1. University of Canberra Advanced Communications Topics
Television Broadcasting into the Digital Era
Lecture 3
Audio/Video
Compression
MPEG-2 Multiplexing
by: Neil Pickford 1

2. 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
Digital TV has Key Technologies that make it possible. Most production within the current TV stations already happens in the digital domain using standards such as Rec 601 digital video. It only becomes analog when it is transmitted over the air to the viewer.
Display technology has not reached the level needed for HDTV to be fully implementable at present.

3. Video Bitrate - HDTV = 1.24416 G bits / sec for Interlace Scan or
2 M pixels * 25 pictures * 3 colours * 8 bits
= G bits / sec for Interlace Scan or
= G bits / sec for Progressive
We need to Compress this a bit!

4. Compression Technology
When low bandwidth analog information is digitised the result is high amounts of digital information.
5 MHz bandwidth analog TV picture º 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
Compression technology is also an enabling thechnology. When you digitise video you end up with massive amounts of real time data. In the above example you would need 20 channels to transmit one digital video signal. We have to compress the television signal so there is less data, allowing it to fit in a normal channel

5. Compression - Types Two types of compression available
Loss-less compression 2 to 5 times
Lossy compression 5 to 250 times
Loss-Less compression produces exactly the same data out as went into the process, like over a telephone modem.
Lossy Compression allows changes to occur in the data such that subtle approximations are made to the images or sounds that the viewer will not be able to notice.

6. Compression - Loss-less Types
Picture differences - temporal
Run length data coding - GIF
= 1 + 4x
21 bits source = 12 bits compressed
Huffman coding - PKZIP
Short codes for common blocks
Longer codes for uncommon blocks
Lookup tables
Temporal Picture Differences rely on the premise that most of the background in pictures does not change from picture to picture, so why transmit it more than once. A difference is calculated by subtraction and only the information that has changed is transmitted.
Run Length encoding exploits repeating sequences or data patterns

7. Compression - Lossy Types
Quantisation - rounding
Motion vectors
Prediction & interpolation
Fractal coding
Discrete cosine transform (DCT)
Quantisation - A 8 or 16 bit signal may not need that level of resolution, 4 or 6 bits may suffice. It might, however make things a bit more fuzzy.
Motion Vectors - is a technique where common pixel blocks are identified from picture to picture and their movement transmitted, instead or retransmitting all the information for the blocks. Eg a hand moving, the pixel blocks displaying the hand are identified and the information transmitted that it they moved X pixels in direction Y. The main problem with this technique is the high level of processor power needed to carry out a pixel block search and match.
Prediction & Interpolation use averaging to determine data between known points without having to transmit it.
DCT is used by MPEG-2 along with Differencing, Motion Vectors, Prediction and Interpolation.

8. Compression - DCT
Here is an example of DCT compression. A simple 8x8 pixel area around the “1” on the calendar has been compressed using the DCT. The original and compressed data values in an 8x8 matrix are shown. Notice the 64 original values have been reduced in this particular example to 4 non zero numbers. The numbers in the DCT matrix represent a frequency distribution of the H & V pixel information in the original picture. The quantisation level of the final numbers can also be reduced with typically half of the matrix being zero or very close to zero not requiring transmission. The reverse process produces a pixel block which is a very close approximation to the original, even when some of the elements have been quantised.
8x8 Pixels

9. MPEG-2 - I, P & B Frames
Uncompressed SDTV Digital Video Stream Mb/s
Picture 830kBytes
Picture 830kBytes
Picture 830kBytes
Picture 830kBytes
I Frame
B Frame
B Frame
P Frame
100 kBytes
12 kBytes
12 kBytes
33 kBytes
MPEG-2 Compressed SDTV Digital Video Stream 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
MPEG has different types of frames which allow interpolation and prediction to be used to reduce the amount of data that needs to be sent.
Three types of frames I, P & B frames.
I frames have around 9 times less data, P frames 25 times less and B frames 70 times less data than the original frame. In this example the total compression is 43 times.
These frames are usually sequenced in a 12 frame Group Of Pictures (GOP) sequence. Typically structured I B B P B B P B B P B B I B B P B B P B B P B B I etc

10. 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 resolution
Low level (LL) by 288 SIF
Main level (ML) by 576 SDTV
High level (HL) 1920 by 1152 HDTV
Profiles and Levels within MPEG-2 define sets of tools or syntaxes for compressing the picture information. If you have a higher level toolbox than you can handle the lower level compression modes, however if you have a middle level tool set you are unable to process images built with the high level tool set.
For SDTV you need Main Level (ML) where as for HDTV you need High Level (HL). If you want to process HDTV you must have a HL decoder. During HL only transmissions a ML decoder will stop decoding and go “Black”. Although HDTV may not be ready at the start of the Digital TV era, the decoders must have a HL decoder otherwise when HDTV is available those decoders will not work. A more inefficient solution which may need to be adopted in Britain, which is installing ML only decoders, is to always transmit a ML signal along with the HDTV. Unfortunately this reduces the data rate available to HDTV by 3-4 Mb/s.

11. MPEG Profiles and Levels
MAX.
BIT-RATE
300 Mbit/s
100 Mbit/s
80 Mbit/s
60 Mbit/s
40 Mbit/s
This diagram is a summary of the profile and levels of MPEG.
“Profile” is what has been used in MPEG to mean a subset of the syntax, for example if you look at the main profile, which is the one most commonly used, the difference between it and the simple profile, is that bidirectionally coded frames can be used. Similarly as we go up the scale of profiles, extra functionality is added. In almost every case, but not in the 4:2:2 case, it’s an onion ring where each one further down the scale is a simple sub-set of the ones further up the scale.
“Levels” is a term that is used for constraints on parameters. Typical parameters that this covers are Video resolution or Bitrate. The diagram is attempting to highlight the bitrate by the height of the cylinders. If we look at the MPEG Main Profile at Main Level the maximum allowed bitrate for that is 15 Mbits/s, the maximum allowed video resolution corresponds to Recommendation 601 in it’s 25 & 50 Hz flavors. The next one in yellow is Main Profile at High Level, this is what has been chosen for High Definition TV. In addition to having the enhanced resolution, you are allowed a higher bit rate which is 80 Mbits/s.
If we look at the 4:2:2 Profile (Green). In addition to being able to have 4:2:2 encoding as well as the 4:2:0 (In other words having full horizontal resolution relative to Rec 601) you are also allowed to have a much higher bit rate. The reason for the much higher bit rate is, in addition, in a contribution application wanting to have the highest possible quality prior to transmission, you may also want to have more frequent occurrences of Intra-coded frames which are the points at which you can make editing. This results in less efficient coding, so therefor you need a higher bit rate to maintain the quality.
20 Mbit/s
HIGH
HIGH-1440
4:2:2
LEVELS
MAIN
HIGH
SPATIALLY
SCALABLE
SNR
SCALABLE
LOW
PROFILES
MAIN
SIMPLE 4

12. MP@ML MP@HL All decoders sold in Australia will
be capable allowing all
viewers access to HD resolution
when it becomes available

13. 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, ….)

14. Multichannel Sound TV
C LFE R L Ls Rs

15. Masking
Both use perceptual audio coding that exploits a psychoacoustic effect known as masking

16. 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 kb/s MPEG 2 5.1

17. MPEG Audio Encoder Audio Bit Stream 32 Subbands O/P Subband Filter
Quantiser & Coder
Frame Packer
Audio In
2 x kb/s
2 x 768 kb/s
Bit Allocation
Coding of Side Information
Psycho- Acoustic Model

18. MPEG Audio Decoder Audio Bit Stream Inverse Subband Filter
Frame Unpacker
De-Quantiser
Inverse Subband Filter
Audio Out
2 x kb/s
2 x 768 kb/s
Decoding of Side Information

19. Multichannel Sound - Dolby AC-3
Dolby AC-3 was developed as a 5.1 channel surround sound system from the beginning.
Compression 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

20. AC-3 Coding Block Diagram

21. AC-3 Multichannel Coder
5.1-ch Encoder
5.1-ch Decoder C LS LS RS RS
LFE
LFE
Encoder
Decoder

22. AC-3 Stereo Decoder 5.1-ch Decoder 5.1-ch Encoder Matrix EncoderL L R R Lo C
5.1-ch Decoder
5.1-ch Encoder C
Matrix Ro LS LS RS RS
LFE
LFE
Encoder
2-channel Decoder

23. 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

24. MPEG-2 Multichannel Coder concept
MPEG-2 Encoder
MPEG-2 Decoder
Down mix R L C LS RS
LFE Ro Lo
MPEG-1 Encoder
MPEG-1 Decoder
Re matrix R L C LS RS
LFE
Extension Encoder
Extension Decoder

25. Low cost 2-channel decoder
MPEG-1 Encoder
MPEG-1 Decoder Lo Lo L Ro Ro R
Down mix C LS RS T2
Extension Encoder T3 T4
LFE
LFE
2-channel Decoder
MPEG-2 Encoder
 Low cost 2-channel decoder

26. Compatibility with Pro LogicS
Surround Encoder
MPEG-1 Encoder
MPEG-1 Decoder R L C S
Pro Logic Decoder Rt Lt Ro Lo

27. Compatibility with Pro Logic (2)LS RS
LFE T2 T3 T4
MPEG-1 Encoder
Extension Encoder
Down mix
MPEG-2 Encoder
MPEG-1 Decoder
Pro Logic Decoder R L C S Rt Lt Ro Lo

28. 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

29. 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

30. Digital Television - Types
Satellite (DBS)
DVB-S
Program interchange
Direct view / pay TV
SMATV
Downlink
Uplink
What are the type of Digital Television?
Satellite uses a central uplink to provide regional or national coverage.
You need to have a satellite dish and down converter. Because accuracy in pointing is required this is a fixed service.

31. Digital Television - Types
Cable
HFC - pay TV
MATV
DVB-C / 16-VSB
Fibre
Main Coax
Tethered cable systems coaxial or fibre. You need to be connected to a permanent cable. Again this is a relatively fixed system.
Spur
Tee
Tap

32. Digital Television - Types
Terrestrial (DTTB)
DVB-T / 8-VSB
Free to air TV (broadcasting)
Narrowcasting/value added services
Untethered - portable reception
DTTB allows both fixed or portable operation.

33. 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
Digital TV has Key Technologies that make it possible. Most production within the current TV stations already happens in the digital domain using standards such as Rec 601 digital video. It only becomes analog when it is transmitted over the air to the viewer.
Display technology has not reached the level needed for HDTV to be fully implementable at present.

34. 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
The functions of MPEG-2 are then as shown in this table.

35. Digital Terrestrial TV - Layers
. . . provide clean interface points
Picture
Layer
Multiple Picture Formats
and Frame Rates
1920 x 1080
1280 x 720
50,25, 24 Hz
Video Compression
Layer
MPEG-2
compression syntax or
Data
Headers
Motion
Vectors
Chroma and Luma
DCT Coefficients
Variable Length Codes
Transport
Layer
MPEG-2 packets
Video packet
Audio packet
Aux data
Packet Headers
Flexible delivery of data
DTTB is about layers.
Picture
Compression
Transport/Multiplexing
Transmission
Transmission
Layer
7 MHz
COFDM / 8-VSB
VHF/UHF TV Channel

36. Digital Television Encode Layers
Picture
Coding
Audio Coding
Data
MPEG-2 or AC-3
MPEG-2
Control
Video
Sound
Program 1 Multiplexer
MPEG Transport Stream Mux
Control Data (PMT)
PES
Bouquet Multiplexer
Program 2
Program 3
Service Mux
Other Data
Control Data (PAT)
Modulator & Transmitter
Error Protection
Control Data
188 byte packets
MPEG Transport Data Stream
What are the inputs to these layers?
Delivery System

37. Digital Television Decode Layers
Data
Mon
Speakers
Audio Decoder
Data
Decoder
Picture
MPEG or AC-3
MPEG-2
MPEG Transport Stream
De-Multiplexer
MPEG DeMux
Transport Stream
Demodulator & Receiver
Error Control
At the receiver we simply select the portions of the existing data stream we wish to decode and throw away the rest.
Delivery System

38. 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)

39. 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
Services can be dynamically reconfigured
DTTB can carry many other things than just television.
Interactive services need a back channel such as the telephone line or a cable/wireless modem.
The analog television we are used to uses a very dumb device to display the pictures.
Digital TV uses a smart box which can be dynamically re-configured. You can choose the change the channel/data structure mid program, upload new operating software with different funtions. No longer will the transmission be totally constrained by the dumb receiver at the other end.

40. 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

41. Examples of DVB Data Containers
Channel bandwidth can be used in different ways:
SDTV 1
SDTV 2
SDTV 3
SDTV 4
SDTV 5
Multiple SDTV programs
Single HDTV program
HDTV 1
SDTV 1
HDTV 1
Simulcast HDTV & SDTV
Focusing on the TV aspects.
There are different ways the channel bandwidth can be used.
As has been done in the UK, you can have a multiplex which has multiple standard definition programs within it.
You can also have a multiplex which contains a single High Definition program which is the plan for many situations in Australia.
There is also the facility to have simulcast, where you cut down a bit on the High Definition to allow the older receivers that are there to decode the Standard Definition variant. This may well be used in the future in the UK when the analog transmissions are turned off. The plan in most of Europe is for High definition to be introduced as a second phase as part of the analog turn off.

42. 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
Digital TV will have a data capacity around 20 Mb/s.
For Sport or high action we can have relatively few services.
Films have high levels of temporal redundancy because both fields are scanned from the same frame. This allows the compression systems to perform higher levels of compression allowing spare data capacity and the ability to have more services.
Generally News & Soapies have lower data requirements so more channels are possible.
You do not ge more channels for nothing.
More Services Means Less Quality.

43. Spare Data Capacity
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.
An example from tests in America.
Other non-real time services can use opportunistic use of the DTV data pipe to transmit data when the full bandwidth of the channel is not required for the main services. These services would be data specifically intended for broadcast application with no need for acknowledgement or a back channel. Teletext or Newspaper type information are good examples.