A Brief Overview of the MPEG2 Standard - PowerPoint Presentation

Slide1

A Brief Overview of the MPEG2 Standard

Dr.

David CorriganSlide2

MPEG-2 the Basics

We have already covered a lot of the background of how video compression (or coding) works inside MPEG2.

Intra-Coding (I-frames) v Prediction Coding (P-frames) v Bi-Directional Prediction Coding (B-frames) & the Group of Pictures (GOP)

Most of the subsequent coding architecture is inherited from JPEG

DCT.

Quantisation.

Variable Length Coding (VLC) –

ie

Huffman + Run Length Coding.

But it is not that simple

motion vectors need to be coded, we may need to include direction prediction information

must deal with interlaced video

plus what about audio & subtitles (either in text or lossless image format)?

and what about streaming

so….Slide3

MPEG-2 the Basics

MPEG 2 is about more than video coding

Part 1 – Systems (describes how audio and video are plugged together)

Part 2 – Video

Part 3 – Audio (an extension of the MPEG 1 audio standards)

Part 4 – conformance testing

Part 5 – software simulation

Part 6 – Digital Storage Media Command and Control – (

eg

. rewind forward

etc

etc

)

Part 7 – Advanced Audio Coding (AAC) – a 2

nd

audio standard

there are even more partsSlide4

Challenges in MPEG2 (besides compression)

Multiplexing

H

ow to combine audio video and text?

They must appear at the same time.

Media

Streams can be stored on a hard drive or DVD

D

ata can be broadcast or streamed on the internet

Sequencing

how to send data so that it will be received in the correct order?

how to

sychronise

the decoder and encoder? Slide5

Challenges in MPEG2 (besides compression)

Error Resilience

like synchronisation in JPEG.

but temporal propagation of errors is a problem in video

.

Rate Control/Bandwidth

need to be able to specify a bit rate given the bandwidth available

need to be able to adaptive adjust the quantisation step size

Scalability/Multiplatform

adaptive quality based on the decoder hardware

can we have one stream for both low and high quality video?Slide6

Scalability in MPEG 2

SNR Scalability

Spatial ScalabilitySlide7

Profiles and Levels

MPEG 2 supports a wide variety of scenarios

eg

. high quality

tv

broadcast, low bit rate internet streaming

etc

decoders can have varying degrees of complexity + plus a decoder for internet streaming should not have to support decoding of digital

tv

signals.

MPEG 2 defines Profiles and Level for streams

Profiles define the required decoder complexity (feature set) to decode the stream

Levels define the maximum allowed resolution frame rate and bit rate. Slide8

Levels in MPEG 2Slide9

Levels in MPEG 2Slide10

Allowed Profile/Level Combinations

4:2:2 profile extends on the main profile but does not support

scalabiltySlide11

Profile/Level Combinations

Standard Definition TV uses the Main Profile and the Main Level

allows bi-directional prediction but not scalability and stream must use 4:2:0

YC

b

C

r

chroma

downsampling

Streams have a max resolution of 720x576, max frame rate of

30 fps and max bit rate of 15 mbits/secondalso used on DVDs

HDTV uses the Main Profile and the High Level

The Main Profile defines the core set of algorithms in MPEG 2. Slide12

MPEG 2 Main Profile (Layers)

MPEG Sequence is organised into a hierarchy of layers, like an onion

The Sequence Layer – the entire video sequence

The GOP Layer – delineating exactly one Group of Pictures (PAL max 15, NTSC max 18 frames)

The Picture Layer – referring to a single I- P- or B-frame.

The Slice Layer – represents a horizontal group of

macroblocks

that do not span multiple rows.

The

Macrobock

Layer – represents unit of data for motion estimation (16x16).

Conists

of blocks for luminance and chrominance. The Block Layer – contains the DCT coefficients for 1 8x8 block of pixels (can be either a luminance of chrominance block).Slide13

The GOP Layer (Frame Ordering)

When using IBBP…. prediction mode we have to reorder frames so that all prediction is “backward” (

ie

. causal)

so if a B-frame requires a subsequent P-frame for forward prediction the p-frame is placed first in the stream.

B-frames from previous GOP come after I frame

P-frame 4 is sent before B-frames 2 and 3Slide14

The Picture Layer (Interlacing)

The odd and even fields can be coded together as if it were a frame or the can be coded independently

if there is no motion then we can combine the two fields into a single image called a “frame-picture.” Better for compression efficiency.

if there is motion then the two fields are coded separately as if they were two pictures called “field-pictures”.

Odd

Field-Picture

Even

Field-Picture

Frame PictureSlide15

The Slice Layer (Synchronisation)

Slices can be of arbitrary length but can not extend onto a new line.

They are the MPEG-2 solution to the problem of spatial synchronisation (errors can not propagate spatially between slices).

Slice length set depending on the error conditions

ie

. shorter when the error rate is high.

Can get temporal propagation of errors too but they can extend longer than 1 GOP because of the prediction strategy.Slide16

The Macroblock Layer

Each

macroblock

contains 4 luminance blocks and 2 chrominance blocks if 4:2:0 (4 chrominance if 4:2:2)

I-frame

macroblocks

contain no vectors, 1 in P-frames and 2 in B-frames. If interlaced then the number of vectors doubles.

Macroblocks

for P- and B-frames can be intra-coded if the prediction error (DFD) is too large.

Motion estimator not specified but the vectors are limited in range and are quantised to 0.5 pixel accuracy. Slide17

Coding of Motion Vectors

Motion Vectors are differentially coded

wrt

the vector for the previous

macroblock

(

ie

. to the left)

PMV – previous motion vector.

MV – motion vector for the current

macroblock

.

Define

.

multiply by 2 as 0.5

pel

quantisation used.

and

are coded separately.

 Slide18

Coding Δ

x

and

Δ

y

The absolute value and sign of each component is coded

seperately

. The absolute value is broken down as

– is called the

motion_code

and ranges from 0 to 16. It is

Huffman Coded

– is called the size and effectively limits the range of motion vector. It ranges from 0 to 8. It is

not Huffman Coded

(four bit binary value).

– is the

motion_residual

. It ranges from 0 to

and is

not Huffman

Coded.

It is a

-bit binary number.

 Slide19

Coding Δ

x

and

Δ

y

A table of how the choice of Size effects the range of difference that can be coded.

Size is set once at the start of each Picture Layer. (

ie

. it is the same over the entire picture).

It is common to choose larger size for P-frames cause motion is bigger.Slide20

Coding Δ

x

and

Δ

y

Size is chosen based on the range of motion vectors.

eg

. say we limit search width to 10. Then we could have a vector [10, 10] and a previous vector [-10 10].

The max

or

is

. Therefore we need to choose

.

Given an MV [4.5, 3] and PMV [5, -1] then

Then for

,

 Slide21

Huffman Codes for motion_code

s is 0 if the component is positive and 1 if negative.

each vector is specified by a (

motion_code

,

motion_residual

) pair.

the Size value is specified at the start of the Picture Layer.

If

then we set the

motion_code

to 0 (

codeword

is 1). There is no

motion_residual

.

 Slide22

Example

if

then the

motion_code

is 1, the sign bit is 1 and the

motion_residual

is 0. Therefore the code

is inserted into the

bitstream

if

then the

motion_code

is

2,

the sign bit is

0

and the

motion_residual

is

3.

Therefore the code

is

inserted into the

bitstream

 Slide23

The Block Layer (Quantisation)

Quantisation step sizes for intra-coded blocks

Quantisation step size for prediction-coded blocks

similar to the matrix used in JPEG

a fixed

Q

step

= 16 for all coefficientsSlide24

The Block Layer (Quantisation)

One of the ways rate control is achieved is by increasing the quantisation step size in blocks which would otherwise have a higher entropy.

We can specify a quantisation scale value that scale the coefficient of the Q matrix.

ie

. the effective step sizes are

This will reduce quality in these areas.

 Slide25

The Block Layer (Scan Order)

Scan Order for Progressive Video

Scan Order for Interlaced VideoSlide26

The Block Layer (Scan Order)

Idea is to maximise length of runs of zeros in the block.

So progressive frames use the

zig-zag

scan like JPEG

Interlaced Frames use an alternative scan because there are likely to be non-zero DCT coefficients toward the bottom left corner of the block.Slide27

Sequencing, Media and Multiplexing

We could have multiple elementary streams (

ie

. video, audio, text etc.). They have to combined into a single non-elementary stream and have to be both decoded and displayed in a certain order in the receiver.

The MPEG 2 Part 1 (Systems) standard specifies two different ways of doing this

Program Stream (PS) – used for reliable media such as DVDs

Transport Stream (TS) – used for Digital TV Transmission over noisy channels.

Note there are other ways of doing this that exist outside of the standard

eg

. the

avi

and

mov

file formats can be used with compressed MPEG2 data.

To do this the notions of time and packets are introduced.

each elementary stream is divided into packets.

T

hey can be of fixed or variable length.

these packets are interleaved by the encoder.

e

ach packet carries a timestamp which tells the decoder the correct order.Slide28

MPEG 2 Program Stream (PS)

Consists of

Packetised

Elementary Stream

(PES) packets.

PES packets contain 2 timestamps

Decoding Time Stamp (DTS) – this tells the decoder when the packet should be decoded. The data is then decoded into the bit stream.

Presentation Time Stamp (PTS) – this tells the decoder when the data should be displayed.

The systems part specifies that the decoder must contain a Systems Clock called the STC

when a decoder’s STC is equal to a packet’s DTS the data in the packet is decoded

when the STC is equal to a packet’s PTS the decoded data is sent to the display device (

eg

. graphics card or sound card)

the state of the encoders clock is placed in the stream at regular intervals. This synchronises the decoder with the encoder. Slide29

MPEG-2 Transport Stream (TS)

The transport stream uses a fixed packet length (188 bytes)

this allows easy decoder/encoder synchronisation.

it also allows error correction codes to be inserted.

Transport Streams can contain packets from a number of Programs

These can be different TV channels or maybe an EPG.

Each program has a unique Packet ID placed in the packet header.

Decoder can discard packets of other programs by checking the PID.