Dr. Nikos Desypris Feb Postgraduate course University of Athens

5 th Lecture on Digital TV Academic Writing (separate presentation) DTV Transmission and Reception System DTV Transmission Systems

DTV Transmission & Reception System  Layered approach to system design  Broadcasting system: includes four subsystems: Presentation / Compression / Transport – Multiplex / Transmission  Distribution channels: Traditional: Terrestrial / Cable / Sattellite New: Internet TV / IPTV / Hand-held devices  DTV receiver: the broadcast sequence is reversed Reception / Transport – Demultiplex / Decoding / Presentation

Layered approach to system design

Broadcasting system: 4 sub-systems  Presentation: image format and sound  Compression: reduces data rate to practical levels  Transport multiplex: packetizes audio, video and data: includes assembly instruction for the DTV receiver  Transmission: adds error correction and modulates symbols for channel distribution

The DTV Receiver: 4 sub-systems The sequence is reversed  Reception: Signal capture, demodulation and recovery of data packets from the transmission channel  Transport demultiplex: extraction of audio, video and data packets and assembly instructions  Decoding: Expansion of compressed audio and video data  Presentation: audio and video are synchronised and the complete program is ready for viewing

Layered approach to system design

Engineering Basics  Voltage levels  Noise  Units of data Bits, Bytes and Words  Repetive signals: frequency and the Hertz  Analogue to Digital Conversion and Quantisation  Data Structure and Processing Packets, Headers and Payloads; Streams; Data Buffer and Parsing

Engineering Basics  Voltage levels For digital systems, discrete voltage level represent information. Two levels are used for 0 and 1; however 3, 4 or more distinct levels can be used  Noise Can be internal or caused from external sources. Becomes a problem if amplified  Units of data bit: 0 or 1 byte: 8bits, 256 levels word: two bytes, 16 bits, values double word: four bytes, 32 bits, 2 32 values in digital audio applications: 18 and 20 bits words in digital video applications: 10 and 12 bit words are common

Engineering Basics  Repetition signals: the notion of the frequency Unit: one cycle per sec referred as Hertz named after the German physicist Heinrich Rudolf Hertz ( ) Definition of 1Hz, 1KHz, 1MHz, 1 GHz Audio signals are between 20 Hz and 20KHz. Video images have spatial (within a frame) and temporal (between successive frames) frequency characteristics

Engineering Basics Analogue to Digital Conversion and Quantisation

Engineering Basics Analogue to Digital Conversion and Quantisation

Engineering Basics Data Structure and Processing Bits and bytes are arranged in groups referred as data structures or object model, which are processed by machines Packets, Headers and Payloads Data is grouped into packets having the actual content of transmission (the payload) and additional header information describing the payload Streams Continuous transmission of data (data stream): can be asynchronous (at random), synchronous (with a defined timing) or isochronous (defined amount of data over a time period) Data Buffer A storage device, often RAM that stores and transmits data at different rates. Parsing Examining a data stream for defined values in a data structure and extracting desired information is called parsing

Broadcasting system: 4 sub-systems  1. Presentation: image format and sound  2. Compression: reduces data rate to practical levels  3. Transport multiplex: packetizes audio, video and data: includes assembly instruction for the DTV receiver  4. Transmission: adds error correction and modulates symbols for channel distribution

Broadcasting System 1: Presentation  General  Visual Perception: Luminance sensitivity, Colour Sensitivity  Video Presentation formats: Aspect Ratio Scanning methods Refresh Rate Resolution and pixel grid Synchronisation Pixel Squareness Sampling and colorimetry  Audio Formats: Audio Perception Audio Presentation Formats

Broadcasting System 1: Presentation  General  Visual Perception  Video Presentation formats  Audio Formats

Broadcasting System 1: Presentation General  Presentation is about Immersive consumption;  HDTV was envisaged as a large-screen theatrical viewing experience  Importance of surround sound in the immersive experience DTV is a mass product at an affordable price based on current technological limits

Broadcasting System 1: Presentation  General  Visual Perception  Video Presentation formats  Audio Formats

Broadcasting System 1: Presentation  Visual Perception General Luminance sensitivity, Colour Sensitivity

Broadcasting System 1: Presentation  Visual Perception General

Broadcasting System 1: Presentation  Visual Perception Luminance sensitivity Y= 0,30 * Red + 0,59 * Green + 0,11 * Blue Colour Sensitivity Combinatorial process of mixing red, green and blue Visible light is between 380 nm and 760 nm Red -> 440 nm Green -> 540 nm Blue -> 700 nm

Broadcasting System 1: Presentation  General  Visual Perception  Video Presentation formats  Audio Formats

Broadcasting System 1: Presentation  Video Presentation formats: General Aspect Ratio Scanning methods Refresh Rate Resolution and pixel grid Synchronisation Pixel Squareness Sampling and colorimetry

Broadcasting System 1: Presentation  Video Presentation formats: General A video format consists of : -Aspect ratio -Raster (pixel grid structure) -Scanning method -Refresh rate Note: Analogue TV had only one video format and one colour space sampling structure. There are 18 different DTV formats in the ATSC standard (many more in satellite and cable!)

Broadcasting System 1: Presentation  Video Presentation formats: Aspect Ratio

Broadcasting System 1: Presentation  Video Presentation formats: Scanning methods

Broadcasting System 1: Presentation  Video Presentation formats: Refresh Rate  Movie industry 24 frames/sec  TV: 30 Hz (half the AC 60 Hz power line frequency)  Other frame rates: 29.97, 30, 60,  Film rates: 24 or 23.98

Broadcasting System 1: Presentation  Video Presentation formats: Resolution and pixel grid  Always defined as pixels per line by pixels per frame  Refresh rate defines the number of complete frames; 60p: 60 progressive frames per second 60i: denotes 30 frames per second, each frame comprising of two interlaced fields (one field odd lines, the other even lines) Important note: The no of pixels and lines in an image format is not a statement of the image resolution which is dependent on the entire signal processing chain

Broadcasting System 1: Presentation  Video Presentation formats: Synchronisation The timing relationship of the scanning process that produces pixels, lines and a display raster (pixel grid) must be unambiguously established pixels x 1080 lines 1280 pixels x 720 lines Define the active pixels and lines in a display, while the actual scanning number for each video presentation format is 2200 pixels x 1125 lines and 1650 pixels x 750 lines respectively

Broadcasting System 1: Presentation  Video Presentation formats: Synchronisation DTV eliminates blanking intervals during compression

Broadcasting System 1: Presentation  Video Presentation formats: Pixel Squareness Square pixels are defined (in DTV standards) as when the number of horizontal to vertical pixels is the same proportion as the display aspect ratio. 640:480 = 4:3, HD formats 1920x1080 and 1280x720 are 16:9, therefore HD formats employ square pixels. However SD, 720x480 and 704x480 are not 4:3 and it is quite important for creating graphics on video. A simple solution is to create a graphics canvas that is the same display format as the intended display

Broadcasting System 1: Presentation  Video Presentation formats: Sampling

Broadcasting System 1: Presentation  Video Presentation formats: C olorimetry

Broadcasting System 1: Presentation  General  Visual Perception  Video Presentation formats  Audio Formats

Broadcasting System 1: Presentation  Audio Formats: Audio Perception Audio Presentation Formats

Broadcasting System 1: Presentation  Audio Formats: Audio Perception Audible sound for young people: 20Hz to 20KHz

Broadcasting System 1: Presentation  Audio Formats: Audio Presentation Formats

Broadcasting system: 4 sub-systems  1. Presentation: image format and sound  2. Compression: reduces data rate to practical levels  3. Transport multiplex: packetizes audio, video and data: includes assembly instruction for the DTV receiver  4. Transmission: adds error correction and modulates symbols for channel distribution

Broadcasting System 2: Compression  Compression  MPEG-2 Video Compression Exploiting Redundant Visual Information Group of Pictures Compression Steps Exploiting Temporal Redundancy  Audio Compression Aural Perception and Compression Algorithms  Masking  Framing

Broadcasting System 2: Compression  Compression  MPEG-2 Video Compression  Audio Compression

Broadcasting System 2: Compression Full bandwidth HDTV picture with 6 audio channels is 1,5 GBps. To fit in a 6MHz channel, data reduction of 50:1 for video and 12:1 for audio are common. Compression generation: an encoding and decoding cycle. Each compression generation creates artefacts Consumer electronics are not to highest fidelity; however they have consumer satisfaction Compression ‘engines’ are divided in two broad categories: Lossless compression reduces the volume of data and, when reconstructed restores it to its original state, perfectly without any loss of information Lossy compression discards data based on auditory and visual sensory characteristics and limits

Broadcasting System 2: Compression Recent new video codecs: VC1 derived from Windows Media and MPEG-4 Part 10 claim to deliver the same quality of MPEG-2 with half the rate MP3 is part of MPEG-1 or MPEG-2 Audio Layer III Advanced Audio Coding (AAC) is also an MPEG standard AC-3 (Audio Codec 3) with support of up to 6 channels was intended for theatrical applications Key data JPEG focuses on still images MPEG (Motion Pictures Expert Group) MPEG1: MPEG2: 1994 MPEG4, Part 10: latest version of mpeg

Broadcasting System 2: Compression  Compression  MPEG-2 Video Compression  Audio Compression

Broadcasting System 2: Compression MPEG-2 Video Compression General Exploiting Redundant Visual Information Group of Pictures Compression Steps Exploiting Temporal Redundancy

Broadcasting System 2: Compression MPEG-2 Video Compression: General Compression is characterised by a ratio (video 50:1, audio 12:1) The higher the ratio the fewer the number of bits Raw bit rates for HD video are about 1,5 Gbps. With 4:2:2 sampling and 50:1 compression, yields 30 Mbps. With 4:2:0 sampling (1 Gbps) and 50:1 compression results 20 Mbps. The higher rate (30Mbps) can survive a generation or two of editing without artefact, while the lower rate (20Mbps) is used for presentation only. For multiple generations of compression and decompression in production environment, video should be at least 100 Mbps for HD and 40 Mbps for SD.

Broadcasting System 2: Compression MPEG-2 : Exploiting Redundant Visual Information Coding only scene differences results in significant compression, but depends on scene complexity, for example: High: Detail -> spatial complexity: rain grounds Motion: Change -> temporal complexity: sports Low: Still = simple: talking head, stationary text and graphics MPEG-2: two attributes for scene complexity: profile and level: Profile compression complexity related to colour sampling: Simple; Main; 4:2:2, SNR, Spatial and High Level specifies picture size in pixels: Low; Main; High 1440 and High

Broadcasting System 2: Compression MPEG-2 : Exploiting Redundant Visual Information SD is HD is Profile LevelLevel

Broadcasting System 2: Compression MPEG-2 Video Compression: Group of Pictures  I frames: Intraframe, encoded independent of other frames  P frames: Predictive, encoding is dependent on  B frames: Bidirectional, encoding is dependent on previous or subsequent I or P frames  A ‘long GOP’ is usually up to 15 frames such as: I B B P B B P B B P B B P B B, corresponding to 0,5 sec on 1080i at 30Hz, or 0,25 sec for 720p at 60Hz (important for switching TV channels)  I frames are called anchor frames

Broadcasting System 2: Compression

MPEG-2 Video Compression: Compression Steps Compression of an I-frame: It is divided into 8x8 pixels blocks Discrete cosine transform (from 8 bits pixels to 11 bits coefficients!) Weighting and Requantisation (to reduce high frequency coeffs) Variable length coding (to assign small codewords to most common symbols as in Morse code where E, T ->. and _) Run length coding (to code a sequence of same digits more efficiently)

Broadcasting System 2: Compression MPEG-2 Video Compression: Exploiting Temporal Redundancy I frame – current frame = P Frame (different pixels not values!) frame 2 – I frame – P frame = B frame

Broadcasting System 2: Compression

P: difference 1 and 3 B: difference 2, I and P> P > B

Broadcasting System 2: Compression  Compression  MPEG-2 Video Compression  Audio Compression

Broadcasting System 2: Compression  Audio Compression Aural Perception and Compression Algorithms, based on psychoacoustic model Masking: When one sound hides another sound, it need not be encoded Framing: Further data reduction is taking place by requantising high frequencies using the psychoacoustic model

Broadcasting system: 4 sub-systems  Presentation: image format and sound  Compression: reduces data rate to practical levels  Transport multiplex: packetizes audio, video and data: includes assembly instruction for the DTV receiver  Transmission: adds error correction and modulates symbols for channel distribution

Broadcasting System 3: Transport Multiplex  Compressed Data Streams  Packetised Transport Multiplex Concepts PSI and Program Assembly  Transport Stream Multiplex  Statistical Multiplexing and Rate Shaping

Broadcasting System 3: Transport Multiplex  Compressed Data Streams  Packetised Transport

Broadcasting System 3: Transport Multiplex  Compressed Data Streams The result of compression process is a continuous flow of audio and video data known as an Elementary Stream (ES) ES become part of Packetized Elementary Streams for not loosing audio/video timing relationship

Broadcasting System 3: Transport Multiplex  Compressed Data Streams  Packetised Transport

Broadcasting System 3: Transport Multiplex  Packetized Transport Two types of MPEG streams: Program and Transport Program streams: are used in environments that rarely corrupt data (such as a DVD player) and contain a single audio and video program Transport streams: Enable robust delivery of data over noisy channels and error prone environments may contain numerous programs packets are much smaller that those in program streams (only 188 bytes)

Broadcasting System 3: Transport Multiplex Packetized Transport: Multiplex concepts

Broadcasting System 3: Transport Multiplex Packetized Transport: PSI and Program Assembly

Broadcasting System 3: Transport Multiplex Packetized Transport: Transport Stream Mulitplex

Broadcasting System 3: Transport Multiplex Packetized Transport: Statistical Multiplexing & Rate Shaping

Broadcasting system: 4 sub-systems  1. Presentation: image format and sound  2. Compression: reduces data rate to practical levels  3. Transport multiplex: packetizes audio, video and data: includes assembly instruction for the DTV receiver  4. Transmission: adds error correction and modulates symbols for channel distribution

Broadcasting System 4: Transmission  Data Protection Randomisation Reed Solomon Encoding Data Interleaving Trellis Encoding  Digital Modulation Modulation  Digital Modulation  Analogue Modulation  Digital Modulation  Vestigial SideBand Modulation (VSB)  Quadrature Amplitude Modulation (QAM)  Quartenary Phase Shift Keying (QPSK)

Broadcasting System 4: Transmission  Data Protection Randomisation:  Spreads data over the full transmission channel  Scrambling the stream avoiding repetitive patterns Reed Solomon Encoding  Use of forward error Correction Code for correcting multiple bit errors, since retransmission is not possible Data Interleaving  Spreading data over time in order to avoid lost sequences due to impulse noise (i.e. lightning) Trellis Encoding  Transforming bits to symbols used to modulate the RF carrier (generally depends on the modulation standard)

Broadcasting System 4: Transmission

Broadcasting System : Summary  Four sub-systems form a DTV broadcasting system: Presentation, compression, transport and transmission  Exploitation of the characteristics and limitations of the sensory perception are used in DTV presentation and compression systems  Packetized audio and video, along with assembly instructions are sent to a DTV receiver  Digital transmission uses sophisticated method of error protection and correction