MPEG DECODER PROVIDING MULTIPLE STANDARD OUTPUT SIGNALS

Abstract: A video decoder compliant with the Advanced Television Systems Standard (ATSC) includes circuitry which, when the decoder is operated in a first mode, decodes a Main Profile, High Level (MP@HL) image to produce a high-definition video output signal and decodes a Main Profile, Main Level (MP@ML) signal to produce a standard definition video signal. In addition, when the decoder is operated in a second mode, circuitry is used which generates a standard definition image from the MP@HL signal. The video decoder includes a frequency-domain filter to reduce the resolution of the MP@HL signal when the decoder is operated in the second mode.

Description: MPEG DECODER PROVIDING MULTIPLE STANDARD OUTPUT SIGNALS This patent application claims the benefit of U. S. Provisional Application number 60/040,517 filed March 12, 1997. The entire disclosure of U.S. Provisional Application No. 60/040,517 filed March 12, 1997 is expressly incorporated herein by reference. FIELD OF THE INVENTION This invention relates to a decoder for receiving, decoding and conversion of frequency domain encoded signals, e.g. MPEG-2 encoded video signals, into standard output video signals, and more specifically to a decoder which formats an encoded high resolution video signal to a decoded output video signal having a user-selected resolution. BACKGROUND OF THE INVENTION In the United States a standard, the Advanced Television System Committee (ATSC) standard defines digital encoding of high definition television (HDTV) signals. A portion of this standard is essentially the same as the MPEG-2 standard, proposed by the Moving Picture Experts Group (MPEG) of the International Organization for Standardization (ISO). The standard is described in an International Standard (IS) publication entitled, "Information Technology - Generic Coding of Moving Pictures and Associated Audio, Recommendation H.626", ISO/IEC 13818-2, IS, 11/94 which is available from the ISO and which is hereby incorporated by reference for its teaching on the MPEG-2 digital video coding standard. The MPEG-2 standard is actually several different standards. In MPEG-2 several different profiles are defined, each corresponding to a different level of complexity of the encoded image. For each profile, different levels are defined, each level corresponding to a different image resolution. One of the MPEG-2 standards, known as Main Profile, Main Level is intended for coding video signals conforming to existing television standards (i.e., NTS C and PAL). Another standard, known as Main Profile, High Level is intended for coding high-definition television images. Images encoded according to the Main Profile, High Level standard may have as many as 1,152 active lines per image frame and 1,920 pixels per line. The Main Profile, Main Level standard, on the other hand, defines a maximum picture size of 720 pixels per line and 567 lines per frame. At a frame rate of 30 frames per second, signals encoded according to this standard have a data rate of 720 * 567 * 30 or 12,247,200 pixels per second. By contrast, images encoded according to the Main Profile, High Level standard have a maximum data rate of 1,152 * 1,920 * 30 or 66,355,200 pixels per second. This data rate is more than five times the data rate of image data encoded according to the Main Profile Main Level standard. The standard for HDTV encoding in the United States is a subset of this standard, having as many as 1,080 lines per frame, 1,920 pixels per line and a maximum frame rate, for this frame size, of 30 frames per second. The maximum data rate for this standard is still far greater than the maximum data rate for the Main Profile, Main Level standard. The MPEG-2 standard defines a complex syntax which contains a mixture of data and control information. Some of this control information is used to enable signals having several different formats to be covered by the standard. These formats define images having differing numbers of picture elements (pixels) per line, differing numbers of lines per frame or field and differing numbers of frames or fields per second. In addition, the basic syntax of the MPEG-2 Main Profile defines the compressed MPEG- 2 bit stream representing a sequence of images in five layers, the sequence layer, the group of pictures layer, the picture layer, the slice layer, and the macroblock layer. Each of these layers is introduced with control information. Finally, other control information, also known as side information, (e.g. frame type, macroblock pattern, image motion vectors, coefficient zig-zag patterns and dequantization information) are interspersed throughout the coded bit stream. Format conversion of encoded high resolution Main Profile, High Level pictures to lower resolution Main Profile, High Level pictures; Main Profile, Main Level pictures, or other lower resolution picture formats, has gained increased importance for a) providing a single decoder for use with multiple existing video formats, b) providing an interface between Main Profile, high level signals and personal computer monitors or existing consumer television receivers, and c) reducing implementation costs of HDTV. For example, conversion allows replacement of expensive high definition monitors used with Main Profile, High Level encoded pictures with inexpensive existing monitors which have a lower picture resolution to support, for example, Main Profile, Main Level encoded pictures, such as NTSC or 525 progressive monitors. One aspect, down conversion, converts a high definition input picture into lower resolution picture for display on the lower resolution monitor. To effectively receive the digital images, a decoder should process the video signal information rapidly. To be optimally effective, the coding systems should be relatively inexpensive and yet have sufficient power to decode these digital signals in real time. Consequently, a decoder supporting multiple formats must minimize processor memory. SUMMARY OF THE INVENTION The present invention is embodied in a digital video decoder which operates in first and second modes. The digital video decoder receives an encoded video signal at a terminal, and includes decoding circuitry which operates in 1) a first mode to decode the received encoded video signal to provide a decoded video signal having a first spatial resolution; and 2) a second mode to provide the decoded video signal having a second spatial resolution which is less than the first spatial resolution. The digital video decoder further includes: formatting circuitry which includes a spatial filter that, when the decoding circuitry operates in the second mode, is responsive to a formatting control signal to process the decoded video signal to change the spatial resolution of the decoded video signal to produce at least one display video signal having a respective display spatial resolution, different from the first and second spatial resolutions; and a controller for switching the decoder between the first and second modes and for providing the formatting control signal to the formatting circuitry. BRIEF DESCRIPTION OF THE DRAWINGS These and other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, wherein: Figure 1 A is a high level block diagram of a video decoding and format conversion system according to an exemplary embodiment of the present invention. Figure IB is a high level block diagram showing the functional blocks of the ATv * Video Decoder including an interface to external Memory as employed in an exemplary embodiment of the present invention. Figure 2A is a high level block diagram of a video decoder as employed by an exemplary embodiment of the present invention. Figure 2B is a high level block diagram of the down conversion system as employed by an exemplary embodiment of the present invention. Figure 2C is a block diagram which illustrates a configuration of the decoder shown in Figure 2A which is used to decode a video signal in 750P format. Figure 2D is a block diagram which illustrates a configuration of the decoder shown in Figure 2B which is used to decode a video signal in 11251 format including a downconversion by factor of 3 to 525P/525I format. Figure 2E is a block diagram which illustrates a configuration of the decoder shown in Figure 2B which is used to decode a video signal in 750P format including a downconversion by factor of 2 to 525P/525I format. Figure 3A is a pixel chart which illustrates subpixel positions and corresponding predicted pixels for the 3: 1 and 2: 1 exemplary embodiments of the present invention. Figure 3B is a flow-chart diagram which shows the upsampling process which is performed for each row of an input macroblock for an exemplary embodiment of the present invention. Figure 4 is a pixel chart which illustrates the multiplication pairs for the first and second output pixel values of an exemplary embodiment of a block mirror filter. Figure 5 is a block diagram which illustrates an exemplary implementation of the filter for down-conversion for a two-dimensional system processing the horizontal and vertical components implemented as cascaded one-dimensional IDCTs. Figure 6A is a macroblock diagram which shows the input and decimated output pixels for 4:2:0 video signal using 3:1 decimation. Figure 6B is a pixel block diagram which shows the input and decimated output pixels for 4:2:0 video signal using 2:1 decimation. Figure 6C is a macroblock diagram which illustrates a merging process of two macroblocks into a single macroblock for storage in memory for downconversion by 2 horizontally. Figure 6D is a macroblock diagram which illustrates a merging process of three macroblocks into a single macroblock for storage in memory for downconversion by 3 horizontally. Figure 7A is a block diagram illustrating a vertical programmable filter of one embodiment of the present invention. Figure 7B is a pixel diagram which illustrates the spatial relationships between vertical filter coefficients and a pixel sample space of lines of the vertical programmable filter of Figure 7A. Figure 8A is a block diagram illustrating a horizontal programmable filter of one embodiment of the present invention. Figure 8B is a pixel diagram which illustrates spatial relationships between horizontal filter coefficients and pixel sample values of one embodiment of the present invention. Figure 9A is a graph of pixel number versus resampling ratio which illustrates a resampling ratio profile of an exemplary embodiment of the present invention. Figure 9B is a graph which shows a first ratio profile for mapping a 4:3 picture onto a 16:9 display. Figure 9C is a graph which shows a second ratio profile for mapping a 4:3 picture onto a 16:9 display. Figure 9D is a graph which shows a first ratio profile for mapping a 16:9 picture onto a 4:3 display. Figure 9E is a graph which shows a second ratio profile for mapping a 16:9 picture onto a 4:3 display. Figure 10 is a chart of image diagrams which illustrates the effect of using resampling ratio profiles according to an exemplary embodiment of the present invention. Figure 11A is a high level block diagram illustrating the display section of the ATV Video Decoder of an exemplary embodiment of the present invention. Figure 1 IB is a block diagram which illustrates a 27 MHz Dual output mode of an exemplary embodiment of the present invention which, for which the video data is 525P or 5251, a first processing chain provides video data to a 27 MHz DAC well as to an NTSC Encoder. Figure 11C is a block diagram which illustrates that, in the 27 MHz single output mode of an exemplary embodiment of the present invention, only a 5251 video signal is provided to a NTSC encoder. Figure 1 ID is a block diagram which illustrates a 74 MHz /27 MHz MHz mode of an exemplary embodiment of the present invention in which the output format matches the input format and the video data is provided to either a 27 MHz DAC or 74 MHz DAC depending on the input format. Figure 12 is a high level block diagram of the video decoder having high bandwidth memory as employed by an exemplary embodiment of the present invention to decode only Main Profile, Main Level MPEG-2 ATSC video signals. Figure 13 A is a top half of a timing diagram which illustrates the process and flow times associated with reading, storing and displaying picture information when the input video is in a field format. Figure 13B is a bottom half of a timing diagram which illustrates the process and flow times associated with reading, storing and displaying picture information when the input video is in a field format. Figure 14A is a top half of a timing diagram which illustrates the process and flow times associated with reading, storing and displaying picture information when the input video is in a frame format. Figure 14B is a bottom half of a timing diagram which illustrates the process and flow times associated with reading, storing and displaying picture information when the input video is in a frame format. Figure 15A is a top half of a timing diagram which illustrates the process and flow times associated with reading, storing and displaying picture information when the input video is in a progressive sequence and the display is in an interlaced sequence. Figure 15B is a bottom half of a timing diagram which illustrates the process and flow times associated with reading, storing and displaying picture information when the input video is in a progressive sequence and the display is in an interlaced sequence. Figure 16A is a top half of a timing diagram which illustrates the process and flow times associated with reading, storing and displaying picture information when the input video is in a progressive sequence and the display is in a progressive sequence. Figure 16B is a bottom half of a timing diagram which illustrates the process and flow times associated with reading, storing and displaying picture information when the input video is in a progressive sequence and the display is in a progressive sequence. Figure 17A is a memory map diagram which shows how the memory is utilized when the decoder is configured to provide a high-definition video signal. Figure 17B is a memory map diagram which shows how a row of image macroblocks maps into a memory row when a type of image is decoded. Figure 18A is a memory map diagram which shows how the memory is utilized when the decoder is configured to provide a standard-definition video signal. Figure 18B is a memory map diagram which shows how a row of image macroblocks maps into a memory row when another type of image is decoded. Figure 19A is a memory map diagram which shows how a reduced memory is utilized when the decoder is configured to provide a high-definition video signal. Figure 19B is a memory map diagram which shows how a row of image macroblocks maps into a memory row when another type of image is decoded. DETAILED DESCRIPTION System Overview The exemplary embodiments of the invention decode conventional HDTV signals which have been encoded according to the MPEG-2 standard and in particular, the Main Profile High Level (MP@HL) and the Main Profile Main Level (MP@ML) MPEG- 2 standards, and provides the decoded signals as video signals having a selected one of multiple formats. The MPEG-2 Main Profile standard defines a sequence of images in five levels: the sequence level, the group of pictures level, the picture level, the slice level, and the macroblock level. Each of these levels may be considered to be a record in a data stream, with the later-listed levels occurring as nested sub-levels in the earlier listed levels. The records for each level include a header section which contains data that is used in decoding its sub-records. Each macroblock of the encoded HDTV signal contains six blocks and each block contains data representing 64 respective coefficient values of a discrete cosine transform (DCT) representation of 64 picture elements (pixels) in the HDTV image. In the encoding process, the pixel data may be subject to motion compensated differential coding prior to the discrete cosine transformation and the blocks of transformed coefficients are further encoded by applying run-length and variable length encoding techniques. A decoder which recovers the image sequence from the data stream reverses the encoding process. This decoder employs an entropy decoder (e.g. a variable length decoder), an inverse discrete cosine transform processor, a motion compensation processor, and an interpolation filter. The video decoder of the present invention is designed to support a number of different picture formats, while requiring a minimum of decoding memory for downconversion of high resolution encoded picture formats, for example, 48 Mb of Concurrent Rambus dynamic random access memory (Concurrent RDRAM). Figure 1 A shows a system employing an exemplary embodiment of the present invention for receiving and decoding encoded video information at MP@HL or at MP@ML, formatting the decoded information to a user selected output video format (which includes both video and audio information), and interfaces for providing the formatted video output signals to display devices. The exemplary embodiments of the present invention are designed to support all ATSC video formats. For simplicity, the operation is divided into a Down Conversion (DC) mode which receives any MPEG Main Profile video bitstream (constrained by FCC standards) and provides a 525P, 5251 or NTSC format picture, and a Full Specification (FS) mode which provides high resolution 10801/ 11251 or 750P format picture from the MPEG Main Profile bitstream. The exemplary system of Figure 1A includes a front end interface 100, a video decoder section 120 and associated Decoder Memory 130, a primary video output interface 140, an audio decoder section 160, an optional computer interface 110, and an optional NTSC video processing section 150. Referring to Figure 1A, the exemplary system includes a front end interface 100, having a transport decoder and processor 102 with associated memory 103. Also included may be an optional multiplexer 101 for selecting received control information and computer generated images from the computer interface 110 at, for example, the IEEE 1394 link layer protocol or for recovering an encoded transport stream from a digital television tuner (not shown). The transport decoder 102 converts the received compressed data bit stream from the communication channel bit stream into compressed video data, which may be, for example, packetized elementary streams (PES) packets according to MPEG-2 standard. The transport decoder may provide either the PES packets directly, or may further convert the PES packets into one or more elementary streams. The video decoder section includes an ATV Video Decoder 121 and digital phase-locked loop (DPLL) 122. The ATV video Decoder 121 receives an elementary stream or video (PES) packets from the front end interface 100, from the front end interface and converts the packets to the elementary stream. A front end picture processor of the ATV Video Decoder 121 then decodes the elementary streams according to the encoding method used, to provide luminance and chrominance pixel information for each image picture. The ATV Video Decoder 121 further includes a memory subsystem to control decoding operations using an external memory which provides image picture information and a display section to process decoded picture information into a desired picture format. The ATV Video Decoder 121 employs the Decoder Memory 130 to process the high resolution encoded video signal. The DPLL 122 is used to generate timing signals for synchronized processing operations between the ATV Decoder 120 and Decoder Memory 130. The Decoder Memory 130 includes a first group of memory units 131, 132 and 133 which may each be a 16 Mb RDRAM memory, and a second group of memory units 134, 135 and 136, which may also each be a 16 Mb RDRAM memory. Exemplary embodiments the present invention are subsequently described with respect to, and preferably implemented within, the video decoder section 120 and Decoder Memory 130. The primary video output interface 140 includes a first Digital to Analog (D/A) converter (DAC) 141 (which actually has three D/A units for the luminance signal and the C R and C B chrominance signals) which may operate at 74 MHz, followed by a filter 142. This interface produces analog video signals having a 11251 or 750P format. The interface 140 and also includes a second (D/A) converter (DAC) 143 (also with three D/A units for luminance signal and C R and C B chrominance signals) which may operate at 27 MHz, followed by a filter 142 to produce for video signals having a 5251 or 525P format. The primary video output interface 140 converts the digitally encoded video signals having a desired format, creates an analog video signal having chrominance and luminance components with the desired format using a (D/A) converter, and filters the analog video signal to remove sampling artifacts of the D/A conversion process. The audio decoder section 160 includes an AC3 Audio decoder 162 which provides audio signals at output ports 163 and 164, and optional 6-2 channel down mixing processor 161 to provide 2 channel audio signals at output port 165. The audio processing of MP@HL MPEG-2 standard audio signal components from encoded digital information to analog output at output ports 163, 164 and 165 is well known in the art, and an audio decoder suitable for use as the decoder 160 is a ZR38500 Six Channel Dolby Digital Surround Processor, available from the Zoran Corporation of Santa Clara, CA. The optional computer interface 110 transmits and receives computer image signals which conform, for example, to the IEEE 1394 standard. The computer interface 110 includes a physical layer processor 111 and link layer processor 112. The physical layer processor 111 converts electrical signals from output port 113 into received computer generated image information and control signals, and provides these signals, for decoding by the link layer processor 112 into IEEE 1394 formatted data. The physical layer processor 111 also converts received control signals encoded by the link layer processor 112 originating from the transport decoder 102 into electrical output signals according to the IEEE 1394 standard. The NTSC video processing section 150 includes an optional ATV-NTSC Downconversion processor 151 which converts the analog HDTV signal provided by the filter 142 into a 525 I signal. This conversion between standards is known in the art and may be accomplished using spatial filtering techniques such as those disclosed in, for example, U.S. Patent 5,613,084 to Hau et al. entitled INTERPOLATION FILTER SELECTION CIRCUIT FOR SAMPLE RATE CONVERSION USING PHASE QUANTIZATION, which is incorporated herein by reference. In the exemplary embodiment of the invention, this processing section is used only when the decoder processes a 10801 or 11251 signal. The NTSC encoder 152 receives a 5251 analog signal either from the processor 151 or directly from the decoder 120, and converts the signal to the NTSC formatted video signal at output ports 153 (S-video) and 154 (composite video). Video Decoder Section Employing Decoder Memory Figure IB is a high level block diagram showing the functional blocks of the ATV Video Decoder 121 including an interface to external Memory 130 as employed in an exemplary embodiment of the present invention. The ATV Video Decoder 121 includes a Picture Processor 171, a Macroblock Decoder 172, a Display section 173, and a Memory subsystem 174. The Picture processor 171 receives, stores and partially decodes the incoming MPEG-2 video bitstream and provides the encoded bitstream, onscreen display data, and motion vectors, which may be stored in memory 130 under the control of the Memory subsystem 174. The Macroblock Decoder 172 receives the encoded bitstream, motion vectors, and stored motion compensation reference image data, if predictive encoding is used, and provides decoded macroblocks of the encoded video image to the memory subsystem 174. The Display Section 173 retrieves the decoded macroblocks from the Memory subsystem 174 and formats these into the video image picture for display. The operation of these sections is described in detail below. a) Main Profile Format Support for Picture Processing The ATV video decoder 121 of the present invention is designed to support all ATSC video formats. For simplicity, the operation of the ATV video decoder 121 is divided into a Down Conversion (DC) mode which receives any MPEG Main Profile video bitstreams shown in Table 1 and provides a 525P, 5251 or NTSC format video signal, and a Full Specification (FS) mode which provides high resolution 10801/11251 or 750P format picture from the MPEG Main Profile bitstream. For the exemplary video decoder of Figure 1 A, in FS mode any High definition or Standard definition television signal (HDTV or SDTV) is decoded and provided at one output port as in the same format as was used when it was encoded. In DC mode, any HDTV or SDTV signal is decoded and a display output signal provided at either of two ports, with port one providing either a progressive or interlaced image, and port two providing an interlaced image. Table 1. Video Bitstream Formats In FS mode, picture information is contained in the header (sequence or picture) and the MPEG standard allows a progressive picture to be displayed as an interlaced picture. Further, to support frame rate-Hz conversion from 30Hz/60Hz to 29.97Hz/59.94Hz, the decoder can drop one picture frame out of every 1001 picture frames received. Table 2 gives the supported FS mode output picture formats for the respective input bitstreams of Table 1: Table 2. FS Supported Video Formats In DC mode, low pass filtering of the high frequency components of the Main Level picture occurs as part of the decoding process to adjust the resolution of the high resolution picture to a format having a lower resolution. This operation includes both horizontal and vertical filtering of the high resolution picture. Note that in DC Mode, the display format conversion may display 16x9 aspect ratio sources on 4x3 displays, and vice-versa. This process is described subsequently with reference to the display section of the video decoder section 120. Table 3 gives the supported primary and secondary output picture formats for the respective input bitstreams of Table 1: Table 3 DC Supported Video Formats b) Decoding. Downconversion and Downsampling I.) Overview Figure 2A is a high level block diagram of a typical video decoding system which processes an MPEG-2 encoded picture. The general methods used to decode an MPEG-2 encoded picture, without subsequent processing, downconversion or format conversion, are specified by the MPEG-2 standard. The video decoding system includes an entropy decoder (ED) 110, which may include variable length decoder (VLD) 210 and run length decoder 212. The system also includes an inverse quantizer 214, and inverse discrete cosine transform (IDCT) processor 218. The exemplary system also includes a Controller 207 which controls the various components of the decoding system responsive to the control information retrieved from the input bit stream by the ED 110. For processing of prediction images, the system further includes a memory 199 having reference frame memory 222, adder 230, and Motion Compensation Processor 206a which may have a motion vector processor 221 and half -pixel generator 228. The ED 110 receives the encoded video image signal, and reverses the encoding process to produce macroblocks of quantized frequency-domain (DCT) coefficient values and control information including motion vectors describing the relative displacement of a matching marcoblock in a previously decoded image which corresponds to a macroblock of the predicted picture that is currently being decoded. The Inverse Quantizer 214 receives the quantized DCT transform coefficients and reconstructs the quantized DCT coefficients for a particular macroblock. The quantization matrix to be used for a particular block is received from the ED 110. The IDCT processor 218 transforms the reconstructed DCT coefficients to pixel values in the spatial domain (for each block of 8 X 8 matrix values representing luminance or chrominance components of the macroblock, and for each block of 8 X 8 matrix values representing the differential luminance or differential chrominance components of the predicted macroblock). If the current macroblock is not predictively encoded, then the output matrix values provided by the IDCT Processor 218 are the pixel values of the corresponding macroblock of the current video image. If the macroblock is interframe encoded, the corresponding macroblock of the previous video picture frame is stored in memory 199 for use by the Motion Compensation processor 206. The Motion Compensation Processor 206 receives a previously decoded macroblock from memory 199 responsive to the motion vector, and then adds the previous macroblock to the current IDCT macroblock (corresponding to a residual component of the present predictively encoded frame) in Adder 230 to produce the corresponding macroblock of pixels for the current video image, which is then stored into the reference frame memory 222 of memory 199. The system of the first exemplary embodiment of Figure 2A also includes a Display Format Block 290 including a Vertical Filter 292 and Horizontal Filter 294. The Display Format Block 290 formats decoded high definition (FS) images into images for display on a particular display unit. Figure 2A as described illustrates decoding of an encoded picture without down conversion. If down conversion is used to provide a lower resolution picture, a DCT filter may be added before the IDCT operation. Figure 2B is a high level block diagram of the down conversion system of one exemplary embodiment of the present invention employing such a DCT filtering operation, and which may be employed by an exemplary embodiment of the present invention in DC mode. As shown in Figure 2B, the down conversion system includes a variable length decoder (VLD) 210, a run-length (R/L) decoder 212, an inverse quantizer 214, and inverse discrete cosine transform (IDCT) processor 218. In addition, the down conversion system includes a Down Conversion filter 216 and Down Sampling processor 232 for decimating decoded pictures. While the following describes the exemplary embodiment for a MP@HL encoded input, the present invention may be practiced with any similarly encoded high-resolution image bit stream. The down conversion system also includes a Motion Compensation Processor 206b including a Motion Vector (MV) Translator 220, a Motion Block Generator 224 including an Up-Sampling Processor 226, Half-Pixel Generator 228, and a Reference Frame Memory 222. The system of the first exemplary embodiment of Figure 2B also includes a Display Conversion Block 280 having a Vertical Programmable Filter (VPF) 282 and Horizontal Programmable Filter (HZPF) 284. The Display Conversion Block 280 converts downsampled images into images for display on a particular display device having a lower resolution than the original image, and is described in detail subsequently in section d)(II) on Display Conversion. The Down Conversion Filter 216 performs a lowpass filtering of the high resolution (e.g. Main Profile, High Level DCT) coefficients in the frequency domain. The Down Sampling Process 232 eliminates spatial pixels by decimation of the filtered Main Profile, High Level picture to produce a set of pixel values which can be displayed on a monitor having lower resolution than that required to display a MP@HL picture. The exemplary Reference Frame Memory 222 stores the spatial pixel values corresponding to at least one previously decoded reference frame having a resolution corresponding to the down-sampled picture. For interframe encoding, the MV Translator 220 scales the motion vectors for each block of the received picture consistent with the reduction in resolution, and the High Resolution Motion Block Generator 224 receives the low resolution motion blocks provided by the Reference Frame Memory 222, upsamples these motion blocks and performs half-pixel interpolation as needed to provide motion blocks which have pixel locations which correspond to the decoded and filtered differential pixel blocks. Note that in the down conversion system of Figure IB the downsampled images are stored rather than high definition images, resulting in a considerable reduction of memory required for storing reference images. The operation of an exemplary embodiment of the down-conversion system of the present invention for intra-frame encoding is now described. The MP@HL bit- stream is received and decoded by VLD 210. In addition to header information used by the HDTV system, the VLD 210 provides DCT coefficients for each block and macroblock, and motion vector information. The DCT coefficients are run length decoded in the R/L decoder 212 and inverse quantized by the inverse quantizer 214. Since the received video image represented by the DCT coefficients is a high resolution picture, the exemplary embodiment of the present invention employs lowpass filtering of the DCT coefficients of each block before decimation of the high resolution video image. The Inverse Quantizer 214 provides the DCT coefficients to the DCT filter 216 which performs a lowpass filtering in the frequency domain by weighting the DCT coefficients with predetermined filter coefficient values before providing them to the IDCT processor 218. For one exemplary embodiment of the present invention, this filter operation is performed on a block by block basis. The IDCT processor 218 provides spatial pixel sample values by performing an inverse discrete cosine transform of the filtered DCT coefficients. The Down Sampling processor 232 reduces the picture sample size by eliminating spatial pixel sample values according to a predetermined decimation ratio; therefore, storing the lower resolution picture uses a smaller frame memory compared to that which would be needed to store the higher resolution MP@HL picture. The operation of an exemplary embodiment of the down-conversion system of the present invention for predicted frames of the encoding standard is now described. In this example, the current received image DCT coefficients represent the DCT coefficients of the residual components of the predicted image macroblocks, which is now referred to as a predicted frame (P-frame) for convenience. In the described exemplary embodiment, the horizontal components of the motion vectors for a predicted frame are scaled since the low resolution reference pictures of previous frames stored in memory do not have the same number of pixels as the high resolution predicted frame (MP@HL). Referring to Figure 2B, the motion vectors of the MP@HL bit stream provided by the VLD 210 are provided to the MV Translator 220 Each motion vector is scaled by the MV Translator 220 to reference the appropriate prediction block of the reference frame of a previous image stored in reference frame memory 222. The size (number of pixel values) in the retrieved block is smaller than block of the corresponding high resolution block used to encode the current image; consequently, the retrieved block is upsampled to form a prediction block having the same number of pixels as the residual block provided by the IDCT Processor 218 before the blocks are combined by the summing network 230. The prediction block is upsampled by the Up-Sampling Processor 226 responsive to a control signal from the MV Translator 220 to generate a block corresponding to the original high resolution block of pixels, and then half pixel values are generated - if indicated by the motion vector for the up-sampled prediction block in the Half Pixel Generator 228 - to ensure proper spatial alignment of the prediction block. The upsampled and aligned prediction block is added in summing network 230 to the current filtered block, which is, for this example, the reduced resolution residual component from the prediction block. All processing is done on a macroblock by macroblock basis. After the motion compensation process is complete for the current high-resolution macroblock, the reconstructed macroblock is decimated accordingly by the Down Sampling Processor 232. This process does not reduce the resolution of the image but simply removes redundant pixels from the low resolution filtered image. Once the downsampled macroblocks for an image are available, the Display Conversion Block 280 adjusts the image for display on a low resolution television display unit by filtering the vertical and horizontal components of the downsampled image in VPF 282 and HZPF 284 respectively. The relationship between the functional blocks of the ATV Video Decoder 121 of Figure 1A and Figure IB is now described. The picture processor 171 of Figure IB receives the video picture information bitstreams. The Macroblock Decoder 172 includes VLD 210, Inverse Quantizer 214, the DCT filter 216, IDCT 218, adder 230, and the motion compensated predictors 206a and 206b. The picture processor 171 may share the VLD 210. External Memory 130 corresponds to memory 199, with 16Mb RDRAM 131-136 containing the reference frame memory 222. For FS mode, decoding of the MPEG 2 stream with 750P format may be performed by storing reference frames with a process using either 96 Mb RDRAM or 48 Mb RDRAM for efficient memory utilization. In FS mode, decoding of the MPEG 2 stream with 11251/1125P/750P formats employs the full 96 Mb memory of the Memory 130 of Figure 1. Figure 2C shows a configuration of the system shown in Figure 1A and IB in which, in FS mode, decoding an MPEG 2 stream having a 750P format is decoded employing 48 Mb memory of the Memory 130 of Figure 1. As shown in Figure 2C, the 750P bitstream is received and decoded as described with reference to Figure 2A, with 1280H pixels and 720V lines stored within memory. For this example, the DCT filter 216 is present but only acts as an all-pass filter, or is simply disabled. Figure 2D illustrates the operation of the system in DC mode, converting an 11251 signal to 525P/525I format. In this scenario, after low pass filtering by DCT filter 216 as described above with reference to Figure 2B, the system down samples the high resolution signal by a factor of 3, and stores the pictures in the 48 Mb memory as 640H and 1080 V, interlaced. For this system, the motion compensation process upsamples the stored pictures by a factor of 3 (as well as translation of the received motion vectors) before motion-predictive decoding is accomplished. Also, the picture is filtered horizontally and vertically for display conversion. Figure 2E similarly illustrates the relationship between DC mode format downconversion from 750P to 525P/525I format. This conversion operates in the same way as the 11251 to 525P/525I conversion except that downsampling for memory storage, and upsampling for motion compensation, is by a factor of 2. π) Macroblock Prediction for Downconversion For the exemplary downconversion process, since the reference frames of the previous images are down sized in the horizontal direction, the received motion vectors pointing to these frames may also be translated according to the conversion ratio. The following describes the motion translation for the luminance block in the horizontal direction. One skilled in the art could easily extend the following discussion to motion translation in the vertical direction if desired. Denoting x and y as the current macroblock address in the original image frame, Dx as the horizontal decimation factor and mv x as the half pixel horizontal motion vector of the original image frame, the address of the top left pixel of the motion block in the original image frame, denoted as XH in the half pixel unit, is given by (1): XH = 2x + mv x (1) The pixel corresponding to the motion block starts in the down-sampled image, and has an address denoted as x* and y* may be determined using equation (2). XH X = - 2-Dx ; y = y (2) The division of equation (2) is an integer division with truncation. Because the exemplary filter 216 and Down Sampling Processor 232 only reduce the horizontal components of the image, the vertical component of the motion vector is not affected. For the chrominance data, the motion vector is one-half of a luminance motion vector in the original picture. Therefore, definitions for translating the chrominance motion vector may also use the two equations (1) and (2). Motion prediction is done by a two step process: first, pixel accuracy motion estimation in the original image frame may be accomplished by upsampling of down-sampled image frame in the Up Sampling Processor 226 of Figures 2A and 2B, then the half pixel Generator 228 performs a half pixel interpolation by averaging of nearest pixel values. The reference image data is added to output data provided by the IDCT processor 218. Since the output values of the Adder 230 correspond to an image having a number of pixels consistant with a high resolution format, these values may be downsampled for display on a display having a lower resolution. Downsampling in the Down Sampling processor 232 is substantially equivalent to subsampling of an image frame, but adjustments may be made based upon the conversion ratio. For example, in the case of 3:1 downsampling, the number of horizontally downsampled pixels are 6 or 5 for each input macroblock, and the first downsampled pixels are not always first pixel in the input macroblock. After acquiring the correct motion prediction block from the down-sampled image, upsampling is used to get the corresponding prediction block in the high resolution picture. Consequently, subpixel accuracy in motion block prediction is desirable in the down sampled picture. For example, using 3:1 decimation, it is desirable to have 1/3 (or 1/6) subpixel accuracy in the down-converted picture for proper motion prediction. The subpixel which is a first pixel required by the motion vector, in addition to the downsampled motion block, is determined. Then, subsequent subpixel positions are determined using modulo arithmetic as described in the following. The subpixel positions are denoted as x s as given in equation (3): where " % " represents modulo division. For example, the ranges of x s are 0, 1, 2 for 3:1 upsampling and 0, 1 for 2:1 upsampling. Figure 3 A shows subpixel positions and corresponding 17 predicted pixels for the 3:1 and 2:1 examples, and Table 4 gives the legend for Figure 3A. Table 4 As previously described, the upsampling filters may be upsampling polyphase filters, and Table 5 gives characteristics of these upsampling polyphase interpolation filters. Table 5: Next two tables, Table 6 and Table 7, show polyphase filter coefficients the exemplary 3:1 and 2:1 upsampling polyphase filters. Table 6: 3:1 Upsampling Filter Table 7: 2:1 Upsampling Filter In a fixed point representation, the numbers in parenthesis of Table 6 and Table 7 are 2' s complement representations in 9 bits with the corresponding double precision numbers are on the left. Depending upon the subpixel position of the motion prediction block in the downsampled reference image frame, one corresponding phase of the polyphase interpolation filter is used. Also, for the exemplary embodiment additional pixels on the left and right are used to interpolate 17 horizontal pixels in the original image frame. For example, in the case of 3:1 decimation, a maximum of 6 horizontally downsampled pixels are produced for each input macroblock. However, when upsampling, 9 horizontal pixels are used to produce the corresponding motion prediction block values because an upsampling filter requires more left and right pixels outside of the boundary for the filter to operate. Since the exemplary embodiment employs half pixel motion estimation, 17 pixels are needed to get 16 half pixels which are the average values of nearest two pixel samples. A half pixel interpolator performs the interpolation operation which provides the block of pixels with half -pixel resolution. Table 8A illustrates an exemplary mapping between subpixel positions and polyphase filter elements, and shows a number of left pixels which are needed in addition to the pixels in the upsampled block for the upsampling process. Table 8A Figure 3B summarizes the upsampling process which is performed for each row of an input macroblock. First, in step 310, the motion vector for the block of the input image frame being processed is received. At step 312, the motion vector is translated to correspond to the downsampled reference frame in memory. At step 314, the scaled motion vector is used to calculate the coordinates of the reference image block stored in memory 130. At step 316 the Subpixel point for the block is determined and the initial polyphase filter values for upsampling are then determined at step 318. The identified pixels for the reference block of the stored downsampled reference frame are then retrieved from memory 130 at step 320. Before the first pass at the filtering step 324, the registers of the filter may be initialized at step 322, which, for the exemplary embodiment includes the step of loading the registers with the initial 3 or 5 pixel values. Then, after filtering step 324, the process determines, at step 326, whether all pixels have been processed, which for the exemplary embodiment is 17 pixels. If all pixels have been processed, the upsampled block is complete. For an exemplary embodiment, a 17 by 9 pixel block is returned as the top or bottom motion block. If all pixels have not been processed, the phase is updated at step 328, and the phase is checked, for the 0 value. If the phase is zero, the registers are updated for the next set of pixel values. Updating the phase at step 328 updates the phase value to 0, 1 , and 2 for the filter loop period for exemplary 3:1 upsampling and to 0, and 1 for the filter loop period for 2: 1 upsampling. Where the leftmost pixel is outside of a boundary of the image picture, the first pixel value in the image picture may be repeated. For an exemplary embodiment, the upsample filtering operation may be implemented in accordance with the following guidelines. First, several factors may be used: 1) the half-pixel motion prediction operation averages two full pixels, and corresponding filter coefficients are also averaged to provide the half-pixel filter coefficient; 2) a fixed number of filter coefficients, five for example, which may be equivalent to the number of filter taps, may be employed regardless of the particular downconversion; 3) five parallel input ports may be provided to the upsampling block for each forward and backward lower and upper block, with five input pixels LWR(0)- LWR(4) for each clock transition for each reference block being combined with corresponding filter coefficients to provide one output pixel, and 4) the sum of filter coefficients h(0)-h(4) combined with respective pixels LWR(0)-LWR(4) provide the output pixel of the sampling block. Filter coefficients are desireably reversed because the multiplication ordering is opposite to the normal ordering of filter coefficients, and it may be desireable to zero some coefficients. Table 8B gives exemplary coefficients for the 3:1 upsampling filter, and Table 8C gives exemplary coefficients for the 2:1 upsampling filter: Table 8B Table 8C In Tables 8B and 8C, x * is the downsampled pixel position defined in equations (1) and (2), and subpixel position, x s , is redefined from equation (3) as equation (3') x s = (XH) (2Dx) (3') For chrominance values of the exemplary implementation, XH is scaled by two and equations (1),(2) and (3') are applied. In one embodiment, phase and half pixel information (coded as two bits and one bit, respectively) is used by motion compensation processor 220 and half-pixel generator 228 of Figure 2B. For example, reference block pixels are provided as U pixels first, V pixels next, and finally Y pixels. U and V pixels are clocked in for 40 cycles and Y pixels are clocked in for 144 cycles. Reference blocks may be provided for 3:1 decimation by providing the first five pixels, repeating twice, shifting the data by one, and repeating until a row is finished. The same method may be used for 2: 1 decimation except that it is repeated once rather than twice. Input pixels are repeated since decimation follows addition of the output from motion compensation and half -pixel generation with the residual value. Consequently, for 3:1 decimation, two of three pixels are deleted, and dummy pixels for these pixel values do not matter. m) DCT Domain Filtering Employing Weighting of DCT Coefficients The exemplary embodiment of the present invention includes the DCT filter 216 of Figure 2 A processing the DCT coefficients in the frequency domain, which replaces a lowpass filter in the spatial domain. There are several advantages in DCT domain filtering instead of spatial domain filtering for DCT coded pictures, such as contemplated by the MPEG or JPEG standards. Most notably, a DCT domain filter is computationally more efficient and requires less hardware than a spatial domain filter applied to the spatial pixel sample values. For example, a spatial filter having N taps may use as many as N additional multiplications and additions for each spatial pixel sample value. This compares to only one additional multiplication in the DCT domain filter. The simplest DCT domain filter is a truncation of the high frequency DCT coefficients. However, truncation of high frequency DCT coefficients does not result in a smooth filter and has drawbacks such as "ringing" near edges in the decoded picture. The DCT domain lowpass filter of the exemplary embodiment of the present invention is derived from a block mirror filter in the spatial domain. The filter coefficient values for the block mirror filter are, for example, optimized by numerical analysis in the spatial domain, and these values are then converted into coefficients of the DCT domain filter. Although the exemplary embodiment shows DCT domain filtering in only the horizontal direction, DCT domain filtering can be done in either horizontal or vertical direction or both by combining horizontal and vertical filters. IV) Derivation of the DCT Domain Filter Coefficients One exemplary filter of the present invention is derived from two constraints: first, that the filter process image data on a block by block basis for each block of the image without using information from previous blocks of a picture; and second, that the filter reduce the visibility of block boundaries which occur when the filter processes boundary pixel values. According to the first constraint, in the DCT based compression of an MPEG image sequence, for example, N X N DCT coefficients yield N X N spatial pixel values. Consequently, the exemplary embodiment of the present invention implements a DCT domain filter which only processes a current block of the received picture. According to the second constraint, if the filter is simply applied to a block of spatial frequency coefficients, there is a transition of the filtering operation at the block boundary which is caused by an insufficient number spatial pixel values beyond the boundary to fill the residual of the filter. That is to say, coefficient values at the edge of a block cannot be properly filtered because the N-tap filter has values for only N/2 taps, the remaining values are beyond the boundary of the block. Several methods of supplying the missing pixel values exist: 1) repeat a predetermined constant pixel value beyond a boundary; 2) repeat the same pixel value as the boundary pixel value; and 3) mirror the pixel values of the block to simulate previous and subsequent blocks of pixel values adjacent to the processed block. Without prior information on the contents of the previous or subsequent block, the mirroring method of repeating pixel values is considered as a preferred method. Therefore, one embodiment of the present invention employs this mirroring method for the filter and is termed a "block mirror filter. " The following describes an exemplary embodiment which implements a horizontal block mirror filter that lowpass filters 8 input spatial pixel sample values of a block. If the size of input block is an 8 X 8 block matrix of pixel sample values, then a horizontal filtering can be done by applying the block mirror filter to each row of 8 pixel sample values. It will be apparent to one skilled in the art that the filtering process can be implemented by applying the filter coefficients columnwise to the block matrix, or that multidimensional filtering may be accomplished by filtering the rows and then filtering the columns of the block matrix. Figure 4 shows an exemplary correspondence between the input pixel values Xn through x 7 (group XO) and filter taps for an exemplary mirror filter for 8 input pixels which employs a 15 tap spatial filter represented by tap values ho through h 14 . The input pixels are mirrored on the left side of group XO, shown as group XI, and on the right side of group XO, shown as group X2. The output pixel value of the filter is the sum of 15 multiplications of the filer tap coefficient values with the corresponding pixel sample values. Figure 4 illustrates the multiplication pairs for the first and second output pixel values. The following shows that the block mirror filter in the spatial domain is equivalent to DCT domain filter. The mirror filtering is related to a circular convolution with 2N points (N= 8). Define the vector x' as shown in equation (4). x'(n) =x(n) +x(2N-l-n); 0 < =n < =2N-1 (4) In the case of N=8, x' =(x0, xl, x2, x3, x4, x5, x6, x7, x7, x6, x5, x4, x3, x2, xl, xO) Rearranging the filter tap values ho through h 14 , and denoting the rearranged values by h' h' =(h7, h8, h9, hlO, hll, hl2, hl3, hl4, 0, hO, hi, h2, h3, h4, h5, h6) Therefore, the mirror filtered output y(n) is a circular convolution of x'(n) and h'(n) which is given by equation (5). y(n)=x'(n) ® h'(n) (5) Which is equivalent to equation (6). 2N - 1 y(n) = ∑ x ' [n- k]- h ' (n) (6) k = 0 where x'[n-k] is a circular modulo of x'(n) and x'[n] =x'(n) for n> =0 x'[n] =x'(n+2N) for n<0. The circular convolution in the spatial domain shown in equation (5) corresponds to the scalar multiplication in the Discrete Fourier Transform (DFT) domain. Defining Y(k) as the DFT of y(n), then equation (5) becomes equation (7) in the DFT domain. Y(k)=X'(k) H'(k) (7) where X'(k) and H'(k) are the DFTs of x'(n) and h'(n) respectively. Equations (4) through (7) are valid for a filter with a number of taps less than 2N. In addition, the filter is limited to be a symmetric filter with an odd number of taps, with these constraints H'(k) is a real number. Therefore, X'(k), the DFT of x'(n), can be weighed with a real number H'(k) in the DFT frequency-domain instead of 2N multiplication and 2N addition operations in the spatial domain to implement the filtering operation. The values of X'(k) are very closely related to the DCT coefficients of the original N-point x(n), because an N-point DCT of x(n) is obtained by the 2N-point DFT of x'(n) which is the joint sequence composed of x(n) and its mirror, x(2N-l-n). The following describes the derivation of the DFT coefficients of the spatial filter, H'(k), by assuming a symmetric filter having an odd number of taps, 2N-1, which is h(n)=h(2N-2-n), and equivalently h'(n)=h'(2N-n) and h'(N)=0. Define H'(k) as in equation (8). where W 2N kn =exp{-2πkn/(2N)}; and H'(k) = H'(2N-k). The inventor has determined that the 2N-point DFT of x'(n), X'(k), can be expressed by its DCT coefficients as shown in equation (9). whereas the DCT coefficient of x (n), C(k), is given by equation (10). . . N-i , πk(2n+l) C(k)= ∑ 2x(n)-cos— i '- = w k/2 for 0<k<N-l n=l 2N 2N X'(k) (10) and C(k)=0 elsewhere. The values of X'(k), the DFT coefficients of x'(n), can be expressed by C(k), the DCT coefficients of x'(n) by the matrix of equation (11). X'(k) = for k = N (11) 2N -k/2 -C(2N-k) for N + l≤k≤2N-l The original spatial pixel sample values, x(n), can be also obtained by IDCT (Inverse Discrete Cosine Transformation) shown in equation (12). where α(k) = l/2 for k=0 and 1 otherwise. The values of y(n) for 0 < =n < =N-1 , are obtained by IDFT of X'(k)H'(k) given in (13): 2N-1 y(n) = k 0 X '(k)-H'(k).W 2N -kn 2N 1 N~1 , s. N ~ {n + l / 2) — ∑ (k) ■ {C{k) ■ H'(k)} ■ cos— -—- 1 (13) N fc=o ' N The values y(n) of equation (13) are the spatial values of the IDCT of C(k)H'(k). Therefore, the spatial filtering can be replaced by the DCT weighting of the input frequency-domain coefficients representing the image block with H'(k) and then performing the IDCT of the weighted values to reconstruct the filtered pixel values in the spatial domain. One embodiment of the exemplary block mirror filtering of the present invention is derived as by the following steps: 1) a one dimensional lowpass symmetric filter is chosen with an odd number of taps, which is less than 2Ν taps; 2) the filter coefficients are increased to 2N values by padding with zero's; 3) the filter coefficients are rearranged so that the original middle coefficient goes to the zeroth position by a left circular shift; 4) the DFT coefficients of the rearranged filter coefficients are determined; 5) the DCT coefficients are multiplied with the real number DFT coefficients of the filter; and 6) an inverse discrete cosine transform (IDCT) of the filtered DCT coefficients is performed to provide a block of lowpass-filtered pixels prepared for decimation. The cutoff frequency of the lowpass filter is determined by the decimation ratio. For one exemplary embodiment, the cutoff frequency is π/3 for a 3: 1 decimation and π/2 for a 2:1 decimation, where π corresponds to one-half of sampling frequency. A DCT domain filter in MPEG and JPEG decoders allows memory requirements to be reduced because the inverse quantizer and IDCT processing of blocks already exists in the decoder of the prior art, and only the additional scalar multiplication of DCT coefficients by the DCT domain filter is required. Therefore, a separate DCT domain filter block multiplication is not physically required in a particular implementation; another embodiment of the present invention simply combines the DCT domain filter coefficients with the IDCT processing coefficients and applies the combined coefficients to the IDCT operation. For the exemplary down conversion system of the present invention, the horizontal filtering and decimations of the DCT coefficients were considered; and the following are two exemplary implementations for: 1. 1920H by 1080V interlace to 640 by 1080 interlace conversion (Horizontal 3:1 decimation). 2. 1280H by 720V progressive to 640 by 720 progressive conversion (Horizontal 2:1 Decimation) Table 9 shows the DCT block mirror filter (weighting) coefficients; in Table 9 the numbers in the parenthesis are 10 bit 2' s complementary representations. The "*" of Table 9 indicates an out of bound value for the 10 bit 2' s complement representation because the value is more than 1 ; however, as is known by one skilled in the art, the multiplication of the column coefficients of the block by the value indicated by the * can be easily implemented by adding the coefficient value to the coefficient multiplied by the fractional value (remainder) of the filter value. Table 9 These horizontal DCT filter coefficients weight each column in the 8 X 8 block of DCT coefficients of the encoded video image. For example, the DCT coefficients of column zero are weighted by H[0], and the DCT coefficients of the first column are weighted by H[l] and so on. The above description illustrates a horizontal filter implementation using one-dimensional DCTs. As is known in the digital signal processing art, such processing can be extended to two-dimensional systems. Equation (12) illustrates the IDCT for the one-dimensional case, consequently, equation (12') gives the more general two dimensional IDCT: 2 ^J ^! (2x + l)uπ (2y + l)vπ / ^ f( χ ,y) = - j 7Z. Z. C(u)C(v)F(u,v) cos κ _ — cos ^ „ (12') N _=0 v=0 2N 2N — j = U, V = 0 where C(u), C(v) are V2 1 otherwise where f(x,y) is the spatial domain representation, x and y are spatial coordinates in the sample domain, and u,v are the coordinates in the transform domain. Since the coefficients C(u), C(v) are known, as are the values of the cosine terms, only the transform domain coefficients need to be provided for the processing algorithms. For a two-dimensional system, the input sequence is now represented as a matrix of values, each representing the respective coordinate in the transform domain, and the matrix may be shown to have sequences periodic in the column sequence with period M, and periodic in the row sequence with period Ν, Ν and M being integers. A two- dimensional DCT can be implemented as a one dimensional DCT performed on the columns of the input sequence, and then a second one dimensional DCT performed on the rows of the DCT processed input sequence. Also, as is known in the art, a two- dimensional IDCT can be implemented as a single process. Figure 5 shows an exemplary implementation of the filter for downconversion for a two-dimensional system processing the horizontal and vertical components implemented as cascaded one-dimensional IDCTs. As shown in Figure 5, the DCT Filter Mask 216 and IDCT 218 of Figure 2 may be implemented by a Vertical Processor 510, containing a Vertical DCT Filter 530 and a Vertical IDCT 540, and a Horizontal Processor 520, containing a horizontal DCT Filter and horizontal IDCT which are the same as those implemented for the vertical components. Since the filtering and IDCT processes are linear, the order of implementing these processes can be rearranged (e.g, horizontal and vertical DCT filtering first and horizontal and vertical IDCTs second, or vise- versa, or Vertical Processor 520 first and Horizontal Processor 510 (second)). In the particular implementation shown in Figure 5, the Vertical Processor 510 is followed by a block Transpose Operator 550, which switches the rows and columns of the block of vertical processed values provided by the Vertical Processor. This operation may be used to increase the efficiency of computation by preparing the block for processing by the Horizontal Processor 520. The encoded video block, for example an 8 X 8 block of matrix values, is received by the Vertical DCT filter 530, which weights each row entry of the block by the DCT filter values corresponding to the desired vertical decimation. Next, the Vertical IDCT 540 performs the inverse DCT for the vertical components of the block. As described previously, since both processes simply perform a matrix multiplication and addition, the DCT LPF coefficients can be combined with the vertical DCT coefficients for matrix multiplications and addition operations. The Vertical Processor 510 then provides the vertically processed blocks to the Transpose Operator 550, which provides the transposed block of vertically processed values to the Horizontal Processor 520. The Transpose Operator 550 is not necessary unless the IDCT operation is only done by row or by column. The Horizontal Processor 520 performs the weighting of each column entry of the block by the DCT filter values corresponding to the desired horizontal filtering, and then performs the inverse DCT for the horizontal components of the block. As described with reference to equation (12'), only coefficients in the transform domain are provided to the processing algorithms; and the operations are linear which allows mathematical operations on these coefficients only. The operations for the IDCT, as is readily apparent from equation (12'), form a sum of products. Consequently, a hardware implementation requires known coefficients to be stored in memory, such as a ROM (not shown), and a group of multiply and add circuits (not shown) which receives these coefficients from the ROM as well as selected coefficients from the matrix of input transform coordinates. For more advanced systems, a ROM-accumulator method may be used if the order of mathmatical operations is modified according to distributed arithmetic to convert from a sum of products implementation to a bit-serial implementation. Such techniques are described in, for example, Stanley A. White, Applications of Distributed Arithmetic to Digital Signal Processing: A Tutorial Review, IEEE ASSP Magazine, July, 1989, which take advantage of symmetries in the computations to reduce a total gate count of the sum of products implementation. In an alternative embodiment of the present invention, the DCT filter operation may be combined with the inverse DCT (IDCT) operation. For such an embodiment, since the filtering and inverse transform operations are linear, the filter coefficients may be combined with the coefficients of the IDCT to form a modified IDCT. As is known in the art, the modified IDCT, and hence the combined IDCT and DCT downconversion filtering, may be performed through a hardware implementation similar to that of the simple IDCT operation. c) Memory Subsystem I) Memory Access and Storage of Bitstream and Picture Data As shown in Figure IB, the exemplary embodiment of the present invention employs an ATV Video Decoder 121 having a Memory Subsystem 174 which controls the storage and reading of information to and from Memory 130. Memory Subsystem 174 provides picture data and bitstream data to Memory 130 for video decoding operations, and in the preferred embodiment, at least 2 pictures, or frames, are used for proper decoding of MPEG-2 encoded video data. An optional On-Screen Display (OSD) section in the Memory 130 may be available to support OSD data. The interface between the Memory Subsystem 174 and Memory 130 may be a Concurrent RDRAM interface providing a 500 Mbps channel, and three RAMBUS channels may be used to support the necessary bandwidth. An embodiment of the present invention having Picture processor 171, Macroblock decoder 172, and Memory subsystem 174 operating with external memory 130 may employ a system as described in U.S. Patent 5,623,311 entitled MPEG VIDEO DECODER HAVING A HIGH BANDWIDTH MEMORY to Phillips et al. , which is incorporated herein by reference. Figure 12 is a high level block diagram of such system of a video decoder having high bandwidth memory as employed by an exemplary embodiment of the present invention to decode MP@ML MPEG-2 pictures. In summary, and described with relation to Figure 1A and Figure IB, U.S. Patent 5,623,311 describes a single, high bandwidth memory having a single memory port. The memory 130 holds input bitstream, first and second reference frames used for motion compensated processing, and image data representing the field currently being decoded. The decoder includes 1) circuitry (picture processor 171) which stores and fetches the bitstream data, 2) circuitry that fetches the reference frame data and stores the image data for the currently decoded field in block format (Macroblock decoder 172), and fetches the image data for conversion to raster-scan format (display section 173). The memory operations are time division multiplexed using a single common memory port with a defined memory access time period, called Macroblock Time (MblkT) for control operations. Table 10 summarizes the picture storage requirements for FS and DC configurations to support multiple formats: Table 10 For DC mode, 1920 x 1080 pictures are reduced by a factor of 3 horizontally, yielding a 640 x 1080 picture; 1280 x 720 pictures are reduced by a factor of 2 horizontally yielding a 640 x 720 picture. The 704 x 480 and 640 x 480 pictures are not reduced in PC mode. For bitstream data, according to the MPEG-2 standard, the minimum required buffer capacity (i.e. VBV Buffer Size), and hence memory storage capacity, is 7,995,392 bits. In the preferred embodiment of the present invention, bitstream storage capacity may be increased to 10,737,258 bits for synchronization and interface processing functions. Optional OSD functionality may be implemented using 4 bits per pixel, 2 bits per pixel for low resolution, or 32 bits per 2 pixels for high resolution. For fullscreen OSD, Table 11 gives exemplary storage requirements: Table 11 As described in U.S. Patent 5,623,311, the previously described memory storage capacity requirements may be implemented using an expandable configuration with three 16 Mb RDRAMS 134, 135, 136 (of Figure 1A) on 3 RAMBUS channels for DC formats expanding with additional three 16 Mb RDRAMS 131, 132, 133 for FS formats. Accommodating multiple DC and FS formatted pictures in Memory 130 also requires supporting respective decoding operations according to corresponding picture display timing. For example, progressive pictures occur at twice the rate of interlaced pictures (60 or 59.94 Hz progressive vs. 30 or 29.97 Hz interlace) and, as a result, progressive pictures are decoded faster than interlaced pictures (60 or 59.94 Frames per second progressive vs. 30 or 29.97 Frames per second interlace). Consequently, the decoding rate is constrained by the display rate for the format, and if the less stringent 59.97 or 29.97 frames per second decoding rates are used rather than the 60 or 30 frames per second, one frame out of every 1001 frames may be dropped from the conversion. For convenience, decoding operations for a format may be measured in units of "Macroblock Time" (MblkT) defined as the period during which all decoding operations for a macroblock may be completed (clock cycles per macroblock decoding). Using this period as a measure, as defined in equation 14, control signals and memory access operations can be defined during the regularly occurring MblkT period. MblkT (clock cycles/macroblock) = system clock rate (clock cycles/sec.) / Frame rate (frames/ sec.) / Picture Size (macroblocks/frame) (14) In addition, a blanking interval may not be used for picture decoding of interlaced pictures, and an 8-line margin to the time period is added to account for decoding 8 lines simultaneously (interlaced) and 16 lines simultaneously (progressive). Therefore, an adjustment factor (AdjFact) may be added to the MblkT, as given in equations (15) and (16). AdjFact (interlace) = (total lines - vertical blank lines -8) / total lines (15) AdjFact (progressive) = (total lines -16) / total lines (16) Table 12 lists MblkT for each of the supported formats: Table 12 In an exemplary embodiment of the present invention, a MblkT of 241 clocks is employed for all formats to meet the requirement of the fastest decode time including a small margin. For such chosen MblkT period, slower format decoding includes periods in which no decoding activities occur; consequently, a counter may be employed to reflect the linear decoding rate with a stall generated to stop decoding in selected MblkT intervals. Referring to Figure IB, the Memory Subsystem 174 may provide internal picture data interfaces to the Macroblock decoder 172 and display section 173. A decoded macroblock interface accepts decoded macroblock data and stores it in correct memory address locations of Memory 130 according to a memory map defined for the given format. Memory addresses may be derived from the macroblock number and picture number. The macroblocks may be received as a macroblock row on three channels, one channel per 16 Mb memory device (131-136 of Figure 1A) at the system clock rate. Each memory device may have two partitions for each picture, each partition using an upper and lower address. For interlaced pictures, the one partition carries Field 0 data and the other partition carries Field 1 data, and for progressive pictures, both upper and lower partitions are treated as a single partition and carry data for the entire frame. Every macroblock is decoded and stored for every picture, except for 3:2 pull down mode where decoding is paused for an entire field time period. In 3:2 pulldown mode, a signal having a frame rate of 24 frames per second is displayed at 60 frames (or fields) per second by displaying one frame twice and the next frame three times. A reference macroblock interface supplies stored, previously decoded picture data to the macroblock decoder 172 for motion compensation. The interface may supply two, one or no macroblocks corresponding to bi-directional predictive (B) encoding, uni-directional predictive (P) encoding or intra (I) encoding. Each reference block is supplied using two channels, and each channel contains one-half of a macroblock. For FS mode, each stored reference half macroblock may be 17x9 (Y), 9x5 (C R ) and 9x5 (C B ) to allow interpolation for half pixel resolution. For DC mode employing a factor of 3, each retrieved half macroblock is 10x6 (Y), 6x4 (C R ) and 6x4 (C B ). For DC mode employing a decimation factor of 2, each retrieved half macroblock is 14x9 (Y), 10x5 (C R ) and 10x5 (C B ) to allow for up-sampling and half-pixel resolution. A display interface provides retrieved pixel data to the display section 173, multiplexing Y, C R , and C B pixel data on a single channel. Two display channels may be provided to support conversion from/to interlaced to/from progressive formats. In DC mode, a first channel may provide up to 4 lines of interlaced or progressive data simultaneously and a second channel may provide up to 4 lines of interlaced data, and in FS mode only the first channel is used to supply a single line of data. As described previously, MPEG-2 encoded video pictures may be decoded by employing up to 3 pictures stored in memory, which allows picture information to be stored in six areas with each area holding one field of picture information. Figures 13 A and 13B illustrate the process and flow times associated with reading, storing and displaying picture information when the input video is in a field format. Figures 14A and 14B illustrate the process and flow times associated with reading, storing and displaying picture information when the input video is in a frame format. Figures 15A and 15B illustrate the process and flow times associated with reading, storing and displaying picture information when the input video is in a progressive sequence and the display is in an interlaced sequence. Figures 16A and 16B illustrate the process and flow times associated with reading, storing and displaying picture information when the input video is in a progressive sequence and the display is in a progressive sequence. In Figures 14A and 14B and Figures 15A and 15B, the label "A" indicates an interval when decoding is suspended during vertical blanking. (II) Memory Map Configurations for Supported Formats As described previously, MPEG-2 encoded video may be decoded using 3 pictures stored in the Memory 130 of Figure 1 A, but different memory capacity may be used depending upon the picture format and whether downconversion is employed. Consequently, external memory mapping may be employed to efficiently utilize available storage with respect to reference macroblock access. Also as described previously, for an exemplary embodiment of the present invention memory capacity for DC mode is 48 Mbits and is expandable to 96 Mbits for FS mode. Further, the external memory mapping may accommodate RDRAMS with 2K or IK page sizes. For the exemplary embodiment of the present invention, the Memory Subsystem 174 employs a memory mapping as specified in U.S. Patent 5,623,311. Figure 17A is a memory map diagram which illustrates the manner in which the bit stream 1730 and the image data fields 1710, 1712, 1714, 1716, 1718 and 1720 are stored into the RDRAM memory in FS mode. Figure 17B is a memory map diagram which shows how a row of image macroblocks maps into a memory row when an exemplary type of 1920 X 1088 FS image is decoded utilizing 96 Mbits RDRAM with 2k page size. In FS mode, the RDRAM includes 96 Mbits of memory. The bit stream 1730 is allocated a memory area at the upper address range of the memory as the maximum VBV buffer size for the MP@HL image. Each of the image data fields is allocated, as shown where the values of X and Y depend on the number of pixels per line and the number of lines per field in the particular MP@HL image which is being decoded. These values are provided in Table 13 for 19201 images; for smaller images, each field buffer partition will be less than the fully utilized value. Table 13 Figure 18A illustrates a general memory map for DC formats employing 48 Mbits RDRAM memory. Figure 18B is a memory map diagram which shows how a row of image macroblocks maps into a memory row when an exemplary type of 1920 X 1088 DC image is decoded with horizontal decimation by 3 and utilizing 48 Mbits RDRAM and 2k page size. The memory map of Figure 18A is described in terms of parameters which are given in table 14A for 5251 and 525P images. Table 14A For a case of a 1280 x 720 image, this image may be stored in FS mode with 48 Mbits of RDRAM. However, in FS mode, the memory storage process may be modified to fit the stored picture into 48Mbits of RDRAM using a 2k page size. Figure 19 A illustrates a general memory map for FS formats employing 48 Mbits RDRAM memory for this case, and Figure 19B illustrates the layout of a pair of half-macro-block rows for this case. Table 14B gives some of the parameters for this case. Table 14B The memory mapping described in U.S. patent 5,623,311 employs a fixed four RDRAM rows per macroblock row to store MP@HL decoded pictures. The exemplary embodiment, however, may be based on Concurrent RDRAM where the two banks may have overlapping requests within the same device, as opposed to base RDRAM in which overlapping requests can only occur to different devices. For the preferred embodiment, banks are interleaved within the same field buffer within the same macroblock row. For example, the number of RDRAM rows employed to store a picture may be variable, depending on thelK/2K page size and the number of horizontal pixels. One method may configure the memory for the largest picture, and smaller pictures may be accommodated within the configuration but not fully utilize the memory space. The exemplary embodiment may also use three RDRAM rows per macroblock row to store MP@HL decoded pictures; this mapping is shown in Figures 19A and 19B. However, the present invention may employ a memory mapping which employs a variable number of RDRAM rows per macroblock row to store MP@HL and MP@ML pictures. As shown, this mapping interleaves the macroblock rows between memory rows from bank A and bank B. As described in the referenced patent, this allows macroblocks which span two or more memory rows to be accessed quickly by the memory subsystem 174 which issues overlapping requests to the different memory banks of the memory 130. This memory mapping applies for FS mode. When used, this mapping of the memory 130 supports full specification decoding of a 1280 x 720 video signal using only 48 Mbit RDRAM with a modification to the memory mapping process. This modification uses 3 RDRAM rows to hold a single macroblock row. In the exemplary embodiment of the present invention, bank interleaving and display start line counters are reset to support 3 RDRAM rows per macroblock. For downconversion, downsampled macroblocks are merged into a single macroblock for storage. The downsampling process of the DC mode is described subsequently with reference to Figure 6A and Figure 6B. Figure 6C illustrates a merging process of two macroblocks into a single macroblock for storage in memory 130 for downconversion by 2 horizontally. Figure 6D illustrates a merging process of three macroblocks into a single macroblock for storage in memory 130 for downconversion by 3 horizontally. d) Downsampling and Display Conversion of the Display Section (I) Down Sampling for Low Resolution Formats Down sampling is accomplished by the Down Sampling process 232 of Figure 2B to reduce the number of pixels in the downconverted image. Figure 6A shows the input and decimated output pixels for a 4:2:0 signal format for 3:1 decimation. Figure 6B shows the input and decimated output pixels for 4:2:0 chrominance type 2:1 decimation. Table 16 gives the legend identification for the Luminance and Chrominance pixels of Figure 6 A and Figure 6B. The pixel positions before and after the down conversion of Figures 6 A and 6B are the interlaced (3:1 decimation) and progressive (2:1 decimation) cases respectively Table 16 For down sampling of the interlaced image, which may be the conversion from a 1920 by 1080 pixel image to a 640 by 1080 pixel horizontally compressed image, two out of every three pixels are decimated on the horizontal axis. For the exemplary 3:1 decimation, there are three different macroblock types after the down conversion process. In Figure 6A, original macroblocks were denoted by MBO, MB1, MB2. The down sampled luminance pixels in MBO start at the first pixel in the original macroblock, but in MB1 and MB2 the down-sampled pixels start at the third and the second pixels. Also the number of down- sampled pixels in each macroblock are not the same. In MBO, there are 6 down-sampled pixels horizontally, but 5 pixels in MBl and MB2. These three MB types are repeating, therefore Modulo 3 arithmetic is to be applied. Table 11 summarizes the number of downsampling pixels and offsets for each input macroblock MBO, MBl, MB2. Table 11: For downsampling of the progressive format image the luminance signal is subsampled for every second sample horizontally. For the chrominance signal, the downsampled pixel has a spatial position that is one-half pixel below the pixel position in the original image. (IT) Display Conversion The display section 173 of the ATV Decoder 121 of Figure IB is used to format the stored picture information (the decoded picture information) for a particular display format. Figure 11A is a high level block diagram illustrating the display section of the ATV Video Decoder 121 for an exemplary embodiment of the present invention. Referring to Figure 11 A, two output video signals are supported, a first output signal VTD 0Utl which supports any selected video format, and a second output signal VID our2 which supports 5251 (CCIR-601) only. Each output signal is processed by separate sets of display processing elements 1101 and 1102, respectively, which perform horizontal and vertical upsampling/downsampling. This configuration may be preferred when the display aspect ratio does not match the aspect ratio of the input picture. An optional On Screen Display (OSD) section 1104 may be included to provide on screen display information to one of the supported output signals VCD outl and VID out2 to form display signals V outl or V out2 . All processing is performed at the internal clock rate except for control of the output signals V outl or V out2 at Output Controllers 1126 and 1128, which is done at the pixel clock rate. For the preferred embodiment, the pixel clock rate may be at the luminance pixel rate or at twice the luminance pixel rate. Because the display sets of processing elements 1101 and 1102 operate similarly, only the operation of the display processing set 1101 is described. Referring to the display processing set 1101 , four lines of pixel data are provided from the memory 130 (shown in Figure 1A) to the vertical processing block 282 (shown in Figure 2B) in raster order. Each line supplies C R ,Y,C B ,Y data 32 bits at a time. Vertical Processing block 282 then filters the four lines down to one line and provides the filtered data in 32 bit C R YC B Y format to horizontal processing block 284 (also shown in Figure 2B). The horizontal processing block 284 provides the correct number of pixels for the selected raster format as formatted pixel data. Consequently, the filtered data rate entering the horizontal processing block 284 is not necessarily equal to the output data rate. In an upsampling case, the input data rate will be lower than the output data rate. In a down sampling case, the input data rate will be higher than the output data rate. The formatted pixel data may have background information inserted by optional background processing block 1110. As would be known to one skilled in the art, the elements of the display section 173 are controlled by a controller 1150, which is set up by parameters read from and written to the microprocessor interface. The controller generates signal CNTRL, and such control is necessary to coordinate and to effect proper circuit operation, loading and transfer of pixels, and signal processing. Data from the horizontal processing block 284, data from a second horizontal processing block 284a, and HD (non processed) video data on HD Bypass 1122 are provided to Multiplexer 118 which selects, under processor control (not shown), one video data stream which is provided to mixer 116 to combine the video data stream and optional OSD data from OSD processor 1104 into mixed output video data. The mixed video output data is then provided to MUXs 1120 and 1124. For the first set of processing elements 1101, MUX 1120 may select from mixed output video data, HD data provided on HD bypass 1122, or data from background insertion block 1110. The selected data is provided to output control processor 1126 which also receives the pixel clock. Output control processor 1126 then changes the data clock rate from the internal processing domain to the pixel clock rate according to the output mode desired. For the second processing elements 1102, MUX 1124 may select from mixed output video data or data from background insertion block 1110a. The selected data is provided to output control processor 1128 which also receives the pixel clock. Output control processor 1128 then changes the data clock rate from the internal processing domain to the pixel clock rate according to the output mode desired. MUX 1132 provides either the received selected data (601 Data Out) of MUX 1124 or optional OSD data from OSD processor 1104. Raster Generation and Control processor 1130 also receives the pixel clock and includes counters (not shown) which generate the raster space, allowing control commands to be sent on a line by line basis to Display Control Processor 1140. Display Control processor 1140 coordinates timing with the external memory 130 and starts the processing for each processing chain 1101 and 1102 on a line by line basis synchronized with the raster lines. Processor 1130 also generates the horizontal, vertical and field synchronization signals (H,V, and F). Figures 11B through 11D relate the output modes provided by Display section 173 shown in Figure 11A of the Video Decoder 121 to the active blocks of Figure 1A. Figure 11B illustrates a 27 MHz Dual output mode which, for which the video data is 525P or 5251, first processor 1101 (shown in Figures 11 A) provides 525P video data to 27 MHz DAC 143 as well as 5251 data (601 Data Out) to NTSC Encoder 152. Figure 11C illustrates that in 27 MHz single output mode, only 5251 data (601 Data Out) is provided to NTSC encoder 152. Figure 11D illustrates a 74 MHz/27 MHz mode in which the output mode matches the input format and the video data is provided to either the 27 MHz DAC 143 or 74 MHz DAC 141 depending on the output format. The 74 Mhz DAC is used for 1920 x 1088 and 1280 x 720 picures; the 27 Mhz DAC is used for all other output formats. Display conversion of the downsampled image frames is used to display the image in a particular format. As noted previously, the Display Conversion block 280 shown in Figure 2B includes the vertical processing block (VPF) 282 and horizontal processing block (HZPF) 284 which adjust the down converted and down sampled images for display on the lower resolution screen. VPF 282 which, for the exemplary embodiment, is a vertical line interpolation processor implemented as a programmable polyphase vertical filter, and HZPF 284 which, for the exemplary embodiment, is a horizontal line interpolation processor also implemented as a programmable horizontal polyphase filter. The filters are programmable, which is a design option in order to accommodate display conversion for a number of display formats. As shown in Figure 2B, four lines of downsampled pixel data enter the VPF 282 in raster order. For the exemplary embodiment this data includes luminance (Y) and chrominance (C R and C B ) pixel pairs which enter VPF 282 32 bits at a time. VPF 282 filters the four lines of data into one line and passes this line to the HZPF 284 as 32 bit values each containing luminance and chrominance data in a YC R YC B , and HZPF 284 then generates the correct number of pixels to match the desired raster format. Figure 7A is a high level block diagram illustrating an exemplary filter suitable for use as the VPF 282 of one embodiment of the present invention. In the following, the VPF 282 is described as processing pairs of input pixels (each pair includes 2 luminance pixels, Y, and a chrominance, C R or C B , pixel) to produce a pair of output pixels. This facilitates processing of the 4:2:0 format because color pixels may be easily associated with their corresponding luminance pixels. One skilled in the art, however, would realize that only luminance pixels or only chrominance pixels may be so processed. Referring to Figure 7A, VPF 282 includes a VPF Controller 702; first muliplexer network including Luminance Pixel MUXs (LP MUXs) 706, 708, 710, and 712 and Chrominance Pixel MUXs (CP MUXs) 714, 716, 718, and 720; second multiplexer network including Luminance Filter MUXs (LF MUXs) 726, 728, 730 and 732 and Chrominance Filter MUXs (CF MUXs) 734, 736, 738 and 740; Luminance Coefficient RAM 704; Chrominance Coefficient RAM 724; Luminance Coefficient Multipliers 742, 744, 746, and 748; Chrominance Coefficient Multipliers 750, 752, 754, and 756; Luminance Adders 760, 762 and 764; Chrominance Adders 766, 768 and 770; Round and Clip processors 772 and 776; Demux/Registers 774 and 778; and Output Register 780. The operation of the VPF 282 is now described. Vertical resampling is accomphshed with two 4-Tap polyphase filters, one for the Luminance pixels and one for the Chrominance pixels. The following details operation of the filter for the Luminance pixels only, since the operation for the Chrominance pixels is similar, but points out those differences in the paths as they occur. Vertical filtering of Luminance pixels can use up to 8 phases in the 4-Tap polyphase filter and filtering of Chrominance pixels can use up to 16 phases in the 4-Tap polyphase filter for the exemplary embodiment. The VPF Controller 702, at the beginning of a field or frame, resets the vertical polyphase filter, provides control timing to the first and second multiplexer networks, selects coefficient sets from Luminance Coefficient RAM 704 and Chrominance Coefficient RAM 724 for the polyphase filter phases, and includes a counter which counts each line of the field or frame as it is processed. The VPF Controller 702, in addition to coordinating the operation of the network of MUXs and the polyphase filters, keeps track of display lines by tracking the integer and fractional parts of the vertical position in the decoded picture. The integer part indicates which lines should be accessed and the fractional part indicates which filter phase should be used. In addition, use of modulo N arithmetic when calculating the fractional part allows, less than 16 phases to be used, which may be efficient for exact downsampling ratios such as 9 to 5. The fractional part is always truncated to one of the modulo N phases that are being used. As shown in Figure 7A, luminance and chrominance pixel pairs from the four image lines are separated into a chrominance path and a luminance path. The 16 bit pixel pair data in the luminance path may be further multiplexed into an 8-bit even (Y- even) and 8-bit odd (Y-odd) format by LP MUXs 706, 708, 710, and 712, and the 16 bit pixel pair in the chrominance path into an 8-bit C R and 8-bit C B format by CP MUXs 714, 716, 718 and 720. The luminance filter MUXs 706, 708, 710 and 712 are are used to repeat pixel values of a line at the top and a line at the bottom at the boundaries of a decoded image in order to allow filter pixel boundary overlap in the polyphase filter operation. Pixel pairs for the four lines corresponding to luminance pixel information and chrominance pixel information are then passed through the respective polyphase filters. Coefficients used by Multipliers 742, 744, 746 and 748 for weighting of pixel values for a filter phase are selected by the VPF Controller 702 based on a programmed up or down sampling factor. After combining the weighted luminance pixel information in Adders 760, 762 and 764, the value is applied to the Round and Clip processor 772 which provides eight bit values (since the coefficient multiplication occurs with higher accuracy). DEMUX register 774 receives the first 8 bit value corresponding to an interpolated 8 bit even (Y-even) luminance value and second 8-bit value corresponding to the interpolated 8-bit odd (Y-odd) value, and provides a vertical filtered luminance pixel pair in 16 bits. Register 780 collects and provides the vertical filtered pixels in the luminance and chrominance paths and provides them as vertically filtered 32 bit values containing a luminance and chrominance pixel pair. Figure 7B shows the spatial relationships between the coefficients and pixel sample space of the lines. The coefficients for the luminance and chrominance polyphase filter paths each have 40 bits allocated to each coefficient set, and there is one coefficient set for each phase. The coefficients are interpreted as fractions with a denominator of 512. The coefficients are placed in the 40-bit word from left to right, CO to C3. CO and C3 are signed ten bit 2's complement values, and Cl and C2 are 10 bits which have a given range, for example, from -256 to 767, which are each subsequently converted to 11 -bit 2's complement values. Figure 7A includes an optional luminance coefficient adjustment 782 and chrominance coefficient adjustment 784. These coefficient adjustments 782 and 784 are used to derive the 11 bit 2's complement number for Cl and C2. If bits 8 and 9(the most significant bit) are both 1, then the sign of the eleven bit number is 1 (negative), otherwise the value is positive. Figure 8A is a high level block diagram illustrating an exemplary filter suitable for use as the HZPF 284 of one embodiment of the present invention. HZPF 284 receives a luminance and chrominance pixel information pair, which may be 32-bit data, from the VPD 282. The HZPF 284 includes a HZPF Controller 802; C R latches 804; C B latches 806; Y latches 808; Selection MUXs 810; Horizontal Filter Coefficient RAM 812; Multiplying network 814; Adding network 816; Round and Clip processor 818, DEMUX register 820 and output register 822. Horizontal resampling is accomplished by employing an 8 tap, 8 phase polyphase filter. Generation of display pixels is coordinated by the HZPF Controller 802 by tracking the integer and fractional parts of the horizontal position in the decoded and downsampled picture. The integer part indicates which pixels are to be accessed and the fractional part indicates which filter phase should be used. Using modulo N arithmetic when calculating the fractional part may allow for less than N phases to be used. For example, this may be useful if an exact downsampling ratio such as 9 to 5 is used. If the down-sampling ratio cannot be expressed as a simple fraction, the fractional part is truncated to one of the N phases. The HZPF 284 of the exemplary embodiment of the present invention filters pixel pairs, and uses alignment on even pixel boundaries to facilitate processing of the 4:2:0 formatted picture and to keep the C R and C B pixels (the color pixels) together with the corresponding Y pixels. The operation of the HZPF 284 is now described with reference to Figure 8 A. The HZPF Controller 802, at the beginning of a horizontal line, resets the horizontal polyphase filter, provides control timing to the first and second multiplexer networks, selects coefficient sets from Horizontal Coefficient RAM 812 for the C R , C B and Y filter coefficients for each of the polyphase filter phases, and selects each set of C R , C B and Y values for processing. In addition, when the horizontal position is near the left or right side of the line, the HZPF Controller 802 forces the edge pixel values to be repeated or set to 0 for use by the 8-tap polyphase filter. Any distortion in the image caused by this simplification is usually hidden in the overscan portion of the displayed image. The pixel data received from the VPF 282 is separated into Y, C R and C B values, and these values are individually latched into C R latches 804; C B latches 806; and Y latches 808 for filtering. The HZPF Controller 802 then selects the Y, C R and C B values by appropriate signal to the selection MUXs 310. In the exemplary embodiment, there are more Y values which use additional latches in the Y luminance latches 808. At the same time, the HZPF Controller 802 selects the appropriate filter coefficients for the filter phase, and for the C R or C B or Y values, based on a programmed upsampling or downsampling value by a control signal to Horizontal Filter Coefficient RAM 812. Horizontal Filter Coefficient RAM 812 then outputs the coefficients to the respective elements of the multiplying Network 814 for multiplication with the input pixel values to produce weighted pixel values, and the weighted pixel values are combined in Adding Network 816 to provide a horizontally filtered C R , C B or Y value. After combining the weighted pixel values in Adding network 816, the horizontally filtered pixel value is applied to the Round and Clip processor which provides eight-bit values (since the coefficient multiplication occurs with higher accuracy). DEMUX register 820 receives a series of 8 bit values corresponding to a C R value, an 8 bit even (Y-even) Y value, an eight-bit C B value, and finally an eight-bit value corresponding to an 8-bit odd (Y-odd) Y value; and the DEMUX register 820 multiplexes the values into a horizontally filtered luminance and chrominance pixel pair having a 32 bit value (Y even, C R , Y odd, C B ). Register 822 stores and provides the pixel pair as a vertically and horizontally filtered 32 bit pixel luminance and chrominance pixel pair. Figure 8B illustrates the spatial relationships between coefficients stored in Horizontal Filter Coefficient RAM 812 and used in the polyphase filter and the pixel sample values of the down sampled image for a horizontal line. The coefficients for the exemplary embodiment are placed in a 64 bit word from left to right, CO to C7. The coefficients CO, Cl, C6 and C7 are signed 7-bit 2's complement values, and C2 and C5 are signed 8-bit 2's complement and C3 and C4 are signed 10-bit 2's complement values representing a range from -256 to 767. The 11-bit 2's complement values for C3 and C4 are derived using an adjustment. If both bit 8 and bit 9 (the most significant bit) are 1, then the sign of the 11 -bit value is 1 (negative), otherwise the value is 0 (positive). All coefficients can be interpreted as fractions with a denominator of 512. Table 12 lists coefficient for the VPF 282 and HZPF 284 for exemplary embodiments of the present invention performing the indicated format conversion. Table 12. Coefficients for 750P to 525P or 750P to 5251 4 tap and 2 polyphase Luminance Vertical Filter Coefficients for 750P to 525P or 750P to 5251 4 tap and 4 polyphase Chrominance Vertical Filter Coefficients for 750P to 5251 4 tap and 2 polyphase Luminance Vertical Filter Coefficients for 750P to 5251 4 tap and 4 polyphase Chrominance Vertical Filter Coefficients for 11251 to 525P 4 tap and 8 polyphase Luminance Vertical Filter Coefficients for 11251 to 525P 4 tap and 16 polyphase Chrominance Vertical Filter In the exemplary embodiments of the display conversion system horizontal conversion is, in part performed by the DCT domain filter 216 and the downsample processor 230, shown in Figure 2B. These provide the same number of horizontal pixels (640) whether the conversion is from 11251 or 750P. Accordingly, the HzPF 284 upsamples these signals to provide 720 active pixels per line and passes 525P or 5251 signals unmodified, as these signals have 720 active pixels per line as set forth above in Tables 1 and 2, the values of the coefficients of the Horizontal Filter do not change for conversion to 480P/480I/525P/525I. These Horizontal filter coefficients are given in Table 13. Table 13 Coefficients for Horizontal Filter In addition, the programmable capability of the HZPF 284 allows for a nonlinear horizontal scan. Figure 9A illustrates a resampling ratio profile which may be employed with the present invention. As shown the resampling ratio of the HZPF 284 may be varied across the horizontal scan line and may be changed in piecewise linear fashion. In the exemplary configuration of Figure 9A, at the beginning of the scan line, the resampling ratio increases (or decreases) linearly until a first point on the scan line, where the resampling ration is held constant until a second point is reached where the resampling ratio decreases (or increases) linearly. Referring to Figure 9 A, h initial resampling ratio is the initial resampling ratio for a picture, h resampling ratio change is the first change per pixel in the resampling ratio, - h_resampling_ratio_change is the second change per pixel in the resampling ratio, and h resampling ratio hold column and h resampling ratio reverse column are the display column pixel points between which the resampling ratio is held constant. The value display width is the last pixel (column) of the picture line. Figures 9B and 9C show ratio profiles for mapping a 4:3 picture onto a 16:9 display. The ratios are defined in terms of input value to output value, so 4/3 is downsampling by 4 to 3 and 1/3 is up sampling 1 to 3. The ratio profiles shown in Figures 9B and 9C map an input picture image having 720 active pixels to a display having 720 active pixels. For example, in Figure 9B mapping a 4:3 aspect ratio display to a 16x9 aspect ratio display uses a 4/3 downsampling, but to fill all the samples of the display requires a 1/1 average across the horizontal line. Consequently, the profile of Figure 9B has the correct aspect ratio in the center between display pixels 240 and 480, while the values at the sides are upsampled to fill the display. Figures 9D and 9E illustrate the profiles used for resizing from a 16x9 display image to a 4:3 display which is the inverse of the profiles shown in Figures 9B and 9C. The effect of using resampling ratio profiles according to an exemplary embodiment of the present invention may be seen pictorially in Figure 10. A video transmission format having either a 16x9 or 4x3 aspect ratio may be displayed as either 16x9 or 4x3, but the original video picture may be adjusted to fit within the display area. Consequently, the original video picture may be shown in full, zoom, squeeze, or variable expand/ shrink. While exemplary embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions will occur to those skilled in the art without departing from the spirit of the invention. Accordingly, it is intended that the appended claims cover all such variations as fall within the scope of the invention.

Claims: What is Claimed: 1. A digital video decoder comprising: a terminal for receiving an encoded video signal; decoding circuitry which operates in a first mode to decode the received encoded video signal to provide a decoded video signal having a first spatial resolution and which operates in a second mode to provide the decoded video signal having a second spatial resolution which is less than the first spatial resolution; formatting circuitry which includes a spatial filter that, when the decoding circuitry operates in the second mode, is responsive to a formatting control signal to process the decoded video signal to change the spatial resolution of the decoded video signal to produce at least one display video signal having a respective display spatial resolution, different from the first and second spatial resolutions; and control means for switching the decoder between the first and second modes and for providing the formatting control signal to the formatting circuitry. 2. A decoder according to claim 1 , wherein the encoded video signal is encoded using a frequency-domain transform; and the decoding circuitry includes circuitry which filters the frequency domain transformed video signal, and circuitry which decodes the encoded video signal according to an inverse frequency-domain transform. 3. A decoder according to claim 1 wherein the encoded video signal applied to the video decoder is one of a high-definition video type and a standard- definition video type each having a respective scan signal type, the scan signal type selected from the group consisting of a progressive format and an interlaced format, further comprising: means, responsive to the encoded video signal, for determining the respective scan signal type of the encoded video signal; means for receiving a programmed scan signal having a respective scan signal type; means for modifying the respective display spatial resolution of each display video signal so as to provide the display video signal having 1) the scan signal type of the encoded video signal when the video decoder is operated in the first mode, and 2) the scan signal type of the programmed scan signal when the video decoder is operated in the second mode. 4. A video decoder according to claim 3, wherein each respective scan signal type further includes a respective display format, the formatting means further including: means, responsive to the encoded video signal, for determining the original display format of the scan signal type; and means for receiving a programmed display format; wherein the means for modifying the respective display spatial resolution further provides the display video signal having 1) the original display format of the encoded video signal when the video decoder is operated in the first mode, and 2) the programmed display format when the video decoder is operated in the second mode. 5. A video decoder according to claim 4, wherein the original and the programmed display format each includes a respective aspect ratio; the video signal is represented by a plurality of pixel groups defined by the original display format; and the means for modifying the respective display spatial resolution, when operated in the second mode, modifies the original display format to the programmed display format by applying a resampling profile to selected ones of the plurality of pixel groups. 6. A video decoder according to claim 5, wherein the resampling profile includes at least a first and a second section, each profile section having a resampling ratio varying between the aspect ratio of the original display format and the aspect ratio of the programmed display format. 7. A video decoder according to claim 4, wherein the video signal is an image represented by a plurality of pixel groups defined by the original display format, and the means for modifying the respective display spatial resolution includes: conversion means for converting, when operated in the second mode, the respective aspect ratio of the original display format of the image to the aspect ratio of the programmed display format by applying a resampling profile to selected ones of the plurality of pixel groups. 8. A video decoder according to claim 7, wherein the resampling profile applied by the conversion means includes at least a first and a second section, each profile section having a resampling ratio varying between the aspect ratio of the original display format and the aspect ratio of the programmed display format, and wherein the conversion means converts the image represented by the plurality of pixel groups having the aspect ratio of the original display format is converted to a new image having the aspect ratio of the programmed display format by resampling of each one of the plurality of pixel groups according to the resampling profile. 9. A video decoder according to claim 7, wherein the resampling profile applied by the conversion means includes at least a first and a second section, each profile section having a resampling ratio varying between the aspect ratio of the original display format and the aspect ratio of the programmed display format, and wherein the conversion means converts the image represented by the plurality of pixel groups having the aspect ratio of the original display format to a new image having the aspect ratio of the programmed display format by resampling of selected ones of the plurality of pixel groups while blanking remaining ones of the plurality of pixel groups. 10. A video decoder according to claim 7, wherein the conversion means converts the respective aspect ratio between 1) the aspect ratio of the original display format having a 16 by 9 ratio, and the aspect ratio of the programmed display format having a ratio of 4 by 3, and 2) the aspect ratio of the original display format having a 4 by 3 ratio, and the aspect ratio of the programmed display format having a ratio of 16 by 9. 11. A video decoder according to claim 1 , further comprising a user interface means for receiving a display type signal, the display type signal corresponding to a display type selected from the group consisting of SDTV, S-video, HDTv " , and computer monitor, and wherein the control means further provides the formatting control signal having the respective display type of the display type signal so that the formatting circuitry formats the display video signal for the corresponding display type. 12. A video decoder according to claim 1, wherein the decoding circuitry includes a memory, the memory having a first and a second section, wherein the decoding circuitry employs both the first and the second section of the memory when the video decoder is operated in the first mode and one of the first and the second section of the memory when the video decoder is operated in the second mode. 13. A digital video decoder which receives and decodes an encoded video signal and is operated in one of a first and a second mode, of which the video decoder provides the video signal as a display video signal having a respective display resolution and a respective display format, the video decoder comprising: a terminal for receiving an encoded video signal; decoding circuitry which operates in the first mode to decode the received encoded video signal to provide a decoded video signal having a first spatial resolution and which operates in the second mode to provide the decoded video signal having a second spatial resolution which is less than the first spatial resolution; and formatting circuitry which includes a spatial filter that, when the decoding circuitry operates in the second mode, is responsive to a formatting control signal to process the decoded video signal to change the spatial resolution of the decoded video signal to at least one display video signal having a respective display spatial resolution, different from the first and second spatial resolutions; and control means for switching the decoder between the first and second modes and for providing the formatting control signal to the formatting circuitry. 14. The video decoder as recited in claim 13, wherein, in the second mode, the video decoder forms the display video signal having the respective display spatial resolution from the encoded video signal representing a video image, the encoded video signal being a frequency-domain transformed high resolution video signal; and the decoding means includes: means for receiving and for providing the encoded video signal as a plurality of high resolution frequency-domain video coefficient values; down-conversion filter means for receiving and weighting selected ones of the plurality of high resolution frequency-domain video coefficient values to form a set of filtered frequency-domain video coefficients; and inverse-transform means for transforming the filtered frequency-domain video coefficients into a set of filtered pixel sample values; and the spatial filter includes decimating means for deleting selected ones of the set of filtered pixel sample values to provide the display video signal having the respective display spatial resolution. 15. The video decoder as recited in claim 14, wherein the frequency- domain transformed video signal is transformed by a discrete cosine transform (DCT) operation so as to provide each of the plurality of high resolution frequency domain values as a respective DCT coefficient, the down-conversion filter means weights each selected one DCT coefficient, and the inverse-transform means transforms the frequency domain video coefficients by an inverse discrete cosine transform (IDCT) operation. 16. The video decoder as recited in claim 14, wherein the down conversion filter means is a lowpass filter having a cutoff frequency determined by a sampling frequency of the encoded video signal divided by a decimation ratio. 17. The video decoder as recited in claim 14, wherein the down conversion filter means includes a plurality of frequency domain coefficients of a lowpass block mirror filter having a predetermined number of taps. 18. The video decoder as recited in claim 14, wherein the down conversion filter means is a lowpass filter represented by a set of frequency domain filter coefficients, and the down conversion means weights the selected ones of the plurality of high resolution frequency-domain video coefficient values by multiplying the set of frequency domain filter coefficients with the plurality of high resolution frequency- domain video coefficient values. 19. The video decoder as recited in claim 14, wherein the decimation means down-samples the set of filtered pixel sample values according to a decimation ratio. 20. The video decoder as recited in claim 13, wherein in the second mode the video decoder forms the display video signal having the respective display spatial resolution from the encoded video signal representing a video image, the encoded video signal being a frequency-domain transformed high resolution video signal, and the decoding means includes: means for providing the encoded video signal as a plurality of frequency- domain video coefficient values; and combining means for combining the plurality of frequency domain video coefficient values with a set of filtering inverse-transform coefficients to produce a set of filtered pixel sample values, wherein the filtering inverse-transform coefficients are a combination of a set of weighting coefficients for down-conversion and a set of inverse- transform coefficients for conversion from the frequency domain to the spatial domain; and the spatial filter includes decimating means for deleting selected ones of the set of filtered pixel sample values to produce a set of decimated pixel sample values; and the video decoder includes means for storing the set of decimated filtered pixel sample values and for providing the stored set of decimated filtered pixel sample values as the the display video signal having the respective display spatial resolution. 21. A digital video decoder which receives an encoded video signal having an encoded video portion and an encoded audio portion, the video decoder comprising: a transport decoder, coupled to receive the encoded video signal, which separates the encoded video signal into the encoded video portion and the encoded audio portion; a video decoder including: a memory having a first and a second section; decoding circuitry which decodes the received encoded video portion to provide the video portion of the video signal having a respective resolution; digital filtering circuitry, coupled to the decoding circuitry, which, when the decoder is operated in a second mode, digitally filters the received encoded video portion to provide a filtered video portion having a respective resolution, the resolution of the filtered video portion being no greater than the resolution of the video portion of the video signal, wherein the decoding circuitry employs the first and the second section of the memory to decode the encoded video portion when the video decoder is operated in the first mode, and the decoding circuitry employs the first section to decode the encoded video portion when the digital filtering circuitry is enabled and the video decoder is operated in the first mode formatting circuitry which formats the video portion of the video signal according to at least one received display format, the formatting circuitry including; means for providing the video portion as at least one display video signal at a respective output port and with a respective display format, the at least one display video signal having the resolution of the video portion of the video signal when the video decoder is operated in a first mode, and means for providing the filtered video portion as at least one display video signal at a respective output port and with a respective display format, the at least one display video signal having the corresponding resolution of the filtered video portion of the video signal when the video decoder is operated in the second mode; and means for switching the decoder between the first and second modes; and an audio decoder which decodes the audio portion and provides the audio portion with each corresponding display video signal 22. The digital video decoder as recited in claim 21, wherein the at least one display video signal at the respective output port is a digital signal which is provided to a digital to analog converter to convert the digital signal to an analog signal, and the analog signal is subsequently provided to a filter, whereby artifacts are filtered from each display video signal. 23. The digital video decoder as recited in claim 21, further including: conversion means for down-conversion of the one display video signal to a signal having an NTSC resolution; and encoding means for encoding the converted one display video signal as an NTSC formated video signal. 24. The digital video decoder as recited in claim 21, wherein the video signal represents a video image, the video image being at least one of a high definition video image and a computer-generated image, which is frequency-domain encoded and then transport encoded; the video transport decoder includes a communication interface which receives and decodes the transport-encoded video image to provide the frequency- domain encoded video image as the encoded video signal to the video decoder. 25. A digital video decoder comprising: a terminal for receiving an encoded video signal; decoding circuitry which decodes the received encoded video signal having a first spatial resolution to provide a decoded video signal having a second spatial resolution which is less than the first spatial resolution; formatting circuitry which includes a spatial filter to process the decoded video signal to change the spatial resolution of the decoded video signal to produce at least one display video signal having a respective display spatial resolution, different from the first and second spatial resolutions. 26. A decoder according to claim 15, wherein the encoded video signal is encoded using a frequency-domain transform; and the decoding circuitry includes circuitry which filters the frequency domain transformed video signal, and circuitry which decodes the encoded video signal according to an inverse frequency-domain transform. 27. A method of decoding an encoded video signal with a digital video decoder comprising the steps of: a) decoding the received encoded video signal to provide i) a decoded video signal having a first spatial resolution when the digital video decoder is operated in a first mode, and ii) the decoded video signal having a second spatial resolution which is less than the first spatial resolution when the digital video decoder is operated in a second mode; b) providing, by a controller, a formatting control signal; c) filtering, according to the formatting control signal, the decoded video signal to change the spatial resolution of the decoded video signal when the digital video decoder is operated in the second mode, the filtered decoded video signal having a respective display spatial resolution, different from the first and second spatial resolutions; and d) producing at least one display video signal from 1) the decoded video signal having the first spatial resolution when the digital video decoder is operated in the first mode, and 2) the filtered decoded video signal when the digital video decoder is operated in the second mode. 28. The method of decoding by the digital video decoder as recited in claim 27, wherein the encoded video signal is a frequency-domain transformed high resolution video signal, the method further including the steps of: providing the encoded video signal as a plurality of high resolution frequency-domain video coefficient values; weighting selected ones of the plurality of high resolution frequency- domain video coefficient values to form a set of filtered frequency-domain video coefficients; transforming the filtered frequency-domain video coefficients into a set of filtered pixel sample values; and deleting selected ones of the set of filtered pixel sample values to provide the display video signal having the respective display spatial resolution. 29. A method of decoding an encoded video signal by a digital video decoder, the encoded video signal encoded using a frequency-domain transform, the method comprising the steps of: a) decoding the received encoded video signal having a first spatial resolution to provide the frequency domain transformed video signal; b) filtering, in the frequency domain, and inverse-transforming the frequency domain transformed video signal to provide the decoded video signal having a second spatial resolution which is less than the first spatial resolution; and c) spatially filtering the decoded video signal to change the spatial resolution of the decoded video signal to produce at least one display video signal having a respective display spatial resolution, different from the first and second spatial resolutions.

Inventors: BROSZ EDWARD, GHANSHYAM DAVE, KIM HEE-YONG, MEYER EDWIN ROBERT, NAIMPALLY SAIPRASAD, PEARSON JERELD, PHILLIPS LARRY, RYAN ROBERT T, SITA RICHARD, BROSZ, EDWARD, GHANSHYAM, DAVE, KIM, HEE-YONG, MEYER, EDWIN, ROBERT, NAIMPALLY, SAIPRASAD, PEARSON, JERELD, PHILLIPS, LARRY, RYAN, ROBERT, T., SITA, RICHARD
Applicants: BROSZ EDWARD, GHANSHYAM DAVE, KIM HEE YONG, MATSUSHITA ELECTRIC IND CO LTD, MEYER EDWIN ROBERT, NAIMPALLY SAIPRASAD, PEARSON JERELD, PHILLIPS LARRY, RYAN ROBERT T, SITA RICHARD, BROSZ, EDWARD, GHANSHYAM, DAVE, KIM, HEE-YONG, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD., MEYER, EDWIN, ROBERT, NAIMPALLY, SAIPRASAD, PEARSON, JERELD, PHILLIPS, LARRY, RYAN, ROBERT, T., SITA, RICHARD
Application year: 1998
Main classification: H04N5
IPC codes: G06T1, G06T3, G06T9, H04N5, H04N7, H04N9