Transcript Digital Television Talk Lecture 3

University of Canberra
Advanced Communications Topics
Television Broadcasting
into the Digital Era
1
Lecture 3
Audio/Video
Compression
MPEG-2 Multiplexing
by: Neil Pickford
Enabling Technologies
 Source
digitisation (Rec 601 digital studio)
 Compression technology (MPEG, AC-3)
 Data multiplexing (MPEG)
 Display technology (large wide screens)
 Transmission technology (modulation)
 Production
2
Video Bitrate - HDTV

2 M pixels * 25 pictures * 3 colours * 8 bits
= 1.24416 G bits / sec for Interlace Scan
or
= 2.4833 G bits / sec for Progressive
We need to Compress this a bit!
3
Compression Technology
When low bandwidth analog information is
digitised the result is high amounts of digital
information.
5 MHz bandwidth analog TV picture

170 - 270 Mb/s digital data stream.
 270 Mb/s would require a bandwidth of
at least 140 MHz to transport
 Compression of the information is required

4
Compression - Types
 Two
types of compression available
Loss-less
compression
2 to 5 times
Lossy
compression
5 to 250 times
5
Compression - Loss-less Types
Picture differences - temporal
 Run length data coding - GIF

 101000100010001001101
= 1 + 4x0100 + 1101
 21 bits source = 12 bits compressed

Huffman coding - PKZIP
 Short
codes for common blocks
 Longer codes for uncommon blocks

6
Lookup tables
Compression - Lossy Types
 Quantisation
- rounding
 Motion vectors
 Prediction & interpolation
 Fractal coding
 Discrete cosine transform (DCT)
7
Compression - DCT
8x8 Pixels
8
MPEG-2 - I, P & B Frames
Uncompressed SDTV Digital Video Stream - 170 Mb/s
Picture 830kBytes
I Frame
100 kBytes
Picture 830kBytes
Picture 830kBytes
B Frame
B Frame
12 kBytes
12 kBytes
Picture 830kBytes
P Frame
33 kBytes
MPEG-2 Compressed SDTV Digital Video Stream - 3.9 Mb/s
 I - intra picture coded without reference to other pictures.
Compressed using spatial redundancy only

P - predictive picture coded using motion compensated
prediction from past I or P frames

B - bidirectionally-predictive picture using both past and
future I or P frames
9
MPEG-2 - Formats ML & HL

MPEG-2 defines profiles & levels
 They
describe sets of compression tools
DTTB uses main profile.
 Choice of levels
 Higher levels include lower levels

Level
Low level (LL)
Main level (ML)
High level (HL)
10
resolution
360 by 288
720 by 576
1920 by 1152
SIF
SDTV
HDTV
MPEG Profiles and Levels
422P@HL
MAX.
BITRATE
300 Mbit/s
HP@HL
100 Mbit/s
80 Mbit/s
MP@HL
60 Mbit/s
40 Mbit/s
HP@H14L
SSP@H14L
MP@H14L
422P@ML
20 Mbit/s
HP@ML
HIGH
SNRP@ML
MP@ML
HIGH-1440
LEVELS
SP@ML
11
4:2:2
SNRP@LL
MAIN
MP@LL
LOW
MAIN
SIMPLE
SNR
SCALABLE
SPATIALLY
SCALABLE
HIGH
PROFILES
MP@ML
MP@HL
All decoders sold in Australia will
be MP@HL capable allowing all
viewers access to HD resolution
when it becomes available
12
Digital Audio - Multichannel

Two sound coding systems exist for Digital TV
 MPEG
1&2
 Dolby AC-3

Cover a wide variety of Audio Applications
 DVB
 VCD
and S-VCD
 DAB, DBS, DVD
 Cinema (Film)
 Computer Operating Systems (Windows)
 Professional (ISDN codecs, tapeless studio, ….)
13
Multichannel Sound
TV
L
Ls
14
C
LFE
R
Rs
Masking

Both use perceptual audio coding that exploits a
psychoacoustic effect known as masking
masker
Inaudible (masked) tone
Level
(dB)
Masked
Threshold
500
1000
2000
4000
8000
Frequency (Hz)
15
Multichannel Sound - MPEG 1/2

MPEG Audio Layer II was developed in
conjunction with the European DVB technology
 Uses
Musicam Compression with 32 sub bands
 MPEG 1 is basic Stereo 2 channel mode
 MPEG 2 adds enhancement information to allow
5.1 or 7.1 channels with full backwards
compatibility with the simple MPEG 1 decoders
 MPEG 1 is compatible with Pro-Logic processing.
 Bitrate 224 kb/s MPEG 1
 Bitrate 480 - 512 kb/s MPEG 2 5.1
16
MPEG Audio Encoder
Audio
Bit
Stream
O/P
32 Subbands
Audio
In
2 x 768
kb/s
Subband
Filter
PsychoAcoustic
Model
17
Quantiser
&
Coder
Frame
Packer
Bit
Allocation
Coding
of Side
Information
2x
32-192
kb/s
MPEG Audio Decoder
Audio
Frame
Bit
Unpacker
Stream
2 x 32-192 kb/s
De-Quantiser
Decoding
of Side
Information
18
Inverse
Subband
Filter
Audio
Out
2 x 768 kb/s
Multichannel Sound - Dolby AC-3

Dolby AC-3 was developed as a 5.1 channel
surround sound system from the beginning.
 Compression
19
Filter bank is 8 x greater
than MPEG 2 (256)
 Must always send full 5.1 channel mix
One bitstream serves everyone
 Decoder provides downmix for
Mono, Stereo or Pro-Logic
 Listener controls the dynamic range,
Audio is sent clean
 Bitrate 384 kb/s or 448 kb/s
 Dialogue level passed in bit-stream
AC-3 Coding Block Diagram
Quantized Samples
Source Audio
Analysis
Filterbank
Bit Allocator
Quantization
Multiplex
Coded
Bit Stream
Core
Bit Allocator
Bit Allocation Side Information
Encode
Envelope
Reconstructed
Audio
20
Synthesis
Filterbank
Encoded Spectral Envelope
De-quantization
Core
Bit Allocator
Demultiplex
Decode
Envelope
Channel
Coded
Bit Stream
AC-3 Multichannel Coder
L
R
C
LS
RS
5.1-ch
Encoder
5.1-ch
Decoder
LFE
LFE
Encoder
21
L
R
C
LS
RS
Decoder
AC-3 Stereo Decoder
L
R
C
LS
RS
5.1-ch
Encoder
Matrix
LFE
LFE
Encoder
22
5.1-ch
Decoder
L
R
C
LS
RS
2-channel Decoder
Lo
Ro
Problem of AC-3
An AC-3 stereo decoder is more complex than an
AC-3 multichannel decoder
 With AC-3, end users with stereo receivers must
pay extra so that others can enjoy multichannel
sound

23
MPEG-2
Multichannel Coder concept
Lo
Ro
L
R
C
LS
RS
MPEG-1
Encoder
MPEG-1
Decoder
Re
matrix
Down
mix
Extension
Encoder
LFE
MPEG-2 Encoder
24
Lo
Ro
Extension
Decoder
MPEG-2 Decoder
L
R
C
LS
RS
LFE
Low cost 2-channel decoder
L
R
C
LS
RS
LFE
Down
mix
Lo
Ro
MPEG-1
Encoder
T2
T3
T4
LFE
Extension
Encoder
MPEG-2 Encoder
 Low cost 2-channel decoder
25
MPEG-1
Decoder
2-channel
Decoder
Lo
Ro
Compatibility with Pro Logic
L
R
C
S
26
Surround
Encoder
Lt
Lo
Rt
Ro
MPEG-1
Encoder
MPEG-1
Decoder
Lo
Lt
Ro
Rt
Pro Logic
Decoder
L
R
C
S
Compatibility with Pro Logic (2)
L
R
C
LS
RS
LFE
Down
mix
Lot
Rot
MPEG-1
Encoder
T2
T3
T4
LFE
Extension
Encoder
MPEG-2 Encoder
27
MPEG-1
Decoder
Lot
Rot
Pro Logic
Decoder
L
R
C
S
Widely Available
All major MPEG-2 Video decoders incorporate
2-channel or 5.1 channel MPEG-2 Audio
 Several dedicated MPEG-2 multichannel
decoders
 More than 100 Million decoders world-wide

28
Studio Multichannel Sound
Present AES3 PCM Audio
does not cater for 5.1 channel surround.
 Dolby has produced a system called Dolby E

 Handles
6-8 audio inputs
 Uses low compression 3-4:1
 Can be transported/stored on 2ch PCM audio
equipment
 Incorporates time stamps and is segmented at the
video frame rate allowing editing on video frame
boundaries
29
Digital Television - Types

Satellite (DBS)
 DVB-S
 Program
interchange
 Direct view / pay TV
 SMATV
Uplink
30
Downlink
Digital Television - Types

Cable
 HFC
- pay TV
 MATV
 DVB-C / 16-VSB
Fibre
Main Coax
Tap
31
Spur
Tee
Digital Television - Types

Terrestrial (DTTB)
 DVB-T
/ 8-VSB
 Free to air TV (broadcasting)
 Narrowcasting/value added services
 Untethered - portable reception
32
Enabling Technologies
 Source
digitisation (Rec 601 digital studio)
 Compression technology (MPEG, AC-3)
 Data multiplexing (MPEG)
 Display technology (large wide screens)
 Transmission technology (modulation)
 Production
33
MPEG-2
Compresses source video, audio & data
 Segments video into I, P & B frames
 Generates system control data
 Packetises elements into data stream
 Multiplexes program elements - services
 Multiplexes services - transport stream
 Organises transport stream data
into 188 byte packets

34
Digital Terrestrial TV - Layers
. . . provide clean interface points. . . .
1920 x 1080
1280 x 720
50,25, 24 Hz
Picture
Layer
Video
Compression
Layer
Data
Headers
Motion
Vectors
Multiple Picture Formats
and Frame Rates
MPEG-2
compression
syntax
ML@MP
or
HL@MP
Chroma and Luma
DCT Coefficients
Variable Length Codes
Flexible delivery of data
Packet Headers
Transport
Layer
Transmission
Layer
Video packet
Audio packet
VHF/UHF TV Channel
7 MHz
35
Video packet
Aux data
MPEG-2
packets
COFDM / 8-VSB
Digital Television Encode Layers
Control
Data
Video
Picture
Coding
MPEG-2
Data
Data
Coding
PES
Control Data
(PMT)
PES
Sound
Audio
Coding
PES
MPEG Transport
Stream Mux
Program 1 Multiplexer
Program 2
Other Data
Control Data
(PAT)
MPEG-2
or AC-3
Program 3
Service
Mux
Bouquet Multiplexer
MPEG Transport Data Stream 188 byte packets
Control Data
Modulator & Transmitter
Delivery
36
System
Error
Protection
Digital Television Decode Layers
MPEG-2
Transport
Stream
Mon
Data
Picture
Decoder
Data
Decoder
MPEG Transport Stream
De-Multiplexer
Demodulator & Receiver
Delivery
37
System
Speakers
Audio
Decoder
MPEG
DeMux
Error
Control
MPEG
or AC-3
Set top Box (STB) - Interfacing
Domestic and Professional interfaces
still to be defined
 Most probably Transport Stream via IEEE 1394
(Firewire)
 Baseband Audio & RGB/YUV Video signals.
 STB can convert between line standards
so you do not have to have a HD display.
 Display and transmitted information must be at
same Frame/Field rate. (25/50)

38
DTTB - Content & Services
DTTB was designed to carry video, audio and
program data for television
 DTTB can carry much more than just TV

 Electronic
program guide, teletext
 Broadband multimedia data, news, weather
 Best of internet service
 Interactive services
 Software updates, games

39
Services can be dynamically reconfigured
DVB Data Containers

MPEG Transport Stream is used to provide DVB
“data containers” which may contain a flexible
mixture of:
 Video
 Audio
 Data

services
Streams with variable data rate requirements can
be Statistically Multiplexed together.
 Allows
Six 2 Mb/s programs to be
placed in a 8 Mb/s channel
40
Examples of DVB Data Containers
Channel bandwidth can be used in different ways:
SDTV 1
SDTV 2
SDTV 3
HDTV 1
SDTV 4
SDTV 5
Multiple
SDTV
programs
41
HDTV 1
SDTV 1
Single
HDTV
program
Simulcast
HDTV &
SDTV
Video Program Capacity
For a payload of around 19 Mb/s
1
HDTV service - sport & high action
 2 HDTV services - both film material
 1 HDTV + 1 or 2 SDTV non action/sport
 3 SDTV for high action & sport video
 6 SDTV for film, news & soap operas
However you do not get more for nothing.
 More services means less quality
42
Spare Data Capacity



43
Spare data capacity is
available even on a fully
loaded channel.
Opportunistic use of
spare data capacity when
available can provide
other non real time data
services.
Example: 51 second
BMW commercial
The Commercial was
shown using 1080 Lines
Interlaced. 60 Mb of data
was transferred during it.
In the Final 3 seconds the
BMW Logo was displayed
allowing 3 Phone Books
of data to be transmitted.