has been configured correctly in 2 above as per test/system requirements, we can determine IRT mode of operation as follows. Apply power: Verify that Manual LED conducts. Tune IRT to desired transponder number: Verify that the DigiCipher LED conducts. Verify that correct Port 1 LED conducts. Disconnect the IRT input signal from Port 1: Verify that both Port 1 LED and DigiCipher LED are off. Change input signal from Port 1 to Port 2: Verify that Port 2 LED conducts and Port 1 LED is off.
Headend Digital Video Processing Page 5.36 4. IRT-1OOO Output Level Measurement Application: Digital video processing at the headend Outline: The IRT has been configured and operational status confirmed. Determine the output level of the IF signal. The IRT output is typically 44 MHz IF. The nominal signal level is +30 dbmv. Connect the IF output port of the IRT to the spectrum analyzer input. Measure the carrier level with a spectrum analyzer. Note: Consult measurement note for details on SA configuration.
Headend Digital Video Processing Page 5.37 Measurement Note This note is written to assist in accurate measurements of the IRT input and output levels. Spectrum Analyzer Configuration: Resolution Bandwidth Video Bandwidth Span RF Attn Video Averaging 300 khz 30 khz 10 MHz Automatic OFF Spectrum Analyzer Measurement Corrections: Measurements are recorded with a resolution bandwidth of 300 khz. The correction factor is necessary because the resolution bandwidth does not equal the signal bandwidth. For readings on an analog spectrum analyzer the following correction figures must be added. +12 db for 64 QAM signals +7 db for QPSK signals These correction figures are derived from the following equation. PA = PM + 10Log(WS / WR) Where, PA = Actual power level PM = Measured power level WS = Signal bandwidth WR = Analyzer Resolution Bandwidth
Headend Digital Video Processing Page 5.38 Glossary ADM Add-Drop Multiplexer. Equipment that adds and drops certain signals from a digital signal stream. Bootp Boot Protocol. Software code used to set equipment parameters when it is initialized (turned on). CAT Conditional Access Table DVB Digital Video Broadcast. A standard for digital video developed in Europe. DVS Digital Video Subcommittee of SCTE. ECM Entitlement Control Message. One of the MPEG-2 message types. EMM Entitlement Management Message. One of the MPEG-2 message types. FEC Forward Error Correction HSTM High-Speed Transport Multiplex IRD Integrated Receiver/Decoder IRT Integrated Receiver/Transcoder PID Packet Identifier PMT Program Map Table RPC Remote Procedure Call Command. Used by a centralized operations controller to configure digital equipment.
Headend Digital Video Processing Page 5.39 TSODA Processor used in Motorola s IRT to either decrypt or encrypt MPEG-2 data streams.