Quantizer control method and apparatus

• US 5,283,646 A
• Filed: 04/09/1992
• Issued: 02/01/1994
• Est. Priority Date: 04/09/1992
• Status: Expired due to Term

• Current Assignee: Polycom Incorporated (Plantronics, Inc.)
• Original Assignee: Picturetel Corporation (Plantronics, Inc.)
• Inventors: Bruder, John E.
• Primary Examiner(s): Coles, Sr., Edward L.
• Assistant Examiner(s): Williams, Kimberly A.
• Application Number:
  US07/866,027
• Time in Patent Office: 663 Days
• Field of Search: 358/136, 358/138, 358/429, 341/76, 341/77, 375/27
• US Class Current: 375/240.12
• CPC Class Codes: H04N 19/124 Quantisation
• CPC Class Codes: H04N 19/126 Details of normalisation or weighting functions, e.g. normalisation matrices or variable uniform quantisers
• CPC Class Codes: H04N 19/196 being specially adapted for the computation of encoding parameters, e.g. by averaging previously computed encoding parameters processing of motion vectors H04N19/513
• CPC Class Codes: H04N 19/197 including determination of the initial value of an encoding parameter H04N19/56 takes precedence
• CPC Class Codes: H04N 19/60 using transform coding
• CPC Class Codes: H04N 19/61 in combination with predictive coding

Abstract

A method and apparatus for enabling a real time video encoding system to accurately deliver the desired number of bits per frame, while coding the image only once, updates the quantization step size used to quantize coefficients which describe, for example, an image to be transmitted over a communications channel. The data is divided into sectors, each sector including a plurality of blocks. The blocks are encoded, for example, using DCT coding, to generate a sequence of coefficients for each block. The coefficients can be quantized, and depending upon the quantization step, the number of bits required to describe the data will vary significantly. At the end of the transmission of each sector of data, the method and apparatus of the invention compare the accumulated actual number of bits expended with the accumulated desired number of bits expended, for a selected number of sectors associated with the particular group of data. The system then readjusts the quantization step size to target a final desired number of data bits for a plurality of sectors, for example describing an image. Various methods are described for updating the quantization step size and determining desired bit allocations.

Claims

1. A method for selecting an initial quantizer step size for use in encoding an image frame, each frame having a plurality of sectors, and each sector being divided into a plurality of blocks, the method comprising the steps of

selecting a quantizer step size equal to the quantizer step size of a previously encoded frame,

adjusting said selected step size by a weighted function of the difference between the actual number of bits used to encode a previous frame and a desired number of bits for coding the present frame, the weighting function being a preselected first constant divided by the number of blocks expected to be encoded, and

adjusting said now modified selected quantizer step size a second weighted function of the logarithm of the ratio of the geometric mean of the total energy of the number of blocks actually to be encoded for this frame multiplied by the number of blocks expected to be encoded, and the total energy of the number of blocks actually encoded for the previous frame multiplied by the number of blocks expected to be encoded in the previous frame.

2. The method of claim 1 wherein the first weighting function is about 24.

3. The method of claim 1 wherein the second weighted function is a constant having a value of about 0.004.

4. A method for transmitting a selected number of bits for representing an encoded image, the number of bits being transmitted being a function of a quantizer step size, said image being comprised of a plurality of sectors, each sector being comprised of a plurality of blocks, said method comprising the steps of

determining a desired number of bits to be allocated to each sector,

transmitting said sectors in a selected sequence, and

associating a quantizer step size with each sector,

said associating step comprising setting the quantizer step size for a sector as a function of the difference in number of bits actually transmitted for all previous sectors of the image and the desired number of bits to be transmitted for all previous sectors of the image.

5. The method of claim 4 further comprising the step of

multiplying said difference by a weighting constant.

6. The method of claim 5 wherein said constant is about 0.005.

7. A method for transmitting a selected number of bits for representing an encoded image, the number of bits being transmitted being a function of a quantizer step size, said image being comprised of a plurality of sectors, each sector being comprised of a plurality of blocks, said apparatus comprising the step of

allocating available bits to blocks of the image to be encoded according to the relationship

8. An apparatus for selecting an initial quantizer step size for use in encoding an image frame, each frame having a plurality of sectors, and each sector being divided into a plurality of blocks, the apparatus comprising

means for selecting a quantizer step size equal to the quantizer step size of a previously encoded frame,

means for adjusting said selected step size by a weighted function of the difference between the actual number of bits used to encode a previous frame and a desired number of bits for coding the present frame, the weighting function being a preselected first constant divided by the number of blocks expected to be encoded, and

means for adjusting said now modified selected quantizer step size by a second weighted function of the logarithm of the ratio of the geometric mean of the total energy of the number of blocks actually to be encoded for this frame multiplied by the number of blocks expected to be encoded, and the total energy of the number of blocks actually encoded for the previous frame multiplied by the number of blocks expected to be encoded in the previous frame.

9. The apparatus of claim 8 wherein the first weighting function is about 24.

10. The apparatus of claim 8 wherein the second weighted function is a constant having a value of about 0.004.

11. An apparatus for transmitting a selected number of bits for representing an encoded image, the number of bits being transmitted being a function of a quantizer step size, said image being comprised of a plurality of sectors, each sector being comprised of a plurality of blocks, said apparatus comprising

means for determining a desired number of bits to be allocated to each sector,

means for transmitting said sectors in a selected sequence, and

means for associating a quantizer step size with each sector,

said associating means comprising means for setting the quantizer step size for a sector as a function of the difference in number of bits actually transmitted for all previous sectors of the image and the desired number of bits to be transmitted for all previous sectors of the image.

12. The apparatus of claim 11 further comprising

means for multiplying said difference by a weighting constant.

13. The apparatus of claim 12 wherein said constant is about 0.005.

14. An apparatus for transmitting a selected number of bits for representing an encoded image, the number of bits being transmitted being a function of a quantizer step size, said image being comprised of a plurality of sectors, each sector being comprised of a plurality of blocks, said apparatus comprising means for allocating available bits to blocks of the image to be encoded according to the relationship