An information source encoding system in which overload noise and block noise are less recognized and quantizing overload and large quantizing distortion can be reduced within an intra-frame mode encoding region even when intra-frame and inter-frame mode encoding regions coexist in an image screen. An orthogonal converter eg. a discrete cosine converter, in a motion compensated prediction unit outputs conversion coefficients in a predetermined spatial frequency region. A coefficient separating unit 11 selectively separates the conversion coefficients. With the separated coefficients from the coefficient separating unit, a power is calculated. The calculated power is compared with a predetermined threshold value by a coefficient power determining unit 12. A step width controller 13 sets a quantizing step width, based on the combination of a power determining ON/OFF signal from the coefficient power determining unit 12 and an adaptive control ON/OFF signal from the encoding controller 14, when the coefficient power is larger than the threshold value in the intra-frame mode, and sets a quantizing step width determined by the quantization characteristic designation signal from the encoding controller 14 in remaining cases. As a result, a combined adaptive quantization characteristic signal having a quantizing step width for combined adaptive control is output to the quantizer. <IMAGE>
Information source encoding apparatus