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Video Compression

H.261

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Contents

H.261 Overview

Motivation Features of H.261

H.261 Coder

Summary References

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H.261 Overview

An !"#! H#series standard app$ica%$e

to videop&one or video conferencin' Video codin' a$'orit&m is desi'ned for

transport usin' t&e Rea$#time !ransport(rotoco$ )R!(*

Operates in rea$#time wit& $imited de$ay !ransmission %it rate is at mu$tip$es of

6+,%it-s

!"#! )nternationa$ !e$ecom "nion*

Recommendation in 1/ (recursor to codin' sc&emes found in

H.260 and M(#1

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H.261 )p 3 6+* Motivation "ncompressed video and audio data are &u'e

Compression ratio of $oss$ess met&ods is not &i'&enou'&

!ar'et networ4s are p 5 6+,%ps 1 

p 

0/ 64Kbps (p=1) < data rate < 1920Kbps (p=30) Covers transmission from ISD base rate (64 Kbps) !p past

t"e #$1 data rate (1%&4 'bps)

'aim!m dea* of 1&0 ms Codin' a$'orit&m is a &y%rid of7

nter#picture prediction # removes tempora red!ndan+*

!ransform codin' # removes t"e spatia red!ndan+*

Motion compensation 8 !ses motion ve+tors to "ep t"e

+ode+ +ompensate for motion

9ata rate can %e set %etween +/ ,%it-s and 2 M%it-s

nput si'na$ format CF )Common ntermediate Format* and :CF ):uarter CF*

avaiabe

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Features of H.261 8 Source format

;it rate

!&e tar'et %it rate is < 6+,%ps to 12/,%ps (icture formats

CF )Common ntermediate Format* 8 =!SC > (A?

:CF ):uarter Common ntermediate Format*

,t 29%9- frames per se+ond .it" 4/2/0 +"rominan+e

s!b$sampin (/C/C)

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Features of H.261 8 Video mu$tip$e3

arran'ement roup of %$oc4s structure (icture 8 coded as $uminance and two

co$or di@erence components ) C; and CR*

roup of %$oc4s )O;*

Macro;$oc4 )M;*

;$oc4 

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H.261 # Video mu$tip$e3arran'ement + $ayers in t&e compressed stream

(icture O; )roup Of ;$oc4* M;)Macro;$oc4* ;$oc4 

CF

:CF

O;

11

0

  

M;C% Cr

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H.261 (icture Synta3 )1-+*

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H.261 (icture Synta3 )2-+*

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H.261 (icture Synta3 )0-+*

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H.261 (icture Synta3 )+-+*

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H.261 Codin' Frame !ypes

ntra#ncoded Frames )#Frames*

Simi$ar to B( compression Spatia$ $terin' 8 sti$$ o%Dects )transform codin'*

3.7 a+ and 5"ite ba+ro!nd pattern

(redicted Frames )(#Frame* (redicted %ased on ear$ier frame

!empora$ $terin' 8 nter#picture prediction 3.7 ,ppi+ation ser

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H.261 # Motion Compensation

Assumption

,s parts of animae moves7 its+oors sta*s most*+onstant

dea 8ind simiar parts in

ot"er imaes n+ode ."ere it

.as fo!nd (i%e%motion ve+tor)

:revio!s de+odedimae ; eferen+eimae

Imae to +ode ; #aret Imae

n+ode t"e

resid!a on*

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H.261 Coder

Out$ine %$oc4 dia'ram for H.261 codec

Source coder!ransmission

%u@er

Codin' contro$

Video mu$tip$e3coder

Source decoderVideo mu$tip$e3

decoderReceivin'

%u@er

Video

Si'na$

Coded

%itstream

3terna$ contro$

a) Video coder 

b) Video decoder 

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H.261 coder structure )1-2*

ncoder

9C!#1

:#1

M

MC

Codin'contro$

V?C:9C!

Frame%u@er

$

ideo in

Intra>inter'ode

?p

ideo o!t

'

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H.261 coder structure )2-2*

9ecoder

'C 8rame'emor*

IDC#

?$1@CDe+oder

!AerrrorCorre+tion

Inp!t data

De+oded data

Intra>inter

'otion e+tor

Step siBe

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9iscrete Cosine !ransformation)9C!* Overview7 6+ 2#9 9C! %asis

functions7

FF! vs. 9C!7

  ?i4e

FF! %ut

appro3imates

$ines wit& fewer

  coeEcients

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9iscrete Cosine !ransformation)9C!* Computation

Factorin' reduces transformation to a

series of 1#9imensiona$ 9C!s

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9emo

Miss America ):CF*

 

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Summary

H.261

sed in video>tee+onferen+in !ec&niues used7

 #.o dimensiona (2$D) DC# to remove intra$frame+orreation

Ei$Ba order to s+an t"e transform +oeF+ients

!n @ent" +odin for Bero$va!ed +oeF+ients afterG!antiBation

'otion estimation is appied to video seG!en+e to improvet"e predi+tion bet.een s!++essive frames

 #ransmission rates +ontro in t"e rane of p 64 Kbps

rror resiien+e in+!din s*n+"roniBation and+on+eament te+"niG!e reG!ired in transmission +ode7 to+over !p +"anne errors

Common Intermediate 8ormat (CI8) and ?!arter CI8(?CI8) for a sine so!tion to diAerent video formats

(#SC > :,@)

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References

Video codec for audiovisua$ services at p 3 6+4%it-s !"#! )nternationa$ !e$ecommunication"nion 8 !e$ecommunication StandardiGationSector* Recommendation H.261 10

Step&en B. So$ari Digital Video and AudioCompression, Mcraw#Hi$$ (rofessiona$ )Marc& 11I*

,. RiD4se JH.261 7 Video Codin' for ?ow#;it#RateCommunicationK IEEE Communications Magazine, pp. 42

 – 45 

H.261 Video Coding 

http://www-mobile.ecs.soton.ac.uk/peter/h261/h261.html