Method for transcoding an MPEG-2 video stream to a new bitrate

• US 20040247030A1
• Filed: 06/09/2003
• Published: 12/09/2004
• Est. Priority Date: 06/09/2003
• Status: Abandoned Application

• Current Assignee: Pinnacle Systems Incorporated (Avid Technology Incorporated)
• Original Assignee: Pinnacle Systems Incorporated (Avid Technology Incorporated)
• Inventors: Wiethoff, Andre
• Application Number:
  US10/458,020
• Publication Number:
  US 20040247030A1
• Time in Patent Office: Days
• US Class Current: 375/240.16
• CPC Class Codes: H04N 19/115 Selection of the code volume for a coding unit prior to coding
• CPC Class Codes: H04N 19/124 Quantisation
• CPC Class Codes: H04N 19/132 Sampling, masking or truncation of coding units, e.g. adaptive resampling, frame skipping, frame interpolation or high-frequency transform coefficient masking
• CPC Class Codes: H04N 19/18 the unit being a set of transform coefficients
• CPC Class Codes: H04N 19/40 using video transcoding, i.e. partial or full decoding of a coded input stream followed by re-encoding of the decoded output stream
• CPC Class Codes: H04N 19/48 using compressed domain processing techniques other than decoding, e.g. modification of transform coefficients, variable length coding [VLC] data or run-length data motion estimation in a transform domain H04N19/547; processing of decoded motion vectors H04N19/513
• CPC Class Codes: H04N 19/51 Motion estimation or motion compensation
• CPC Class Codes: H04N 19/86 involving reduction of coding artifacts, e.g. of blockiness

Abstract

A method for transcoding an MPEG-2 video stream to a new bitrate using the motion vectors from the original stream. A desired bitrate is chosen and the macroblocks of the target frames are requantized accordingly in the transcoder. In order to adjust the motion compensation in the target frames, the difference between the original and target reference frames is added on a pixel-by-pixel basis to the target frame'"'"'s prediction error, or correction matrix. An ideal quantization value is determined using a perceptive algorithm that reduces image quality in high-activity areas where the human visual system does not perceive quality reduction and enhances image quality in areas where noise is noticeable. The new correction matrix is transformed to a frequency domain by a DCT. A coefficient threshold algorithm then identifies those coefficients that would be set to zero using the ideal quantization value and sets them to zero. The number of zeroed coefficients for each macroblock are counted and a formula (in one embodiment, a lookup table) used to determine a new, lower quantization value. The macroblock is then quantized using this lower quantization value.

Claims

1. A method for transcoding an MPEG-2 video stream to a new bitrate using motion vectors in the original stream comprising:

a) correcting motion compensation in each target macroblock having motion compensation;

b) determining an ideal quantization value for each target macroblock by using a perceptive algorithm;

c) applying a discrete cosine transform algorithm to each target macroblock;

d) performing a coefficient threshold algorithm on each target macroblock to determine how many quantization steps can be reduced, thereby setting a new quantization factor; and

e) quantizing each target macroblock using the new quantization factor.

2. The method of claim 1 further comprising determining a desired bitrate for the target stream.

3. The method of claim 1 further comprising dequantizing and performing an inverse discrete cosine transform algorithm on the macroblocks of original stream reference frames.

4. The method of claim 1 wherein correcting the motion compensation in the target macroblock having motion compensation is accomplished by determining a difference between the original and target reference frames.

5. The method of claim 4 further comprising adding the difference between the original and target reference frames to an original correction matrix of the target macroblock having motion compensation to create a new correction matrix.

6. The method of claim 5 wherein adding the difference between the original and target reference frames to the original correction matrix is accomplished by performing an inverse discrete cosine transform algorithm on the original correction matrix and adding pixel values based on the difference between original and target reference frames to the original correction matrix to produce the new correction matrix.

7. The method of claim 1 further comprising dequantizing and performing an inverse discrete cosine transform algorithm on the macroblocks of target reference frames to create a new target reference frame for following P- and B-frames.

8. A method for transcoding an MPEG-2 video stream to a new bitrate using motion vectors in the original video stream comprising:

a) receiving an encoded MPEG-2 video stream at a transcoder;

b) dequantizing and performing an inverse discrete cosine transform algorithm on the macroblocks of original stream reference frames;

c) creating a new prediction error for each target macroblock having motion compensation based on a determined difference between the original and target video reference frames;

d) determining an ideal quantization value for each target macroblock by using a perceptive algorithm;

e) applying a discrete cosine transform algorithm to each target macroblock;

f) performing a coefficient threshold algorithm on each target macroblock to determine how many quantization steps can be reduced, thereby setting a new quantization factor;

g) quantizing each target macroblock using the new quantization factor; and

h) dequantizing and performing an inverse discrete cosine transform on the macroblocks of target reference frames to create a new target reference frame for following P- and B-frames.

9. The method of claim 8 wherein the new prediction error is created by adding pixel values based on the determined difference between the original and target video reference frames to the target macroblock.

10. A computer-readable storage medium storing instructions that, when executed, cause the computer to perform a method for transcoding an MPEG-2 video stream to a new bitrate using motion vectors in the original stream, the method comprising:

a) correcting motion compensation in each target macroblock having motion compensation;

b) determining an ideal quantization value for each target macroblock by using a perceptive algorithm;

c) applying the discrete cosine transform algorithm to each target macroblock;

d) performing the coefficient threshold algorithm on each target macroblock to determine how many quantization steps can be reduced, thereby setting a new quantization factor; and

e) quantizing each target macroblock using the new quantization factor.

11. The computer-readable storage medium of claim 10, the method further comprising determining a desired bitrate for the target stream.

12. The computer-readable storage medium of claim 10, the method further comprising dequantizing and performing an inverse discrete cosine transform algorithm on the macroblocks of original stream reference frames.

13. The computer-readable storage medium of claim 10, wherein correcting the motion compensation in the target macroblock having motion compensation is accomplished by determining a difference between the original and target reference frames.

14. The computer-readable storage medium of claim 13, the method further comprising adding the difference between the original and target reference frames to an original correction matrix of the target macroblock having motion compensation to create a new correction matrix.

15. The computer-readable storage medium of claim 14, wherein adding the difference between the original and target reference frames to the original correction matrix is accomplished by adding pixel values based on the difference between original and target reference frames to the original correction matrix to produce the new correction matrix.

16. The computer-readable storage medium of claim 10, the method further comprising dequantizing and performing an inverse discrete cosine transform on the macroblocks of target reference frames to create a new reference frame for following P- and B-frames.