Rate control with panic mode

Claim 1

In an encoder for coding an input sequence of video frames to generate a compressed bitstream, each frame comprising a plurality of macroblocks, a method for coding a frame comprising the steps of: (1) after coding one or more macroblocks in a frame by said encoder, initiating a panic mode if a physical output buffer of said encoder has a number of bits less than k(N.sub.max -R.sub.eff /f.sub.eff K) where: K is the total number of macroblocks in the frame, k is the number of macroblocks in the frame remaining to be encoded, R eff is the effective rate at which bits are removed from the physical encoder output buffer, f eff is the effective frame rate of the input video sequence, and N max is the maximum number of bits used by said encoder to encode a macroblock when a panic mode is initiated, where N max is a selectable number depending on whether the frame is an I, P or B frame, and (2) in said panic mode, coding macroblocks remaining in said frame by providing no coded block patterns for inter-macroblocks and retaining at least a DC discrete cosine transfer coefficient and a selected number of AC discrete cosine transform coefficients in said coded bitstream for intra-macroblocks, such that the value of said N max further depending on the number of AC coefficients retained for each of said remaining intra-macroblocks.