Currently, dual channel blood flow measurement using Doppler ultrasound systems appeares in the field of diagnostic ultrasound. Such systems have a problem of velocity-range limitation. To address this problem, we use a mathematical model obtained by system identification. This model inputs the electrocardiographic (ECG) waveform and outputs the short time Fourier transform (STFT) image parameters. With this model, we developed a system that fills the 100 ms gap. Simulation results also demonstrate the effectiveness of the proposed method.