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, 24PStandard Definition 480 704 16:9 60I, 60P,

30P, 24PExtended Definition 720 1280 16:9 60P,

30P, 24PHigh Definition 1080 1920 16:9 60I, 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 Mbps166

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

Sub-channels6 MHz frequency can be divided up into

at most 4 sub-channelsUsed to broadcast multiple

programming options or multimedia channels

Good for programming that does not require high resolutions