Picturetel Corporation

John E Bruder

John C Sievers

A method and apparatus for selectively pre-filtering signal representations of images representing data to be transmitted and substantially maintaining the frame rate of transmission by reducing the number of expected bits needed to transmit the selected block portions of an image frame. The pre-filter uses the quantizer state value representative of the number of expected bits required for each block portion of the image as well as a weighted energy error value based on the preselected coding method of the block of the image being transmitted to determine a level of prefiltering required. The prefilter, for example, may be a multi-tap filter having a center tap and outer taps with amplitudes based on the quantizer state and weighted error energy values.

Spatially adaptive blur filter

A method and apparatus for selecting blocks for intraframe encoding in a way which avoids intraframe encoding blocks which have not diverged. An inter-energy value is calculated which represents the approximate number of bits required to encode the block of data using a best interframe encoding technique. An intra-energy value is calculated which represents the approximate number of bits required to encode the block of data using intraframe encoding. The block is transmitted using intraframe encoding if the intra-energy is approximately less than the inter-energy. Otherwise, the block is either a) transmitted using an interframe encoding technique or b) not transmitted, thereby implying that the block from the previous frame in the sequence should be repeated. An encoder determines the number of times K that the block has been interframe encoded since it was last intraframe encoded. A weighting factor w(K) is calculated which decreases with the value of K. A weighted energy value is then calculated by multiplying the intra-energy plus an offset S by the weighting factor w(k). If the weighted energy value is approximately less than the inter-energy value, the block is transmitted using intraframe encoding.

Method and apparatus for efficiently transmitting forced updates in a

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.

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Arlington, MA, United States of America

John E Bruder