An encoder for encoding a sequence of pictures as a plurality of block transform coefficients to meet network traffic model restrictions, the encoder comprising an iterative loop for selecting one of a plurality of quantization parameter values for each picture, the iterative loop comprising: pre-encoding means for pre-encoding the sequence of pictures for each of a plurality of quantization parameter values; the encoder being characterized in that it comprises: selecting means for selecting, for each picture of the sequence of pictures, one of the plurality of quantization parameter values responsive to the quantization parameter values and bitrate operating points of neighboring pictures in a sliding time window, said selecting means further including means for determining a number of bits to encode the next N pictures, including a current picture, the current picture being said each picture, starting with a lowest quantization parameter value, and selecting the one of the plurality of the quantization parameter values for which the number of bits to encode the next N pictures comes closest to, but does not exceed a target number of bits, and also the number of bits to encode the previous N pictures, including the current picture, does not exceed the target number of bits; and encoding means for encoding each picture of the sequence using the quantization parameter value selected for that picture. pre-encoding means for pre-encoding the sequence of pictures for each of a plurality of quantization parameter values; the encoder being characterized in that it comprises: selecting means for selecting, for each picture of the sequence of pictures, one of the plurality of quantization parameter values responsive to the quantization parameter values and bitrate operating points of neighboring pictures in a sliding time window, said selecting means further including means for determining a number of bits to encode the next N pictures, including a current picture, the current picture being said each picture, starting with a lowest quantization parameter value, and selecting the one of the plurality of the quantization parameter values for which the number of bits to encode the next N pictures comes closest to, but does not exceed a target number of bits, and also the number of bits to encode the previous N pictures, including the current picture, does not exceed the target number of bits; and encoding means for encoding each picture of the sequence using the quantization parameter value selected for that picture. selecting means for selecting, for each picture of the sequence of pictures, one of the plurality of quantization parameter values responsive to the quantization parameter values and bitrate operating points of neighboring pictures in a sliding time window, said selecting means further including means for determining a number of bits to encode the next N pictures, including a current picture, the current picture being said each picture, starting with a lowest quantization parameter value, and selecting the one of the plurality of the quantization parameter values for which the number of bits to encode the next N pictures comes closest to, but does not exceed a target number of bits, and also the number of bits to encode the previous N pictures, including the current picture, does not exceed the target number of bits; and encoding means for encoding each picture of the sequence using the quantization parameter value selected for that picture.