DigiPoints Volume 2. Leader Guide. Module 5 Headend Digital Video Processing

Transcription

1 Headend Digital Video Processing Page 5.i DigiPoints Volume 2 Module 5 Headend Digital Video Processing Summary In this module, students will learn engineering theory and operational information about Headend Digital Video Processing. Outcomes When they complete this module, students will be more knowledgeable about Digital Video Processing at the system headend, and as a consequence will be more able to operate, maintain, and troubleshoot this kind of system. Module Objectives Upon successful completion of this module, students 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 Students should have read Chapter 5 of Digipoints, Volume 2. Length 3 hours

2 Headend Digital Video Processing Page 5.ii Materials/Preparation for Instructor One workbook per student The instructor should read Chapter 5 of, and complete review questions from the chapter. System-specific information about digital video processing, as available, including: Kind of Equipment used Processing methods used Supplies/Equipment Flip chart or board, markers Overhead projector or LCD Audience The intended audience will be mid- to senior-level technicians or other associates who are seeking an understanding of digital basics.

3 Headend Digital Video Processing Page 5.iii Module Outline This is an introductory level module that will provide a review of the following topics: Objectives...1 Introduction...2 What is Headend Digital Video?...3 Brief Overview of MPEG2 Transport Layer Multiplexing...6 Integrated Program and System Information (IPSI) Protocol...7 Generic Equipment Functions...11 The General Instrument Solution: An Example of a Digital Headend...16 Practical Digital...29 Summary...31 Appendix...32

4 Headend Digital Video Processing Page 5.1 EXPLAIN REFER TO WB 5.1 Objectives Tell students that when they have completed this module, they will be able to accomplish these objectives. 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.

5 Headend Digital Video Processing Page 5.2 DISCUSS Introduction Ask the students the following questions: What do they know about digital video processing? Who has worked with it? How much of their local system is equipped for digital video? Tell students that this module will increase their knowledge of how digital video processing is implemented at the headend and will describe operational and maintenance concerns. Use local information to build on students answers. Flip chart their answers. EXPLAIN Overview Explain that this session will investigate the concepts that support headend digital video processing such as: Digital signal reception and distribution Headend equipment functionality MPEG-2 video standard Signal encryption & decryption Add-Drop Multiplexing Point out that headend personnel will need to understand: Basic digital video technology Digital signal flow at the headend Installation, function and maintenance of equipment Lab Exercise #1 will provide hands-on experience with digital video signal processing by including an IRT as part of the lab

6 Headend Digital Video Processing Page 5.3 EXPLAIN DISPLAY VA 5.1 REFER TO WB 5.3 REMIND STUDENTS TO COMPLETE WB 5.3 AS YOU DISCUSS THE FUNCTIONS OF RELEVANT DIGITAL HEAD END EQUIPMENT. REMIND STUDENTS TO CONTINUE FILLING IN WB 5.4 ASK IF THERE ARE ANY QUESTIONS. What is Headend Digital Video? Signal Flow: Some History and a Review Explain that digital signals entering the headend are combined at the multiplexer, processed and re-modulated prior to cable distribution. These digital signals can originate from Satellite feeds Various land links Local origination Video servers The Integrated Receiver/Decoder (IRD) or Integrated Receiver/Transcoder (IRT) receives the satellite feed, tuning to the center frequency of the satellite carrier The functionality of the receiver in the IRD and IRT is similar to any RF Receiver Front End Down-converting the RF signal to an IF, then Demodulating the IF to provide a baseband signal The Out-of-Band Modulator is used to process and extract control information that will be sent to set-top boxes. The multiplexer provides a way to add local programming to digital streams. The modulator places the multiplexed signal onto an RF carrier. The combiner is the means of placing several signals onto common distribution plant. The Decoder in the IRT or IRD decrypts the encrypted baseband signal using locally generated keys. Refer to VA 5.2 and trace the signal flow from the dish to the RF Out. Inform the students that Chapter 7 will cover further details on encryption & decryption methods and the implementation of keys. Explain that a key is a process that changes the order of 1s and 0s in a digital transmission.

7 Headend Digital Video Processing Page 5.4 Point out that signal security technology has been an evolution. Some early techniques: Line shuffling on each frame of video The sequence of video lines is shuffled pseudo-randomly at the transmitter. The decoder received the shuffled video plus data that enabled the video line to be decoded to its original sequence. Requires a large amount of storage to hold lines prior to resequencing A later method involved a technique called VideoCipher where IRDs tuned to a single analog satellite channel, removed the sync information signal and replaced it with the digitally encoded audio before distribution to the subscriber. The subscriber s decoder re-inserted the sync signal to the video, removed and decoded the audio signal. A video frame = two fields, even & odd, 525 lines per frame. Difficult to implement line shuffling, prior to the availability of low cost, high speed storage medium. Barker channel can provide information on how to subscribe. REFER TO VA 5.2 Point out that digital systems use more sophisticated encryption techniques than their analog counterparts, DigiCipher is an example. It employs IRDs to tune a single digital program from a multi-program digital stream. MPEG-2 is employed as the transport mechanism of the encrypted signal. The systems section of the MPEG-2 standard specifies transport that permits the multiplexing of individual data streams. Forward Error Correction (FEC) minimizes data errors introduced during transmission that negatively impact on signal quality. VA 5.2 illustrates the relationship of FEC to other digital signal processes employed in the distribution of the digital signal. Chapter 8 in DigiPoints, Volume 1, discusses the MPEG-2 standard, in particular the compression algorithms employed. MPEG-2 Transport Layer Multiplexing is discussed in the Appendix.

8 Headend Digital Video Processing Page 5.5 ASK EXPLAIN DISPLAY VA 5.2 WB EXERCISE #1, WB What factors are likely to impact the quality of reception from a satellite downlink such that FEC is required? Answer: Signal degradation caused by precipitation, fade and shadowing Inform the class that to maximize signal security during transmission, the MPEG-2 data stream can be encrypted prior to FEC. The decryption of the MPEG-2 stream at the subscriber terminal is typically achieved by using decryption keys. Derived from the received data stream Derived from the smart card installed at the set-top Refer students to Workbook Exercise 1. Have the students complete the exercise by labeling the components in the diagram as you identify them. Chapter 7 will provide further details on encryption, decryption & smart cards. Explain that the decryption key restores the order of 1s and 0s that the encryption key had changed. Motorola systems use a key derived from the data stream. DVB systems employ smart cards. Answers to workbook exercises are at the end of the

9 Headend Digital Video Processing Page 5.6 DISPLAY VA 5.3 REFER STUDENTS TO THE CORRESPONDING FIGURE 9 ON WB 5.7 REMIND STUDENTS TO DEFINE THE ACRYONYMS ON WB 5.8 EXPLAIN Chapter Appendix: Brief Overview of MPEG-2 Transport Layer Multiplexing Explain that an MPEG-2 signal consists of a number of separate data packets, each with individual headers. These packets are often referred to as messages. Each header contains information that identifies the type of information represented by the packet or message. A set of packets from a given signal are termed streams. For example: Video streams Audio streams Text streams Data streams All packets are contained in a multiplex transport stream along with a fixed packet identifier (PID) in a hierarchical, tabular format that identifies each individual stream. The MPEG-2 standard specifies that the PID number 0 is assigned to the PAT for an MPEG-2 encoded transport multiplex. The Program Association Table (PAT) Provides the associations between the packets Lists all the services (programs) in the transport multiplex Declares the PID numbers of the Service Control Channels (SCCs) Point out that SCCs contain a Program Map Table (PMT) that lists: The PID numbers of every packet stream making up that service (e.g., audio, video, and data). The PID number of the packet stream containing access control information if the service is encrypted. Termed the Entitlement Control Message (ECM) stream Introduce the chapter Appendix material at this point to clarify MPEG-2 processing. Sketch a packet on the board, showing the header and payload (Refer to DigiPoints, Volume 1, p.197). Multiple packets = set of packets VA 5.10 details the relationship between tables and ordering of messages in an MPEG-2 data stream.

10 Headend Digital Video Processing Page 5.7 REFER TO WB 5.9 REFER TO VA 5.4 REMIND STUDENTS TO LIST THE FOUR TABLES IN WB 5.9, AND PROVIDE A DESCRIPTION FOR EACH. Integrated Program and System Information (IPSI) Protocol Explain that the traditional channel bandwidth (6 MHz) for analog TV broadcast can, under the ATSC standard for digital TV, support 20 Mbps of data throughput. Employing MPEG-2 compression algorithms, provides 3-6 digital TV channels within the 6 MHz bandwidth. Standard resolution can be compressed to sustainable rates of 2-6 Mbps. The digital channel can also support additional low-bandwidth services such as: Data broadcasting (weather reports, stock indices, headline news) Software downloads (for games or enhanced applications) Image-driven classified ads Home shopping Pay-per-view information Future improvements to technology may see the traditional 6 MHz bandwidth support further channels, emerging services and applications. Point out that the IPSI protocol tracks all information that each 6 MHz physical channel carries. This information from both analog and digital channels is hierarchically tabulated. Each table describes particular aspects of typical digital TV services. There are four base tables, contained in packets with a base PID (B-PID): System Time Table: One packet data structure containing time of day references used for timing advertised events in the programming guide. Refer to ANSI/SCTE for further details on this topic. PID = packet identifiers

11 Headend Digital Video Processing Page 5.8 DISPLAY VA 5.4 REFER STUDENTS TO WB 5.10 Rating Region Table: Compiles program-rating standards such as MPAA Accommodates regional/country specific definitions. Master Guide Table Provides general information on the tables that compose the IPSI protocol Defines table sizes for memory allocation during decoding Provides version numbers to identify tables that require updating. Enumerates location identifiers (PIDs) for the EIT and ETTs Virtual Channel Table: Contains a list of all channels that are or will be on-line plus their attributes such as: Channel name Navigation identifiers Stream components Types Point out that the Event Information Tables (EITs) are a second level of tables whose locations or packet identifiers are defined in the MGT. EITs describe events/programs associated with each channel in the VCT. A maximum of 16 days of programming can be advertised in advance consisting of 64 EITs containing information for a 6 hour span Two EITs are normally present in every transport stream. EIT-0 current time span EIT-1 next 6 hours To facilitate remultiplexing where EIT from different transport streams may be grouped together, start times for EIT are limited to the following: 0:00, 6:00,12:00 and 18:00 If this time constraint were not imposed, remultiplexing would be a difficult task requiring the EIT to be parsed in real time. MPAA =Motion Picture Association of America V-chip technology utilizes the RRT. Emphasize the hierarchical nature of the tables.

12 Headend Digital Video Processing Page 5.9 EXPLAIN DISPLAY VA 5.5 REMIND STUDENTS TO FILL IN WB 5.10 Explain that the Extended Text Tables (ETTs) are a third set of optional tables whose packet identifiers are also defined in the MGT. ETTs carry text messages. For example, for channels listed in the VCT, the text will describe: Channel Info Cost Coming attractions For movies, the text will contain a short description of the movie. MGT= Master Guide Table Identify the MGT in VA 5.5. SUMMARY To summarize the IPSI protocol, the minimum amount of information required by the ATSC terrestrial digital transport stream is the four base tables and the first two EITs. ASK ASK DISPLAY VA 5.6 REFER TO FIG. 5 ON WB 5.11 Ask the students to list the essential tables. Answer: VCT, MGT, RRT, STT, EIT-0, and EIT-1 Ask the students to identify the exception to the summary. Answer: When all channels in a transport will be off the air at a certain time. In this case, no EIT-1 will exist in the hours prior to shutting the system off, and thus, only EIT-0 will be transmitted. Each of these tables fit into the MPEG-2 transport stream as standard packets where typically: The MGT, STT and VCT and each instance of the RRT and EIT will have one or at most a few sections. Each section is appended one after the other and then segmented into a standard 184-byte packet. MPEG is discussed in the Appendix.

13 Headend Digital Video Processing Page 5.10 A four byte header is added and the packets are multiplexed at constant bit rates with other carriers containing video, data etc.

14 Headend Digital Video Processing Page 5.11 CONDUCT GROUP ACTIVITY: REFER STUDENTS TO WB ALLOW 15 MINUTES CONTINUE WITH DISCUSSION OF GROUP ACTIVITY REFER TO WB 5.12 Generic Equipment Functions Integrated Receiver/Decoder (IRD) Ask class to break into four groups. Assign each group one of the following pieces of equipment: IRT, IRD, multiplexer, and modulator. Ask each group to discuss the functions of their assigned piece of equipment and to fill in the appropriate sections of the workbook. After 15 minutes, review the functions of each piece of equipment using the discussion points below. Inform the class that an IRD receives QPSK modulated signals arriving from a satellite downlink. It is capable of being tuned to the desired carrier frequency within a range of transponder frequencies. The output of the device typically provides one program channel containing: Composite video in NTSC or PAL formats Two baseband audio components In addition to demodulating the carrier, the IRD facilitates: Software downloads to upgrade device performance ( fix bugs ) Status monitoring via Ethernet headend links Signal decryption using companion smart cards Monitoring the signal quality of received channel QPSK = Quaternary (4) Phase Shift Keying Smart cards contain decryption key.

15 Headend Digital Video Processing Page 5.12 DISCUSS REFER TO WB 5.12 Integrated Receiver/Transcoder (IRT) Explain that although similar in front-end functionality to the IRD, the IRT: Accepts satellite signal in QPSK format and transcodes the signal to a QAM format for redistribution in the CATV network. Provides a compressed, multiplexed output consisting of several program channels. Satellite Tx format = QPSK. Cable Tx format = QAM QAM = Quadrature Amplitude Modulation

16 Headend Digital Video Processing Page 5.13 DISCUSS REFER TO WB 5.13 ASK EXPLAIN DISPLAY VA 5.7 Multiplexer Tell the class that a multiplexer combines several lower speed signals into a single higher speed output. Once multiplexed, how does one re-gain access to an individual signal? Answer: Early versions required the signal to be de-multiplexed to its lowest bandwidth components. The process of multiplexing and demultiplexing requires hardware for each step and as such becomes expensive. A better alternative is provided by Add-Drop Multiplexing (ADM), which, without the need to demultiplex, enables: Direct access to the individual data stream The addition of new signals to an existing data stream In the digital headend, the received signal is remultiplexed by: Selecting services for output Dropping unwanted services Multiplexing the selected services into a new data stream The ADM is used to change the mix of the program. For example: Add local programming Customize a premium service for a given market Inform the class that to accomplish adddrop The ADM processes the MPEG-2 stream to obtain the PAT by reading the PID-0 packet. The PAT provides information that determines the packet streams with the program maps contained in the SCC for all services. ADM is an attribute of the MPEG-2 standard. SONET standard for Optical Transmission also provides add-drop capability. PAT = Packet Association Table SCC = Service Control Stream

17 Headend Digital Video Processing Page 5.14 REFER TO WB 5.14 EXPLAIN The ADM then obtains the PMTs by reading the SCC which provide details of all the service packet streams included in the multiplex The ADM can then operate on each packet stream: Adding and dropping packets as specified Adjusting the PIDs, related tables and timing to provide a new MPEG- 2 output multiplex Point out that a key role for the technician in the implementation of ADM is bandwidth management and ensuring that the maximum bandwidth is not exceeded to avoid data lost in dropped packets. The maximum output bandwidth is the sum of the following: Maximum input PMT PID stream bandwidth Tolerances for both input and output clocks will also impact on PMT PID bandwidth such that, for calculations, use the fastest input data rate and slowest output data rate. Maximum bandwidth of any unique conditional access streams listed in the PMT Maximum bandwidth of each elementary PID stream listed in the PMT PMT = Program Map Tables VA 5.7 provides an example of the Add-Drop process. PID = Packet Identifier

18 Headend Digital Video Processing Page 5.15 DISCUSS AND REFER STUDENTS TO WB 5.15 WORKBOOK EXERCISE #2, WB 5.16 Modulator Explain that the functionality of a modulator is often included in headend digital processing equipment. A modulator accepts baseband video, audio and data in digital or analog formats, and provides a RF modulated output in various formats. In IRTs described earlier, the QPSK satellite signal is first demodulated to a baseband data stream and then re-modulated to a QAM signal. In headends where ADM is employed, the ADM output can feed directly to the modulator input of the IRT or to a stand-alone modulator. Control signals for both forward and reverse path are also modulated on carriers in digital systems. Inform the class that there are a few standard modulation schemes within the broadcast, satellite and CATV industries. QAM is common in CATV. In noisy environments, such as satellite and cable return paths, QPSK is used. In digital broadcast, Vestigial Sideband (VSB) is the format employed. Refer students to workbook exercise #2. Answers are in the Workbook glossary, WB Review answers as a group. Refer to DigiPoints, Volume 1, Chapter 5 for further details on QAM. Allow ten minutes for completion of exercise.

19 Headend Digital Video Processing Page 5.16 EXPLAIN DISPLAY VA 5.8 REFER TO WB 5.17 Motorola Solution: An Example of a Digital Headend Explain that we will use the Motorola digital headend system as a practical example for our discussion today on digital video processing at the headend. Follow the digital signal, beginning at the Integrated Receiver/Transcoder (IRT), and continuing through to the cable distribution network The IRT is one of two systems offered by Motorola. The Modular Processing System is the other. Refer to VA 5.8 and trace the signal path. Note: C6U is a Motorola upconverter used to convert intermediate frequency (IF) signal to RF.

20 Headend Digital Video Processing Page 5.17 REMIND THE STUDENTS TO SUMMARIZE THE SIGNAL ACQUISITION PROCESS IN THEIR WORKBOOKS ON WB 5.17 WRITE THE FOUR FUNCTIONS ON THE BOARD OR EASEL. REFER TO THEM AS YOU CONTINUE WITH THE DETAILS. IRT Signal Acquisition Tell the class that the IRT accepts a FEC encoded, QPSK modulated carrier between MHz. The QPSK signal bandwidth is dependent on the satellite transponder and is typically 24 or 36 MHz wide. The IRT tunes to the desired carrier frequency, demodulates the QPSK satellite signal, and decodes the FEC encoding. The resultant MPEG-2 data stream is termed a High-Speed Transport Multiplex (HSTM) which consists of: Several separate information streams containing programs Control information for processing the network services The HSTM PID identifies each individual signal component and determines the appropriate processing required. Four functions are performed on the input signal: Decryption of the encoded satellite signal Removal of obsolete downstream information Insertion of information that will be needed for downstream processing Re-encryption of the signal with new security and access control for distribution to the subscriber L band = MHz Programs = network services

21 Headend Digital Video Processing Page 5.18 POINT OUT THAT THIS IS THE FIRST OF THE FOUR FUNCTIONS PERFORMED BY THE IRT ON THE INPUT SIGNAL. REMIND THE STUDENTS TO FOLLOW THE KEY POINTS OF THE DESCRIPTION OF IRT FUNCTIONS IN THE WORKBOOK, BEGINNING ON WB WB DISPLAY VA 5.9 REFER TO WB 5.20 POINT OUT THAT THE BOXES, LETTERS, AND KEYS ARE ANALOGIES TO HELP REMEMBER WHAT HAPPENS AT EACH STEP OF DECRYPTION. Decryption Explain that the IRT employs six processors (TSODAs), connected in series to decrypt the input signal. The decryption key is derived from one of two sources: The input data stream Data generated locally from a headend key list server A total of six information streams can be processed from a combination of no more than two services. Point out that when the keys are derived from the input data stream, the IRT obtains authorization to decrypt the data in the following manner. CAT messages are examined for: PID numbers of the Entitlement Management Message (EMM) streams in the multiplex Provider identification number associated with each EMM stream The IRT searches the incoming multiplex for the EMM stream that matches the PID numbers. It then searches for an EMM provider identification value that matches its configured provider identification. If found, the IRT: Extracts all EMM messages addressed to its decryption processors and delivers each EMM to the appropriate 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. IRTs can be cascaded or looped to increase processing capacity CAT = Conditional Access Table.

22 Headend Digital Video Processing Page 5.19 POINT OUT THAT THIS IS THE SECOND FUNCTION PERFORMED ON THE INPUT SIGNAL. REFER TO WB Clearing Packets from the Transport Multiplex Explain that as the data contained in the transport multiplex is accessed and then processed: The system access controller informs the IRT to null the contents of the PID1 and EMM packet values. It is necessary to clear the redundant data to make room for new PID1 and EMM packet streams, used for encrypting information destined for the subscriber.

23 Headend Digital Video Processing Page 5.20 POINT OUT THAT THIS IS THE THIRD FUNCTION PERFORMED BY THE IRT ON THE INPUT SIGNAL. REFER TO THE FUNCTIONS YOU WROTE ON THE BOARD, AND TO WB 5.22 ASK CHOOSE FROM VOLUNTEERS TO GIVE AN ANSWER Inserting Messages into the Transport Multiplex Explain that the IRT then adds new messages that are required to process the signal at the subscriber s terminal. For data encryption of the signal both PAT and CAT messages must be inserted. PAT messages can be 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 Generated from a local system access controller Program-Specific Information (PSI) messages deliver special features to the set-top, such as a violence rating: Generated from a local system access controller Why does the headend remove one form of encryption from the satellite signal only to re-encrypt for cable distribution? Answer: The program streams to the subscriber are different than those originating at the satellite uplink. The operator has the option of adding and removing programs, or adding locally generated content. Some operators may also choose to include certain programs as part of basic service, while others may treat them as premium, requiring some encryption (The Disney Channel is a good example.) From the perspective of encryption, it is therefore necessary to treat the signal to the subscriber as an entirely different transport stream. PAT = Program Association Table. CAT = Conditional Access Table

24 Headend Digital Video Processing Page 5.21 POINT OUT THAT THIS IS PART OF THE FOURTH FUNCTION THE IRT PERFORMS ON THE INPUT SIGNAL. REFER TO WB 5.23 Encrypting the Transport Multiplex Inform the class that there are six encryption processors in the IRT. Each can process a maximum of six data streams from no more than two services. A processor receives an encryption EMM from the transport stream and extracts the access control key. Using the map of services to search the multiplex to find the PID numbers of the streams making up the service (e.g., video, audio, data, text). The IRT instructs a processor to apply the key to those streams providing the encrypted MPEG-2 data stream Do not confuse with the decryption processors, although they are also called TSODAs. EXPLAIN REFER TO WB 5.24 Upconverting the Bit Rate Explain that the input signal to an IRT may be a split multiplex which contains: Two separate signals, I and Q whose data rate is 23.6 Mbps. In such cases, the appropriate signal must be specified for access control processing. After encryption, an Up Rate Converter circuit inserts null packets into the multiplex stream to increase the information rate to Mbps, as required by the FEC encoder. In cases where a combined multiplex is the input, the information rate is Mbps and no packet insertion is required.

25 Headend Digital Video Processing Page 5.22 REFER TO WB 5.25 Generating the Output Signal Explain that FEC is applied to the signal prior to QAM modulation. FEC employs Reed-Solomon encoding, interleaving and convolutional encoding. The resultant data rate is Megasymbols per second The signal is then upconverted to a center frequency of 44 MHz within an IF bandwidth of MHz. Position of FEC in signal flow is illustrated in VA 5.3 Review the reasons for using FEC by asking why it is needed. (Answer: there is opportunity for errors at several points in the signal processing.) Refer to ANSI/SCTE for further details on Reed Solomon encoding. ASK FOR EXAMPLES OF PARAMETERS THAT MIGHT BE SET. DISPLAY VA 5.10, TABLE 1 REFER TO WB FOR ANSWERS Parameters that a Technician Sets Explain that the system technician may be required to set certain operational parameters during installation and system updates. For example, the following parameters of a satellite receiver require setting: Input port selection Transponder number L-Band frequency Split multiplex, I or Q Refer to VA 5.10 for further examples.

26 Headend Digital Video Processing Page 5.23 EXPLAIN THAT THE MPS PERFORMS ALL THE FUNCTIONS OF THE IRT, AND IS ONE WAY TO PROVIDE FLEXIBILITY FOR AN OPERATOR. REFER TO WB 5.28 Modular Processing System (MPS) Explain that Motorola has added the MPS to its product line as a more flexible design for processing digital signals. Configurable backplane Multi-card chassis Field upgradeable

27 Headend Digital Video Processing Page 5.24 STIMULATE DISCUSSION BY ASKING WHAT INFORMATION OTHER THAN PROGRAMS AND ENCRYPTION MUST BE COMMUNICATED BETWEEN SUBSCRIBER EQUIPMENT AND THE HEADEND. REFER TO WB 5.29 Digital Modulators Tell the class that we will describe two examples of stand-alone equipment that operates on an analog carrier to either add or remove digital information. The Out-of-Band Modulator The Return Path Demodulator

28 Headend Digital Video Processing Page 5.25 EXPLAIN AND REMIND STUDENTS TO TAKE NOTES IN WB 5.29 EXPLAIN Out-of-Band Modulator (OM) Explain that the OM employs a QPSK formatted RF carrier to transmit in the forward path, control signal such as: Commands and authorization messages for set-top terminals Electronic Program Guide information Emergency Alert Messages Data services Executable code downloads to fix/ upgrade cable terminal operation The OM multiplexes the MPEG-2 messages in a similar manner to the ADM, reassigning PID numbers to avoid possible duplication. The combined output is limited to 2.005Mbps and to meet that data rate such packets may be dropped or null packets added as required. The bandwidth of the QPSK modulated data is 1.5 MHz wide and is upconverted to a carrier in the range MHz. The default output frequency is MHz Point out that input signals can arrive at the OM modulator on either serial or Ethernet ports. The Ethernet port typically accepts: Set-top control streams containing commands and authorization messages that are generated by: A local access controller A national access controller The IRT extracts the national center generated control streams, which are embedded in a Mbps data stream via the satellite downlink. Configuration and control data for the OM modulator Boot information and status monitoring OM transmits downstream to the subscriber. Ethernet facilitates twoway communications

29 Headend Digital Video Processing Page 5.26 The serial port typically accepts: Program guide information Code downloads Set-top terminal control information. EXPLAIN Point out that Emergency Alert Signal (EAS) processing is a special function of the OM. These control messages force tune the converter to a predetermined channel to receive the alert and instructions. Two input connections are required to process these signals. A control processor providing terminal control messages via an Ethernet link for insertion into the output transport multiplex A relay connection to an emergency alert remote control unit which, when energized, transmits control messages to the OM packet multiplexer which routes the emergency messages to the RF modulator

30 Headend Digital Video Processing Page 5.27 REMIND STUDENTS TO CONTINUE TAKING NOTES IN WB 5.29 ASK QPSK Return Path Demodulator/Multiplexer (RPD 1000) Inform the class that the QPSK RPD demodulates ATM, rather than MPEG-2 signals that originate in subscriber set-top terminals, and provides: Billing information for PPV purchases Opinion poll responses At the subscriber set-top, the modem: Generates an ATM signal that modulates a QPSK carrier for upstream transmission The carrier is frequency agile and can transmit a 54-byte data packet at an instantaneous data rate of 256 Kbps in an 8-12 MHz bandwidth. In addition to demodulating the QPSK formatted carrier the demodulator also calibrates modem transmit levels across the upstream bandwidth to determine if power level adjustments are required at the set-top terminals. Why is QPSK the format employed for upstream transmission and not QAM? Answer Upstream is a noisy environment where all distribution branches are combined. QPSK is more resilient than QAM in such conditions, with improved bit error rates. Point out that an Ethernet port provides two-way communications capability for: Upstream data transfer Device configuration Initialization using boot protocol functions ATM = Asynchronous Transfer Mode. Note that modulation occurs at the set top terminal. RPD transmits upstream to the headend.

31 Headend Digital Video Processing Page 5.28 POINT OUT THAT OPERATIONS SYSTEMS MAKE IT EASIER TO PROVISION AND MAINTAIN SYSTEMS. Operations Systems Explain that we will be discussing operations systems in Chapter 10. A point to remember in our discussions thus far is that all of the digital headend equipment contain Ethernet connections to a headend LAN. The headend LAN connects to an operations support terminal that monitors and updates equipment status.

32 Headend Digital Video Processing Page 5.29 DISCUSS THE POSSIBLE REASONS FOR SYSTEM MAINTENANCE. REMIND THE STUDENTS TO LIST TYPICAL TECHNICIAN RESPONSIBILITIES IN THE WB 5.30 Practical Digital Point out that in general terms, once a digital system has been installed and configured, it is reliable and remains functional, unless there is a: Power outage Channel/system reconfiguration Physical disaster that causes damage to the headend It follows that the system operation is most vulnerable during installation, the technician s key responsibilities at this time are detailed below. Bootup software and procedure is critical for system configuration. Each parameter detailed in Table 1 having bootup as a setting must be correctly specified. Measure and adjust the modulator output to a set level. To ensure the MPEG-2 data stream or out-of-band control data arrives at the set-top within specified levels. Verify correct selection of the EIA frequency lineup. In systems where access control is not provided locally, the technician has additional responsibilities. Verify that the IRT is properly authorized by providing correct decrypt processor (TSODA) addresses to the satellite uplink controller. Configure the IRTs as Primary or Secondary The Primary IRT processes control signals and forwards them to the Out-of-band Modulator. Secondary IRTs use control signals processed by the Primary IRT.

33 Headend Digital Video Processing Page 5.30 INFORM The technician must also be aware of system parameters that are impacted by external events such as daylight saving times. Time zone variables (program offsets) and program skews may need to be set from the addressable controller. Point out that the industry trend is to centralize the monitoring of equipment status where Ethernet based LANs will provide a bidirectional network monitoring capability. Centralized maintenance and monitoring of equipment will be discussed in a future module.

34 Headend Digital Video Processing Page 5.31 DISCUSS QUESTION Summary Tell the class that in today s discussion on headend digital video processing: We have traced the input digital signal from its entry on a satellite downlink, through the demodulation and decryption processes, and watched it become part of a remultiplexed MPEG-2 data stream, upconverted to RF, ready for cable distribution to the subscriber. During the process the signal may have changed format from a QPSK satellite input or a VSB broadcast input to a QAM cable output. The MPEG-2 transport layer is a critical part of processing a signal through a digital headend. We have discussed several components of the MPEG-2 transport layer and their inter-relationships, such as: PAT PID CAT EMM ECM The Motorola headend model provided a practical example of a working digital headend. It provided an introduction to the equipment required to process the digital data stream: The Integrated Receiver/Decoder (IRD) and the Integrated Receiver/Transcoder (IRT) Multiplexer Modulator Future discussions will deal with the implementation of operating systems to both control and monitor the headend equipment. Ask the student to identify each acronym (Note: Motorola system components are proprietary implementations of digital signal processing) WORKBOOK EXERCISE #3, WB Wrap Up Ask students to complete the Test Your Knowledge section, Workbook Exercise #3. Review answers as a group. Pair students for the Review Questions.

35 Headend Digital Video Processing Page 5.32 Appendix Workbook Exercise # 1 Complete Workbook Exercise #1 by labeling the functions applied to the Digital Signal Processing as your leader identifies them. Figure 2 Digital Signal Processing T Source: Digital Video Standard DVS-031 Transmitter: MPEG-2 Transport MPEG Framing FEC Encoder QAM Modulator Channel Receiver: QAM Demodulator FEC Decoder MPEG-2 Framing MPEG-2 Transport

36 Headend Digital Video Processing Page 5.33 Workbook Exercise # 2 Write a short description of the following Digital Video Processing keywords. Answers are taken from the Learning Enough to be Dangerous: Glossary Bootup: ECM: FEC: HSTM: IRT: PID: PMT: Boot Protocol. Software code used to set equipment parameters when it is initialized (turned on) Entitlement Control Message. One of the MPEG-2 message types. Forward Error Correction High-Speed Transport Multiplex Integrated Receiver/Decoder Packet Identifier Program Map Table

37 Headend Digital Video Processing Page 5.34 Workbook Exercise # 3 Answers to the questions at end of student workbooks. 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. An IRD is one type of receiver for modulated signals arriving on a satellite downlink. When used in digital systems, the output of an IRD is typically one program channel containing one video and two audio components. IRDs are capable of being tuned to a frequency within a range of transponder frequency values. Composite outputs can be NTSC or PAL format video. Most IRDs can be upgraded by software downloads, and have operations monitoring data outputs (typically Ethernet connections) to a headend LAN. Signal decryption is typically accomplished using keys contained on a smart card inserted into the unit. Because the IRD has an output that can be viewed on a standard television set, it is often used as a way to visually monitor the digital signal quality as it enters the headend for processing. An IRT receives satellite signals in a digital system. Major differences between an IRD and an IRT are the transcoding function of the IRT, which converts the QPSK input signal format to a QAM signal output, and the compressed, multiplexed output of an IRT. Typically, IRT output consists of several program channels. 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? The signal must be demodulated and decrypted. If the receiving device is an IRD, the digital signal is processed into its audio and video components and output as baseband. If the receiving device is an IRT, the signal forward error correction is applied to the signal before it is re-encrypted and re-modulated as a QAM output. 3. How does MPEG-2 multiplexing differ from digital multiplexing in the North American Digital Signal hierarchy? How does it differ from SONET? MPEG-2 uses a 188-byte packet as the basic building block, and allows individual program signals to be extracted from a signal stream by add-drop demultiplexing. Overhead is in a header at the front of the packet, where Program Identifiers track individual programs in the overall stream of data.

38 Headend Digital Video Processing Page 5.35 Digital multiplexing in the North American Digital Signal hierarchy is bit-oriented, and uses bit stuffing in the multiplexing process, resulting in a system where higher rate signals must be demultiplexed down in stages to the lowest signal rate (DS0) to access an individual low rate channel (No add-drop multiplexing). Overhead consists of the stuffing bits, and framing bits inserted after each sample of 24 DS0 channels. The basic building block is an 8-bit word. SONET, like MPEG-2, is byte-oriented, but has a different data structure. SONET consists of a 9 by 90 byte envelope. There are three types of SONET overhead. Two of them are in the first three columns of the SONET envelope, and the third one is part of the SONET payload. SONET, like MPEG-2, allows add-drop multiplexing. 4. What are possible signal sources for a digital headend? Satellite downlink, land line links, and local origination 5. Where does the system get the information it needs to decrypt signals? Where are the encrypted signals received? The keys for removing the encryption of the satellite signal can be locally generated at the headend, or generated at a controller at the satellite uplink, and received with the signal. The same holds true for the keys that remove encryption of the signal that goes to the subscriber. Encrypted signals are received by satellite downlinks and by digital set-top terminals. 6. What are the two ways that Motorola digital headend equipment can be configured? At control panels on the equipment, or from a remote controller 7. How does MPEG keep track of program streams? An MPEG-2 signal consists of a number of separate packets, with individual headers. Each header contains information that identifies the type of message represented by the packet. Each packet or message contains a fixed packet identifier (PID) as part of its header. The set of these packets containing information from a given signal is called a stream. There are separate video streams, audio streams, text streams, and data streams, all contained in the umbrella transport stream, or multiplex. The multiplex also contains the tables that reference the PIDs of each underlying stream so that they can be found and operated upon within the overall transport stream.

39 Headend Digital Video Processing Page 5.36 The highest level table that provides the associations between the packets is called the Program Association Table (PAT). It lists all the services (programs) in the transport multiplex. The MPEG-2 standard specifies that the stream carrying the PAT for an MPEG-2 encoded transport multiplex must be assigned PID number 0. In addition to declaring all the services carried in the multiplex, the PAT also declares the PID numbers of packet streams in which more detailed descriptions of each service can be found. These packet streams are called Service Control Channels (SCCs). The SCC for a service contains a Program Map Table (PMT) that lists the PID numbers of every packet stream making up that service, including the program streams for audio, video, and data. The PMT also lists the PID number of the packet stream containing access control information if the service is encrypted. This stream is called the Entitlement Control Message (ECM) stream.

40 Headend Digital Video Processing Page 5.37 Laboratory Exercise #1 Refer to WB 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 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. Instructor Note: The following examples are possible exercises that will provide the student with practical experience in IRT installation and maintenance issues. Some of these tasks may not be applicable to the IRT at your location, so you will need to review its operating manual or talk to the responsible headend technician to choose appropriate tasks for the lab.

41 Headend Digital Video Processing Page 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 ( 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. Lab is written with the following assumptions. Input port = 1 Transponder Channel = DigiCipher signal 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/ Msps 1/2 36 MHz

42 Headend Digital Video Processing Page 5.39 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. Transponder Number L-Band Frequency (MHz) Transponder Number L-Band Frequency (MHz) 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: Manual LED: LED conducts when the IRT has locked onto a valid input signal. 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

43 Headend Digital Video Processing Page 5.40 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. 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 for details on SA configuration.