University of Canberra Advanced Communications Topics Television Broadcasting into the Digital Era 1 Lecture 4 Error Correction, DTTB Planning & System Information by: Neil Pickford 64-QAM - Perfect & Failure 2 COFDM DTTB Block Diagram Error Correction 3 Forward Error Correction (FEC) Broadcast transmission
Forward Error Correction is a technique used to improve the accuracy of data transmission Extra redundant bits are added to the data stream Error correction algorithms in the demodulator use the extra FEC bits to correct data errors C OFDM uses a Convolutional FEC code N bits 4 One way process - Tx to Rx Not possible to repeat any errored data Encode Tx/Rx N+Code Decode N+Code+Error N bits Convolutional Coder 1111001
0 Y Output 1011011 5 Puncturing Codes (FEC) 6 The X and Y outputs of the Convolutional coder are selected in a Puncturing pattern Inner Coding Convolutional coder generates the X & Y codes Puncturing operation selects X & Y in sequence Result then scrambled with an interleaver X Data 7
Convolutional Puncturing Y Encoder Interleaver Coded Data Viterbi Decoder 8 A special type of data decoder designed to work with convolutional FEC codes Uses the past history of the data to identify valid future data values Element in the Receiver Only Reed Solomon (RS) RS is a Block data correcting Code Hamming type cyclic Polynomial sequence
Code Generator Polynomial: g(x) = (x+0)(x+1)(x+2)...(x+15), =02 Hex Field Generator Polynomial: p(x) = x8 + x4 + x3 + x2 + 1 Has special ability to correct multiple bursts of errors in a code block DVB-T uses 204 bytes for each 188 byte Packet (ATSC uses 207 bytes for each 187 byte Packet) 9 Can correct 8 bytes in each 204 byte packet Error Protection - Order 188 Bytes Data Input 204 Bytes 1
Outer Code RS (204,188) Interleaver 204 Bytes Inner Code FEC (2/3) 306 Bytes Interleaver 2448 Bits Error Mapper Protected Data 6 bits x 1512 Carriers 64 QAM 6 bits x 6048 Carriers DVB-T - Bit Rates [2k] 7 MHz D/Tu = 1/4 64 us D/Tu = 1/8 32 us
General Parameters - Aust Tests Parameter DVB-T Data Payload Carriers ATSC 19.35 Mb/s 19.39 Mb/s 1705 1 Symbol Time 256 us 93 ns Time Interleaving 1 Symbol 4 ms Reed Solomon code rate IF Bandwidth (3 dB) 1 188/204 187/207 6.67 MHz 5.38 MHz 19 8VSB vs COFDM Latest 35 True COFDM
Advantage 30 25 True ATSC Advantage 19 15 0 1 8VSB -3 Realistic COFDM perform -15 Multipath Level (dB) Max Analog Ghost 7 MHz COFDM Modulator Spectrum Power Spectrum Density (dB) 7 MHz Theoretical DVB Transmission signal spectrum 00 -10 -10
1 33 44 55 66 77 88 Frequency Planning Fundamental Matter - Scarce Resource Analogue Rules set limit to more Services No NEW TV Spectrum is Available Digital Transmission changes Rules Signals have different behaviour Digital Signals can occupy unused space Taboos Digital Needs to fit in with Existing PAL Eventually Digital Only - but long wait?? 1
Digital Has to Fit In With PAL World TV channel bandwidths vary USA / Japan 6 MHz Australian 7 MHz 28 30 Europeans 8 MHz 31 Affects:- tuning, filtering, interference & system performance 28 28 1
34 35 Channel Spacing Existing analog TV channels are spaced so they do not interfere with each other. Gap between PAL TV services VHF 1 channel UHF 2 channels Digital TV can make use of these gaps Ch 6 Taboo 1 Ch 7 Ch 8 Taboo VHF Television Spectrum
Ch 9 Ch 9A Taboo Digital Challenges Digital TV must co-exist with existing PAL services DTV operates at lower power DTV copes higher interference levels Share transmission infra-structure DTV needs different planning methods Ch 6 8-VSB 2 Ch 7 Ch 8 COFDM VHF Television Spectrum Ch 9
Ch 9A DTTB & PAL 2 UHF Channels: London Photograph by courtesy and BBC R&D Planning Issues Channel Disturbances: Antenna Pattern? Static Roof Top? Directional? Wideband? CCIR Antenna Rec BT-419-3 Portable Receivers? No Antenna?
Frequency Re-Use Distances 2 Noise, at edge of area with NO interference Interference, Co Channel Interference and Adjacent Channel Interference Multipath, Echoes:How Many, How Large, Moving? Terrain Data Propagation Models Protection Ratios Signal Strength SIGNAL STRENGTH, MicroVolts REGION OF SERVICE FAILURES FOR PERCENTAGE OF TIME MEAN RECEIVER C/N LIMIT TIME 2 Digital Service Area Planning
Analog TV has a slow gradual failure Digital TV has a cliff edge failure 2 Existing PAL service was planned for: 50 % availability at 50 % of locations Digital TV needs planning for: 90-99 % availability at 90-99 % of locations TV System Failure Characteristic Good Quality Edge of Service Area Rotten Close 2 Distance
Far TV System Failure Characteristic Good Quality Edge of Service Area Rotten Close 2 Distance Far TV System Failure Characteristic Good HDTV PAL Quality Edge of Service Area Rotten Close 2
Distance SDTV Far Service Area Planning PICTURE QUALITY DIGITAL 5 4 Modulation Dependent Variation ANALOGUE 3 THRESHOLD OF ACCEPTABILITY Typical Choice of C/N 2 1
NO 10 SERVICE 2 15 ANALOGUE FAILURE 20 25 30 35 40 45 C/N Service Areas - Current 50 - 100 KILOMETRES TRANSMITTER A 3 TRANSMITTER B
Service Areas - SFN 50 - 100 KILOMETRES TRANSMITTER B 3 Digital Provides New Concepts Single frequency networks (SFNs) can help solve difficult coverage situations 3 SFNs allow the reuse of a transmission frequency many times in the same area so long as exactly the same program is carried Allows lower power operation Better shaping of coverage Improved service availability Better spectrum efficiency MPEG Packet PACKET START CODE PREFIX
STREAM ID 24 8 OPTIONAL PES HEADER PES PACKET LENGTH 16 PESP PSC DAI CY 2 11111111111 DATA BYTES 8 OOC PTSDTSF
ES RATE 42 22 PESSC = Packet Elementary Stream Scrambling Control PESP = Packet Elementary Stream Priority DAI = Data Alignment Indicator CY = Copyright OOC = Original or Copy PTSDTSF = PTS & DTS Flags ESCRF = ESCR Flag ESRFES = Rate Flag DSMTMF = Trick Mode Flag PES PRIVATE ACIF = Additional Copy Info. Flag DATA PESCRCF = PES Extension Flag PESHDL = PES Header Data Length PTS = Presentation Time Stamp DTS = Date Time Stamp 3 ADDITIONAL COPY INFO 8
OPTIONAL FIELDS PES CRC 16 5 FLAGS PES EXTENSION 11111 PACK HEADER FIELD PROGRAM PACKET SEQUENCE COUNTER 8 P-STD BUFFER PES FIELD EXTENSION System Level Multiplexing Approaches Video Video Data Data
Video Video Encoder Encoder Packetizer Packetizer Audio Audio Data Data Audio Audio Encoder Encoder Packetizer Packetizer 3 Video PES PES Video Audio AudioPES PES PS
PS Mux Mux Program Stream Stream Program TS TS Mux Transport TransportStream Stream Packetisation Approaches Fixed Length Audio Video Video Audio Audio Video Video Audio Audio Video Video Video Audio Video Audio Audio Audio Audio Audio Audio Video Video Video Video Audio Audio Video Audio
Video Audio Transport Transport Stream Stream Audio Audio Video Video Audio Audio Program Stream Stream Program Variable Length 3 Video Video Transport Stream 188 188Bytes Bytes Adaptation (Variable Length)
AdaptationHeader Header (Variable Length) 44 Bytes Bytes Payload Payload notto toscale scale not Link Header Header Link Header Format sync_byte (0x47) 13 bit PID 1 bit: transport_priority 1 bit: payload_unit_start_indicator 1 bit: transport_packet_error_indicator 3 or Adaptation header packet payload
4 bit: continuity_counter 2 bit: adaptation_field_control 2 bit: transport_scrambling_control System Information (SI) 3 Required for : Automatic Tuning of receiver upon selection Program location EPG (Electronic Program Guide) API (Application Programming Interface) CA (Conditional Access) DVB SI Model Networks Satellite Terrestrial
Cable Transport Streams Transponder Transponder Transponder Transponder Transponder Transponder Transponder Transponder 1 2 3 T 1 2 3 T Services Channel Channel 1 1 Channel Channel C C Channel Channel 1 1 Channel
Channel 2 2 Channel Channel C C Bouquet Service Service Service Service Service Service Service Service 11 22 33 SS Service Service Service Service Service Service 11 22 SS Components Video Video
3 Channel Channel 2 2 Audio Audio11 Audio Audio22 Data Data Service Service Service Service Service Service 11 22 SS System Information 3 The DVB SI structure has its derivation in MPEG ISO/IEC 13818-1 and is defined in a set of tables. The primary link between DVB SI and MPEG
is thePSI (Program Specific Information) in MPEG and is contained primarily in the PAT, PMT and CAT set of tables What is SI? SI data provides information for: Automatic tuning to transport stream User Information for: Service selection Event selection Component selection PSI data provides information for: Configuration of decoder for selected Service DVB extensions for non-MPEG components 4 PSI and DVB SI Tables DVB OPTIONAL
MPEG DVB MANDATORY PID=0x0000 PID=P PID=Ox0001 PID=0X0002 4 PAT NIT PID=0x0010 OTHER Delivery Sys. ACTUAL Delivery Sys. PMT CAT TSDT NIT PID=0x0010
BAT PID=Ox0011 SDT PID=Ox0011 EIT PID=0X0012 ACTUAL TS ACTUAL TS PCR TDT PROGRAMME CLOCK REF. TIME AND DATE. PID=0X0012 ACTUAL TS
RUNNING STATUS. SERVICE DESCRIPTION. OTHER TS EIT RST BOUQUET INFORMATION. SDT PID=Ox0011 EIT OTHER TS TOT TIME OFFSET. NETWORK INFORMATION. EVENT
INFORMATION. ST STUFFING TABLE. MPEG Program PIDs What is a program ? 4 MPEG has a definition which is different to that normally understood. A program in broadcasting is a collection of elements with a common time base and the same start and stop times. A program in MPEG is a collection of elements with a common time base only. That is a collection of elementary streams with same PCR_PID and referenced to the same program_number Virtual Channels & PCR Timing
A conventional Broadcaster of a TV channel or service having one program would be composed of a series of broadcaster programs or events with the same program_number and a common PCR_PID. In other words the PCR time base effectively creates a virtual channel which may be associated with a single or multiple program_numbers. A TV channel having multiple programs would have multiple program_numbers with either single or multiple PCR_PID between program streams. NOTE : Services with different program_numbers may draw upon the same video as with the case of multilingual services. 4 Decoding the Program Decoding the correct program (ie channel) ? Where there are several Transport Streams available to a decoder, in order to successfully demultiplex a program, the decoder must be notified of both the transport_stream_id (to find the correct multiplex) and the program_number of the service (to find the correct program within the multiplex). Note again the program here refers to the channel not the event or actual broadcast program.
4 Now to the various main table purposes : PAT, PMT & CAT Tables PAT (Program Association Table) PMT (Program Map Table) when pointed to from the PAT, the PMT provides the associated group of elements (video, audio etc) with the program_number. CAT (Conditional Access Table) 4 provides the link between the transport_stream_id, the program_number and the program_map_id (PMT). provides the association between CA system(s) and their EMM (Entitlement Management Messages) streams and any special parameters associated with
them. DVB SI Features Data structured as several Tables Structures use fixed format for essential data, and descriptors for optional or variable-length data (similar to PSI) Efficient data transmission Extensible while maintaining compatibility Support for private extensions Can provide standard EPG data-stream Look and Feel determined by receiver software Resident or Downloaded 4 SI Features: NIT Network Information Table
Identification of transmission as a member of a group of multiplexes - Network Network Name Tuning parameters with support for various delivery media 4 List of additional frequencies for terrestrial transmission Designed for simple transcoding of transport streams SI Features: SDT & BAT Service Description Table Identifies all Service names and Service types in TS Linked Services Pointer to MPEG Program in PSI
Bouquet Association Table (Optional) 4 Service_id = MPEG Program Number Groupings of Services May convey logical channel number SI Features: EIT Present/Following Schedule (optional) Up to 64 days ahead - ordered by service and time Event Information
4 Information on current and next events Title, short description Start time & duration Content classification & parental rating Longer text description Information on components SI Features: TDT,TOT, RST Time and Date Table Time Offset Table (optional) Transmission of time offset by zone - both current, and next offset values, with date at which next occurs Running Status Table (optional) 5 Transmission of current time for automatic setting of
receiver clock Mechanism for signalling status transitions with greater timing precision Electronic Program Guide - EPG EPG (Electronic Program Guide) 5 Combining primarily the EIT and the SDT, both the time and description is provided to the viewer via some form of EPG ranging from vanilla EPGs, simple eye plate style displays to full blown EPGs, either from Receiver manufacturers designs or downloaded EPGs with GUI interfaces designed by the Networks. This information so constructed and displayed can be used to provide a Parental Guidance lock function through PIN number access. Example EPG using DVB SI 5 Example: Event Details 5
Example: Selection by Genre 5 Example: Selection by Genre 5 Application Program Interface API API (Application Programming Interface) some form of API must be used to allow the control by the viewer or installer of the decoder / receiver. The API software provides the connection between the applications (eg. EPG) and the hardware. Some APIs may employ MHEG-5 multimedia support and Java programming language for EPG generation. 5 CA in practice is reliant upon EPGs and the APIs. DVB - Conditional Access
CA (Conditional Access) Access to the EMM (Entitlement Management Message) is provided by the CAT. The EMM allows a single decoder to view the program material which is scrambled via a DVB common scrambling algorithm by providing the key to the code word which is involved in the scrambling. The code word is sent via the ECM (Entitlement Control Message). 5 Typical Conditional Access System 5 A Future Digital System Concept MMDS Satellite Hypermedia Integrated Receiver Decoder (IRD) Terrestrial Cable Broadcast
Interactivity B-ISDN XDSL 5 CD, DVD DVC DTTB Systems Doppler Performance Limits for current implementations 300 250 DOPPLER SHIFT (Hz) UHF 200 VHF - Band III COFDM 2K, 3dB degrade 140 COFDM 2K
AIRCRAFT Over Cities COFDM implementations will inherently handle post and pre-ghosts equally within the selected guard interval. 1000 Main Results - Lab Tests C/N ATSC 4 dB better than DVB-T. This Advantage offset by Poor Noise Figure DVB-T is better than ATSC for Multipath ATSC is better than DVB-T for Impulse Noise ATSC cannot handle Flutter or Doppler Echoes ATSC is very sensitive to Transmission system impairments and IF translation DVB-T is better at handling Co-channel PAL DVB-T is better rejecting on channel interference (CW) 13 6
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