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.nr LL 40.5P
.nr ll 40.5P
.nr HM 3P
.nr FM 6P
.nr PO 4P
.nr PD 9p
.po 4P

.rs
\v | 5i'
.LP
\fB2\fR \fBCodecs not requiring separate television standards conversion 
when used on interregional connections\fR 
.sp 1P
.RT
.LP
	\fBA codec for 525\(hyline, 60 fields/s and 1544 kbit/s transmission
for intra\(hyregional use\fR \fBand capable of interworking with the codec of
\(sc 1\fR 
.sp 2P
.LP
2.1
	\fIIntroduction\fR 
.sp 1P
.RT
.PP
Section 2 indicates the changes and additions which must be made
to the text of \(sc\ 1 in order to define the version of the codec for use with
525\(hyline, 60\ fields/s television standards and transmission at 1544\ 
kbit/s. The two versions are capable of interworking via a re\(hymultiplexing 
unit which can convert the Recommendation\ G.704, \(sc\ 2.1 compatible 
frame structure on one side to the Recommendation\ G.704 , \(sc\ 2.3 compatible 
frame structure (with 6\ time 
slots empty) on the other side.
.PP
The two versions of the codec are identical in most respects, the
important differences (apart from the obvious ones arising from different 
input and output signals) being confined to the digital pre\(hy and post\(hyfilters 
and the signals for the control of the buffers. Moreover, the detailed 
algorithms of 
the pre\(hy and post\(hyfilters do not need to be specified to permit interworking. 
Only an outline of their mode of operation together with the few necessary 
specifications are therefore provided.
.RT
.sp 2P
.LP
2.2\fR 
	\fIBrief specification\fR 
.sp 1P
.RT
.sp 1P
.LP
2.2.1 
	\fIVideo input/output\fR 
.sp 9p
.RT
.PP
The video input and output are standard 525\(hyline, 60\ fields/s
colour or monochrome television signals. The colour signals are in component
form. Colour and monochrome operation are fully compatible.
.RT
.sp 1P
.LP
2.2.2
	\fIDigital output/input\fR 
.sp 9p
.RT
.PP
The digital output and input are at 1544\ kbit/s, compatible with
the frame structure of Recommendation\ G.704.
.RT
.sp 1P
.LP
2.2.3 
	\fISampling frequency\fR 
.sp 9p
.RT
.PP
The video sampling frequency and 1544\ kbit/s network clock are
asynchronous.
.RT
.sp 1P
.LP
2.2.4
	\fICoding techniques\fR 
.sp 9p
.RT
.PP
Conditional replenishment coding supplemented by adaptive digital filtering, 
differential PCM and variable\(hylength coding are used to achieve low 
bit\(hyrate transmission. 
.RT
.sp 1P
.LP
2.2.5
	\fIAudio channel\fR 
.sp 9p
.RT
.PP
An audio channel using 64 kbit/s is included. At present, coding is A\(hylaw 
according to Recommendation\ G.711, but provision is made for future use 
of more efficient coding. 
.RT
.sp 1P
.LP
2.2.6
	\fIMode of operation\fR 
.sp 9p
.RT
.PP
The normal mode of operation is full duplex.
.RT
.sp 1P
.LP
2.2.7
	\fICodec\(hyto\(hynetwork signalling\fR 
.sp 9p
.RT
.PP
An optional channel for codec\(hyto\(hynetwork signalling is
included.
.RT
.sp 1P
.LP
2.2.8
	\fIData channels\fR 
.sp 9p
.RT
.PP
Optional 2 \(mu 64 kbit/s and 1 \(mu 32 kbit/s data channels are
available. These are used for video if not required for data.
.bp
.RT
.sp 1P
.LP
2.2.9
	\fIForward error correction\fR 
.sp 9p
.RT
.PP
Optional forward error correction is available. This is required
only if the long\(hyterm error rate of the channel is worse than\ 1 in\ 
10\u6\d. 
.RT
.sp 1P
.LP
2.2.10
	\fIAdditional facilities\fR 
.sp 9p
.RT
.PP
Provision is made in the digital frame structure for the future
introduction of encryption, a graphic mode and multipoint facilities.
.RT
.PP
2.2.11
When the coder buffer is empty and the decoder buffer full, the
coder delay is 31\ \(+-\ 5\ ms and the decoder delay is 176\ \(+-\ 31\ ms
.FS
These are
typical figures. The delays depend upon the detailed implementation
used.
.FE
.
.sp 9p
.RT
.sp 1P
.LP
2.3
	\fIVideo interface\fR 
.sp 9p
.RT
.PP
The normal video input is a 525\(hyline, 60\ fields/s signal in
accordance with CCIR Report\ 624. When colour is being transmitted, the input
(and output) video signals are in component form. The luminance and
colour\(hydifference components, E`\dY\u, (E`\dR\u\ \(em\ E`\dY\u) and 
(E`\dB\u\ \(em\ E`\dY\u) are as defined in CCIR Report\ 624. The video 
interface is as recommended 
in CCIR Recommendation\ 567.
.RT
.sp 2P
.LP
2.4
	\fISource coder\fR 
.sp 1P
.RT
.sp 1P
.LP
2.4.1
	\fILuminance component or monochrome\fR 
.sp 9p
.RT
.sp 1P
.LP
2.4.1.1
	\fIAnalogue\(hyto\(hydigital conversion\fR 
.sp 9p
.RT
.PP
The signal is sampled to produce 256 picture samples per active
line (320\ samples per complete line). The sampling pattern is orthogonal and
line, field and picture repetitive. For the 525\(hyline input, the sampling
frequency is 5.0\ MHz, locked to the video waveform.
.PP
Uniformly quantized PCM with 8 bits/sample is used.
.PP
Black level corresponds to level 16 (00010000).
.PP
White level corresponds to level 239 (11101111).
.PP
PCM code words outside this range are forbidden (the codes being used for 
other purposes). For the purposes of prediction and interpolation, the 
final picture element in each active line (i.e.\ picture element\ 255) 
is set to level\ 128 in both encoder and decoder. 
.PP
In all arithmetic operations, 8\(hybit arithmetic is used and the bits
below the binary point are truncated at each stage of division.
.RT
.sp 2P
.LP
2.4.1.2
	\fIPre\(hy and post\(hyfiltering\fR 
.sp 1P
.RT
.sp 1P
.LP
2.4.1.2.1
	\fISpatial filtering\fR 
.sp 9p
.RT
.PP
A digital filter reduces the 242\(12\ active lines\(hyper\(hyfield of the
525\(hyline signal to 143\ lines\(hyper\(hyfield, the same number as in 
the 625\(hyline 
version of the codec. In the decoder, the digital post\(hyfilter uses
interpolation to restore the signal to 525\(hylines per picture.
.RT
.sp 1P
.LP
2.4.1.2.2
	\fITemporal filtering\fR 
.sp 9p
.RT
.PP
A 
recursive temporal pre\(hyfilter
with non\(hylinear transfer
characteristics is used in the coder to reduce noise in the signal and
increase coding efficiency. The frame store used in this filter can also be
used as the storage element of a frame interpolator with variable coefficients 
which is used to reduce the transmitted frame rate to a value less than 
that of the input video signal. In 525\(hyline to 525\(hyline transmission, 
the transmitted frame frequency is locked to the video clock and is approximately 
29.67\ Hz 
(29.97\ Hz times 3057/3088) instead of the nominal video rate of 29.97\ Hz. In
525\(hyline to 625\(hyline transmission, the transmitted frame frequency is
nominally 25\ Hz and is locked to the channel clock.
.bp
.PP
Because the (television) frames are leaving the coder more slowly than 
they are entering, the coding process is suspended for one frame every 
\fIN\fR th 
input frame. \fIN\fR is approximately 100\ for 525\(hyline to 525\(hyline 
operation and 
approximately 6\ for 525\(hyline to 625\(hyline operation.
.PP
In the decoder, the digital post\(hyfilter incorporates a frame store in 
some versions of the 625\(hyline codec where it is used in the line interpolation 
process. In the 525\(hyline version, in addition to its use for line 
interpolation, it is used as a temporal interpolator with variable
coefficients to provide an extra output frame during those periods when the
decoding is temporarily suspended.
.RT
.sp 2P
.LP
2.5
	\fIVideo multiplex coding\fR 
.sp 1P
.RT
.sp 1P
.LP
2.5.1
	\fIBuffer store\fR 
.sp 9p
.RT
.PP
The size of the buffer store is defined at the transmitting end
only and is 160\ kbits. Of this, 96\ kbits is used for smoothing the video 
data in the face\(hyto\(hyface mode and the remainder is used to accomodate 
the action of the frame interpolator (see \(sc\ 2.5.1.1 below) and the 
requirements of the 
graphics mode.
.PP
At the receiving end, the buffer must be at least this length but in some 
implementations of the decoder, it may be longer. 
.RT
.sp 1P
.LP
2.5.1.1
	\fIBuffer control\fR 
.sp 9p
.RT
.PP
The amount to which the transmitting buffer is filled is used to
control various coding algorithms (subsampling,\ etc.) and is signalled 
to the decoder to enable it correctly to interpret the received signals. 
In the 
525\(hyline codec, the transmission rate is less than the video input rate and
hence the buffer tends to fill more rapidly than would be determined by the
movement in the picture, only to empty again when the interpolator suspends 
the coding process. 
.PP
To avoid incorrect changes in coding algorithms, the 
buffer\(hystate  signal
is modified to take account of the progressively changing
coefficients of the interpolator in the pre\(hyfilter. The buffer then 
operates as though the data is coming from a video source whose frame rate 
is uniform and the same as the transmitted frame rate. 
.RT
.sp 1P
.LP
2.6
	\fITransmission coding\fR 
.sp 9p
.RT
.PP
The transmission coder assembles the video, audio, signalling and optional 
data channels into a 1544\ kbit/s frame structure which is compatible with 
Recommendation\ G.704. 
.RT
.sp 1P
.LP
2.6.1
	\fISerial data\fR 
.sp 9p
.RT
.PP
See \(sc 1.6.1.
.RT
.sp 1P
.LP
2.6.2
	\fIAudio\fR 
.sp 9p
.RT
.PP
See \(sc 1.6.2.
.RT
.sp 1P
.LP
2.6.3
	\fITransmission framing\fR 
.sp 9p
.RT
.PP
The frame structure, compatible with Recommendation\ G.704 and also compatible 
with that of the 625\(hyline version in \(sc\ 1, is given in \(sc\ 2 of 
Recommendation\ H.130.
.RT
.sp 1P
.LP
2.6.3.1
	\fIGeneral\fR 
.sp 9p
.RT
.PP
See \(sc 1.6.3.1.
.RT
.sp 1P
.LP
2.6.3.2
	\fIUse of certain bits in each octet in the odd frames of time\fR 
\fIslot 2\fR 
.sp 9p
.RT
.PP
The use of certain of the bits in time slot 2 (odd) differs
slightly from that given for the codec in \(sc\ 1. The differences are as
follows:
.RT
.LP
	\fIBit 1\ \(em\ For clock justification\fR 
.LP
	This bit is disregarded in 525\(hyline decoders.
.bp
.LP
	To permit interworking with the 626\(hyline codecs of \(sc 1, the
525\(hyline coders must transmit a fixed bit\(hypattern which is used to
control the frequency of the video clock in 625\(hyline decoders. The
exact form of the repetitive pattern need not be specified but it
must contain seven \*Qones\*U and four \*Qzeros\*U in 11\ bits, e.g.:
.sp 1P
.ce 1000
1\ 0\ 1\ 1\ 0\ 1\ 0\ 1\ 1\ 0\ 1
.ce 0
.sp 1P
.LP
	\fIBit 2\ \(em\ To signal buffer state\fR 
.LP
	The degree to which the encoder buffer is filled, after correction
for the interpolator (see \(sc\ 2.5.1.1), is measured in increments of 1\ K
(1\ K\ =\ 1024\ bits), and signalled using an 8\(hybit binary code. When
working to a 525\(hyline decoder, the buffer state is sampled every
3057\ channel\(hyclock periods. When working to a 625\(hyline decoder, the
buffer state is sampled 10\ times during every 525\(hyline field period.
When the buffer input is suspended for a frame period, the buffer
sampling is stopped. The sampled values of the buffer state are stored
prior to transmission. The store may hold between zero and 23\ values
which have been modified to take account of the interpolator
coefficients at the times of sampling. The modified sample values are
read out [as bit\ 2 of TS2 (odd)] at a uniform rate; the most
significant bit (MSB) in frame\ 1 of the multiframe, the second MSB
in frame\ 2,\ etc.
.LP
	\fIBit 3.7\ \(em\ Fast update request\fR 
.LP
	On receipt of this bit set to 1, the transmitter buffer is forced
to decrease its full and stabilise to a modified state of less than
6\ K by preventing coded picture elements from entering the buffer.
Bit\ A is set to\ 1 in the next FST. The two following fields are
treated as complete moving areas and the encoder uses an arrangement
for control of the sub\(hysampling modes to make the buffer overflow
condition unlikely.
.sp 2P
.LP
\fB3\fR \fBA codec for 525\(hylines, 60 fields/s and 1544 kbit/s transmission 
for intra\(hyregional use\fR 
.sp 1P
.RT
.sp 1P
.LP
3.1
	\fIIntroduction\fR 
.sp 9p
.RT
.PP
A 1.5 Mbit/s interframe codec described under \(sc\ 3, is capable of
transmitting and receiving a single NTSC video signal and audio signal 
using an adaptive predictive coding technique with 
motion\(hycompensated
prediction
, 
background prediction
and 
intraframe
prediction
.
.PP
The aim of this codec is to effectively transmit video telephone and video 
conferencing signals which have relatively small movements. The video 
interface of the codec is a 525\(hyline, 60\ fields/s standard analogue 
television signal corresponding to the \*QClass\ \fIa\fR \*U standard of 
Recommendation\ H.100. 
.RT
.sp 1P
.LP
3.2
	\fIOutline of codec\fR 
.sp 9p
.RT
.PP
The essential parts of the codec block diagram are shown in
Figure\ 7/H.120. The coder consists of three basic functional blocks, that 
is, pre\(hyprocessing, video source coding and transmission coding. 
.PP
In the pre\(hyprocessor, the input analogue NTSC video signal is
digitized and colour decoded into one luminance component and two chrominance 
components. These three components are time division multiplexed into a 
digital video form, whose noise and unnecessary signal components are removed 
by the 
pre\(hyfilter.
.PP
In the video source coder, the digital video signal is fed to the
predictive coder where interframe and intraframe predictive coding
techniques are fully utilized for minimizing prediction errors  to be
transmitted. The prediction error signal is next entropy\(hycoded using its
statistical properties to reduce redundancies. Since the coded error
information is generated in irregularly spaced bursts, a buffer is used. 
If the buffer becomes full, the number of prediction error quantizing levels 
and/or 
picture elements to be coded is reduced to prevent any overflow.
.PP
In the transmission coder, coded video and audio signals are first
encrypted on an optional basis. The coded video signal is then forward error
correction coded and scrambled. The three signals, coded video, coded audio 
and optional data signals are multiplexed into a 1544\ kbit/s digital format 
with a frame structure as defined in Recommendation\ H.130. 
.PP
The decoder carries out a reverse operation.
.bp
.RT
.LP
.rs
.sp 30P
.ad r
\fBFigure 7/H.120, p.\fR 
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
3.3
	\fIBrief specification\fR 
.sp 1P
.RT
.sp 1P
.LP
3.3.1
	\fIVideo input/output\fR 
.sp 9p
.RT
.PP
NTSC signals are used for the video input/output signal, with
monochrome signals being additionally applicable.
.RT
.sp 1P
.LP
3.3.2
	\fIDigital output/input\fR 
.sp 9p
.RT
.PP
The interface conditions for the digital output/input signal
satisfy Recommendation\ G.703 specifications. The signal transmission rate is
1544\ kbit/s.
.RT
.sp 1P
.LP
3.3.3
	\fISampling frequency\fR 
.sp 9p
.RT
.PP
The video sampling frequency is four times the colour sub\(hycarrier frequency 
(\fIf\fR\d\fIS\fR\\d\fIC\fR\u) and asynchronous with the 1544\ kHz network 
clock.
.RT
.sp 1P
.LP
3.3.4
	\fITime division multiplexed  (TDM) digital video format\fR 
.sp 9p
.RT
.PP
An NTSC signal is separated into a luminance component (Y) and two chrominance 
components (C\d1\uand\ C\d2\u). A time division multiplexed signal 
composed of\ Y and time\(hycompressed\ C\d1\uand\ C\d2\uis employed in 
the source 
coding as the standard digital video format.
.bp
.RT
.sp 1P
.LP
3.3.5
	\fICoding algorithm\fR 
.sp 9p
.RT
.PP
Adaptive predictive coding
supplemented by 
variable
word\(hylength coding
is used to achieve low bit rate transmission. The
following three predictions are carried out adaptively on a 
pel
\(hyby\(hypel  basis:
.RT
.LP
	a)
	motion\(hycompensated interframe prediction for a still or
slowly moving area,
.LP
	b)
	background prediction for an uncovered background area,
and
.LP
	c)
	intraframe prediction for a rapidly moving area.
.PP
Prediction errors for video signals and motion vectors are both
entropy\(hycoded using the following two techniques:
.LP
	i)
	variable word\(hylength coding for non\(hyzero errors, and
.LP
	ii)
	run\(hylength coding for zero errors.
.sp 1P
.LP
3.3.6
	\fIAudio channel\fR 
.sp 9p
.RT
.PP
An audio channel using 64 kbit/s is included. The audio coding
algorithm complies with Recommendation\ G.722.
.RT
.sp 1P
.LP
3.3.7
	\fIData channel\fR 
.sp 9p
.RT
.PP
An optional 64 kbit/s data channel is available, which is used for video 
if not required for data. 
.RT
.sp 1P
.LP
3.3.8
	\fIMode of operation\fR 
.sp 9p
.RT
.PP
The normal mode of operation is full duplex, with other modes,
e.g.\ the one\(hyway broadcasting operation mode, also taken into account.
.RT
.sp 1P
.LP
3.3.9
	\fITransmission error protection\fR 
.sp 9p
.RT
.PP
A BCH error correcting code is used along with a demand refreshing method 
to prevent uncorrected errors from degrading the picture quality. 
.RT
.sp 1P
.LP
3.3.10
	\fIAdditional facilities\fR 
.sp 9p
.RT
.PP
Provision is made in the digital frame structure for the future
introduction of such facilities as encryption, graphics transmission and
multipoint communication.
.RT
.sp 1P
.LP
3.3.11
	\fIProcessing delay\fR 
.sp 9p
.RT
.PP
The coder plus decoder delay is about 165 ms without that of a
pre\(hyfilter and a post\(hyfilter.
.RT
.sp 1P
.LP
3.4
	\fIVideo interface\fR 
.sp 9p
.RT
.PP
The video input/output signal of the codec is an analogue NTSC
signal (System\ M) in accordance with CCIR Report\ 624.
.RT
.sp 2P
.LP
3.5
	\fIPre\(hy and post\(hyprocessing\fR 
.sp 1P
.RT
.sp 1P
.LP
3.5.1
	\fIAnalogue\(hyto\(hydigital and digital\(hyto\(hyanalogue conversion\fR 
.sp 9p
.RT
.PP
An NTSC signal band\(hylimited to 4.5 MHz is sampled at a rate of
14.3\ MHz, four times the colour sub\(hycarrier frequency (\fIf\fR\d\fIS\fR\\d\fIC\fR\u), 
and converted to an 8\(hybit linear PCM signal. The sampling clock is locked 
to the 
horizontal synchronization of the NTSC signal. Since the sampling frequency 
is asynchronous with the network clock, the justification information is 
coded and transmitted from the coder to the decoder. 
.PP
The digital video data is expressed in two's complement form. The
input level to the A/D converter is defined as follows:
.RT
.LP
	\(em
	sinc tip level (\(em40 IRE) corresponds to \(em124 (10000100);
.LP
	\(em
	white level (100 IRE) corresponds to 72 (01001000).
.LP
	(IRE: Institute of Radio Engineers)
.bp
.PP
As a national option, a pad can be inserted before the A/D
converter if a level fluctuation should be taken into account at analogue
transmission lines connecting terminal equipment and codec.
.PP
At the decoder, the NTSC signal is reproduced by converting the 8\(hybit 
PCM signal to an analogue signal. 
.RT
.sp 1P
.LP
3.5.2
	\fIColour decoding and encoding\fR 
.sp 9p
.RT
.PP
The digitized NTSC signal is separated into the luminance component (Y) 
and the carrier band chrominance component (C) by digital filtering. The 
two baseband chrominance signals (C\d1\uand\ C\d2\u) are obtained by digitally
demodulating the separated carrier band chrominance component. The effective
sampling frequency after colour decoding is converted to 7.2\ MHz
(2\ \fIf\fR\d\fIS\fR\\d\fIC\fR\u) and 1.2\ MHz (1/3\ \fIf\fR\d\fIS\fR\\d\fIC\fR\u) 
for the luminance 
signal and chrominance signals respectively.
.PP
The replica of the NTSC signal is obtained by digitally modulating the 
C\d1\uand\ C\d2\usignals and adding to the Y\ signal at the decoder. 
.PP
Filter characteristics for colour decoding and encoding are left to
each hardware implementation since they do not affect interworking between
different design codecs. Examples of recommended characteristics are described 
in Annex\ E. 
.RT
.sp 1P
.LP
3.5.3
	\fITDM signal\fR 
.sp 9p
.RT
.PP
A time division multiplexing (TDM) signal is constructed from the separated 
component signals. 
.PP
First, the C\d1\uand C\d2\usignals are time\(hycompressed to 1/6. Next, 
each of the time compressed C\d1\uand\ C\d2\usignals, with their horizontal 
blanking parts removed, is inserted into the Y\ signal horizontal blanking
interval on alternate lines. C\d1\uis inserted on the first line of the 
first field and on every other line following throughout the frame, while\ 
C\d2\uis 
inserted on the second line of the first field and on every other line
following throughtout the frame.
.PP
Active samples for the Y signal are 384 samples/line and
64\ samples/line for the C\d1\uand C\d2\usignals. The TDM signal is constructed 
with these active samples and 7\ colour burst samples (B), which are inserted 
into the top of the TDM signal.
.PP
As shown in Figure 8/H.120, the C\d1\uand C\d2\usignal sampling points 
coincide with that of the Y\ signal on every sixth sample. The C\d1\uand 
C\d2\usignals of only the odd lines are transmitted to the decoder. 
.PP
At the decoder, each component signal is again demultiplexed from the TDM 
signal, and time\(hyexpansion processing of 6\ times is carried out for 
the 
C\d1\uand C\d2\usignals.
.PP
\fINote\fR \ \(em\ When a pad is inserted before the A/D converter as described 
in \(sc\ 3.5.1, pre\(hyemphasis (de\(hyemphasis) with a compensating gain 
for the\ C\d1\u, \d2\uand colour burst signals is recommended at the source 
coder input 
(decoder output) to obtain better picture reproduction in coloured parts.
.RT
.sp 1P
.LP
3.5.4
	\fIPre\(hy and post\(hyfiltering\fR 
.sp 9p
.RT
.PP
In addition to conventional anti\(hyaliasing filtering prior to
analogue\(hyto\(hydigital conversion, the following two filtering processes 
should be used as pre\(hyfiltering for source coding: 
.RT
.LP
	a)
	temporal filtering to reduce random noise included in the
input video signal;
.LP
	b)
	spatial filtering to reduce aliasing distortion in
subsampling.
.PP
At the decoder, the following three filtering processes should be used 
as post\(hyfiltering in addition to conventional low pass filtering after 
digital\(hyto\(hyanalogue conversion:
.LP
	i)
	spatial filtering to interpolate the omitted picture
elements in subsampling;
.LP
	ii)
	spatio\(hytemporal filtering to interpolate the omitted fields
in field repetition;
.LP
	iii)
	temporal filtering to reduce noise generated in the course
of source coding.
.PP
Although these filtering processes are important for improving
reproduced picture quality, their characteristics are independent of
interworking between different design codecs. Hence, pre\(hy and post\(hyfiltering 
is left to each hardware implementation. 
.bp
.LP
.rs
.sp 40P
.ad r
\fBFigure 8/H.120, p.\fR 
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
3.6
	\fISource coding\fR 
.sp 1P
.RT
.sp 1P
.LP
3.6.1
	\fIConfiguration of source coder and decoder\fR 
.sp 9p
.RT
.PP
The video source coder and decoder configuration of this codec is outlined 
in Figure\ 9/H.120. 
.PP
The predictive encoder converts the input video signal \fIx\fR into the
prediction error signal\ \fIe\fR , using the motion vector\ \fIv\fR . This 
conversion is 
controlled by the coding mode\ \fIm\fR .
.PP
The variable word\(hylength (VWL) coder codes\ \fIe\fR and \fIv\fR into the
compressed data\ \fIC\fR using the variable length coding method. The transmission 
buffer memory (BM) smoothes out the irregularly spaced data\ \fIC\fR . 
The coding mode\ \fIm\fR is also coded. 
.bp
.PP
The 
frame memory parity information
\fIp\fR is used to check the identity of coder and decoder frame memory 
contents. If any parity error is 
detected, frame memories of both coder and decoder are reset by the demand
refresh information (DR) and the demand refresh confirmation information
(DDR).
.PP
At the decoder, the variable word\(hylength (VWL) decoder decodes \fIe\fR ,
\fIv\fR , \fIm\fR and\ \fIp\fR , and the predictive decoder reproduces 
the video 
signal\ \fIx\fR `.
.RT
.LP
.rs
.sp 37P
.ad r
\fBFigure 9/H.120, p.\fR 
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
3.6.2
	\fIPredictive coding\fR 
.sp 1P
.RT
.sp 1P
.LP
3.6.2.1
	\fICoding modes\fR 
.sp 9p
.RT
.PP
Five coding modes as summarized in Table 3/H.120 are provided. All of the 
samples are coded and transmitted in normal mode, while half of the 
samples are omitted in subsampling mode. In field repetition mode, one 
or more consecutive fields are omitted (called multi\(hyfield repetition, 
see Note\ 1). If field repetition mode and subsampling mode are used in 
combination, only a 
quarter or less of the original picture elements are coded and transmitted.
.bp
.PP
Subsampling is carried out in a quincunx way, namely by transmitting only 
odd\(hynumbered pels on odd\(hynumbered lines and even\(hynumbered pels 
on 
even\(hynumbered lines in each block\(hyline (see Note\ 2).
.PP
In field repetition mode, either the odd or even fields are omitted. For 
the omitted fields, both the prediction error\ \fIe\fR and the motion vector\ 
\fIv\fR are set to\ 0. 
.PP
\fINote\ 1\fR \ \(em\ If odd fields and even fields are mixed after field
omission, a severe picture degradation takes place. Hence, 1\ out of 2, 3 out
of\ 4 or\ 5 out of\ 6 field omission is recommended.
.PP
\fINote\ 2\fR \ \(em\ Each block\(hyline consists of 8 lines as defined in
\(sc\ 3.6.2.5.
.RT
.LP
.sp 1
.ce
\fBH.T. [T7.120]\fR 
.ce
TABLE\ 3/H.120
.ce
\fBCoding modes\fR 
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(66p) | cw(36p) | cw(78p) .
Coding modes	Abbreviation	Operation
_
.T&
cw(12p) | lw(54p) | cw(36p) | lw(78p) .
1	Normal	NRM	Full sampling
_
.T&
cw(12p) | lw(54p) | cw(36p) | lw(78p) .
2	Field repetition	FRP	One or more fields omission
_
.T&
cw(12p) | lw(54p) | cw(36p) | lw(78p) .
3	Subsampling	SBS	2: 1 per omission
_
.T&
cw(12p) | lw(54p) | cw(36p) | lw(78p) .
4	Stop	STP	Suspension of coding
_
.T&
cw(12p) | lw(54p) | cw(36p) | lw(78p) .
5	Refresh	RFS	Renewal of frame memory
_
.TE
.nr PS 9
.RT
.ad r
\fBTableau 3/H.120 [T7.120], p.\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.sp 5
.sp 1P
.LP
3.6.2.2
	\fIAdaptive prediction\fR 
.sp 9p
.RT
.PP
Prediction functions are adaptively selected on a pel\(hyby\(hypel basis 
as shown in Figure\ 10/H.120. The selection is carried out so as to minimize 
probable prediction errors. This is accomplished using the two prediction
status signals, which are determined by prediction reference signals, for 
the preceding pels located on the previous and the present lines. 
.PP
When subsampling and/or field repetition are operated, omitted pels
are interpolated in the prediction loop.
.PP
The notations defined for the i\(hynumbered pel are as follows:
.RT
.LP
	\fIX\fR\d\fIi\fR\u | 
	local decoder output,
.LP
	\fIY\fR\d\fIi\fR\u | 
	interpolator output,
.LP
	\fIM\fR\d\fIi\fR\u | 
	motion compensated 
interframe prediction
value,
.LP
	\fIB\fR\d\fIi\fR\u | 
	background prediction
value,
.LP
	\fII\fR\d\fIi\fR\u | 
	intraframe prediction
value,
.LP
	* | 
	logical product, and
.LP
	+ | 
	logical sum.
.bp
.LP
.rs
.sp 32P
.ad r
\fBFigure 10/H.120, p.\fR 
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
3.6.2.2.1
	\fIMotion\(hycompensated interframe prediction/background\fR 
\fIprediction\fR 
.sp 9p
.RT
.PP
Prediction status signal \fIS\fR\d1\u\fI\fI\d\fIi\fR\u | for pel \fIi\fR 
 | is determined as 
\v'6p'
.RT
.ad r
.ad b
.RT
.LP
where prediction reference signal \fIR\fR\d1\u(\fIi\fR ) is
\v'6p'
.ad r
.ad b
.RT
.PP
Based on \fIS\fR\d1\u\fI\fI\d\fIi\fR\u, prediction signal \fIX\fR\d1\u\fI\fI\d\fIi\fR\u | 
is given as 
\v'6p'
.ad r
.ad b
.RT
.PP
If pel \fIi\fR  | is either omitted due to subsampling and/or field
repetition or forced intraframe coded or in burst\ \fIB\fR , its
corresponding\ \fIR\fR\d\fIi\fR\u\ (\fIi\fR ) is set to\ 0 regardless of
equation\ (3\(hy2).
.bp
.sp 1P
.LP
3.6.2.2.2
	\fIInterframe prediction/intraframe prediction\fR 
.sp 9p
.RT
.PP
Prediction status signal \fIS\fR\d2\u\fI\fI\d\fIi\fR\u | for pel \fIi\fR 
 | is determined as 
\v'6p'
.RT
.ad r
.ad b
.RT
.LP
where prediction reference signal \fIR\fR\d2\u\ (\fIi\fR ) is
\v'6p'
.ad r
.ad b
.RT
.PP
Based on \fIS\fR\d2\u\fI\fI\d\fIi\fR\u, prediction signal \fIX\fR\d2\u\fI\fI\d\fIi\fR\u | 
is given as 
\v'6p'
.ad r
.ad b
.RT
.PP
If pel (\fIi\fR \ \(em\ 1) is omitted due to subsampling, \fIR\fR\d2\u(\fIi\fR 
\ \(em\ 2) is used instead of \fIR\fR\d2\u\ (\fIi\fR \ \(em\ 1). On the 
other hand, if pel\ (\fIi\fR \ \(em\ 455) is omitted, \fIR\fR\d2\u(\fIi\fR 
\ \(em\ 454)\ 
*
\ \fIR\fR\d2\u\ (\fIi\fR \ \(em\ 456) is used instead of \fIR\fR\d2\u\ 
(\fIi\fR \ \(em\ 455). If pel \fIi\fR is forced intraframe\(hycoded, its 
corresponding \fIR\fR\d2\u\ (\fIi\fR ) is set to\ 1 regardless of equation\ 
(3\(hy5). 
.PP
\fI\fR If pel \fIi\fR  | is omitted due to field repetition, its corresponding
\fIR\fR\d2\u\ (\fIi\fR ) is set to\ 0 regardless of equation\ (3\(hy5). 
When pel\ \fIi\fR is not 
forced\(hyintraframe coded, \fIR\fR\d2\u\ (\fIi\fR ) in burst \fIB\fR \ 
is set to\ 0. 
.RT
.sp 1P
.LP
3.6.2.3
	\fIBackground generation\fR 
.sp 9p
.RT
.PP
The background prediction value is generated scene adaptively
as
\v'6p'
.RT
.ad r
.ad b
.RT
.LP
where
\v'6p'
.ad r
.ad b
.RT
[Formula Deleted]
.LP