Abstract:
The present invention relates to a method of and an apparatus for multiplexing a number of video signals, in which to save circuits for compressors the video signals are reduced in their information, combined into a single video signal, and then the video signal is compressed by one compressor.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of multiplexing a number of video signals with a compressor. 
     BACKGROUND INFORMATION 
     In order to be able to transmit a video signal digitally, a standard has been defined by ISO/IEC and laid down as MPEG2 Standard ISO/IEC 13818 for video compression, audio compression and multimedia multiplexing. European Published Patent Application 0 855 840, which refers to a system with a digital signal multiplexer and a digital signal demultiplexer. In the multiplexer of this system a first number of digital video signals are first multiplexed into a multiplexed digital signal in order to send this to the demultiplexer of this system. The digital video signals are all television signals for broadcast programmes. Each programme is alternatively also referred to as a channel. Each digital video signal contains either a digital audio signal or a data signal or both. Digital signals coming from video sources are coded, packed, multiplexed in the multiplexer and then passed as a serial data stream to a transfer section. On the other side of the transfer section a signal of a television channel is filtered out from the serial data stream. The multiplexer, however, is not explained in detail here and no process of compression is specified. 
     SUMMARY OF THE INVENTION 
     The present invention includes an exemplary method of compression, in which a multiplexer is provided having a computer structure of a memory and control. 
     According to a first exemplary embodiment of the present invention, the video signals are reduced as regards their information, then combined into a single video signal, and the video signal is then compressed by the one compressor, which may save circuits for compressors. 
     According to a second exemplary embodiment of the present invention, television pictures of the video signals are stored alternately in a memory and then the television pictures are read out serially in blocks from the memory and compressed by a compressor. To achieve a high video compression rate, the correlation between successive images should be as high as possible. This, however, may only be possible if only one camera is connected to the compressor. If several cameras are connected via an input switching device, the image after switching to another camera may be totally different from the preceding image. In this case the compression rate may be too low, resulting in a high data flow, which may be avoided with the storage and subsequent compression of successive images of a channel in blocks. 
     According to a third exemplary embodiment of the present invention, a first television picture of a sequence of several video signals is compressed by a compressor each time and stored as an intermediate image in a memory, whereupon subsequent images of the sequences are compressed alternately by the compressor in dependence on the intermediate image associated with the video signal in each case. Thus, the television pictures of various video signals or various channels reach the compressor alternately, while nevertheless a high compression rate may be achieved by a time-delayed access to the associated intermediate images. 
     According to a fourth exemplary embodiment of the present invention, a television picture of sequences of several video signals from a compressor is stored as a compressed intermediate image in a memory, and then successive images are compressed alternately by a second compressor in dependence on the relevant intermediate image. This process modification proposes a two-stage compression in which a single-image compression takes place in a first compressor in a first stage and a motion compression in a second compressor in a second stage. The compression may be performed alternately by the two compressors for several video signals or video channels. In the second or motion compressor, the preceding image to be compared with the subsequent image of a channel is read from the memory with a time delay, so that a high compression rate may be achieved for the subsequent image to be compressed in each case. 
     In an exemplary embodiment, a multiplexer for the conversion of video signals into a serial digital data stream with a compressor comprises a partial image generator with a circuit designed to reduce video signal information and a further circuit designed to combine several video signals into one video signal. Thus, only one video signal is compressed. 
     In a further exemplary embodiment, a memory is arranged between an input switching device and a compressor in a multiplexer for converting a number of video signals into a serial digital data stream. The memory includes an arrangement for alternate writing of digitized video signals and for reading of the digitized video signals serially in blocks. A block compression and hence a high data rate may be achieved by using the memory. 
     In an exemplary embodiment, the memory has two memories parts. While data are written into the one memory part, data are read from the second. This process is repeated alternately for the two memories parts. 
     In a further exemplary embodiment, a compressor in a multiplexer for converting a number of video signals into one serial digital data stream has a memory for images of several video signals. The compressor stores the intermediate results in the memory and processes data several times to achieve a high compression rate. 
     In a further exemplary embodiment, to achieve a high data compression the compressor in a multiplexer for converting a number of video signals into one serial digital data stream has a series arrangement of a first intermediate image compressor designed to generate an intermediate image, a memory for storing intermediate images, and a second or motion compressor for motion compression. The memory has a small memory capacity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of the circuit of an exemplary multiplexer which reduces television pictures generated by four cameras and outputs each television picture as a partial image in a quadrant of a single television picture in a multiplexed format. 
         FIG. 2A  is a block diagram of the circuit of an exemplary multiplexer with an intermediate memory in which television pictures generated by four cameras are intermediate-stored, and with a compressor which compresses the images channel by channel. 
         FIG. 2B  shows a frame with a sequence of television pictures upstream of the intermediate memory. 
         FIG. 2C  shows a frame with a sequence of television pictures downstream of the intermediate memory. 
         FIG. 3A  is a block diagram of the circuit of an exemplary multiplexer with a compressor and with an intermediate memory in which compressed television pictures generated by four cameras and the compression settings are stored. 
         FIG. 3B  shows a frame with a sequence of television pictures upstream of the compressor. 
         FIG. 4  is a block diagram of the circuit of an exemplary multiplexer with a compressor which has an intermediate image compressor, an intermediate image memory, and a motion compressor connected in series. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows four television cameras  1  to  4  connected via signal cables  5  to  8  to a partial image generator  9  of an exemplary multiplexing apparatus  10 , also referred to below as a multiplexer. Here the partial image generator  9  has four connections  11  to  14  for the signal cables  5  to  8 . Pictures are taken with cameras  1  to  4 . The cameras  1  to  4  generate electrical signals for the pictures which are output to the partial image generator  9  as video signals via the signal cables  5  to  8 . The television cameras  1  to  4  act as video sources and produce digital video signals which are used as television signals in broadcast programs. Each program is also referred to alternatively as a channel, table, or frame. Each digital video signal contains either a digital audio signal, or a data signal, or both. The partial image generator  9  contains a reducing circuit  15  to reduce video signal information and a combination circuit  16  to combine several video signals into one video signal. A signal from the partial image generator  9  is given to a compressor  17  which compresses the signal. The compressed signal is given by the compressor  17  via a switch  18  either to a transmitter  19  and hence to the transmission section  20  or to a digital video recorder  21  for recording. 
     The function of the multiplexer  10  is as follows: the four television signals generated by the cameras  1  to  4  are reduced to one quarter of their size in the reducing circuit  15  of the partial image generator  9  by blanking out of information, and then combined into a single television signal in the combination circuit  16  of the partial image generator  9 . Thus the television pictures taken by the four cameras  1  to  4  may be shown simultaneously on a screen each in one quadrant of the single television picture. The television signal of the single television picture is given to the compressor  17  by the partial image generator  9  and compressed in the compressor  17  in accordance with MPEG2 compression. 
     The undesired feature of this process is that on replay of a program the image is reproduced unsharply as information has been lost in the combining of the four partial images into a single picture. 
       FIG. 2A  shows an exemplary multiplexer  30  with an input switching device  31 , an intermediate memory  32 , and a compressor  33 . The input switching device  31  has four connections  34  to  37  for four cameras, the four television signals are sent to the input switching device  31 . The signals of all four television pictures are transferred to the intermediate memory  32 , which comprises two partial memories  38  and  39  each with a memory space for 4×10 television pictures. The intermediate memory acts as a buffer shift register. Each 10 successive television pictures from all four cameras can be temporarily stored in one of the two partial memories  38  and  39 . 
     The function of the multiplexer  30  is as follows: the input switching device  31  synchronizes the incoming television pictures and transfers the images into the first partial memory  38 , while switching takes place after the image of one channel each time, so that the images are transferred alternately. When the first partial memory  38  is filled, switching takes place between the two partial memories, and the second partial memory  39  is filled with television pictures by the input switching device  31 . While the second partial memory  39  is being filled, the first partial memory is emptied. Hence, the first partial memory  38  outputs its images, 10 television pictures of a channel each time, in succession and hence serially in blocks, to the compressor  33 . The compressor  33  compresses the images. As the images for each channel reach the compressor  33  serially, a high compression rate can be achieved. Every 10th image, i.e. image numbers  1 ,  11 ,  21 , etc., is then an intermediate image from which compression can be performed forwards or backwards. 
       FIG. 2B  shows a frame  40  with a sequence of television pictures as given by the input switching device  31  to the partial memories  38  and  39 . The images are written alternately into the memory  32 . A first natural number before the decimal point defines the camera, and a second natural number after the decimal point the sequence of images as sent out by the corresponding camera. 
       FIG. 2C  shows the sequence of television pictures in a frame  41  in which these are output from one of the partial memories  38  or  39  to the compressor  33 . Each partial memory  38  and  39  thus outputs the 10 images of the first channel, then the 10 images of the second channel, then the 10 images of the third channel, and finally the 10 images of the fourth channel serially in blocks to the compressor  33 . 
       FIG. 3A  shows a multiplexer  50  with an input switching device  51 , a compressor  52 , and an intermediate memory  53  which reads out the values written first as well, i.e. first in, first out or FIFO. The input switching device  51  has four connections  54  to  57  for four cameras, which deliver four television signals to the input switching device  51 . Output signals of the compressor  52  are given to a switch  58 . The compressor  52  processes data and stores the data in the memory  53 , the same stored data are then transported to the compressor  52  again for further processing. 
       FIG. 3B  shows a frame  59  with reference to which the functions of the multiplexer  50  will be explained: the input switching device  51  synchronizes the incoming video signals and gives these to the compressor  52 . First the first picture from the first camera, then the first picture from the second camera, then the first picture from the third camera, and then the first picture from the fourth camera are given to the compressor  52 . Then follows the second picture from the first camera, the second picture from the second camera, the second picture from the third camera, and the second picture from the fourth camera. The compressor  52  compresses the first picture from the first camera and passes this compressed image as an output signal to switch  58  and also to the memory  53 . Then the first picture from the second camera is compressed in the compressor  53  and both passed as an output signal to switch  58  and at the same time written to memory  53 . The first picture from the third camera is also compressed and output to switch  58  and written into the memory  53 . The same happens with the first picture from the fourth camera. At the start of the second cycle, the compressed first picture from the first camera is loaded from the memory  53  into the compressor  52  and compared with the second picture from the first camera which is given to the compressor  52  by the input switching device  51 . An MPEG2 compression takes place in the compressor  52  on the basis of an image comparison. The corresponding settings are stored in the memory  53  and at the same time output to switch  58 . The same happens for the second pictures from the second, third, and fourth cameras. A third cycle then takes place with the third pictures from the first, second, third and fourth cameras. After every ten or twelve pictures from a camera, a new intermediate image is set from which compression can take place in forward or backward direction. 
       FIG. 4  shows a multiplexer  70  with an input switching device  71  and a compressor  72 . The input switching device  71  has four connections  73  to  76  for four cameras, which output four television signals to the input switching device  71 . The signals from all four television pictures are transferred to the compressor  72 , as shown in frame  59 . The compressor  72  has a first intermediate image compressor  77 , an intermediate memory  78 , and a second motion compressor  79  and acts as an MPEG2 compressor, typically compressing every tenth or twelfth image from a camera into a compressed intermediate image known as an intra-frame or I-frame for short, i.e. a JPEG compressed single image. In the MPEG2 process, the I-frames are sent as intermediate images at regular intervals, preferably after ten or twelve transferred images. By using the intermediate images, a new intermediate image may be provided as a starting image in the case of failure of the transfer section after the tenth or twelfth image at the latest. On cross-fading, too, a new intermediate image is sent as a start image. Only the changes with respect to the previous image or previous I-frame are stored so as to achieve a high data rate on the digital video recorder or transfer section, in addition to the JPEG compression method of the other intermediate images. Changes within an image may occur only if an object is moved. The data quantity or data rate may thus be reduced. Between these I-frames, data are only transferred for moved image segments. In the MPEG process, therefore, only those data are transferred which identify the modified image segments. The input switching device  71  constantly supplies all pictures from all four cameras to the first compressor  72 , which for each camera produces an I-frame after ten or twelve images or after cross-fading. The I-frame is temporarily stored in the intermediate memory  78  and retained there for a period of ten or twelve images if no cross-fading occurs. Furthermore, compression settings for subsequent images are also temporarily stored. The memory  78  is accordingly dimensioned such that four compressed television pictures, one for each camera, and compression settings can be stored. If an MPEG2 process with ten or twelve images is assumed, an I-frame will be produced for every tenth or twelfth image of a camera, i.e. sequential image numbers  1 ,  11 ,  21 ,  31 , etc. or  1 ,  13 ,  25 ,  37 , etc. The I-frame and the compression settings are supplied constantly to the second compressor  79  over a period of ten or twelve images and simultaneously all intermediate images  2  to  10 ,  12  to  20 ,  22  to  30  or  2  to  12 ,  24  to  24 ,  26  to  36 , etc. of each camera are looped through to the motion compressor  79 . The second compressor  79  compares the I-frame with the new image with regard to compression settings of the camera concerned, and produces only those data which identify changes with respect to the preceding image or I-frame for images between the I-frames. An I-frame is delivered to a switch  80  for each camera every ten or twelve images, and in between those data relating to the other images which contain changes of an image in relation to the preceding image or I-frame.