Abstract:
An apparatus and method for performing spatial scalable compression of video information captured in a plurality of frames is disclosed. A base layer encoder uses a first coding standard to encode a bitstream. An enhancement layer encoder uses a second coding standard to encode a residual signal, wherein the residual signal being the difference between the original frames and the upscaled frames from the base layer.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a video encoder/decoder.  
         BACKGROUND OF THE INVENTION  
         [0002]    Because of the massive amounts of data inherent in digital video, the transmission of full-motion, high-definition digital video signals is a significant problem in the development of high-definition television. More particularly, each digital image frame is a still image formed from an array of pixels according to the display resolution of a particular system. As a result, the amounts of raw digital information included in high-resolution video sequences are massive. In order to reduce the amount of data that must be sent, compression schemes are used to compress the data. Various video compression standards or processes have been established, including, MPEG-2, MPEG-4, H.263, and H26L.  
           [0003]    Many applications are enabled where video is available at various resolutions and/or qualities in one stream. Methods to accomplish this are loosely referred to as scalability techniques. There are three axes on which one can deploy scalability. The first is scalability on the time axis, often referred to as temporal scalability. Secondly, there is scalability on the quality axis (quantization), often referred to as signal-to-noise (SNR) scalability or fine-grain scalability. The third axis is the resolution axis (number of pixels in image) often referred to as spatial scalability. In layered coding, the bitstream is divided into two or more bitstreams, or layers. Each layer can be combined to form a single high quality signal. For example, the base layer may provide a lower quality video signal, while the enhancement layer provides additional information that can enhance the base layer image.  
           [0004]    In particular, spatial scalability can provide compatibility between different video standards or decoder capabilities. With spatial scalability, the base layer video may have a lower resolution than the input video sequence, in which case the enhancement layer carries information which can restore the resolution of the base layer to the input sequence level.  
           [0005]    [0005]FIG. 1 illustrates a known spatial scalable video encoder  100 . The depicted encoding system  100  accomplishes layer compression, whereby a portion of the channel is used for providing a low resolution base layer and the remaining portion is used for transmitting enhancement information, whereby the two signals may be recombined to bring the system up to high-resolution. A high resolution video input Hi-Res is split by splitter  102  whereby the data is sent to a low pass filter  104  and a subtraction circuit  106 . The low pass filter  104  reduces the resolution of the video data, which is then fed to a base encoder  108 . In general, low pass filters and encoders are well known in the art and are not described in detail herein for purposes of simplicity. The encoder  108  produces a lower resolution base stream which can be broadcast, received and via a decoder, displayed as is, although the base stream does not provide a resolution which would be considered as high-definition.  
           [0006]    The output of the encoder  108  is also fed to a decoder  112  within the system  100 . From there, the decoded signal is fed into an interpolate and upsample circuit  114 . In general, the interpolate and upsample circuit  114  reconstructs the filtered out resolution from the decoded video stream and provides a video data stream having the same resolution as the high-resolution input. However, because of the filtering and the losses resulting from the encoding and decoding, loss of information is present in the reconstructed stream. The loss is determined in the subtraction circuit  106  by subtracting the reconstructed high-resolution stream from the original, unmodified high-resolution stream. The output of the subtraction circuit  106  is fed to an enhancement encoder  116  which outputs a reasonable quality enhancement stream.  
         SUMMARY OF THE INVENTION  
         [0007]    Although the known layered compression schemes can be made to work quite well, these schemes still have a problem in that the enhancement layer needs a high bitrate. Normally, the bitrate of the enhancement layer is equal to or higher than the bitrate of the base layer. However, the desire to store or broadcast high definition video signals calls for lower bitrates than can normally be delivered by common compression standards. This can make it difficult to introduce high definition on existing standard definition systems, because the recording/playing time becomes too small or the required bandwidth becomes too large. Thus, there is a need for a more efficient spatial scalable compression scheme which reduces the bitrate of the enhancement layer. The invention overcomes at least part of the deficiencies of other known layered compression schemes by using different coding standards in the base encoder and the enhancement encoder.  
           [0008]    According to one embodiment of the invention, an apparatus and method for performing spatial scalable compression of video information captured in a plurality of frames is disclosed. A base layer encoder uses a first coding standard to encode a bitstream. An enhancement layer encoder uses a second coding standard to encode a residual signal, wherein the residual signal being the difference between the original frames and the upscaled frames from the base layer. It is preferred that the input to the enhancement coder is modified into a signal with a signal level range of a normal video input signal. Such a modification can be performed by adding a DC-offset, preferably such that the pixel values of the enhancement coder input are shifted to the middle of a predetermined input range.  
           [0009]    According to another embodiment of the invention, a method and apparatus for providing spatial scalable compression of a video stream is disclosed. The video stream is downsampled to reduce the resolution of the video stream. The downsampled video stream is encoded using a first encoding standard to produce a base stream. The base stream is decoded and upconverted to produce a reconstructed video stream. The reconstructed video stream is subtracted from the video stream to produce a residual stream. The residual stream is encoded using a second encoding standard and outputs an enhancement stream.  
           [0010]    According to another embodiment of the invention, a method and apparatus for decoding compressed video information received in a base stream and an enhancement stream is disclosed. The base stream is decoded using a first encoding standard. The decoded base stream is upconverted to increase the resolution of the decoded base stream. The enhancement stream is decoded using a second encoding standard. The upconverted decoded base stream with the decoded enhancement stream are combined to produce a video output.  
           [0011]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereafter. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The invention will now be described, by way of example, with reference to the accompanying drawings, wherein:  
         [0013]    [0013]FIG. 1 is a block diagram representing a known layered video encoder;  
         [0014]    [0014]FIG. 2 is a block diagram of a layered video encoder according to one embodiment of the invention;  
         [0015]    [0015]FIG. 3 is a block diagram of a layered video decoder according to one embodiment of the invention; and  
         [0016]    [0016]FIG. 4 is a block diagram of a section of an encoder according to one embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    According to one embodiment of the invention, spatial scalable compression is achieved in a layered encoder by using a first coding standard for the base layer and a second coding standard for the enhancement layer. FIG. 2 illustrates a layered encoder  200  which can be used to implement the invention. It will be understood by those skilled in the art that other layered encoders can also be used to implement the invention and the invention is not limited thereto.  
         [0018]    The depicted encoding system  200  accomplishes layer compression, whereby a portion of the channel is used for providing a low resolution base layer and the remaining portion is used for transmitting edge enhancement information, whereby the two signals may be recombined to bring the system up to high-resolution. A high resolution video input Hi-RES is split by a splitter  202  whereby the data is sent to a low pass filter  204  and a subtraction circuit  206 . The low pass filter  204  reduces the resolution of the video data, which is then fed to a base encoder  208 . In general, low pass filters and encoders are well known in the art and are not described in detail herein for purposes of simplicity. The encoder  208  uses a first coding standard to produce a lower resolution base stream BS which can be broadcast, received and via a decoder, displayed as is, although the base stream does not provide a resolution which would be considered as high-definition. The first coding standard can be any video compression scheme such as MPEG-2, MPEG-4, H263, H26L, etc., but the invention is not limited thereto.  
         [0019]    The output of the encoder  208  is also fed to a decoder  212  within the system  200 . From there, the decoded signal is fed into an interpolate and upsample circuit  214 . In general, the interpolate and upsample circuit  214  reconstructs the filtered out resolution from the decoded video stream and provides a video data stream having the same resolution as the high-resolution input. However, because of the filtering and the losses resulting from the encoding and decoding, loss of information is present in the reconstructed stream. The loss is determined in the subtraction circuit  206  by subtracting the reconstructed high-resolution stream from the original, unmodified high-resolution stream to produce a residual signal. The output of the subtraction circuit  206  is fed to an enhancement encoder  216 . The enhancement encoder  216  uses a second coding standard, which is different from the first coding standard to encode the residual signal and outputs a reasonable quality enhancement stream ES. The second coding standard can be any video compression scheme such as MPEG-1, MPEG-2, MPEG-4, H263, H26L, H264, proprietary video coding methods, etc, and the invention is not limited thereto. This embodiment offers the possibility to provide a base stream which is compatible with a first coding standard and an enhancement stream which is compatible with a second standard, e.g. an advantageous new standard. In the particular example where an MPEG encoder is used for the base layer and a H26L encoder is used for the enhancement layer, a factor of at least 2 can be gained on the bitrate of the enhancement stream.  
         [0020]    [0020]FIG. 3 illustrates a decoder  300  for decoding the encoded signals produced by the layered encoder  200 . The base stream is decoded in a decoder  302  using the first coding standard. The output of the decoder  302  is a SDTV output. The enhancement stream is decoded in a decoder  304  using the second coding standard. The output of the decoder is combined with the decoded base stream which has been upconverted in an upconverted  306  in an addition unit  308 . The output of the addition unit  308  is an HDTV output.  
         [0021]    According to another embodiment of the invention, different quantization schemes can also be used in the base encoder and the enhancement encoder. FIG. 4 illustrates a section of an encoder  400  which can be used in both the base encoder and the enhancement encoder. The encoder  400  comprises, among other features, a DCT circuit  402 , a quantizer  404  and a variable length encoder  406 . The DCT circuit  402  performs DCT processing on the input signal so as to obtain DCT coefficients which are supplied to the quantizer  404 . The quantizer  404  sets a quantization step (quantization scale) in accordance with the data storage quantity in a buffer (not illustrated) received as a feedback and quantizes the DCT coefficients from the DCT circuit  402  using the quantization step. The quantized DCT coefficients are supplied to the VLC unit  406  along with the set quantization step. According to one embodiment of the invention, a first quantization scheme is used by the quantizer in the base encoder and a second quantization scheme, which is different from the first quantization scheme, is used by the quantizer in the enhancement encoder. For example, an adaptive (non-uniform within the macroblock of a frame) quantization scheme is used for the base encoder (which is using MPEG-2 encoding) and a uniform (within the macroblock of one frame) quantization scheme is used for the enhancement encoder (which is using H26L encoding).  
         [0022]    The above-described embodiments of the invention can be applied to two layer DVDs where the first layer is the SD base layer and the first plus second layer make up the HD-sequence. This method could also be used to gradually introduce HD broadcast in Europe and China, with extending the SD-DVB signal with an enhancement layer. This method could also be applied to store programs layered on a disk for elastic storage.  
         [0023]    It will be understood that the different embodiments of the invention are not limited to the exact order of the above-described steps as the timing of some steps can be interchanged without affecting the overall operation of the invention. Furthermore, the term “comprising” does not exclude other elements or steps, the terms “a” and “an” do not exclude a plurality and a single processor or-other unit may fulfill the functions of several of the units or circuits recited in the claims.