ATSC Digital Television Advanced Television Systems Committee standards for broadcast digital television Presented by Andrew Sonnek

Objectives of DTV Increase picture resolution by up to 5x Support AC-3 5.1 channel audio (standard of Dolby Digital) Maximize bandwidth allocation by using complex video and audio compression Reduce the bit rate by a factor of 50 or higher Preserve the high level of quality required for the application

DTV Picture Resolution Standards Vertical Lines Horizontal Pixels Aspect Ratio Picture Rate Standard Definition 480 640 4:3 60I, 60P, 30P, 24P 704 16:9 Extended Definition 720 1280 60P, 30P, 24P High Definition 1080 1920 60I, 30P, 24P

Uncompressed Video Assuming 480p @ 30 frames/sec. Y 720 pixels / line 480 lines / frame 30 frames / sec 8 bits / pixel = 83 Mbps CRCB 360 480 30 16 166 Mbps * See: ftp://ftp.jacweb.jvc.com/jvcpro/justbitsfree.pdf

The Challenge The ATSC allocates 6 MHz for a single channel So… Over the air 6 MHz =~ 19 Mbps Over coaxial Cable TV line =~ 38 Mbps From our last calculations… Uncompressed 480p signal = 166 Mbps!! When in distress… compress!

Video Compression Film mode encoding – encoder recognizes a film source of less than 60 fps and will only encode at a rate of 30 fps or 24 fps Uses source-adaptive processing – maximize compression techniques depending on spatial resolution, temporal resolution, and scanning rate.

Video Compression Uses the MPEG-2 compression layers as a basis for the DTV compression standard Because of limitations of MPEG-2 standard, the DTV standard is only based on MPEG-2 standards. A DTV compressed video stream is NOT an MPEG-2 video stream so a MPEG-2 decoder will not decode DTV signals

Audio Compression Audio is compressed using the AC-3 (Digital Audio Compression Level 3) Standard developed by Dolby Labs. AC-3 is commonly known as Dolby Digital. AC-3 supports 1 mono channel to 5.1 channels per service with multi service support up to 640 kbps total Frequency range from 20Hz – 20kHz Sample rate of 48kHz @ 16 bits/sample

DTV Subsystems Based on International Telecommunications Union – Task Group 11/3 Diagram from ATSC Doc A/54

Source Coding and Compression Subsystem Responsible for minimizing the number of bits needed to represent the video, audio, and control data through: - Video compression - Audio compression - Encoding ancillary data

Ancillary Data Includes: - Control data - Conditional access control - Audio & video associated data such as closed captioning - Independent program services - Program guides - Text based emergency messages

Service Multiplex and Transport Subsystem Based on the MPEG standard for fixed-length transport stream packetization Responsible for multiplexing all video, audio, and ancillary data streams, creating one data stream compatible with terrestrial and cable transmission, and inserting local programming using flags

Advantages of MPEG Transport Standards Easy to detect and correct errors while broadcasting moderately long fixed-length packets over air or cable Can accommodate video, audio, and data Expandable for future services Operational with other media and standards Packets can be easily partitioned for transfer in a link layer that supports ATM transmission

PID (Packet Identifier) Used in the packet header to identify separate audio, video, and data packets in a multiplexed stream. Do not need to be specified in advance! Allows for the allocation for the entire channel so data to be sent in bursts e.g. – send out an encryption key to thousands of subscribers of a pay-per-view at one time or download program related software to a “smart receiver”

Variable length Adaptation Header Transport Packet Variable length Adaptation Header 4 Byte Header Data 188 Byte Packet Header includes – synchronization bit, PID, error handling counter Adaptation Header can include – blank bits for packet sizing, keys for conditional access control, and local programming flags

RF/Transmission Subsystem Responsible for: - Channel coding - Modulation Channel coding inserts additional data for the receiver to reconstruct the original data stream Modulation converts the digital data stream into a transmittable signal

High Data Rate Mode Twice the bandwidth as terrestrial mode Divides the amplitude into more data levels Does not work as well over long distances Terrestrial mode used for most over-the-air broadcasts

Sub-channels 6 MHz frequency can be divided up into at most 4 sub-channels Used to broadcast multiple programming options or multimedia channels Good for programming that does not require high resolutions