Transcript ATSC Digital Television - University of St. Thomas

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 Horizontal
Lines
Pixels
Standard
Definition
Standard
Definition
Extended
Definition
High
Definition
480
640
480
704
720
1280
1080
1920
Aspect Picture
Ratio Rate
60I, 60P,
4:3
30P, 24P
60I, 60P,
16:9
30P, 24P
60P, 30P,
16:9
24P
60I, 30P,
16:9
24P
Uncompressed Video
 Assuming
Y
CRCB
720
pixels /
line
x
360 x
480p @ 30 frames/sec.
480
lines /
frame
480
x
x
30
frames /
sec
30
x
x
8
bits /
pixel
83
=
Mbps
16
=
83
Mbps
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
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.
 Film
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 MPEG2 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 fixedlength 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”
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-theair 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