Frequency domain filtering for down conversion of a DCT encoded picture

Claims

What is claimed:

1. An apparatus for forming a low resolution video signal from an encoded video signal representing a video image, the encoded video signal being a frequency-domain transformed high resolution video signal, the apparatus comprising:

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, wherein the down conversion filter means is a lowpass filter represented by a set of frequency domain filter coefficients, and the down conversion filter 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;

inverse-transform means for receiving and transforming the filtered frequency-domain video coefficients into a set of filtered pixel sample values; and

decimating means for deleting selected ones of the set of filtered pixel sample values to provide the low resolution video signal.

2. Apparatus for forming a low resolution video signal as recited in claim 1, wherein the frequency-domain transformed high resolution video signal is transformed by a discrete cosine transform (DCT) operation, and the inverse-transform means transforms the frequency domain video coefficients by an inverse discrete cosine transform (IDCT) operation.

3. Apparatus for forming a low resolution video signal as recited in claim 1, 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.

4. Apparatus for forming a low resolution video signal as recited in claim 1, 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.

5. The apparatus for forming a low resolution video signal as recited in claim 1, wherein the decimating means down-samples the set of filtered pixel sample values according to a decimation ratio.

6. An apparatus for forming a low resolution video signal from an encoded video signal representing a video image, the encoded video signal being a discrete cosine transformed (DCT) high resolution video signal, the apparatus comprising:

means for receiving and for providing the encoded video signal as a plurality of DCT video coefficient values;

inverse-transform means including

means for weighting a set of the plurality of discrete cosine transform (DCT) coefficient values with a set of multi-bit down-conversion filtering coefficients by multiplying each DCT coefficient in the set of DCT coefficients with a respectively different one of the set of multi-bit down-conversion filtering coefficients to form a set of weighted DCT coefficients; and

means for transforming, by an inverse DCT (IDCT) operation from a DCT domain to a spatial domain, the weighted DCT coefficients into a set of filtered pixel sample values; and

decimating means for deleting selected ones of the set of filtered pixel sample values to provide the low resolution video signal.

7. A method of forming a lower resolution video signal from an encoded video signal representing a video image, the encoded video signal being a frequency-domain transformed video signal, comprising the steps of:

a) providing the encoded video signal as a plurality of discrete cosine transform (DCT) coefficient values;

b) weighting selected ones of the plurality of DCT coefficient values with a plurality of frequency domain coefficients representing a lowpass block mirror filter having a predetermined number of taps to form a set of filtered DCT coefficient values;

c) transforming the filtered DCT coefficient values according to an inverse discrete cosine transform (IDCT) operation to obtain a set of filtered pixel sample values; and

d) retaining selected ones of the set of filtered pixel sample values to provide the lower resolution video signal.

8. An apparatus for forming a lower resolution video signal from an encoded video signal representing a video image, the encoded video signal being a frequency-domain transformed video signal, comprising:

means for receiving and for providing the encoded video signal as a plurality of frequency-domain video coefficient values;

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 formed by multiplying 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;

decimating means for deleting selected ones of the set of filtered pixel sample values to produce a set of decimated pixel sample values; and

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 lower resolution video signal.

9. A method of receiving an encoded video signal representing a video image, the encoded video signal being a frequency-domain transformed video signal, and forming a lower resolution video signal, the method comprising the steps of:

a) providing the encoded video signal as a plurality of frequency-domain video coefficient values;

b) combining the plurality of frequency domain video coefficient values with a set of filtering inverse-transform coefficients to produce a set of low resolution pixel sample values, wherein the filtering inverse-transform coefficients are formed by multiplying a set of weighting coefficients for low-pass filtering and a set of inverse-transform coefficients for conversion from the frequency domain to the spatial domain;

c) decimating selected ones of the set of low resolution pixel sample values; and

d) storing the selected ones of the set of low resolution pixel sample values to provide the stored pixel sample values as the lower resolution video signal.

10. Apparatus for receiving an encoded video signal representing a video image, the encoded video signal being a compressed frequency-domain transformed video signal, and forming a lower resolution video signal, the apparatus comprising:

means for providing the encoded video signal as a plurality of DCT coefficient values and a motion vector;

down-conversion filter means for receiving and weighting, based on a decimation value, selected ones of the plurality of DCT coefficient values with a plurality of frequency domain coefficients representing a lowpass block mirror filter having a predetermined number of taps to form a set of filtered DCT coefficients;

inverse-transform means for receiving and transforming the filtered DCT coefficients using an inverse discrete cosine transform (IDCT) operation to obtain a set of filtered compressed pixel sample values;

translation means for receiving the motion vector and scaling the motion vector based on the decimation value;

prediction block generating means for receiving the scaled motion vector and a previous set of filtered pixel sample values, and forming a set of prediction pixel sample values;

combining means for combining the set of filtered compressed pixel sample values with the set of prediction pixel sample values to form a set of filtered pixel sample values; and

decimating means for receiving and for retaining selected ones of the set of filtered pixel sample values based on the decimation value, wherein the decimating means provides the selected ones of the set of filtered pixel sample values as the lower resolution video signal.

11. Apparatus for forming a low resolution video signal as recited in claim 10, wherein the down conversion filter means is a lowpass filter having a cutoff frequency proportional to a sampling frequency of the encoded video signal divided by the decimation value.

12. Apparatus for forming a lower resolution video signal as recited in claim 10, wherein the down conversion filter means is a lowpass filter represented by a set of frequency-domain filter coefficient values, and the down conversion filter means weights the selected ones of the plurality of DCT coefficient values by multiplying the set of frequency-domain filter coefficients with respective ones of the plurality of DCT coefficient values.

13. Apparatus for forming a lower resolution video signal as recited in claim 10, wherein the prediction block generating means further includes:

memory means for storing at least one reference frame, the reference frame being a previously decoded video signal represented as the previous set of filtered pixel sample values,

up-sampling means for receiving and up-sampling the reference frame, the up-sampling means and the memory means being responsive to the scaled motion vector; and

half-pixel generating means for generating a plurality of half-pixel interpolated values from the up-sampled reference frame, the half-pixel generating means providing the plurality of half-pixel interpolated values as the set of prediction pixel sample values.

14. A method of receiving an encoded video signal representing a video image, the encoded video signal being a compressed frequency-domain transformed video signal, and forming a low resolution video signal, the method comprising the steps of:

a) providing the encoded video signal as a plurality of compressed high resolution DCT coefficient values and a motion vector;

b) weighting, based on a decimation value, selected ones of the plurality of compressed high resolution DCT coefficient values with a plurality of frequency domain coefficients representing a lowpass block mirror filter having a predetermined number of taps to form a set of filtered compressed DCT coefficient values;

c) transforming the filtered compressed DCT coefficient values using an inverse discrete cosine transform (IDCT) operation to obtain a set of filtered compressed pixel sample values;

d) scaling the motion vector based on the decimation value; and

e) forming a set of prediction pixel sample values from the scaled motion vector and a previous set of filtered pixel sample values;

f) combining the set of filtered compressed pixel sample values with the set of prediction pixel sample values to form a set of filtered pixel sample values;

g) deleting selected ones of the set of filtered pixel sample values based on the decimation value to form the lower resolution video signal; and

h) storing the pixel sample values of the low resolution video signal.

FIELD OF THE INVENTION

This invention relates to a decoder having a filter for down conversion of frequency domain encoded signals, e.g. MPEG-2 encoded video signals, and more specifically to a decoder which converts a high resolution video signal to a low resolution video signal by filtering the frequency domain signals.

BACKGROUND OF THE INVENTION

In the United States a standard has been proposed for digitally encoded high definition television signals (HDTV). 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., NTSC 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 proposed 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 proposed 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 six layers, the sequence layer, the group of pictures layer, the picture layer, the slice layer, the macroblock layer, and the block 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.

Down-conversion of high resolution Main Profile, High Level pictures to Main Level, Main Level pictures, or other lower resolution picture formats, has gained increased importance for reducing implementation costs of HDTV. Down 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. 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.

One method of down conversion of the prior art simply low pass filters and decimates the decoded high resolution, Main Profile, High Level picture to form an image suitable for display on a conventional television receiver. Consequently, using existing techniques, a decoder employing down-conversion may be implemented using a single processor having a complex design, considerable memory, and operating on the spatial domain image at a high data rate to perform this function. The high resolution, and high data rate, however, requires very expensive circuitry, which would be contrary to the implementation of a decoder in a consumer television receiver in which cost is a major factor.

SUMMARY OF THE INVENTION

An apparatus for forming a decimated video signal receives an encoded video signal representing a video image, the encoded video signal being a frequency-domain transformed video signal. The apparatus includes means for providing the encoded video signal as a plurality of high resolution frequency-domain video coefficient values. The apparatus further includes 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 receiving and transforming the filtered frequency-domain video coefficients into a set of low resolution pixel sample values. The apparatus also includes a decimating processor for receiving and retaining selected ones of the set of low resolution pixel sample values to provide the decimated video signal.

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:

FIG. 1 is a high level block diagram of a video decoding system of the prior art.

FIG. 2A is a high level block diagram of the down conversion system of one exemplary embodiment of the present invention.

FIG. 2B is a high level block diagram of the down conversion system of a second exemplary embodiment of the present invention employing an inexpensive horizontal and vertical filtering implementation.

FIG. 3A illustrates subpixel positions and corresponding predicted pixels for the 3:1 and 2:1 exemplary embodiments of the present invention.

FIG. 3B shows the upsampling process which is performed for each row of an input macroblock for an exemplary embodiment of the present invention.

FIG. 4 illustrates the multiplication pairs for the first and second output pixel values of an exemplary embodiment of a block mirror filter.

FIG. 5 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.

FIG. 6A shows the input and decimated output pixels for 4:2:0 video signal using 3:1 decimation.

FIG. 6B shows the input and decimated output pixels for 4:2:0 video signal using 2:1 decimation.

FIG. 7A is a high level block diagram illustrating a vertical programmable filter of one embodiment of the present invention.

FIG. 7B illustrates the spatial relationships between coefficients and pixel sample space of lines of the vertical programmable filter of FIG. 7A.

FIG. 8A is a high level block diagram illustrating a horizontal programmable filter of one embodiment of the present invention.

FIG. 8B illustrates spatial relationships between