Image encoding method and apparatus for controlling the number of bits

Abstract

An image encoding method and apparatus for controlling a bit generation quantity using a quantization activity accurately controls a bit generation quantity of an encoding frame. The image encoding apparatus includes a discrete cosine transformer for discrete-cosine-transforming a difference signal between an image signal of a current frame and an image signal of a restored previous frame in units of a block and outputting the transformation coefficients, a quantizer and a variable-length-encoder for quantizing and variable-length-encoding the transformation coefficients according to a quantization stepsize, respectively, a buffer memory for temporarily storing the variable-length-encoded data and transmitting the stored data at a constant bit rate, a quantization activity calculator portion for calculating quantization activities by counting the number of the quantization coefficients which are not zero among the quantization coefficients to which each quantization stepsizes is applied, a quantization controller for predicting a desired bit generation quantity via a fullness of a buffer memory and a transmission rate, and determining a quantization stepsize of the quantization activity to generate the closest bit generation quantity to the predicted bit generation quantity using a linear proportional relationship between the calculated quantization activity and the variable-length-encoded data generation quantity. Thus, an overflow and an underflow of a buffer memory can be surely prevented and a transmission channel can be effectively used.