A video encoder for encoding video signal data for a group of picture GOP formed of I, P and B pictures, the encoder comprising an adaptive quantizer for applying an adaptive quantizer parameter responsive to a current bit rate, characterized in that said adaptive quantizer comprises: • first calculating means for calculating a target number of bits for said GOP based on said current bit rate; • allocating means for allocating corresponding target number of bits to I, P and B picture in said GOP based on said target number of bits for said GOP, the number of P and B pictures in said GOP and a constant that is representative of the number of bits of I and P pictures relatively to a B picture; • second calculating means for calculating a quantization parameter for I picture by dividing a quantization parameter of the previous GOP by a constant representative of the number of P and B pictures in said GOP, and by adding the division result to a differential quantization parameter dqp, wherein the quantization parameter of said previous GOP is a weighted average of quantization parameters for I, P, B pictures of said previous GOP; said differential quantization parameter dqp is determined by a logarithmic function applied to a ratio of a number of bits consumed in said previous GOP to a number of available bits for said GOP, and wherein said number of available bits for said GOP is determined by said target number of bits for said GOP, the number of bits consumed in said previous GOP and a difference of bits between said target number of bits for said previous GOP and a number of consumed bits in said previous GOP; • third calculating means for calculating a quantization parameter for P picture, by adding a quantization parameter for a previous P picture of said GOP to a differential quantization parameter for said P picture; wherein said differential quantization parameter dqp P is determined by a logarithmic function applied to a ratio of the number of available bits for said P picture to said allocated target number of bits for said P picture, said number of available bits for said P picture is determined from a weighted average of the number of available bits for said previous P picture and the number of available bits for the previous B pictures and • fourth calculating means for calculating a quantization parameter for B picture, by adding a quantization parameter for a previous B picture of the GOP to a differential quantization parameter for a B picture dqp B, wherein said differential quantization parameter dqp B is determined by a logarithmic function applied to a ratio of the number of available bits for said B picture to the allocated target number of bits for B picture, said number of available bits for said B picture is determined from another weighted average of the number of bits for said previous P picture and the number of available bits for said previous B pictures. • first calculating means for calculating a target number of bits for said GOP based on said current bit rate; • allocating means for allocating corresponding target number of bits to I, P and B picture in said GOP based on said target number of bits for said GOP, the number of P and B pictures in said GOP and a constant that is representative of the number of bits of I and P pictures relatively to a B picture; • second calculating means for calculating a quantization parameter for I picture by dividing a quantization parameter of the previous GOP by a constant representative of the number of P and B pictures in said GOP, and by adding the division result to a differential quantization parameter dqp, wherein the quantization parameter of said previous GOP is a weighted average of quantization parameters for I, P, B pictures of said previous GOP; said differential quantization parameter dqp is determined by a logarithmic function applied to a ratio of a number of bits consumed in said previous GOP to a number of available bits for said GOP, and wherein said number of available bits for said GOP is determined by said target number of bits for said GOP, the number of bits consumed in said previous GOP and a difference of bits between said target number of bits for said previous GOP and a number of consumed bits in said previous GOP; • third calculating means for calculating a quantization parameter for P picture, by adding a quantization parameter for a previous P picture of said GOP to a differential quantization parameter for said P picture; wherein said differential quantization parameter dqp P is determined by a logarithmic function applied to a ratio of the number of available bits for said P picture to said allocated target number of bits for said P picture, said number of available bits for said P picture is determined from a weighted average of the number of available bits for said previous P picture and the number of available bits for the previous B pictures and • fourth calculating means for calculating a quantization parameter for B picture, by adding a quantization parameter for a previous B picture of the GOP to a differential quantization parameter for a B picture dqp B, wherein said differential quantization parameter dqp B is determined by a logarithmic function applied to a ratio of the number of available bits for said B picture to the allocated target number of bits for B picture, said number of available bits for said B picture is determined from another weighted average of the number of bits for said previous P picture and the number of available bits for said previous B pictures.