H.261_技术文档

INTERNATIONAL TELECOMMUNICATION UNION

H.261

ITU-T

TELECOMMUNICATION

STANDARDIZATION SECTOR

OF ITU

(03/93)

{This document has included corrections to typographical errors listed in Annex

5 to COM 15R 16-E dated June 1994. - Sakae OKUBO}

LINE TRANSMISSION OF NON-TELEPHONE

SIGNALS

VIDEO CODEC FOR AUDIOVISUAL

SERVICES AT

p × 64 kbit/s

ITU-T Recommendation H.261

(Previously “CCITT Recommendation”)

FOREWORD

The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of the International Telecommunication

Union. The ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them

with a view to standardizing telecommunications on a worldwide basis.

The World Telecommunication Standardization Conference (WTSC), which meets every four years, established the topics for

study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics.

ITU-T Recommendation H.261 was revised by the ITU-T Study Group XV (1988-1993) and was approved by the WTSC

(Helsinki, March 1-12, 1993).

___________________

NOTES

As a consequence of a reform process within the International Telecommunication Union (ITU), the CCITT ceased

1

to exist as of 28 February 1993. In its place, the ITU Telecommunication Standardization Sector (ITU-T) was created as of 1

March 1993. Similarly, in this reform process, the CCIR and the IFRB have been replaced by the Radiocommunication

Sector.

In order not to delay publication of this Recommendation, no change has been made in the text to references containing the

acronyms “CCITT, CCIR or IFRB” or their associated entities such as Plenary Assembly, Secretariat, etc. Future editions of

this Recommendation will contain the proper terminology related to the new ITU structure.

2

In this Recommendation, the expression “Administration” is used for conciseness to indicate both a

telecommunication administration and a recognized operating agency.

ITU 1994

mechanical, including photocopying and microfilm, without permission in writing from the ITU.

CONTENTS

Recommendation H.261 (03/93)

Page

1

2

Scope..............................................................................................................................................................

Brief specification ..........................................................................................................................................

1

2

3

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

Video input and output......................................................................................................................

Digital output and input ....................................................................................................................

Sampling frequency ..........................................................................................................................

Source coding algorithm ...................................................................................................................

Bit rate ..............................................................................................................................................

Symmetry of transmission................................................................................................................

Error handling...................................................................................................................................

Multipoint operation .........................................................................................................................

3

Source coder...................................................................................................................................................

3

6

3.1

3.2

3.3

3.4

Source format....................................................................................................................................

Video source coding algorithm.........................................................................................................

Coding control ..................................................................................................................................

Forced updating ................................................................................................................................

4

5

Video multiplex coder....................................................................................................................................

7

4.1

4.2

4.3

Data structure....................................................................................................................................

Video multiplex arrangement............................................................................................................

Multipoint considerations .................................................................................................................

18

Transmission coder.........................................................................................................................................

19

20

5.1

5.2

5.3

5.4

Bit rate ..............................................................................................................................................

Video data buffering .........................................................................................................................

Video coding delay ...........................................................................................................................

Forward error correction for coded video signal...............................................................................

Annex A – Inverse transform accuracy specification...............................................................................................

Annex B – Hypothetical reference decoder .............................................................................................................

Annex C – Codec delay measurement method........................................................................................................

Annex D – Still image transmission........................................................................................................................

21

22

23

24

Recommendation H.261 (03/93)

i

Recommendation H.261

Recommendation H.261 (03/93)

VIDEO CODEC FOR AUDIOVISUAL SERVICES AT X 64 kbit/s

p

(Geneva, 1990; revised at Helsinki, 1993)

The CCITT,

considering

(a)

that there is significant customer demand for videophone, videoconference and other audiovisual services;

that circuits to meet this demand can be provided by digital transmission using the B, H rates or their multiples up

(b)

0

to the primary rate or H /H rates;

11 12

(c)

that ISDNs are likely to be available in some countries that provide a switched transmission service at the B, H or

0

H /H rate;

11 12

(d)

that the existence of different digital hierarchies and different television standards in different parts of the world

complicates the problems of specifying coding and transmission standards for international connections;

(e)

that a number of audiovisual services are likely to appear using basic and primary rate ISDN accesses and that some

means of intercommunication among these terminals should be possible;

(f) that the video codec provides an essential element of the infrastructure for audiovisual services which allows such

intercommunication in the framework of Recommendation H.200;

(g) that Recommendation H.120 for videoconferencing using primary digital group transmission was the first in an

evolving series of Recommendations,

appreciating

that advances have been made in research and development of video coding and bit rate reduction techniques which lead to

the use of lower bit rates down to 64 kbit/s so that this may be considered as the second in the evolving series of

Recommendations,

and noting

that it is the basic objective of the CCITT to recommend unique solutions for international connections,

recommends

that in addition to those codecs complying to Recommendation H.120, codecs having signal processing and transmission

coding characteristics described below should be used for international audiovisual services.

NOTES

1

2

Codecs of this type are also suitable for some television services where full broadcast quality is not required.

Equipment for transcoding from and to codecs according to Recommendation H.120 is under study.

1

Scope

This Recommendation describes the video coding and decoding methods for the moving picture component of audiovisual

services at the rates of p × 64 kbit/s, where p is in the range 1 to 30.

2

Brief specification

An outline block diagram of the codec is given in Figure 1.

Recommendation H.261 (03/93)

1

Extern al con tro l

Cod in g contro l

S ource

co der

V id eo mu ltiplex

Transm ission

bu ffe r

Tra nsm ission

cod er

co de r

Vid eo

sign al

Cod ed

bit s tre am

a) Video coder

So urce

decod er

Receivin g

buffer

Rece iving

de cod er

Vid eo mu ltiplex

de co de r

b) Video decoder

T 1502 430-90/d01

FIGURE 1/H.261

Outline block diagramof the video codec

FIGURE 1/H.261...[D01] = 9 CM

2.1

Video input and output

To permit a single Recommendation to cover use in and between regions using 625- and 525-line television standards, the

source coder operates on pictures based on a common intermediate format (CIF). The standards of the input and output

television signals, which may, for example, be composite or component, analogue or digital and the methods of performing

any necessary conversion to and from the source coding format are not subject to Recommendation.

2.2

Digital output and input

The video coder provides a self-contained digital bit stream which may be combined with other multi-facility signals (for

example as defined in Recommendation H.221). The video decoder performs the reverse process.

2.3

Sampling frequency

Pictures are sampled at an integer multiple of the video line rate. This sampling clock and the digital network clock are

asynchronous.

2.4

Source coding algorithm

A hybrid of inter-picture prediction to utilize temporal redundancy and transform coding of the remaining signal to reduce

spatial redundancy is adopted. The decoder has motion compensation capability, allowing optional incorporation of this

technique in the coder.

2.5

Bit rate

This Recommendation is primarily intended for use at video bit rates between approximately 40 kbit/s and 2 Mbit/s.

2.6

Symmetry of transmission

The codec may be used for bidirectional or unidirectional visual communication.

2

Recommendation H.261 (03/93)

2.7

Error handling

The transmitted bit-stream contains a BCH code (Bose, Chaudhuri and Hocquengham) (511,493) forward error correction

code. Use of this by the decoder is optional.

2.8

Multipoint operation

Features necessary to support switched multipoint operation are included.

3

Source coder

Source format

3.1

The source coder operates on non-interlaced pictures occurring 30 000/1001 (approximately 29.97) times per second. The

tolerance on picture frequency is ± 50 ppm.

Pictures are coded as luminance and two colour difference components (Y, C and C ). These components and the codes

B

R

representing their sampled values are as defined in CCIR Recommendation 601.

Black = 16

White = 235

Zero colour difference = 128

Peak colour difference = 16 and 240.

These values are nominal ones and the coding algorithm functions with input values of 1 through to 254.

Two picture scanning formats are specified.

In the first format (CIF), the luminance sampling structure is 352 pels per line, 288 lines per picture in an orthogonal

arrangement. Sampling of each of the two colour difference components is at 176 pels per line, 144 lines per picture,

orthogonal. Colour difference samples are sited such that their block boundaries coincide with luminance block boundaries as

shown in Figure 2. The picture area covered by these numbers of pels and lines has an aspect ratio of 4:3 and corresponds to

the active portion of the local standard video input.

NOTE – The number of pels per line is compatible with sampling the active portions of the luminance and colour difference

signals from 525- or 625-line sources at 6.75 and 3.375 MHz, respectively. These frequencies have a simple relationship to those in CCIR

Recommendation 601.

The second format, quarter-CIF (QCIF), has half the number of pels and half the number of lines stated above. All codecs

must be able to operate using QCIF. Some codecs can also operate with CIF.

Means shall be provided to restrict the maximum picture rate of encoders by having at least 0, 1, 2 or 3 non-transmitted

pictures between transmitted ones. Selection of this minimum number and CIF or QCIF shall be by external means (for

example via Recommendation H.221).

3.2

Video source coding algorithm

The source coder is shown in generalized form in Figure 3. The main elements are prediction, block transformation and

quantization.

The prediction error (INTER mode) or the input picture (INTRA mode) is subdivided into 8 pel by 8 line blocks which are

segmented as transmitted or non-transmitted. Further, four luminance blocks and the two spatially corresponding colour

difference blocks are combined to form a macroblock as shown in Figure 10.

Recommendation H.261 (03/93)

3

T 150 818 0-92 /d0 2

Lum inan ce sa mple

Ch romin ance sam ple

Block ed ge

FIGURE2/H.261

Positioning of luminance and chrominance samples

FIGURE 2/H.261...[D02] = 10 CM

The criteria for choice of mode and transmitting a block are not subject to recommendation and may be varied dynamically as

part of the coding control strategy. Transmitted blocks are transformed and resulting coefficients are quantized and variable

length coded.

3.2.1

The prediction is inter-picture and may be augmented by motion compensation (see 3.2.2) and a spatial filter (see 3.2.3).

3.2.2 Motion compensation

Prediction

Motion compensation (MC) is optional in the encoder. The decoder will accept one vector per macroblock. Both horizontal

and vertical components of these motion vectors have integer values not exceeding ± 15. The vector is used for all four

luminance blocks in the macroblock. The motion vector for both colour difference blocks is derived by halving the

component values of the macroblock vector and truncating the magnitude parts towards zero to yield integer components.

A positive value of the horizontal or vertical component of the motion vector signifies that the prediction is formed from pels

in the previous picture which are spatially to the right or below the pels being predicted.

Motion vectors are restricted such that all pels referenced by them are within the coded picture area.

4

Recommendation H.261 (03/93)

p

t

CC

q z

q

T

Q

V id e o

in

Q^{- 1}

To vide o

m ultiplex

co de r

T ^{- 1}

F

P

v

f

T 150 24 41-9 0/d0 3

T

Q

P

Tra ns fo rm

Q ua ntizer

P ictu re m e m ory with m o tio n com p en sa ted variab le de lay

L o op filte r

F

CC Co d in g co ntro l

p

t

Flag fo r INTRA /INTE R

Flag fo r transm itte d o r not

Q ua ntizer ind ic ation

Q ua ntizing in de x for tra n sform coe fficien ts

M otion v ector

q z

q

v

f

S witch in g o n/off of the lo op filte r

FIGURE 3/H.261

Source coder

FIGURE 3/H.261...[D03] = 16 CM

3.2.3

Loop filter

The prediction process may be modified by a two-dimensional spatial filter (FIL) which operates on pels within a predicted 8

by 8 block.

The filter is separable into one-dimensional horizontal and vertical functions. Both are non-recursive with coefficients of 1/4,

1/2, 1/4 except at block edges where one of the taps would fall outside the block. In such cases the 1-D filter is changed to

have coefficients of 0, 1, 0. Full arithmetic precision is retained with rounding to 8 bit integer values at the 2-D filter output.

Values whose fractional part is one half are rounded up.

Recommendation H.261 (03/93)

5

The filter is switched on/off for all six blocks in a macroblock according to the macroblock type (see 4.2.3, MTYPE).

3.2.4 Transformer

Transmitted blocks are first processed by a separable two-dimensional discrete cosine transform of size 8 by 8. The output

from the inverse transform ranges from –256 to +255 after clipping to be represented with 9 bits. The transfer function of the

inverse transform is given by:

7

1

f (x, y) =

C(u) C(v) F(u,v)cos[π (2x + 1) u /16]cos[π (2y + 1) v/16]

∑ ∑

4

=

0

u

0

v

with

u, v, x, y = 0, 1, 2, . . ., 7

where

x,y = spatial coordinates in the pel domain,

u,v = coordinates in the transform domain,

=

C(u)

1 / 2

1/ 2

for u = 0; otherwise 1,

=

C(v)

for v = 0; otherwise 1.

NOTE – Within the block being transformed, x = 0 and y = 0 refer to the pel nearest the left and top edges of the picture,

respectively.

The arithmetic procedures for computing the transforms are not defined, but the inverse one should meet the error tolerance

specified in Annex A.

3.2.5

Quantization

The number of quantizers is 1 for the INTRA dc coefficient and 31 for all other coefficients. Within a macroblock the same

quantizer is used for all coefficients except the INTRA dc one. The decision levels are not defined. The INTRA dc

coefficient is nominally the transform value linearly quantized with a stepsize of 8 and no dead-zone. Each of the other 31

quantizers is also nominally linear but with a central dead-zone around zero and with a step size of an even value in the range

2 to 62.

The reconstruction levels are as defined in 4.2.4.

NOTE – For the smaller quantization step sizes, the full dynamic range of the transform coefficients cannot be represented.

3.2.6

Clipping of reconstructed picture

To prevent quantization distortion of transform coefficient amplitudes causing arithmetic overflow in the encoder and

decoder loops, clipping functions are inserted. The clipping function is applied to the reconstructed picture which is formed

by summing the prediction and the prediction error as modified by the coding process. This clipper operates on resulting pel

values less than 0 or greater than 255, changing them to 0 and 255, respectively.

3.3

Coding control

Several parameters may be varied to control the rate of generation of coded video data. These include processing prior to the

source coder, the quantizer, block significance criterion and temporal sub-sampling. The proportions of such measures in the

overall control strategy are not subject to recommendation.

When invoked, temporal sub-sampling is performed by discarding complete pictures.

3.4

Forced updating

This function is achieved by forcing the use of the INTRA mode of the coding algorithm. The update pattern is not defined.

For control of accumulation of inverse transform mismatch error a macroblock should be forcibly updated at least once per

every 132 times it is transmitted.

6

Recommendation H.261 (03/93)

4

Video multiplex coder

Data structure

4.1

Unless specified otherwise the most significant bit is transmitted first. This is bit 1 and is the leftmost bit in the code tables in

this Recommendation. Unless specified otherwise all unused or spare bits are set to “1”. Spare bits must not be used until

their functions are specified by the CCITT.

4.2

Video multiplex arrangement

The video multiplex is arranged in a hierarchical structure with four layers. From top to bottom the layers are:

–

picture;

Group of blocks (GOB);

Macroblock (MB);

Block.

A syntax diagram of the video multiplex coder is shown in Figure 4. Abbreviations are defined in later subclauses.

4.2.1 Picture layer

Data for each picture consists of a picture header followed by data for GOBs. The structure is shown in Figure 5. Picture

headers for dropped pictures are not transmitted.

4.2.1.1 Picture start code (PSC) (20 bits)

A word of 20 bits. Its value is 0000 0000 0000 0001 0000.

4.2.1.2 Temporal reference (TR) (5 bits)

A 5-bit number which can have 32 possible values. It is formed by incrementing its value in the previously transmitted picture

header by one plus the number of non-transmitted pictures (at 29.97 Hz) since that last transmitted one. The arithmetic is

performed with only the five LSBs.

4.2.1.3 Type information (PTYPE) (6 bits)

Information about the complete picture:

Bit 1

Bit 2

Bit 3

Bit 4

Bit 5

Bit 6

Split screen indicator, “0” off, “1” on;

Document camera indicator, “0” off, “1” on;

Freeze picture release, “0” off, “1” on;

Source format, “0” QCIF, “1” CIF;

Optional still image mode HI_RES defined in Annex D; “0” on, “1” off;

Spare.

4.2.1.4 Extra insertion information (PEI) (1 bit)

A bit which when set to “1” signals the presence of the following optional data field.

4.2.1.5 Spare information (PSPARE) (0/8/16 . . . bits)

If PEI is set to “1”, then 9 bits follow consisting of 8 bits of data (PSPARE) and then another PEI bit to indicate if a further 9

bits follow and so on. Encoders must not insert PSPARE until specified by the CCITT. Decoders must be designed to discard

PSPARE if PEI is set to 1. This will allow the CCITT to specify future backward compatible additions in PSPARE.

Recommendation H.261 (03/93)

7

P icture layer

P SC

TR

PTYP E

PE I

P SP ARE

GO B layer

G OB lay er

G BS C

GN

GQ UANT

GE I

GS PA RE

M B layer

M V D

B lock layer

M BA

M TYPE

M QUA NT

CBP

CB P

M B A stuffing

B lock laye r

TCOE FF

EOB

T15 02 45 1-9 0/d0 4

Fixed length

V ariable len gth

FIGURE 4/H.261

Syntax diagramfor the video multiplex coder

FIGURE 4/H.261...[D04] = 21 CM PAGE PLEINE

8

Recommendation H.261 (03/93)

GOB data

PSC

T R

PTYPE

PE I

PSPA RE

PEI

T1 5142 30-93/d0 5

FIGURE 5/H.261

Structureof picturelayer

FIGURE 5/H.261...[D05] = 3 CM

4.2.2

Group of blocks layer

Each picture is divided into groups of blocks (GOBs). A group of blocks (GOB) comprises one twelfth of the CIF or one

third of the QCIF picture areas (see Figure 6). A GOB relates to 176 pels by 48 lines of Y and the spatially corresponding 88

pels by 24 lines of each of C and C .

B

R

Data for each group of blocks consists of a GOB header followed by data for macroblocks. The structure is shown in Figure

7. Each GOB header is transmitted once between picture start codes in the CIF or QCIF sequence numbered in Figure 6, even

if no macroblock data is present in that GOB.

1

3

2

4

1

3

5

6

5

7

8

QCIF

9

10

12

11

CIF

FIGURE 6/H.261

Arrangement of GOBs in a picture

Recommendation H.261 (03/93)

9

GBS C

GN

GQUANT

GEI

G SPA RE

G EI

M B d ata

T151 42 40- 93/d0 6

FIGURE 7/H.261

Structureof groupofblocks layer

FIGURE 7/H.261...[D06] = 3 CM

4.2.2.1 Group of blocks start code (GBSC) (16 bits)

A word of 16 bits, 0000 0000 0000 0001.

4.2.2.2 Group number (GN) (4 bits)

Four bits indicating the position of the group of blocks. The bits are the binary representation of the number in Figure 6.

Group numbers 13, 14 and 15 are reserved for future use. Group number 0 is used in the PSC.

4.2.2.3 Quantizer information (GQUANT) (5 bits)

A fixed length codeword of 5 bits which indicates the quantizer to be used in the group of blocks until overridden by any

subsequent MQUANT. The codewords are the natural binary representations of the values of QUANT (see 4.2.4) which,

being half the step sizes, range from 1 to 31.

4.2.2.4 Extra insertion information (GEI) (1 bit)

A bit which when set to “1” signals the presence of the following optional data field.

4.2.2.5 Spare information (GSPARE) (0/8/16 . . . bits)

If GEI is set to “1”, then 9 bits follow consisting of 8 bits of data (GSPARE) and then another GEI bit to indicate if a further

9 bits follow and so on. Encoders must not insert GSPARE until specified by the CCITT. Decoders must be designed to

discard GSPARE if GEI is set to 1. This will allow the CCITT to specify future “backward” compatible additions in

GSPARE.

NOTE – Emulation of start codes may occur if the future specification of GSPARE has no restrictions on the final GSPARE

data bits.

4.2.3

Macroblock layer

Each GOB is divided into 33 macroblocks as shown in Figure 8. A macroblock relates to 16 pels by 16 lines of Y and the

spatially corresponding 8 pels by 8 lines of each of C and C .

B

R

Data for a macroblock consists of an MB header followed by data for blocks (see Figure 9). MQUANT, MVD and CBP are

present when indicated by MTYPE.

10

Recommendation H.261 (03/93)

1

2

3

4

5

6

7

8

9

10

21

32

11

22

33

12

23

13

24

14

25

15

26

16

27

17

28

18

29

19

30

20

31

FIGURE 8/H.261

Arrangement of macroblocks in a GOB

MBA

MTYPE

MQUANT

MVD

CBP

Block data

FIGURE 9/H.261

Structure of macroblock layer

4.2.3.1 Macroblock address (MBA) (Variable length)

A variable length codeword indicating the position of a macroblock within a group of blocks. The transmission order is as

shown in Figure 8. For the first transmitted macroblock in a GOB, MBA is the absolute address in Figure 8. For subsequent

macroblocks, MBA is the difference between the absolute addresses of the macroblock and the last transmitted macroblock.

The code table for MBA is given in Table 1.

An extra codeword is available in the table for bit stuffing immediately after a GOB header or a coded macroblock (MBA

stuffing). This codeword should be discarded by decoders.

The VLC for start code is also shown in Table 1.

MBA is always included in transmitted macroblocks.

Macroblocks are not transmitted when they contain no information for that part of the picture.

Recommendation H.261 (03/93)

11

TABLE 1/H.261

VLC table for macroblock addressing

MBA

Code

MBA

Code

11

12

13

14

15

16

17

18

19

10

11

12

13

14

15

16

1

011

010

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

0000

0101

10

01

00

11

0101

0100

0011

0001

0000

0011

0010

0001

0000

10

1

0

011

010

001

000

111

110

101

100

011

010

001

000

111

0000 0001

111

110

1011

1010

1001

1000

0111

0110

0101

11

MBA stuffing

Start code

4.2.3.2 Type information (MTYPE) (Variable length)

Variable length codewords giving information about the macroblock and which data elements are present. Macroblock types,

included elements and VLC words are listed in Table 2.

MTYPE is always included in transmitted macroblocks.

4.2.3.3 Quantizer (MQUANT) (5 bits)

MQUANT is present only if so indicated by MTYPE.

A codeword of 5 bits signifying the quantizer to be used for this and any following blocks in the group of blocks until

overridden by any subsequent MQUANT.

Codewords for MQUANT are the same as for GQUANT.

12

Recommendation H.261 (03/93)

TABLE 2/H.261

VLC table for MTYPE

MQUANT

MVD

CBP

TCOEFF

VLC

Prediction

x

0001

Intra

Inter

x

0000 001

1

x

0000 1

x

0000 0000

0000 0001

1

Inter + MC

x

Inter + MC

x

0000 0000 01

Inter + MC

001

Inter + MC + FIL

x

01

0000 01

NOTES

1

2

“x” means that the item is present in the macroblock.

It is possible to apply the filter in a non-motion compensated macroblock by declaring it as MC + FIL but

with a zero vector.

4.2.3.4 Motion vector data (MVD) (Variable length)

Motion vector data is included for all MC macroblocks. MVD is obtained from the macroblock vector by subtracting the

vector of the preceding macroblock. For this calculation the vector of the preceding macroblock is regarded as zero in the

following three situations:

1) evaluating MVD for macroblocks 1, 12 and 23;

2) evaluating MVD for macroblocks in which MBA does not represent a difference of 1;

3) MTYPE of the previous macroblock was not MC.

MVD consists of a variable length codeword for the horizontal component followed by a variable length codeword for the

vertical component. Variable length codes are given in Table 3.

Advantage is taken of the fact that the range of motion vector values is constrained. Each VLC word represents a pair of

difference values. Only one of the pair will yield a macroblock vector falling within the permitted range.

4.2.3.5 Coded block pattern (CBP) (Variable length)

CBP is present if indicated by MTYPE. The codeword gives a pattern number signifying those blocks in the macroblock for

which at least one transform coefficient is transmitted. The pattern number is given by:

32 · P + 16 · P + 8 · P + 4 · P + 2 · P + P

6

1

2

3

4

5

where P = 1 if any coefficient is present for block n, else 0. Block numbering is given in Figure 10.

n

The codewords for CBP are given in Table 4.

Recommendation H.261 (03/93)

13

TABLE 3/H.261

VLC table for MVD

MVD

Code

–16 & 16

–15 & 17

–14 & 18

–13 & 19

–12 & 20

–11 & 21

–10 & 22

o–9 & 23

o–8 & 24

o–7 & 25

o–6 & 26

o–5 & 27

o–4 & 28

o–3 & 29

o–2 & 30

o–1 & 23

o–0 & 23

o–1 & 23

o–2 & –30

o–3 & –29

o–4 & –28

o–5 & –27

o–6 & –26

o–7 & –25

o–8 & –24

o–9 & –23

–10 & –22

–11 & –21

–12 & –20

–13 & –19

–14 & –18

–15 & –17

0000 0011 001

0000 0011 011

0000 0011 101

0000 0011 111

0000 0100 001

0000 0100 011

0000 0100 111

0000 0101 011

0000 0101 111

0000 0111 111

0000 1001 110

0000 1011 110

0000 1111 110

0001 1111 110

0011 1111 110

0111 0100 110

1000 0100 110

01000 0100

1

0010 1111 111

0001 0111 111

0000 1101 111

0000 1010 111

0000 1000 110

0000 0110 110

0000 0101 100

0000 0101 001

0000 0100 101

0000 0100 010

0000 0100 000

0000 0011 110

0000 0011 100

0000 0011 010

4.2.4

Block layer

A macroblock comprises four luminance blocks and one of each of the two colour difference blocks (see Figure 10).

Data for a block consists of codewords for transform coefficients followed by an end of block marker (see Figure 11). The

order of block transmission is as in Figure 10.

4.2.4.1 Transform coefficients (TCOEFF)

Transform coefficient data is always present for all six blocks in a macroblock when MTYPE indicates INTRA. In other

cases MTYPE and CBP signal which blocks have coefficient data transmitted for them. The quantized transform coefficients

are sequentially transmitted according to the sequence given in Figure 12.

The most commonly occurring combinations of successive zeros (RUN) and the following value (LEVEL) are encoded with

variable length codes. Other combinations of (RUN, LEVEL) are encoded with a 20-bit word consisting of 6 bits ESCAPE, 6

bits RUN and 8 bits LEVEL. For the variable length encoding there are two code tables, one being used for the first

transmitted LEVEL in INTER, INTER+MC and INTER+MC+FIL blocks, the second for all other LEVELs except the first

one in INTRA blocks which is fixed length coded with 8 bits.

14

Recommendation H.261 (03/93)

TABLE 4/H.261

VLC table for CBP

CBP

Code

CBP

Code

0001 1100 0

0001 1011 0

0001 1010 0

0001 1001 0

0001 1000 0

0001 0111 0

0001 0110 0

0001 0101 0

0001 0100 0

0001 0011 0

0001 0010 0

0001 0001 0

0001 0000 0

0000 1111 0

0000 1110 0

0000 1101 0

0000 1100 0

0000 1011 0

0000 1010 0

0000 1001 0

0000 1000 0

0000 0111 0

0000 0110 0

0000 0101 0

0000 0100 0

0000 0011 1

0000 0011 0

0000 0010 1

0000 0010 0

0000 0001 1

0000 0001 0

60

64

68

16

32

12

48

20

40

28

44

52

56

61

62

24

36

63

65

69

17

33

66

10

18

34

37

11

19

1110 0000

1101 0000

1100 0000

1011 0000

1010 1000

1001 1000

1001 0000

1000 1000

1000 0000

0111 1000

0111 0000

0110 1000

0110 0000

0101 1000

0101 0000

0100 1000

0100 0000

0011 1100

0011 1000

0011 0100

0011 0000

0010 1110

0010 1100

0010 1010

0010 1000

0010 0110

0010 0100

0010 0010

0010 0000

0001 1111

0001 1110

0001 1101

35

13

49

21

41

14

50

22

42

15

51

23

43

25

37

26

38

29

45

53

57

30

46

54

58

31

47

55

59

27

39

1

3

2

4

5

6

Y

C

B

C

R

FIGURE 10/H.261

Arrangement of blocks in a macroblock

Recommendation H.261 (03/93)

15

TCOEFF

EOB

FIGURE 11/H.261

Structure of block layer

Increas ing cycles

pe r picture width

1

3

2

5

6

7

15

17

26

32

40

47

1 6

2 7

3 1

4 1

4 6

5 2

28

30

42

45

53

56

29

43

44

54

55

61

8

14

18

25

33

39

4

9

1 3

1 9

2 4

3 4

Increasin g cycles

p er picture heigh t

10

11

21

12

20

23

22

36

35

37

3 8

4 9

48

50

51

58

5 7

5 9

60

63

62

64

T1 51 410 0-9 3/d07

FIGURE 12/H.261

Transmission order for transformcoefficients

FIGURE 12/H.261...[D07] = 6 CM

Codes are given in Table 5.

The most commonly occurring combinations of zero-run and the following value are encoded with variable length codes as

listed in the table 5. End of block (EOB) is in this set. Because CBP indicates those blocks with no coefficient data, EOB

cannot occur as the first coefficient. Hence EOB can be removed from the VLC table for the first coefficient.

The last bit “s” denotes the sign of the level, “0” for positive and “1” for negative.

The remaining combinations of (run, level) are encoded with a 20-bit word consisting of 6 bits escape, 6 bits run and 8 bits

level. Use of this 20-bit word form encoding the combinations listed in the VLC table is not prohibited.

16

Recommendation H.261 (03/93)

TABLE 5/H.261

VLC table for TCOEFF

Recommendation H.261 (03/93)

17

Run

Level

Code

10

1s

EOB

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

Escape

^a)If first coefficient in block

11s Not first coefficient in block

0100 s

1

2

3

4

5

6

7

8

0010 1s

0000 110s

0010 0110 s

0010 0001 s

0000 0010 10s

0000 0001 1101 s

0000 0001 1000 s

0000 0001 0011 s

0000 0001 0000 s

0000 0000 1101 0s

0000 0000 1100 1s

0000 0000 1100 0s

0000 0000 1011 1s

011s

0001 10s

0010 0101 s

0000 0011 00s

0000 0001 1011 s

0000 0000 1011 0s

0000 0000 1010 1s

0101 s

9

10

11

12

13

14

15

1

2

3

4

5

6

7

1

2

3

4

5

1

2

3

4

1

2

3

1

2

3

1

2

1

2

1

2

1

2

1

2

1

0000 100s

0000 0010 11s

0000 0001 0100 s

0000 0000 1010 0s

0011 1s

0010 0100 s

0000 0001 1100 s

0000 0000 1001 1s

0011 0s

0000 0011 11s

0000 0001 0010 s

0001 11s

0000 0010 01s

0000 0000 1001 0s

0001 01s

0000 0001 1110 s

0001 00s

0000 0001 0101 s

0000 111s

0000 0001 0001 s

0000 101s

0000 0000 1000 1s

0010 0111 s

0000 0000 1000 0s

0010 0011 s

0010 0010 s

0010 0000 s

0000 0011 10s

0000 0011 01s

0000 0010 00s

0000 0001 1111 s

0000 0001 1010 s

0000 0001 1001 s

0000 0001 0111 s

0000 0001 0110 s

0000 0000 1111 1s

0000 0000 1111 0s

0000 0000 1110 1s

0000 0000 1110 0s

0000 0000 1101 1s

0000 01

1

a)

Never used in INTRA macroblocks.

18

Recommendation H.261 (03/93)

Run is a 6 bit fixed length code

Level is an 8 bit fixed length code

Run

Code

Level

Code

0

1

0000 00

0000 01

0000 10

ξ

−128

−127

ξ

FORBIDDEN

1000 0001

ξ

2

ξ

00−2

00−1

0000

1111 1110

1111 1111

FORBIDDEN

ξ

63

1111 11

0001

0002

ξ

0000 0001

0000 0010

ξ

0127

0111 1111

For all coefficients other than the INTRA dc one, the reconstruction levels (REC) are in the range −2048 to 2047 and are

given by clipping the results of the following formuls:

REC = QUANT • (2 • level + 1); level > 0

U

QUANT =

“odd”

V

=

−

<

REC QUANT • (2 • level 1); level

⁰W

REC = QUANT • (2 • level + 1) − 1; level > 0

U

QUANT =

“even”

V

=

−

+

<

REC QUANT • (2 • level 1) 1; level

⁰W

REC = 0; level = 0

NOTE – QUANT ranges from 1 to 31 and is transmitted by either GQUANT or MQUANT.

Recommendation H.261 (03/93)

19

Reconstruction levels (REC)

QUANT

8

Level

−127

1

2

3

4

ξ

9

ξ

17

18

ξ

30

31

−255

−253

−509

−505

−765

−759

−1019

−1011

ξ

−2039

−2023

−2048

ξ

−2048

ξ

−2048

−126

ξ

00−2

00−1

0000

0001

0002

0003

0004

0005

00−5

00−3

0000

0003

0005

0007

0009

0011

00−9

00−5

0000

0005

0009

0013

0017

0021

0−15

00−9

0000

0009

0015

0021

0027

0033

00−19

00−11

00000

00011

00019

00027

00035

00043

ξ

00−39

00−23

00000

00023

00039

00055

00071

00087

00−45

00−27

00000

00027

00045

00063

00081

00099

ξ

00−85

00−51

00000

00051

00085

00119

00153

00187

00−89

00−53

00000

00053

00089

00125

00161

00197

ξ

0−149

00−89

00000

00089

00149

00209

00269

00329

0−155

00−93

00000

00093

00155

00217

00279

00341

ξ

0056

0057

0058

0059

0060

0113

0115

0117

0119

0121

0225

0229

0233

0237

0241

0339

0345

0351

0357

0363

00451

00459

00467

00475

00483

ξ

00903

00919

00935

00951

00967

01017

01035

01053

01071

01089

ξ

01921

01955

01989

02023

02047

02033

02047

ξ

02047

ξ

0125

0126

0127

0251

0253

0255

0501

0505

0509

0753

0759

0765

01003

01011

01019

ξ

02007

02023

02039

02047

ξ

02047

ξ

02047

NOTE – Reconstruction levels are symmetrical with respect to the sign of level except for 2047/−2048.

For INTRA blocks the first coefficient is nominally the transform dc value linearly quantized with a step size of 8 and no

dead-zone. The resulting values are represented with 8 bits. A nominally black block will give 0001 0000 and a nominally

white one 1110 1011. The code 0000 0000 is not used. The code 1000 0000 is not used, the reconstruction level of 1024

being coded as 1111 1111 (see Table 6).

Coefficients after the last non-zero one are not transmitted. EOB (end of block code) is always the last item in blocks for

which coefficients are transmitted.

4.3

Multipoint considerations

The following facilities are provided to support switched multipoint operation.

4.3.1 Freeze picture request

Causes the decoder to freeze its displayed picture until a freeze picture release signal is received or a timeout period of at

least six seconds has expired. The transmission of this signal is via external means (for example by Recommendation H.221).

4.3.2

Fast update request

Causes the encoder to encode its next picture in INTRA mode with coding parameters such as to avoid buffer overflow. The

transmission method for this signal is via external means (for example by Recommendation H.221).

20

Recommendation H.261 (03/93)

TABLE 6/H.261

Reconstruction levels for INTRA-mode dc coefficient

FLC

Reconstruction level into

inverse transform

0000 0001 (1)

0000 0010 (2)

0000 0011 (3)

ξ

0008

0016

0024

ξ

0111 1111 (127)

1111 1111 (255)

1000 0001 (129)

ξ

1016

1024

1032

ξ

1111 1101 (253)

1111 1110 (254)

2024

2032

NOTE – The decoded value corresponding to FLC “n” is 8n except FLC 255 gives 1024.

4.3.3

Freeze picture release

A signal from an encoder which has responded to a fast update request and allows a decoder to exit from its freeze picture

mode and display decoded pictures in the normal manner. This signal is transmitted by bit 3 of PTYPE (see 4.2.1) in the

picture header of the first picture coded in response to the fast update request.

5

Transmission coder

Bit rate

5.1

The transmission clock is provided externally (for example from an I.420 interface).

5.2

Video data buffering

The encoder must control its output bitstream to comply with the requirements of the hypothetical reference decoder defined

in Annex B.

When operating with CIF the number of bits created by coding any single picture must not exceed 256 Kbits. K=1024.

When operating with QCIF the number of bits created by coding any single picture must not exceed 64 Kbits.

In both the above cases the bit count includes the picture start code and all other data related to that picture including

PSPARE, GSPARE and MBA stuffing. The bit count does not include error correction framing bits, fill indicator (Fi), fill

bits or error correction parity information described in 5.4.

Video data must be provided on every valid clock cycle. This can be ensured by the use of either the fill bit indicator (Fi) and

subsequent fill all 1's bits in the error corrector block framing (see Figure 13) or MBA stuffing (see 4.2.3) or both.

Recommendation H.261 (03/93)

21

(S₁S₂S₃S₄S₅S₆S ₇S₈

)

= (00 011 01 1)

Tran smissio n order

S ₁

S ₂

S₃

S ₇

S ₈

T150 246 0-90/d0 8

S ₁

Da ta

Parity

18

1

493

Fi

1

Coded data

0

1

Fill (all “1”)

49 2

FIGURE 13/H.261

Error correcting frame

FIGURE 13/2H.261...[D08] = 9.5

5.3

Video coding delay

This item is included in this Recommendation because the video encoder and video decoder delays need to be known to allow

audio compensation delays to be fixed when H.261 is used to form part of a conversational service. This will allow lip

synchronization to be maintained. Annex C recommends a method by which the delay figures are established. Other delay

measurement methods may be used but they must be designed in a way to produce similar results to the method given in

Annex C.

5.4

Forward error correction for coded video signal

Error correcting code

5.4.1

The transmitted bitstream contains a BCH (511,493) forward error correction code. Use of this by the decoder is optional.

5.4.2

Generator polynomial

9

4

9

6

4

3

g(x) = (x + x + 1) (x + x + x + x + 1)

Example: For the input data of “01111 . . . 11” (493 bits) the resulting correction parity bits are

“011011010100011011” (18 bits).

5.4.3

Error correction framing

To allow the video data and error correction parity information to be identified by a decoder an error correction framing

pattern is included. This consists of a multiframe of eight frames, each frame comprising 1 bit framing, 1 bit fill indicator

(Fi), 492 bits of coded data (or fill all 1s) and 18 bits parity. The frame alignment pattern is:

(S S S S S S S S ) = (00011011).

1 2 3 4 5 6 7 8

See Figure 13 for the frame arrangement. The parity is calculated against the 493-bits including fill indicator (Fi).

The fill indicator (Fi) can be set to zero by an encoder. In this case only 492 consecutive fill bits (fill all 1s) plus parity are

sent and no coded data is transmitted. This may be used to meet the requirement in 5.2 to provide video data on every valid

clock cycle.

22

Recommendation H.261 (03/93)

5.4.4

Relock time for error corrector framing

Three consecutive error correction framing sequences (24 bits) should be received before frame lock is deemed to have been

achieved. The decoder should be designed such that frame lock will be re-established within 34 000 bits after an error

corrector framing phase change.

NOTE – This assumes that the video data does not contain three correctly phased emulations of the error correction framing

sequence during the relocking period.

Recommendation H.261 (03/93)

23

Annex A

Inverse transform accuracy specification

(This annex forms an integral part of this Recommendation)

A.1

Generate random integer pel data values in the range −L to +H according to the random number generator given

below (“C” version). Arrange into 8 by 8 blocks. Data set of 10 000 blocks should each be generated for (L = 256, H = 255),

(L = H = 5) and (L = H = 300).

A.2

For each 8 by 8 block, perform a separable, orthonormal, matrix multiply, forward discrete cosine transform using

at least 64-bit floating point accuracy.

7

1

F(u, v) =

C(u)C(v)

f (x, y) cos[π (2x + 1)u/16]cos[π (2y + 1) v/16]

∑ ∑

4

=

0

x

0

y

with

u, v, x, y = 0, 1, 2, . . .,7

where

x,y = spatial coordinates in the pel domain,

u,v = coordinates in the transform domain,

C(u) = 1/ 2 for u = 0; otherwise 1,

C(v) = 1/ 2 for v = 0; otherwise 1.

A.3

For each block, round the 64 resulting transformed coefficients to the nearest integer values. Then clip them to the

range –2048 to +2047. This is the 12-bit input data to the inverse transform.

A.4 For each 8 by 8 block of 12-bit data produced by A.3, perform a separable, orthonormal, matrix multiply, inverse

discrete transform (IDCT) using at least 64-bit floating point accuracy. Round the resulting pels to the nearest integer and clip

to the range −256 to +255. These blocks of 8 × 8 pels are the reference IDCT input data.

A.5

For each 8 by 8 block produced by A.3, apply the IDCT under test and clip the output to the range –256 to +255.

These blocks of 8 × 8 pels are the test IDCT output data.

A.6

For each of the 64 IDCT output pels, and for each of the 10,000 block data sets generated above, measure the peak,

mean and mean square error between the reference and the test data.

A.7 For any pel, the peak error should not exceed 1 in magnitude.

For any pel, the mean square error should not exceed 0.06.

Overall, the mean square error should not exceed 0.02.

For any pel, the mean error should not exceed 0.015 in magnitude.

Overall, the mean error should not exceed 0.0015 in magnitude.

A.8

A.9

All zeros in must produce all zeros out.

Re-run the measurements using exactly the same data values of A 1, but change the sign on each pel.

“C” program for random number generation

/* L and H must be long, that is 32 bits */

long rand

long

{

(L,H)

L,H;

static long randx = 1;

/* long is 32 bits */

static double z = (double) 0x7fffffff;

24

Recommendation H.261 (03/93)

long i,j;

double x;

/* double is 64 bits */

randx = (randx 1103515245) + 12345;

*

i = randx & 0x7ffffffe;

x = ( (double)i ) / z;

x * = (L+H+1);

j = x;

/* keep 30 bits */

/* range 0 to 0.99999 ... */

/* range 0 to < L+H+1 */

/* truncate to integer */

/* range −L to H */

return( j – L);

}

Annex B

Hypothetical reference decoder

(This annex forms an integral part of this Recommendation)

The hypothetical reference decoder (HRD) is defined as follows:

B.1 The HRD and the encoder have the same clock frequency as well as the same CIF rate, and are operated

synchronously.

B.2

The HRD receiving buffer size is (B + 256 kbits). The value of B is defined as follows:

B = 4R /29.97 where R is the maximum video bit rate to be used in the connection.

max

B.3

B.4

The HRD buffer is initially empty.

≈

The HRD buffer is examined at CIF intervals ( 33 ms). If at least one complete coded picture is in the buffer then

all the data for the earliest picture is instantaneously removed (e.g. at t

in Figure B.1). Immediately after removing the

n+1

above data the buffer occupancy must be less than B. This is a requirement on the coder output bitstream including coded

picture data and MBA stuffing but not error correction framing bits, fill indicator (Fi), fill bits or error correction parity

information described in 5.4.

To meet this requirement the number of bits for the (n+1)th coded picture d

must satisfy:

n+1

t _n

+

1

≥

+

−

R(t)dt

d_n

b_n

B

1

+

z

t _n

where

b is buffer occupancy just after the time t ;

n

t is the time the nth coded picture is removed from the HRD buffer;

n

R(t) is the video bit rate at the time t.

Recommendation H.261 (03/93)

25

HRD buffe r

occu pancy

(b it)

t_{n+ 1}

( )d

R t

t

t_n

d_n+1

B

b_n

b_n+1

Time

(CIF in terva l)

t _n

t _n+1

T 1 502 470- 90/d09

NOTE - Time (

-

) is aninteger number of CIF picture periods(1/29.97, 2/29.97, 3/29.97, ...).

t_n+1t_n

FIGURE B.1/H.261

HRDbuffer occupancy

FIGURE B.1/H.261...[D09] = 9CM

Annex C

Codec delay measurement method

(This annex forms an integral part of this Recommendation)

The video encoder and video decoder delays will vary depending on implementation. The delay will also depend on the

picture format (QCIF, CIF) and data rate in use. This annex specifies the method by which the delay figures are established

for a particular design. To allow correct audio delay compensation the overall video delay needs to be established from a user

perception point of view under typical viewing conditions.

Point A is the video input to the video coder. Point B is the channel output from the video terminal (i.e. including any FEC,

channel framing, etc.). Point C is the video output from the decoder.

A video sequence lasting more than 100 seconds is connected to the video coder input (point A) in Figure C.1 above. The

video sequence should have the following characteristics:

–

it should contain a typical moving scene consistent with the intended purpose of the video codec;

it should produce a minimum coded picture rate of 7.5 Hz at the bit rate in use;

it should contain a visible identification mark at intervals throughout the length of the sequence. The visible

identification should change every 97 video input frames and be located within the picture area represented by

the first GOB in the picture. For example, the first block in the picture could change from black to white at

intervals of 97 video frame periods. The identification mark should be chosen so that it can be detected at

point B and does not significantly contribute to the overall coding performance.

The codec and video sequence should be arranged so that the bitstream contains less than 10% stuffing (MBA stuffing +

error correction fill bits).

The encoder delay is obtained by measuring the time from when the visible identification changes at point A to the time that

the change is detected at point B. Similarly, the decoder delay is obtained by taking measurements at points B and C.

26

Recommendation H.261 (03/93)

Several measurements should be made during the sequence length and the average period obtained. Several tests should be

made to ensure that a consistent average figure can be obtained for both encoder and decoder delay times.

Average results should be obtained for each combination of picture format and bit rate within the capability of the particular

codec design.

NOTE – Due to pre- and post-temporal processing it may be necessary to take a mid-level for establishing the transition of the

identification mark at points B and C.

B

Video

co der

V id eo

de cod er

A

C

T1 50248 0-90/d10

FIGURE C.1/H.261

Measuring points

FIGURE C.1/H.261...[D10] = 6CM

Annex D

Still image transmission

(This annex forms an integrat part of this Recommendation)

D.1

Introduction

This annex describes the procedure for transmitting still images within the framework of this Recommendation. This

procedure enables an H.261 video coder to transmit still images at four times the normal video resolution by temporarily

stopping the motion video. Administrations may use this optional procedure as a simple and inexpensive method to transmit

still images. However, Recommendation T.81 (JPEG) is preferred when the procedures for using T.81 within audiovisual

systems are standardized.

This procedure can provide high quality image transmission with effects similar to those of progressive and hierarchical

schemes. Minimal changes to H.261 (low cost), backward compatibility with existing terminals, and flexibility in image

quality versus transmission speed were the key considerations in its development.

NOTE – The encoder would set a previously unused bit in PTYPE to “0” when it transmits a still image (unused bits should be

set to “1”). A decoder that ignores this bit would receive the image as normal video. A decoder that goes into an error condition when this

bit is “0” would most likely freeze the previous video frame, and resume when this bit is reset to “1”. A decoder having this new capability

could display the image in a higher resolution, transfer the image to a separate graphics display and hold the image when video resumes,

print and/or save the image, etc.

D.2

Still image format

The still image format is four times the currently transmitted video format. If the video format is QCIF, then the still image is

a CIF frame. If the video format is CIF, which contains 352 × 288 luminance samples, then the still image contains 704 × 576

luminance samples, and a corresponding increase in the number of chrominance samples (a CCIR-601 frame).

Recommendation H.261 (03/93)

27

For transmission using H.261, the still image is sub-sampled 2:1 horizontally and vertically into four sub-images in

the currently transmitted video format. Figure D.1 shows the sub-sampling pattern on the still image. The samples labelled 0,

1, 2 and 3 form the four sub-images 0, 1, 2 and 3, respectively.

0

1

0

1

0

1

3

2

3

2

3

2

0

1

0

1

0

1

3

2

3

2

3

2

0

1

0

1

0

1

3

2

3

2

3

2

0

1

0

1

0

1

3

2

3

2

3

2

FIGURE D.1/H.261

Sub-sampling pattern

D.3

Picture layer multiplex

When HI_RES is “0”, the two lower bits of the temporal reference (TR) identify one of the four sub-images 0, 1, 2 or 3. The

three higher bits of the TR shall be set to “0”.

The encoder transmits a still image by setting HI_RES to “0” and transmitting the four sub-images 0, 1, 2 and 3 in sequential

order. It is allowed to transmit more than one frame for each sub-image, but should not go back once it starts transmitting the

next sub-image. The encoder is allowed to resume motion video at any time by setting HI_RES back to “1”.

NOTE – The reference memory for the current frame is always the previous frame, regardless of whether a frame is motion

video or still image.

D.4

Multipoint considerations

A still image transmitted within the video bit-stream can be broadcast on a multipoint connection by broadcasting the video.

The MCV (multipoint command visualization-forcing) and Cancel-MCV commands defined in Recommendation H.230

provide for this capability. A terminal could force an MCU to broadcast its video by sending MCV, and then return to the

previous mode of operation by sending Cancel-MCV. MCUs are required to implement these commands, but they are

optional for terminals.

D.5

Other considerations

–

All the video coding modes are allowed (intra-frame, inter-frame, motion compensation, etc.);

the multiplex arrangement below the picture layer remains the same (group of blocks, macroblocks, etc.);

the maximum number of bits allowed per frame (sub-image) should not be exceeded (256 Kbits for CIF and 64

Kbits for QCIF);

–

forward error correction is not affected.

28

Recommendation H.261 (03/93)


型号：	H.261
厂家：	ETC
描述：	LINE TRANSMISSION OF NON-TELEPHONE SIGNALS 非电话信号线传输电话
文件：	总31页 (文件大小：125K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

H.261 [ETC]

相关型号：

H.DI-0420R-100

H.DI-0420R-150

H.DI-0420R-1R5

H.DI-0420R-220

H.DI-0420R-2R2

H.DI-0420R-6R8

H.DI-0520-100

H.DI-0520-101

H.DI-0520-150

H.DI-0520-1R2

H.DI-0520-220

H.DI-0520-2R2