AU607636B2 - Method for encoding/transmitting images - Google Patents

Description

_1 i C3- 41 ft,

AUSTRALIA

PATENTS ACT 1952 COMPLETE SPECIFICATION Form

(ORIGINAL)

FOR OFFICE USE Short Title: Int. Cl: Application Number: Lodged:

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C

CD

Complete Specification-Lodged: Accepted: Lapsed: Published: Priority: Related Art: TO BE COMPLETED BY APPLICANT Name of Applicant: Address of Applicant: MITSUBISHI DENKI KABUSHIKI

KAISHA

2-3 MARUNOUCHI 2 CHOME

CHIYODA-KU

TOKYO 100

JAPAN

GRIFFITH HACK CO., 601 St. Kilda Road, Melbourne, Victoria 3004, Australia.

Actual Inventor: 2 sa f Address for Service: Complete Specification for the invention entitled: METHOD FOR ENCODING/TRANSMITTING IMAGES The following statement is a full description of this invention including the best method of performing it known to me:- -1 METHOD FOR ENCODING/TRANSMITTING IMAGES BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a technology for transmitting image signals obtained by encoding image information which utilizes the so-called vector quantization technique and which is applicable to the fields such as the television (TV) conference system and the TV telephone system.

Sccr tt This application is a divisional of Australian iZ "10 Patent Application No. 73379/87, the contents of which are incorporated herein by reference. There are also other

PE

C t divisional applications of the noted parent application.

Description of the Prior Art As a result of the remarkable advance of the *15 image processing technology in recent years, there have been made various attempts to put, for example, the TV 6o 6 conference system and the TV telephone system to the practical use by mutually and bidirectionally transmitting 44, the image information. In such a technological field, the eo44 20 quantization technique has been used in which the image signals as the analog quantity are classified into a finite number of levels changing in a discrete fashion within a fixed width and a unique value is assigned to each of these levels. Particularly, there has been a considerable advance in the vector quantization technique in which a plurality of samples of the image signals are grouped in blocks and each block thereof is mapped onto a pattern most similar thereto in a 1Amultidimensional signal space; thereby accomplishing the quantization.

The study of the vector quantization technology has been described in the following reference materials, for example.

"An Algorithm for'Vector Quantizer. Design" by Y. Linde, A. Buzo, and R. M. Gray (IEEE TRANSACTION ON COMMUNICATIONS, Vol. COM.28, No. 1, January 1980, pp.

84 *06* 10 "On the Structure of Vector Quantizers" by A. Gersho o (IEEE TRANSACTION ON INFORMATION THEORY, Vol. IT28, No. 2, SMarch 1982, pp. 157 166) "Speech Coding Based Upon Vector Quantization" by A. Buzo, A. H. Gray Jr., R. M. Gray and J. D. Markel (IEEE TRANSACTION ON ACOUSTICS, SPEECH, AND SIGNAL PROCESSING, Vol. ASSP28, No. 5, October 1980, pp. 562 i 574) e S Moreover, the following U.S. Patents have been obtained by the assignee of the present invention.

20 U.S.P.N. 4,558,350 "VECTOR QUANTIZER", Murakami t U.S.P.N. 4,560,977 "VECTOR QUANTIZER", Murakami et al.

2 ii *ttf 0 .OV *06 4 Ga 00 a.a 0 agoG, a 004 a *009 Referring now to FIGS. 1 3B, the prior art technology of the present invention will be described.

The conventional image encoding/transmitting apparatus, as shown in FIG. 1, includes a subtractor 1 for obtaining a difference between an input signal S 1 such as an image signal and an estimation signal S 9 and for outputting an estimated error signal S 2F a movement dete( ting circuit 2 for comparing a threshold value T with the estimated error signal S 2 to detect a movement or a change and for 10 generating and outputting a movement or change detect signal S 3 and a differential signal S 4 a quantization circuit 3 for quantizing the movement or change detect signal S53 and the differential signal S 4 to output a quantization signal a variable length encoder 4 for 615 generating from the quantization signal S 5 an encoded £5 signal S 6 with a variable length and for outputting the encoded signal S 6 a transmission data buffer circuit for temporarily storing the encoded signal S 6 and for outputting the encoded signal S 6 to the transmission side, C20 a local decoding circuit 6 for generating a reproduced di4fferential signial S7 from the quantization signal delivered from the quantization circuit 3 and outputting the reproduced or regenerated differential signal S 7 an adder 7 for achieving an addition on the reproduced differential signal S57 and the estimation signal S 9 and for outputting a reproduced input signal an estimating circuit 8 for outputting an estimation signal S 9 based on a# oa 0 S .5 00 a a 00

S

0O 5.54 eGo.

*0 a a 3- _111 11 ii(_ i 1(I l 1( I_ the reproduced input signal and a threshold generating circuit 9 for monitoring the amount of the encoded signal S6 accumulated in the transmission data buffer circuit 5 and for generating an appropriate threshold value T.

The movement detecting circuit 2 comprises, as shown in FIG. 2, an absolute value circuit 10 for -calculating the absolute value IS21 of the estimated error signal S 2 o I

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0 00 a comparing circuit 11 for effecting a comparison between 0O.10 the absolute value I 21 of the estimated error signal S 2 0 and the threshold value T and for outputting the movement o~oo or change detect signal 53, and a zero allocator 12 for allotting 0 and outputting 0 as the differential signal

S

4 when the movement or change is not detected as a 0o66-15 result of the comparison in the comparing circuit 11.

.0a" The movement detect signal S3 is converted into a running record R by use of the running length encode table 4a to 6 64 generate serial data. In addition, only when the movement detect signal S 3 is indicating the validness, the 6 """020 quantization signal S 5 is converted into a variable-length It record through the variable-length encode table 4b to generate serial data (FIGS. 2B 2C). Reference numeral 4C indicates a multiplex operation control section.

In contrast to the configuration on the transmission side of FIGS. 1 2B, the configuration on the reception side is shown in FIGS. 3A 3B. In FIG. 3A, the equipment on the reception side includes a receiving data 4 -I buffer circuit 13 for receiving and for temporarily storing the encoded signal S 6 delivered from the transmission data buffer circuit 5 on the transmission side, a variable length decoder 14 for decoding the encoded signal S6 stored in the receiving data buffer 13 to output a reproduced quantization signal Sll a local decoding circuit 15 for outputting a reproduced differential signal S 1 2 based on the reproduced quantization signal Sll, an adder circuit 16 for obtaining 10 the difference between the reproduced differential signal S12 and the reproduced estimation signal S 1 3 and for reproducing the input signal S 1 4 which corresponds to the C reproduced input signal S8 on the transmission side, and an estimating circuit 17 for outputting the reproduced estimation signal S13.

After the encoded signal S6 undergone the r t multiplexing in the variable-length encode circuit 4 is received by the receive buffer circuit 13, the data is i" distributed to the respective decode tables of variable codes under control of the multiplex separation control circuit 14a. As a result of the decoding, the movement detect signal and the quantization signal are attained.

Moreover, when the decoded movement detect signal indicates the invalidness the quantization signal is reset to by the flip-flop 14c, thereby outputting the output Sl (FIG. 3B), 5 l. Cr) 6 6l Ct;

,ICCC

i Next, the operation on the transmission side will be described with reference to FIGS. 1 2.

Assuming first the non-effective error in the movement detecting circuit 2 to be d, the estimation coefficient to be applied to the reproduced input signal

S

8 in the estimating circuit 8 to be A, and the delay of the time t to be Z the following relationships are satisfied.

S

2 S 1

S

9 10 S 4

S

2 d

S

7

S

4

Q

S

8

S

7

S

9

S

1 Q d -t S A S Z The subtractor 1 calculates the estimated error signal S 2 representing the -difference between the input signal S 1 and the estimated signal S 9 whereas the movement t detecting circuit 2 outputs the movement or change detection signal S 3 and the differential signal S 4 based on the estimated error signal S 2 calculated by the subtractor 1.

C A detailed description will be given of the operation of the movement detecting circuit 2 by referring to FIG. 2. The allotting absolute value circuit 10 obtains the absolute value of the estimated error signal S 2 and then the comparison circuit 11 achieves a comparison between the absolute value IS21 of the estimated error signal S 2 and the threshold value T generated by the threshold value generating circuit 9.

666 6 6 66 66 6 6 616 Ct 6 -1 The movement detection signal S 3 is output in conformity with the following conditions.

S

3 0 (invalid) S121 T S 1 (valid) S21 T When the movement or change is not detected, namely, for "S3 zero allocator 12 outputs ifor the differential signal S4.

On the other hand, the quantization 'circuit 3 converts the inputted differential signal S 4 according to 10 an arbitrary characteristic. The variable encoding ov, circuit 4 receives the quantization signal S5 only when 00 the movement detection signal S 3 is valid, namely, for "S 1" and, foi example, conducts a run-length encoding 3 on the movement detection signal S. For the quantization signal S 5 a code having a smaller code length is assigned to a value in the neighborhood of for which the 00 sa generation frequency is high and then the code is stored 0 B in the transmission data buffer circuit 5. The a a S* transmission data buffer circuit 5 outputs the accumulated datum as the encoded signal S 6 to a 6a.

transmission line. The threshold generating circuit 9 monitors the accumulated amount of the transmission data buffer circuit 5 and further controls the generation amount of the encoded data by generating an appropriate threshold value.

Next, the operation on the reception side will be described with reference to FIG. 3. The receiving data 7 buffer circuit 13 first receives the encoded signal S 6 undergone the variable length encoding on the transmission signal and outputs the signal S 6 to the variable length decoder 14. Only when the movement detection signal S 3 undergone the decoding operation indicates the validness, the variable length decoder 14 outputs the reproduced quantization signal'.,S 11 If the movement detection signal S 3 indicates the invalidness, the variable length decoder 14 outputs Next, the o*«S 10 local decoding circuit 15 decodes the reproduced quantization signal S11 and outputs the reproduced t differential signal S12 to the adder 16. The adder 1i adds the reproduced differential signal S 1 2 to the reproduced estimation signal S 1 3 from the estimation 15 circuit 17 thereby to reproduce the input signal S 14 C t a The operation to effect the data compression and C @9 transmission by use of the differential signal is referred to as the differential pulse code modulation (to be abbreviated as DPCM herebelow) system.

However, in the image encoding/transmnitting apparatus 'r t using the DPCM system, the variable length encoding is achieved on the datum which is judged to be effective at the step of the variable length encoding; consequently, as the threshold value increases, the code having a short code length to be assigned in the neighborhood of "0" cannot be generated and hence the efficiency of the encoding is deteriorated; moreover, there has been a 8 problem that as the threshold value becomes greater, the precision of the quantization not improved for the quantization characteristic of the quantization circuit in the circuitry on the transmission side even when the dynamic range of the effective datum is narrowed.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an image encoding/transmitting apparatus and a method; thereof in which a control can be effected 10 according to an appropriate encoding control parameter.

0o 2eoding to on acspet of the invention teese- 0 0 0 e i4 ovided an image encoding/transmitting method 0 S° comprising: o a preprocessing step for effecting an analog-to-digital conversion on an image input signal to 0a0, generate a digital signal and for storing the digital c0 o 0 signal in a frame memory for each frame; o 0a an encoding processing step for effecting a

O

0 smoothing operation on the digital signal based on an encoding control parameter such as a threshold value and 0o"* for encoding the smoothed digital signal; 0* 00 a transmission control processing step for temporarily storing the encoded signal for each frame in a transmission data buffer and for sending the encoded signal to a transmission control unit; an encoding control parameter control step for controlling the encoding control parameter based on~e Ti4A, amount of the encoded signals temporarily stored in the a transmission data buffer associated with a previous frame; ^and 9 an auxiliary encoding control parameter control step for extracting a portion of a current digital signal stored in the frame memory, for effecting a pseudo encoding on the portion based on the encoding control parameter calculated depending on the amount of the encoded signals of the preceding frame, and for correcting the encoding control parameter based on a-Aamount of the pseudo encoded signals undergone the pseudo encoding, thereby causing the encoding control parameter to an 004* ooo0 10 optimum value.

o OM 001 o BRIEF DESCRIPTION OF THE DRAWINGS o o 6 In order that the invention may be more clearly ascertained, an example of a preferred embodiment will now o o be described with reference to the accompanying drawings wherein: FIG. 1 is a block diagram showing the 0a 0o configuration On the transmission side of the prior art o oo image encoding/transmitting apparatus utilizing the DPCM 0 o a system; FIG. 2A is a block diagram illustrating a "a detailed configuration of the movement detecting circuit 000 go 4 o o I of FIG. 1, FIG. 2B is a block diagram illustrating a detailed configuration of the variable-length encode circuit 4, FIG. 2C is a schematic diagram illustrating an example of the multiplexing of the circuit of FIG. 1; FIG. 3A is a block configuration diagram depicting the reception side of the prior art image 10 3 encoding/transmitting apparatus, FIG. 3B is a block diagram showing the details of the variable-length decsie circuit of FIG. 3A; FIG. 4 is a block construction diagram showing the image encoding/transmitting apparatus as a general example and as a basis of an embodiment of the present invention; FIG. 5 is a block diagram of the transmitting apparatus of the embodiment according to the present gOOo 000 S000 10 invention; 6000 0 ."00 FIG. 6 is an explanatory diagram depicting an 0 00 0 0 0 example of extraction of datum undergone the pseudo oe000o 0 0 ,oo, encoding; 0 0 000 FIG. 6(a) is a graph showing the encoding processing time associated with the image 0oo. encoding/transmitting apparatus in the general example of a 0 0 o00 0 o 00 FIG. 4; FIG. 6(b) is a graph illustrating the processing 0000 0 00 time of the embodiment; and FIG. 7 is an explanatory diagram showing the 6000 00ooo0 time-lapse control.

00 0 i. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior to the description of the concrete contents of the embodiment, the general example of the technology adopted as the basis of the embodiment will be described.

FIG. 4 shows the image encoding/transmitting apparatus as the basis of the embodiment.

11 I In the analog/digital converting section (to be referred to as an A/D Converting section) 51 of FIG. 4, the pixels obtained by effecting the analog/digital conversion on the image input signals are processed to generate groups each containing k pixels being in the neighborhood of each other in the image, so that for each block, a k-dimensional vector input signal is generated.

A frame of vector signals are then stored in the frame memory 52, thereafter the vector signals are transmitted s 10 to the encoding section 53 in the block-by-block fashion.

ep S 0 6 0 0/ poa o"/ A it *0/ i/ 12 Incidentally, the encoding section 53 is provided with a previous frame memory (not shown) in which the image signals associated with the previous frame with respect to the current image signal are stored in blocks.

The frame memory 52 is supplied with a signal from the read address generating section 57. The apparatus control section 59 outputs control signals to tNe respective sections.

Tafl ®In the encoding section 53, using as reference 10 blocks a plurality of blocks each including the current o @4 *06 0vector signal and the block stored in the previous frame

S

.ao, memory at a position associated with the block corresponding to the current vector signal, the vector signal and the block position information of the a 15 reference block are inputted and then the image 41 5~ information is compressed and encoded as will be 6 40 described later; thereafter, the encoded image signals %oe.

4 are sent to the transmission data buffer 54.

Next, a brief description will be given of the as 6 04,20 operations to compress and encode the image information.

6 £First, the distortion (such as the Euclid distortion or the absolute distortion) between the current vector signal and the reference vector signal is computed to select a block having the minimum distortion from the reference blocks and then the selected block position information is stored.

13 ii _e i ii A subtraction is then effected between the current .ector signal and the vector signal of the selected reference block to calculate the differential vector signal.

The average m and the variance a of the differential vector signal are thereafter obtained and the validness/ invalidness is judged according to the following expressions by use of the threshold value T 1 for the 9 0 0 0 as eoo *00009 9 9 06 el 0 average and the threshold value T 2 for the variance 10 controlling the compression amount of the information volume.

m T 1 and a T 2 Invalid m T or a T: Valid If the judgment results in the invalidness, the current block is assumed to be identical with the selected block and only the selection block position information and the information indicating the invalid block ar, encoded, thereby temporarily storing the resultant signals in the transmission data buffer 54.

20 On the other hand, if the judgment results in the validness, the differential vector signal as the datum to be transmitted is normalized according to the following formula. Assuming the differential vector signal to be E El, E2' Ek and the vector signal after the normalization to be x x x 2 xk, then 0099 0 a a o0 «a *o 1 X- m) a 14 n k where, m k i=l E

I

Y k lei -iml Next, the normalized vector signal is subjected to the vector quantization to output the index code I. For details about the vector quantization, refer to the "Image Dynamic Multistage Vector Quantization", the journal of the institute of electronics and communication 000 o.oo engineer, IE84-18. The average value and the variance c00@ *o are output by effecting the quantizing and encoding o 00 00 o operations on the m and a. If the result indicates the a °0 10 validness, the information notifying the valid block, the o index code, and the average and the'variance undergone the quantizing and encoding operations are temporarily stored in the transmission data buffer 54.

0 From the transmission data buffer '54, the image encoded signals thus encoded are transmitted in the frame-by-frame.fashion.

In the encode controlling parameter control section *BO. 55, the threshold values are controlled according to the amount of the image encoded signals stored in the transfer data buffer 54 associated with the previous frame. If the amount of the signals is great, the threshold values are set to the greater values; whereas, if the signal amount is small, the threshold values are set to the smaller values, thereby controlling the degree of the compression for each frame.

15 Moreover, the compression of the image input signal in the conventional image encoding/transmitting method has been achieved by applying the block generation of the image, the differential modulation, and the threshold value control as described above; however, when transmitting an image including a considerable change therein, namely, when transmitting an image having many blocks for which there exists a great difference between .o00* the current frame image and the preceding frame image and 10 hence the threshold judgment results in the validness (the transmission information is to be required), the *0 compression of the encoded information volume cannot be satisfactorily effected in same cases.

In such a case, a long period of time is necessary to transmit a frame due to the great amount of the o information; consequently, the time-lapse control is b 4 effected to thin out the input of the image signals for 0o each frame, thereby, controlling the information volume.

FIG. 7 shows an example of the time-lapse control.

'20 As shown in FIG. 7 the input image frames are o0 V input in the sequence of A, B, C, D, E, and F. If the frame A contains a great amount of encoded information, the transmission of the encode information takes a long period of time, and hence in the actual transmission of the encoded information, the frames B and C are omitted, namely, the encoded information is transmitted in the sequence of A, D, E, and F.

16 According to the image encoding/transmitting method of FIG. 4 as described above, the control of the encoding control parameter such as the threshold values for achieving the smoothing operation of the information generation volume is effected depending on the volume of the encoded signals contained in the preceding frame; consequently, in a case where an abrupt change takes a0 place between frames, the optimum encoding control o0000 o00 parameter cannot be attained, which leads to a problem 0000 o o0 10 that the appropriate value cannot be obtained for the information generation.

0000 0 00° To overcome this difficulty, the embodiment is implemented to solve the problem that the image transmission is attended with the visual unfamiliarity 0":15 due to the delay of the image transmission time and the *0 0 *0 time-lapse control associated with the increase of the o06::o information generation volume.

0 Next, the image encoding/transmitting apparatus of ,00 the embodiment according to the present invention o000 :o will'be described with reference to FIG. 0 04 FIG. 5 is a block configuration diagram illustrating the image encoding/transmitting apparatus of the -fi embodiment in which the same reference numerals are assigned to the same components as those of the prior art technology shown in FIG. 4 and the description thereof will be omitted.

17 I- I~ In this apparatus, there is provided a pseudo encoding/actual encoding control circuit 56 for accomplishing the control of the pseudo encoding/actual encoding. This system further includes a read address counter 57 for extracting the input datum for pseudo encoding operation from the digital signals stored in the frame memory 52 and a pseudo encoded signal volume counter 58 for counting the pseudo encoded signal volume.

a Next, the flow of the signal will be described.

0 oo 10 First, the image input signal 500 is converted into 0 a digital signal 501 by the A/D converting section 51 and the obtained digital signal 501 is stored in the frame memory 52 in the frame-by-frame fashion.

Next, the pseudo encoding is achieved as follows.

*°0:15 First, prior to the actual encoding, the pseudo 00 o 00o encoding control signal 506 indicating an execution of a 000 pseudo encoding is read from the pseudo/actual encoding control circuit 56 and is output to the read address counter 57. The read address counter 57 generates a read b o 00ooo00 o0 20 address for the pseudo encoding and then the pseudo encoding data 502 is extracted from the frame memory 52, for example, for an image constituted by 30 block lines per frame as shown in FIG. 5 namely, the 5-th block line, the 15-th block line, the 25-th block line, etc., thereby sending the extracted signals to the encoding section 53.

18

I

I'

.4)4 Referring now to FIG. 4 the inside of the pseudo/ actual encode control section 56 will be described.

The change-over timing generating section 561 receives as an input (not shown) a control signal 509 delivered from the apparatus control section 59 controlling the entire apparatus, recognizes the timing to effect the pseudo or actual encoding based on the control signal 509, and outputs a timing signal 661.

The change-over signal generating section 562, based on the timing signal 661, generates the change-over signalpseudo encode control signal 506 indicating whether the pseudo encoding or the actual encoding is to be effected.

Referring next to FIG. 4 the inside of the read address generating section 57 will be described.

,15 When receiving the virtual encode control signal 506 from the pseudo/actual encode contrbl section 56 for the pseudo encoding, the pseudo encode read address generating section 571 generates a pseudo encode read address 571 for the pseudo encoding, whereas the actual 20 encode address generating section 572, when indicated to It effect the actual encoding by use of the pseudo encode control signal 506, generates an actual decode read address 572 for the actual decoding. The pseudo decode read address 571 or the actual decode read address 572 is selected according to the pseudo encode control signal 506 and is then outputted as a frame memory read address 507.

I I

I

$4 I S S 4., 14*~ 4I 44 *6 19 I -F I~I~-W13- L -C I-I L 1 In the encoding section 53, a predetermined encoding operation is conducted on the pseudo encoding datum 502 by using the encoding control parameter 505 determined according to the amount of the encoding signals of the preceding frame and then the pseudo encoded signal 503 is sent to the pseudo encoded signal volume counter.

Thereafter, the pseudo encoded signal volume counter 58 counts the signal volume in the pseudo encoding and then based on the output 508, namely, the e9 0 10 pseudo encoded signal volume, the encoding control o9 o 0 9 parameter control section 55 corrects the encoding control parameter 505 to calculate the encoding control e° parameter 505, thereby finishing the pseudo encoding operation.

Referring here to FIG. 4 the inside of the encoding control parameter control section 55 will be described.

Based on the inputted actual encode signal volume 504, the encoding control parameter generating section 551 generates the encoding control parameter 651 for the t t next encoding operation. The encoding control parameter 651 is directly outputted through the correcting section 552 for the pseudo encoding; whereas for the actual encoding, the correcting section 552 corrects the encoding control parameter 651 according to the inputted pseudo encoded signal volume 508 for the optimal encoding and then the resultant parameter is outputted.

20 I I. i Next, a pseudo encoding control signal 506 indicating to execute the actual encoding is delivered from the pseudo/actual encoding control circuit 56, and based on the encoding control parameter 505 thus corrected, the actual encoding operation is achieved on all digital signals stored in the frame memory 52 in the optimum fashion.

According to the embodiment, the datum is extracted from the input frame, the data is once subjected to the *900 w00 10 pseudo encoding operation, the encoding control parameter o ooa is corrected, the actual encoding is executed, and the 0 image is transmitted; however, the image transmission time 0000

C

is the same as that of the prior art technology.

That is, referring now to FIG. 6 illustrating 15 the transmission time of the image encoding/transmitting co00 0° 0 operations in the configuration of FIG; 4 the encoding °w 0 0 time for a frame is from 60 to 70 ms, whereas the line transmission speed is about 100 ms per frame, namely, an idle time of 30 to 40 ms exists before the encoding 20 operation is initiated for the next frame.

*000 0°C o Consequently, when the pseudo encoding is achieved during the idle time, the image encoding and transmission can be efficiently executed.

In the embodiment, although the description has been given to an example in which the same encoding method applies to the pseudo encoding and the actual encoding, the same effect can be developed by applying a simplified encoding method to the actual encoding.

21

PW--

-L -I.

Moreover, in the example of the embodiment above, although the correction of the encoding control param'ter is achieved through an execution of the pseudo encoding, the same effect or the improved effect can be attained by repetitiously effecting the pseudo encoding and the parameter correction several times.

As described above, according to the embodiment, the datum is extracted from the digital signals contained in an input frame, the pseudo encoding 10 is accomplished on the datum, and the encoding control S parameter is corrected depending on the amount of the pseudo encoded signals, which enables to obtain the optimum amount of the encoding signals and to stabilize the image transmission.

4 4 4 $14 t 22

Claims (1)

1. 2. A method as claimed in claim 1, and substantially as herein described with reference to any one of the examples shown in Figures 5 to 7 of the accompanying drawings. DATED THIS 20TH DAY OF SEPTEMBER, 1989 MITSUBISHI DENKI KABUSHI KAISHA e By Its Patent Attorneys V GRIFFITH HACK CO Fellows Institute of Patent Attorneys of Australia Ott 0 ^a a c I 4 t s t( 24