1. A quantization control circuit for determining an optimum scaling factor for quantizing orthogonal transformation coefficient data corresponding to a predetermined period of time of a digital video signal, comprising: determining means for determining a first scaling factor from said orthogonal transformation coefficient data as a function of whether a bit rate determined by quantizing said orthogonal transformation coefficient data using an initial scaling factor is no greater than a target bit rate; and correcting means for determining a corrected scaling factor by calculating a plurality of second scaling factors each derived from said first scaling factor, calculating a plurality of error values each derived from a respective one of said second scaling factors and from said orthogonal transformation coefficient data, and then selecting a respective one of said plurality of second scaling factors corresponding to a respective one of said plurality of error values having a minimum value as said corrected scaling factor.