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 4:360I, 60P, 30P, 24P
Standard Definition
480 704 16:960I, 60P, 30P, 24P
Extended Definition
720 1280 16:960P,
30P, 24P
High Definition
1080 1920 16:960I, 30P,
24P
Uncompressed VideoAssuming 480p @ 30 frames/sec.
* See: ftp://ftp.jacweb.jvc.com/jvcpro/justbitsfree.pdf
Y720 pixels /
linex
480 lines / frame
x30
frames / sec
x8
bits / pixel
=83
Mbps
CRCB 360 x 480 x 30 x 16 = 83 Mbps
166 Mbps
The ChallengeThe ATSC allocates 6 MHz for a single
channelSo…
Over the air 6 MHz =~ 19 MbpsOver coaxial Cable TV line =~ 38 Mbps
From our last calculations…Uncompressed 480p signal = 166 Mbps!!
When in distress… compress!
Video CompressionFilm 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 SubsystemsBased on International Telecommunications Union – Task Group 11/3
Diagram from ATSC Doc A/54
Source Coding and Compression SubsystemResponsible 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 SubsystemBased on the MPEG standard for fixed-
length transport stream packetizationResponsible 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”
Transport Packet
4 Byte Header Data
Variable length Adaptation Header
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 SubsystemResponsible 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 ModeTwice the bandwidth as terrestrial modeDivides the amplitude into more data
levelsDoes not work as well over long
distancesTerrestrial mode used for most over-
the-air broadcasts