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Abstract

A method for coding frames of video wherein a coding circuit (14,16,18) includes a processor (30) for performing an orthogonal transform such as a discrete cosine transform and a quantizer (32) for quantizing the resulting transform coefficients. The coding circuit codes the video frames using intra-frame, predictive or interpolative coding to generate code bits at a variable rate. The code bits are stored at a variable rate in a rate buffer (22), which transmits the code bits into a communication channel (24) at a pseudo-constant rate, i.e. a rate which is constant in every time interval of one frame. To maintain the contents of the rate buffer (22) within predetermined limits, the quantization parameters utilized by the quantizer (32) are periodically adjusted to increase or decrease the amount of code bits generated by the coding circuit. The quantization parameters are changed on a global SGOP level to avoid changes of quantization parameters and corresponding changes in decoded image quality within particular frames. The change in quantization parameters for coding the next SGOP is determined by a deviation measure between the actual number of code bits generated by the coding circuit (14,16,18) for the previous SGOP and an estimate of the number of code bits for the previous SGOP. The estimated number of code bits is determined based on the contents of the rate buffer (22) such that the rate buffer (22) will be emptied in a predetermined time period.

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