SIST ETS 300 142 E1:2003

Integrated Services Digital Network (ISDN) and other digital telecommunications networks; Line transmission of non-telephone signals; Video codec for audiovisual services at p x 64 kbit/s [ITU-T Recommendation H.261 (1993), modified]

see title

Digitalno omrežje z integriranimi storitvami (ISDN) in druga digitalna telekomunikacijska omrežja – Vodovni prenos netelefonskih signalov – Video kodek za avdiovizualne storitve za hitrosti p x 64 kbit/s [priporočilo ITU-T H.261 (1993), spremenjeno]

General Information

• Status: Published
• Publication Date: 30-Nov-2003
• ICS: 33.080 - Integrated Services Digital Network (ISDN)
• Technical Committee: SS EIT - Technical Board for the electrotechnical, information technology and telecommunications fields
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 01-Dec-2003
• Due Date: 01-Dec-2003
• Completion Date: 01-Dec-2003
• Ref Project: ETSI ETS 300 142 ed.1 (1998-06) - Integrated Services Digital Network (ISDN) and other digital telecommunications networks; Line transmission of non-telephone signals; Video codec for audiovisual services at p x 64 kbit/s [ITU-T Recommendation H.261 (1993), modified]

Standards Content (Sample)

SLOVENSKI STANDARD
SIST ETS 300 142 E1:2003
01-december-2003
'LJLWDOQRRPUHåMH]LQWHJULUDQLPLVWRULWYDPL,6'1LQGUXJDGLJLWDOQD
WHOHNRPXQLNDFLMVNDRPUHåMD±9RGRYQLSUHQRVQHWHOHIRQVNLKVLJQDORY±9LGHR
NRGHN]DDYGLRYL]XDOQHVWRULWYH]DKLWURVWLS[NELWV>SULSRURþLOR,787+
VSUHPHQMHQR@
Integrated Services Digital Network (ISDN) and other digital telecommunications
networks; Line transmission of non-telephone signals; Video codec for audiovisual
services at p x 64 kbit/s [ITU-T Recommendation H.261 (1993), modified]
Ta slovenski standard je istoveten z: ETS 300 142 Edition 1
ICS:
33.080 Digitalno omrežje z Integrated Services Digital
integriranimi storitvami Network (ISDN)
(ISDN)
SIST ETS 300 142 E1:2003 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST ETS 300 142 E1:2003

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SIST ETS 300 142 E1:2003
EUROPEAN ETS 300 142
TELECOMMUNICATION June 1998
STANDARD
Source: NA Reference: DE/NA-051108
ICS: 33.020
Key words: Multichannel, codec, teleservice, ISDN, video
Integrated Services Digital Network (ISDN) and
other digital telecommunications networks;
Line transmission of non-telephone signals;
Video codec for audiovisual services at p x 64 kbit/s
[ITU-T Recommendation H.261 (1993), modified]
ETSI
European Telecommunications Standards Institute
ETSI Secretariat
Postal address: F-06921 Sophia Antipolis CEDEX - FRANCE
Office address: 650 Route des Lucioles - Sophia Antipolis - Valbonne - FRANCE
Internet: [e-mail] - http://www.etsi.fr - http://www.etsi.org
Tel.: [phone] - Fax: [phone]
Copyright Notification: No part may be reproduced except as authorized by written permission. The copyright and the
foregoing restriction extend to reproduction in all media.
© European Telecommunications Standards Institute 1998. All rights reserved.

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ETS 300 142: June 1998
Whilst every care has been taken in the preparation and publication of this document, errors in content,
typographical or otherwise, may occur. If you have comments concerning its accuracy, please write to
"ETSI Standards Making Support Dept." at the address shown on the title page.

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ETS 300 142: June 1998
Foreword
This European Telecommunication Standard (ETS) has been produced by the Network Aspects (NA)
Technical Committee of the European Telecommunications Standards Institute (ETSI).
Transposition dates
Date of adoption of this ETS: 19 June 1998
Date of latest announcement of this ETS (doa): 30 September 1998
Date of latest publication of new National Standard
or endorsement of this ETS (dop/e): 31 March 1999
Date of withdrawal of any conflicting National Standard (dow): 31 March 1999
Endorsement notice
The text of ITU-T Recommendation H.261 (1993) was approved by ETSI as an ETS with agreed
modifications as given below.
NOTE: New or modified text is indicated using sidebars. In addition, underlining and/or strike-
out are used to highlight detailed modifications where necessary.

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ETS 300 142: June 1998
Page 1
Replace the scope clause with the following two clauses (scope and normative references):
1      Scope
This ETS describes the video coding and decoding methods for the moving picture component of
audiovisual services at the rates of p x 64 kbit/s, where p is in the range 1 to 30.
The standard includes conformance test specifications.
Normative references
This ETS incorporates by dated and undated reference, provisions from other publications. These
normative references are cited at the appropriate places in the text and the publications are listed
hereafter. For dated references, subsequent amendments to or revisions of any of these publications
apply to this ETS only when incorporated in it by amendments or revision. For undated references the
latest edition of the publication referred to applies.
[1]                   ITU-T Recommendation H.120: "Codecs for videoconferencing using primary
digital group transmission".
[2]                   ITU-T Recommendation H.200: "Framework for Recommendations for
audiovisual services".
[3]                   ITU-T Recommendation H.221: "Frame structure for a 64 to 1 920 kbit/s
channel in audiovisual teleservices".
[4]                   ITU-T Recommendation H.230: "Frame-synchronous control and indication
signals for audiovisual systems".
[5]                   ITU-T Recommendation H.242: "System for establishing communication
between audiovisual terminals using digital channels up to 2 Mbit/s".
[6]                   ITU-T Recommendation T.81: "Information technology - Digital compression
and coding of continuous-tone still images - Requirements and guidelines".
Page 19, subclause 5.2, second and third paragraphs
When operating with CIF the number of bits created by coding any single picture shall not exceed
256 kbits (K = 1 024).
When operating with QCIF the number of bits created by coding any single picture shall not exceed
64 kbits (K = 1 024).
Page 25, clause D.5, third bullet
– the maximum number of bits allowed per frame (sub-image) should not be exceeded
(256 kbit/s for CIF and 64 kbit/s for QCIF, where K is equal to 1 024);
Addition of annex ZA
Add a new annex ZA as follows:

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ETS 300 142: June 1998
Annex ZA (normative): Conformance testing of ITU-T Recommendation H.261
(March 1993)
ZA.1 General principles adopted for the ITU-T Recommendation H.261
conformance tests
1) The conformance test should aim to test for compliance to the respective ETS to the extent which it
is necessary to ensure compatible interworking of different suppliers equipment.
2) The only inputs and outputs of the equipment under test in the conformance verification are those
normally provided to the user and the test shall not rely on directly accessing additional internal test
modes in the equipment.
3) The specification is constrained to only testing the ITU-T Recommendation H.261 bitstream which
can be assumed to have been correctly isolated from the ITU-T Recommendation H.221 framing,
audio, etc.
4) The tests should be constructed to allow for devices which only have visual inputs and outputs (e.g.
a video phone). The existence of composite (or other) electrical interfaces cannot be assumed.
5) If electrical video inputs and outputs do exist (e.g. composite interfaces) then they should be fully
tested in respect of transmitted/received active picture area, etc.
6) The testing of decoders is likely to be more exhaustive than testing of the encoders due to the fact
that decoders will always have a digital channel input. Assuming that the encoder and decoder of
the same manufacturer perform well together, then advantages will be taken of sophisticated
decoder tests. An indication of the correct behaviour of the decoder, will likely implicate that the
complementary part in the encoder is correct as well.
7) Examples of input sequences (video stimulus etc.) will be given as annexes to the conformance
specification. These examples will be given to aid the construction of conformance test suites.
Tests will not be constrained to only using these examples.
Table ZA.1: List of tests
Items Encoder Decoder
1 Video levels E D
2 Geometrical distortion E
3 Frame rates E D
4 QCIF support E D
5 Valid motion vectors E D
6 Loop filter D
7 Inverse DCT D
8 Clipping D
9 Forced updating E
10 Video multiplex E
11 Video demultiplex D
12 Scanning order D
13 Freeze picture D
14 Fast update request E
15 Maximum Bits per picture E D
16 Error correction E D
17 FEC stuffing (optional) E D
18 Arithmetic loop conformance D
19 HRD compliance E D
20 Extra Insertion Information E D

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ZA.2 Description of tests
ZA.2.1 Video levels (ITU-T Recommendation H.261 § 3.1)
PURPOSE:
As the way to produce digital input signals is not a part of ITU-T Recommendation H.261, the only
purpose of this test is to verify that the encoder supports input values from 1 to 254 for luminance and the
two colour difference components. This test can only be performed when digital I/O are available.
A complete test of the video levels should take place in a complete system quality conformance (black
level/white level/linearity).
ZA.2.1.1 Encoder
METHOD:
Connect a digital source pattern which contains all the permitted values to the encoder under test and
connect the output of the encoder under test to the reference decoder.
RESULTS:
Check the pictures decoded with the reference decoder. Inputs from black to white shall imply outputs to
black to white.
ZA.2.1.2 Decoder
METHOD:
Encode the same digital source pattern with the reference encoder and decode the produced bitstream
with the decoder under test. Digital source pattern: uniform picture with programmable values Y, CB and
CR from 1 to 254.
RESULTS:
Check the pictures decoded with the decoder under test. Pattern values corresponding to black to white
shall imply outputs to black to white. Value 128 shall give grey picture.
ZA.2.2 Geometrical Distortion (ITU-T Recommendation H.261 § 3.1)
ZA.2.2.1 Encoder
PURPOSE:
A CIF encoder shall take luminance and colour difference samples in accordance with the requirements of
ITU-T Recommendation H.261 § 3.2 and shall maintain an aspect ratio of 4:3. The purpose of this test is
to verify that sampling and aspect ratio.
METHOD:
If the encoder under test is capable of operating at CIF, then, with the encoder operating at that resolution,
provide an image containing a square, circle and cross all centred on the centre of the image (see
figure ZA.1). Connect the output of the encoder under test to the reference decoder.
RESULT:
Check the pictures decoded with the reference decoder. Verify that there has been no geometrical
distortion of the image.

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Figure ZA.1
ZA.2.3 Frame rates (ITU-T Recommendation H.261 § 3.1)
ZA.2.3.1 Encoder
PURPOSE:
An encoder shall control its output bitstream to comply with the requirements of the Hypothetical
Reference Decoder (HRD) defined in ITU-T Recommendation H.261, annex B.
In ITU-T Recommendation H.261 § 3.1 is stated: "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
ITU-T Recommendation H.221)".
The purpose of the test is to verify that an encoder never sends more pictures per second than signalled
by the decoder.
METHOD:
At the encoder provide an image sequence which contains the following, low detail stills, long periods of
high motion, panning, zooming, scene cuts, and high detail stills. The output bitstream
(ITU-T Recommendation H.261) is stored for off-line analysis. A minimum of 55 minutes of picture date
should be stored.
NOTE: To reduce storage requirements the following could be stored instead:
- accurate time stamps for the Picture Start Codes (PSCs);
- temporal reference count;
- the number of bits between each PSC.
All allowable bit rates which are within the encoders declared range should be tested.
Input this video sequence to an encoder. Signal to the encoder the decoder capabilities of 0, 1, 2, 3 non
transmitted pictures between transmitted ones. Supervise the Temporal Reference in the picture layer.
Supervise the hypothetical buffer verifier. Perform the test for QCIF and CIF (if the encoder claims to
encode also CIF).

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RESULTS:
An encoder can only be said to be compliant if the mean rate of Picture Start Codes is
29,97 Hz – 50 ppm and it satisfies the HRD.
a) The exchange of decoder capabilities (ITU-T Recommendations H.221 and H.242) results in
allowed values for the Temporal Reference (TR) in the picture layer.
The allowed maximum encoder picture rate is (29,97 Hz – 50 ppm) / (n+1); n = 0, 1, 2, 3.
Then the corresponding modulo 32 difference between an actual TR and the TR of the last
encoded picture can have all integer values between n+1, n+2, . 31.
b) In all cases the buffer of the Hypothetical Reference Decoder (ITU-T Recommendation H.261,
annex B) shall not overflow (the data stream shall conform to the hypothetical buffer verifier).
EXAMPLE: Signalled is n = 2; then the maximum allowed picture rate is 29,97 Hz divided by
3 (approximately 10 Hz). The modulo 32 difference between an actual TR and
the TR of the last received picture can have all values between 3 and 31.
ZA.2.3.2 Decoder
PURPOSE:
Purpose of the test is to verify that a decoder which claims to have capability to decode sequences with
encoded picture rates of:
(29,97 Hz – 50 ppm) / (n+1); n = 0, 1, 2, 3
really does it for all picture material.
METHOD:
Input a data stream conforming to ITU-T Recommendation H.261 describing a video sequence (as the
one used in subclause ZA.2.3.1) to a decoder at the lowest claimed value of n for at least 55 minutes.
Care should be taken to ensure the data stream describes a sequence encoded at the claimed value of n
(i.e. there is no picture dropping).
RESULTS:
The decoded pictures should be the same as on the reference decoder.
EXAMPLE: Most critical should be, e.g. the following sequence:
- a synthetic picture sequence consisting of pictures with diagonal stripes
(high saturation, diagonal frequency corresponding to approx. 1 MHz), the
stripes being unmoved for some time and then being moved with 1 pel
per encoded frame both horizontally and vertically.

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ZA.2.4 QCIF Support (ITU-T Recommendation H.261 § 3.1)
PURPOSE:
The ITU-T Recommendation H.261 indicates that two formats are allowed as input to the video coder: the
Common Intermediate Format (CIF) and another format known as Quarter-CIF (QCIF) having half the
number of pel and half the number of lines of the former. 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 144 lines, 176 pixels per line, orthogonal.
ITU-T Recommendation H.261 also specifies that the picture area covered by this number of pixels and
lines has an aspect ratio of 4:3. In addition ITU-T Recommendation H.261 also states that all codecs shall
be able to operate using QCIF. This statement prevents the possibility for an encoder to always transmit
the entire picture in the CIF form at and leaving to a decoder, only able to operate on QCIF, the task of
selecting from the video stream the portion corresponding to its need. From this consideration the need
arises of testing the QCIF support for both encoder and decoder. In fact when an encoder operates with
the QCIF format the decoder shall be able to show a correct picture and conversely if a decoder is only
supporting the QCIF the encoder shall be able to transmit a coded bitstream containing only the
information related to a QCIF picture.
ZA.2.4.1 Encoder
METHOD:
The encoder is set in QCIF mode and the video encoder output is connected to a testing system
implementing a reference decoder (either real time or not) enriched by the capability of analysing the
bitstream at the Group Of Block (GOB) layer and at the picture layer. Bit 4 of the type information in the
picture layer (PTYPE) may only have the value of 0. The range of permitted values for the Group Number
(GN) is limited to 1, 3, 5. Other values of PTYPE or GN are not allowed.
Other features of the coded and displayed images shall be checked through the conformance testing
involving the active picture area and the block structure.
RESULTS:
If analyzing the values of the GN contained in the coded bitstream, the presence of values different from
1, 3 or 5 is detected, this certainly reveals a non correct coding of the QCIF picture.
If the image obtained by a reference decoder which is decoding any image coming from the coder under
test, doesn't appear as a rectangle and/or it doesn't have an aspect ratio of 4:3, this again emphasizes
that the encoder is not correctly handling the QCIF picture.
ZA.2.4.2 Decoder
METHOD:
A suitable test pattern (e.g. a regular sequence of black MBs interlaced with white MBs), exactly covering
an entire QCIF image, is transmitted by a reference encoder or by a test pattern generator to the decoder
under test and a visual verification is performed.
The verification concerns the MBs which have been reconstructed. Their number shall exactly correspond
to the number of MBs contained in a QCIF picture, while concerning content and position of each MB
reference should be made to tests of those specific topics.
RESULTS:
The regular pattern transmitted by the reference encoder shall be reconstructed by the decoder under test
with the same number of MBs (11 MBs per line, 9 MBs per row) forming a rectangle having an aspect
ratio of 4:3.

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ZA.2.5 Valid motion vectors (ITU-T Recommendation H.261 § 3.2.2)
ZA.2.5.1 Encoder
PURPOSE:
Differential motion vector test.
METHOD:
Input a sequence with increasing motion from left to right within GOB's.
RESULTS:
The MV's resulting from the differential MV's should not exceed – 15.
PURPOSE:
The motion estimation/motion vector range of the encoder.
METHOD:
The test input should include test of all 4 directions, that is a white macro-block moving on a dark
background should enter and leave through the left, right, top and bottom edge of the picture. A criteria for
when a vector is pointing outside the picture can be as follows:
- monitor the produced bitstream and extract the motion vectors, summing differential values to
obtain absolute vectors. Eight different restrictions applies to vector magnitudes depending on
which MB the vector belongs to, see figure ZA.2.
1: All MB along picture left edge
53 6 2: All MB along picture right edge
3: All MB along picture top edge
4: All MB along picture bottom edge
10 2 5: Top left corner MB
6: Top right corner MB
7: Bottom left corner MB
74 8 8: Bottom right corner MB
0: All other, no restrictions
Restrictions:
1: x ‡ 0 5: x ‡ 0, y ‡ 0
2: x £ 0 6: x £ 0, y ‡ 0
3: y ‡ 0 7: x ‡ 0, y £ 0
4: y £ 0 8: x £ 0, y £ 0
Figure ZA.2
RESULTS:
The motion estimater should never produce vectors pointing outside the active pixel area. Fail test if pels
are referenced outside the coded picture area.

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PURPOSE:
Check reset of MV's:
The motion vector data that is transmitted is a differential vector except for three cases:
- mb 1, 12 and 23;
- MBA is not equal to 1. For the first transmitted macro block in GOB the Macro Block Address (MBA)
is an absolute address. For subsequent MB's it is a differential address;
- MTYPE of the previous block was not MC.
The purpose of the test is to verify this reset in the encoder motion vector data range which is between
+15 and -15.
METHOD:
Input several sequences and monitor the output bitstream.
RESULTS:
Fail test if a vector exceeds +15 or -15 for any macro block which mb = 1, 12 or 23, MBA not equal to 1 or
MTYPE the previous macro block was not MC.
ZA.2.5.2 Decoder
PURPOSE:
Decoder motion vector range.
METHOD:
Move (a part of) a scene around by supplying motion vectors, no transform coefficients and filter off to the
decoder. For both components of the motion vector, values of -15, 0 and +15 should be used, giving 8
possible combinations/directions.
RESULTS:
The moved part of the scene should stay intact.
PURPOSE:
Decoder loop test: motion compensation.
METHOD:
Input a bitstream representing a frame with two grey blocks on a black background; then move these
blocks around using motion vectors, no coefficients and filter off such that they overlap exactly after a
number of movements.
RESULTS:
If the blocks match exactly, the motion compensation is said to pass.
PURPOSE:
Decoder loop test: motion compensation reset of differential MV's.

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METHOD:
Input a bitstream representing a test pattern. Then input a bitstream with critical conditions for the reset of
the differential motion vectors.
The motion vector data that is transmitted is a differential vector except for three cases:
- mb 1, 12 and 23;
- MBA is not equal to 1. For the first transmitted macro block in GOB the Macro Block Address (MBA)
is an absolute address. For subsequent MB's it is a differential address;
- MTYPE of the previous block was not MC.
The purpose of the test is to verify this reset in the decoder motion vector data range which is between
+15 and -15.
RESULTS:
The test pattern should stay intact.
ZA.2.6 Loop filter (ITU-T Recommendation H.261 § 3.2.3)
ZA.2.6.1 Decoder
PURPOSE:
The loop filter is part of the prediction of the hybrid coding scheme specified in
ITU-T Recommendation H.261. It can be applied to all inter-MC MBs regardless of the value of the motion
vector and is controlled on a MB basis by the MB type "MTYPE". For conformance testing the following
items should be checked:
- controllability of the filter on MB basis;
- characteristic of the two-dimensional filter;
- precision of the arithmetic of the filter.
REMARK:
The only interfaces available at all ITU-T Recommendation H.261 video decoders are the line interface
and the monitor for displaying the received and decoded pictures. Methods for testing the conformance to
ITU-T Recommendation H.261 shall make use of these two interfaces. A compensation method with
visible results is proposed for the test of the loop filter. This method shows the controllability and the
principal characteristic of the filter but is not able to provide exact information about the precision of the
arithmetic of the filter.
The principal process for testing the loop filter in the decoder is demonstrated with a single block. The
method can easily be adapted to the real needs of a conformance test.
METHOD:
The test of the filter starts with the decoding of a intra-coded block comprising only a DC value and one
big AC coefficient in the middle horizontal frequency region (see figure ZA.3).
RESULTS:
This results in a basic picture of the IDCT showing a vertical wave, clearly visible on the screen of the
monitor (see figure ZA.4).

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METHOD:
In the next step this block is processed by the filter in the loop signalling a MB type INTER+MC+FILTER
(TCOEFF = O; MV = O,O). The block is modified only by the filter in the loop.
RESULTS:
The lights and darks of the wave are dimmed, the block becomes more grey (see figure ZA.5).
METHOD:
This dimmed wave is suppressed by an opposite coded block in the following step. Therefore, all pixel
values of the dimmed wave are calculated and the discrete cosine transform is performed on it. Inverting
the signs of all AC-coefficients results in an inverse pattern (see figures ZA.6 and ZA.7).
The inverse AC-pattern is provided to the decoder as an INTER-coded block.
RESULTS:
The result of the decoding of the inverse AC-pattern and the addition to the predicted block is a grey block
without any visible pattern. The structure of the predicted block has been compensated by the opposite
structure of an inter-coded block, which incorporates the effect of the filter process of the filter in the loop
(see figure ZA.8).
Filters violating the standard will result in a visible pattern because of the non complete compensation by
the "inverse" block.
This described the principal method for testing single elements of the decoding loop. More complex block
patterns can be used for gaining more significant results.
f
x x
x
f
y y
y
figure ZA.3 figure ZA.4 figure ZA.5
f x x
x
f
y y y
figure ZA.6 figure ZA.7 figure ZA.8

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ZA.2.7 Inverse DCT (ITU-T Recommendation H.261 § 3.2.4)
ZA.2.7.1 Decoder
PURPOSE:
ITU-T Recommendation H.261 specifies the usage of an 8 x 8 inverse DCT for the transformation of the
transmitted coefficients into the spatial domain.
The purpose of this test is to ensure that an 8 x 8 inverse DCT is used for processing the transmitted
coefficients. This test is closely related to the test of the loop filter performance and the IDCT mismatch.
METHOD:
The test involves a coded bitstream according to ITU-T Recommendation H.261 as stimulus and the
decoded and displayed picture as measure. Only rough estimations about the compliance can be
obtained.
For getting visible results with good confidence the basic pattern of an IDCT produced by a particular
coefficient can be applied. Each basic pattern shows the picture produced by one of the 64 coefficient of
the IDCT; this test being performed using intra mode.
RESULTS:
The 64 basic patterns of the tested IDCT are visually compared with the result of a reference IDCT. The
pictures of the tested device and the reference should be looking similarly though differences might be
caused by the optional post processing and the display.
ZA.2.8 Clipping (ITU-T Recommendation H.261 § 3.2.6)
ZA.2.8.1 Decoder
PURPOSE:
Check that in the decoder loop a clipping function is applied to the reconstructed pictures.
METHOD:
A test bitstream is sent to the decoder under test.
The bitstream contains a saturated green (respectively purple) uniform picture encoded in INTRA mode,
which DC = 00000001 (respectively DC = 1111 1110), followed by several frames encoded in INTER
mode in which the DC component is equal to a programmable negative (respectively positive) value.
RESULTS:
The decoded pictures shall always be saturated normally-green (respectively normally-purple).
(Else: error message: no valid clipping).

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ZA.2.9 Forced updating (ITU-T Recommendation H.261 § 3.4)
ZA.2.9.1 Encoder
PURPOSE:
ITU-T Recommendation H.261 specifies that a MB should be forcibly updated at least once per every 132
times it is transmitted, in order to control the accumulation of the inverse transform mismatch error. This
function is achieved by forcing the use of the INTRA mode of the coding algorithm.
An easy and convenient way by which the updating can be implemented is through the use of a regular
pattern of intra coded MBs, but it is also possible to use a free scheme taking into account the specific
need of each MB. Consequently, the recommendation does not define any particular update pattern.
The aim of this conformance testing is to verify that the encoder satisfies this request of updating. Several
possibilities exist on how to get information on this encoder capability. In the following a method using an
off-line analysis of the bitstream will be described.
METHOD:
The application of this method is constrained by the recording capability of the testing system. If the
analysis of a longer bitstream is needed, real time testing is required.
The starting point for this analysis is the availability of the ITU-T Recommendation H.261 bitstream alone
either directly provided by the encoder or obtained from a protocol tester applied to the
ITU-T Recommendation H.221 output of the encoder under test.
Moving input material, like a panning sequence, should be analysed to ensure the occurrence of the event
of updating for each MB. This output from the encoder shall be recorded (e.g. on a PC hard disk) and later
used as input to a software programme performing the actions described hereafter:
- a byte variable is assigned to each Macro Block (MB) (396 for CIF image format or 99 for QCIF)
and at the start these variables are initialized to zero in order to be used afterwards as counters;
- a Boolean variable for each MB is initialized to FALSE and it will be used to have an indication
about the test significance;
- the bitstream is parsed identifying the header of each transmitted MB. For each one of them an
index referring to the counters is assigned derived from the value of the Group Number (GN) and
from the value of the Macro Block Address (MBA). This index ranges from 1 to 396 for CIF pictures
and from 1 to 99 for QCIF pictures;
- for each MB the associated MTYPE information is also extracted;
- each time MTYPE is INTRA the counter corresponding to the MB is reset, and the associated
Boolean variable becomes TRUE;
- otherwise the value of the counter is incremented by one and afterwards the counter value is
compared to 132; if a value greater or equal to 132 is detected an error flag is set;
- this sequence of operations is performed until the bitstream has been completely parsed.
RESULTS:
The pass condition is obtained when no error flags have been set during the parsing of the whole
bitstream. On the contrary if an error message has been produced, this means that a MB has been inter
frame transmitted (even just motion compensated) for more than 132 times without the requested refresh.
At the end of the test the Boolean
...