GB2157912A - Video signal transmission systems - Google Patents

Abstract

A "slow-scan" television system, e.g. for surveillance, in which sets of information are transmitted in respect of successive video frames, each such information set defining (1) movement of the current frame 61 relative to the preceding frame and (2) infill of space 62 which would otherwise be created by the movement. Information may also be transmitted in respect of a portion 63 of a complete frame to act as a refresher of the receiving picture. <IMAGE>

Description

SPECIFICATION Video signal transmission systems The present invention relates to video signal transmission systems and more particularly, but not exclusively to such a system for transmitting "slow-scan" television pictures.

In some applications such as surveillance the change in image between successive frames of a television transmission is relatively small. Nevertheless such systems still require a large bandwidth for signal transmission to maintain the image presented at the television receiver.

Some bandwidth reduction techniques already in widespread use include that known as "frame-rate reduction" in which fewer frames than those normally used are transmitted and higher persistence television screens are used to maintain the image between transmitted frames.

Another method used to reduce the bandwidth requirement is "resolution reduction" in which each transmitted frame comprises fewer scan lines and/or lines comprising fewer pixels than would normally be used which leads to a picture having lower visual quality.

These frame rate and resolution reductions are usually limited by the operational requirements of the system user.

It is one object of the present invention to provide a method of and apparatus for video signal transmission in which the bandwidth may be further reduced in comparison with previous systems providing an equivalent quality of transmitted image.

According to one aspect of the present invention in a method of transmitting signals defining successive frames of a sequence of video frames, a signal set is transmitted in respect of each frame and each signal set comprises first signals which define the movement (if any) of the current frame relative to the preceding frame and second signals which define image infill of space which would otherwise be created by said movement (if any).

Preferably each said signal set also includes third signals which define a portion of the complete frame.

According to a second aspect of the present invention in a transmitter for use in transmitting signals defining successive frames of a sequence of video frames, data defining an input video signal is stored in a frame store, analyser means is arranged to determine by correlation between data held in respect of a preceding frame and the data stored in the frame store parameters defining movement (if any) of the current frame with respect to the preceding frame and control means is arranged to cause first signals defining the movement (if any) and second signals defining image infill of portions of the frame from which such movement occurs to be transmitted.

Preferably said analyser means selects data defining a block of n X m pixels of a complete frame (where n and m are small in comparison with the number of pixels in the complete frame) in respect of each received frame and is arranged to correlate said selected data from an immediately preceding frame with data defining a plurality of nX m pixel blocks of the present frame, the analyser selecting said parameters defining movement in dependance upon the peak correlation between the two frames.

A plurality of data blocks each defining a respective n X m pixel blocks and each selected from a differing area of the frame may be used, the analyser means being arranged to correlate each such block from the preceding frame with blocks of the current frame and to determine the movement parameters from an average of the respective peak correlation of each block.

Said control means may be arranged to cause the transmission of third signals in respect of each frame, said third signals defining the image in a portion of the current frame.

According to a third aspect of the present invention in a receiver which is for use in a video signal transmission system and which is adapted to receive signals defining a sequence of video frames which signals comprise in respect of each such frame a signal set comprising first signals defining movement (if any) of the current frame relative to the preceding frame and second signals defining image infill of space which would otherwise be created by such movement, a plurality of frame stores are provided a first of which stores data defining a displayed frame output by the receiver and a second of which is for storing data defining a succeeding frame and control means is responsive to received signal sets in respect of each frame to transfer data from addressed positions of said first frame store to different addressed positions of said second frame store in dependance upon first signals of a received signal set which define movement of the succeeding frame with respect to the displayed frame and is responsive to second signals of a received signal set to write data defining image infill in addressed positions of said second frame store to which no such data transfer has occurred and is arraged to cause subsequent selection of data from said second frame store as data defining the displayed frame output by the receiver.

Preferably said control means is also responsive to third signals of a received signal set to overwrite data at some addressed positions of said second frame store.

The output frame rate of the receiver may be higher than the frame rate of the received signal in which case the control means may implement movement and infill defined by said first and second signals in proportion to the input and output frame rates.

A transmission system comprising a transmitter and receiver in accordance with the invention and using the transmission method of the invention will now be described by way of example only with reference to the accompanying drawings of which: Figure 1 is a block schematic diagram of the transmitter; Figure 2 is a flow chart showing the motion analysis used by an analyser of the transmitter of Fig. 1; Figures 3A Et B are schematic diagrams of hypothetical successive frames held in a frame store of the transmitter of Fig. 1; Figure 4 is a block schematic diagram of the receiver; and Figures 5A 8 B are schematic diagrams showing the interchange of pixel data between the frame stores of Fig. 4.

Referring to Fig. 1 the transmitter receives at an input 1 a standard format 625 line, twenty-five frames per second (fps) composite video signal from a television camera (not shown) such as that known as an isocon camera or a vidicon camera.

A system master clock signal and 10 megaherz sampling frequency are generated by a frequency synthesiser 2 which is locked to the video line frequency. The frequency synthesiser 2 includes a synchronisation separator which determines from the signal at the input 1 the field and line synchronisation of the input signal.

After filtering, the input signal is applied to an analogue to digital convertor 3 which converts the input signal to an eight bit signal per pixel. The eight bit signal is applied to a series in/parallel out convertor 4 which forwards the signal in a four pixel by eight bit format on an input bus 5.

The transmitter is controlled by a microprocessor 6 which directs the operation of the system through a microprogram control unit 7 and which writes to and reads from registers (not separately shown) in other system blocks.

Various functions performed by the microprocessor 6 are described in detail hereinafter.

However it will assist explanation of the system if some of these functions are introduced briefly.

In particular, the microprocessor 6 reads data from the register of a motion analyser 8 and performs all the necessary arithmetical and logical computation necessary to determine motion vectors relating to interframe movement. These motion parameters are coded by the microprocessor 6 and are forwarded to an output buffer 9 for transmission.

The microprocessor 6 also provides start address and stop address data to a read address sequencer 10 and an input address sequencer 11 and reads data relating to the current video input address. It will also be realised that data transfer to the output buffer 9 is also controlled by the microprocessor 6.

The microprocessor 6 may be implemented using a National Semiconductor microprocessor board (type BCL 8086/12B) which uses an Intel 8086 processor.

The system operation will now be described in greater detail. Under control of the input address sequencer 11, a frame store 1 2 which is a two port memory of the kind known as a random-access-memory (RAM) and which has a capacity of 256 kilobytes accepts a frame every eighty milliseconds from the video input bus 5. The frame store 12 accordingly has a frame rate of twelveand-a-half f.p.s. The input address sequencer 11 generates addressing in response to the previously derived frame synchronisation and line synchronisation signals and in response to clock signals received by way of a processor bus 14 from the microprogram control unit 7.

It is here noted that during a video input frame the addressing of the frame store 12 alternates between read and write addresses respectively provided by the read address sequencer 10 and the input address sequencer 11.

Having described the reception of frames by the frame store 1 2 it is necessary to consider the analysis of successive frames to determine the movement parameters. For motion analysis the motion analyser 8 selects a number of n X n blocks of pixels of each frame. The blocks selected may be in fixed positions within a frame or may be in differing positions for each frame. In practice it is preferable to make the number of blocks as high as possible and preferably not less than five blocks are selected each of which is eight pixels by eight pixels.

For each of the blocks in one frame the mean "grey level" is determined in known manner. The mean grey level is then used as a threshold value against which each pixel is compared to generate a binary map indicating whether each pixel has a higher or lower grey level than the mean. The threshold value and binary map are then stored for each such block.

When the next frame is received the mean grey level of the preceding frame is again used and thresholded against each pixel in eight by eight pixel blocks shifted by (i, j) pixels from the corresponding block in the preceding frame where i and j vary respectively in the x and y directions of the frame store.

The thresholding is carried out for a number of i and j values for each block assuming in the present system that movement between frames is limited to sixteen by sixteen pixels.

For each i and j position the respective binary map may now be correlated with the binary map stored for the preceding frame and the i and j which give the highest correlation between the frames is determined.

A set of i and j values are thus obtained which provide a local movement map. Averaging of the set of i and j values provides the movement vectors in the x and y directions of transmission. A flow chart showing the operations of the motion analyser is shown in Fig.

2 in which a Motion Store is shown reference 8', the motion store being a part of the motion analyser 8.

Referring to Figs. 3A and B in which Fig.

3A represents a first frame and Fig. 3B a subsequent frame and consider first the upper schematic diagrams which show a full 512 X 512 pixel frame. Within the frame five blocks of eight by eight pixels 31-35 have been selected for motion analysis, these being selected to allow consideration of movement near to the edges of the frame and near to the centre. In the first frame (Fig. 3A) the motion analyser (8 of Fig. 1) will take from the frame store (12 of Fig. 1) the data representing the five blocks 31-35 and will determine the mean grey level for each of the blocks. For example it will be seen from the expanded representation of the block 34 that the pixel having an 'X' co-ordinate (horizontal) of 64 and a 'Y' co-ordinate (vertical) of 432 being denser together with three others will raise the mean grey level of the block 34.Thresholding as hereinbefore described is now carried out and the mean grey level and binary maps stored in the motion store for each of the five blocks.

Referring also to Fig. 2 in the subsequent frame in which the blocks 31 to 35 have been shown shifted by sixteen pixels to the right and sixteen pixels downwards (the assumed maximum shift) the motion analyser 8 will carry out the operations shown in the flow chart thus: For each of the five blocks (k = 1 ,m) (at 21) the previously stored mean grey level value is retrieved (22) from the motion store 8' and is converted (23) to a threshold value. If, as has been assumed, the maximum interframe shift is sixteen pixels, for each value of i and between plus and minus sixteen (24) an eight by eight pixel block shifted by the appropriate amount is retrieved (25) from the frame store 12, thresholded by mean grey level (26) and compared (27) with the previous frame.Once the operation has been carried out for all values of i and j, the i and j values for peak correlation (27) are determined (28) and when all five blocks 31-35 have been so compared a local movement map is created (29).

From the local movement map average and j values may be determined as the x and y movement co-ordinates. However, to increase confidence in the co-ordinates so obtained any block peak correlation value which is substantially out of line with the other correlation values may be eliminated. Supposing for example that another block of pixels 34' having a similar mean grey level to the block 34 falls within the movement range of the block in the second frame then a high degree of correlation may be found (as shown in the figure) (e.g.) i and j equal to zero indicating that no movement had occurred.

However since for the other blocks 31, 32, 33 and 35 the peak of correlation would be found when i and j both equal sixteen, the result for the block 34 could be eliminated.

Referring again to Fig. 1, having determined the movement co-ordinates the microprocessor 6 causes these to be transmitted by way of the output buffer 9 and then by use of the read address sequencer 10 causes appropriate infill data relating to pixels at the edges of the frame from which movement has occurred to be transferred by way of an image bus 1 5 to the output buffer 9.

Once all the essential data (the movement coordinates and infill data) have been transferred to the output buffer 9 any remaining time in the transmission frame is used to forward "refresh" image data relating to the current frame. Refresh data is used to counteract image degradation in the receiver.

It is here noted that the output buffer 9 comprises a first in, first out (FIFO) register which has two 20 kilobit storage areas. One of these areas stores coded data from the previous frame which data is currently being transmitted whilst the other area receives high speed data from the Image bus 1 5 and the processor bus 14 for the subsequent frame.

At the end of each frame period, the functions of the two areas are interchanged.

It is here noted that the data sent by the transmitter is equivalent to twenty kilobytes per frame at twelve and a half f.p.s. showing a substantial compression from the 256 kilobytes of data per 51 2 X 51 2 pixel frame. The actual transmitter data rate used is two megabits per second.

Referring now to Fig. 4 in the receiver overall system control is again undertaken by a microprocessor 41 which controls data transfers throughout the system.

For providing a video display at an output 42 a display address sequencer 42 selects the output alternatively from two frame stores 43 and 44 to provide an output at twenty-five f.p.s. The output of the frame stores 43 and 44 is in parallel form of eight pixels by eight bits to respective parallel in-serial out registers 45 and 46 the outputs of which are selected in turn under control of the display address sequencer 42 by way of a video output multiplexer 47. The selected output signal is converted to analogue by a digital to analogue converter 48 and line synchronisation and frame synchronisation signals from a synchronisation generator 50 are introduced to the analogue signal in a summing circuit 49.

For the purposes of the description which follows it will be assumed that the frame store 43 is providing the video-output and is thereby storing the current frame whilst the subsequent frame is being assembled in the frame store 44. It will be appreciated that the function of the two frame stores 43 and 44 alternates at the frame rate.

Whilst the frame store 43 is providing a display the microprocessor 41 receives the movement parameters i and j from an input buffer 51. Using these parameters the microprocessor 41 causes a read address to be set up in a read/write address sequencer 53 and a write address (which is the read address shifted by i and j) to be set up in a read/write address sequencer 54. The microprocessor 41 by use of a microprogram control unit 52 now causes data to be read from the frame store 43 by way of a pixel bus 55 to the new address in the frame store 44.When all of the appropriate pixel blocks have been transferred in this manner the microprocessor 41 sets up addresses relating to unwritten positions of the frame store 44 in the read/write address sequencer 54 and causes the transfer of the infill data from the input buffer 51 to those addresses and then effects transfer of any "refresh" data to other positions in that frame stroe.

It is here noted that the input buffer 51 will receive from a channel receiver (not shown) a clock signal locked to the transmitter signal, a frame synchronisation signal indicating the start of coded information relating to the next frame and the coded data.

For the avoidance of doubt, whilst herein it has been stated that one set of movement parameters are determined for each complete frame it will be realised that separate movement parameters for various blocks within a frame may be transmitted, the receiver moving each block in accordance with the differing parameters and infilling any spaces created either with infill data, with data taken from the edge of an adjacent block or with the refresh data.

In this manner the system may be operated to take account of enlargement or contraction of the received image such as would be caused by "zooming" of the camera toward or away from the surveyed scene.

If the displayed frame rate is greater than the received frame rate, the receiver may be arranged to apply the movement parameters received to the interchange of data between the two frame stores 43 and 44 over a number of frames to prevent apparent "jerks" in the displayed image.

To emphasise the manner in which a subsequent frame is generated from a current frame reference is now made to Figs. 5A and B. Fig.

5A represents schematically the data held throughout the frame store. If the received movement parameters i, and j indicate that the image is moving to the right horizontally and down vertically then the data 61 enclosed within the chain dashed line is transferred accordingly. The edges from which movement occurred are now filled with infill data 62 received from the transmitter and received refresh data 63 is written into appropriate address space of the next frame store.

The subsequent frame data is then complete as shown in Fig. 5B.

Claims (11)

1. A method of transmitting signals defining successive frames of a sequence of video frames wherein a signal set is transmitted in respect of each frame and each signal set comprises first signals which define the movement (if any) of the current frame relative to the preceding frame and second signals which define image infill of space which would otherwise be created by said movement (if any).

2. A method according to Claim 1 wherein each said signal set also includes third signals which define a portion of the complete frame.

3. A transmitter for use in transmitting signals defining successive frames of a sequence of video frames wherein data defining an input video signal is stored in a frame store, analyser means is arranged to determine by correlation between data held in respect of a preceding frame and the data stored in the frame store parameters defining movement (if any) of the current frame with respect to the preceding frame and control means is arranged to cause first signals defining the movement (if any) and second signals defining image infill of portions of the frame from which such movement occurs to be transmitted.

4. A transmitter according to Claim 3 wherein said analyser means selects data defining a block of n X m pixels of a complete frame (where n and m are small in comparison with the number of pixels in the complete frame) in respect of each received frame and is arranged to correlate said selected data from an immediately preceding frame with data defining a plurality of n X m pixel blocks of the present frame, the analyser selecting said parameters defining movement in dependance upon the peak correlation between the two frames.

5. A transmitter according to Claim 3 or Claim 4 wherein the analyser means operates in respect of a plurality of data blocks each defining a respective n X m pixel blocks and each selected from a differing area of the frame, the analyser means being arranged to correlate each such block from the preceding frame with blocks of the current frame and to determine the movement parameters from an average of the respective peak correlation of each block.

6. A transmitter according to Claim 3, Claim 4 or Claim 5 wherein said control means is arranged to cause the transmission of third signals in respect of each frame, said third signals defining the image in a portion of the current frame.

7. A receiver which is for use in a video signal transmission system and which is adapted to receive signals defining a sequence of video frames which signals comprise in respect of each such frame a signal set comprising first signals defining movement (if any) of the current frame relative to the preceding frame and second signals defining image infill of space which would otherwise be created by such movement wherein said receiver comprises a pluralty of frame stores a first of which stores data defining a displayed output by the receiver and a second of which is for storing data defining a succeeding frame and control means responsive to received signal sets in respect of each frame to transfer data from addressed positions of said first frame store to different addressed positions of said second frame store in dependance upon first signals of a received signal set which define movement of the succeeding frame with respect to the displayed frame and is responsive to second signals of a received signal set to write data defining image infill in addressed positions of said second frame store to which no such data transfer has occurred and is arranged to cause subsequent selection of data from said second frame store as data defining the displayed frame output by the receiver.

8. A receiver according to Claim 7 wherein said control means is also responsive to third signals of a received signal set to overwrite data at some addressed positions of said second frame store.

9. A receiver according to Claim 8 wherein the output frame rate of the receiver is higher than the frame rate of the received signal, the control means implementing movement and infill defined by said first and second signals in proportion to the input and output frame rates.

10. A method of transmitting signals defining successive frames of a sequence of video frames substantially as hereinbefore described with reference to the accompanying drawings.

11. A transmitter for use in transmitting signals defining successive frames of a sequence of video frames substantially as hereinbefore described with reference to Figs. 1, 2 and 3 of the accompanying drawings.

1 2. A receiver for use in a video signal transmission system substantially as hereinbefore described with reference to Figs. 4 and 5 of the accompanying drawings.