Figure 4. A quadrature detector is used to determine the direction (forward or reverse) of the Doppler shift frequencies. The Doppler signal is split into two channels, then mixed with a reference frequency (fref). On one side, this reference signal is shifted by 90° (one quarter of a period). The resulting signals are filtered with a low-pass filter. The relative phases of the two sides are compared to produce forward (fDfor) and reverse (fDrev) flow channels. These two channels may be output to left and right speakers to allow the operator to hear the Doppler shift frequencies in stereo.