**Frequency resolution:**

216 Applications of Digital Signal Processing

The digitization of Doppler ultrasound system had progressed in recent years, and the digital signal processing using DSP etc. can realize complex processing easily from the conventional analog-circuit. We made the target performances of the direction separating process of digital Doppler audio, and evaluated six kinds of digital-signal processing ideas

item target

flat as possible

For the digitization, the target performance is investigated and taken up to Table 1.

1. time-delay bellow 20ms (PRF 4kHz) 2. direction separation above 30dB

4. frequency resolution fs/100 5. calculation volume light as possible

Table 1. Requirement specification of Doppler audio direction separation

to store the Doppler audio is about one frame cycle at maximum in a tomogram.

the frequency characteristic flat in the region of r *fs* /128 to r63 /128 *fs* range.

It has been reported that human's direction distinction requires a right-and-left signal difference of 15 to 20 dB or lager. In an actual Doppler ultrasound system, considering that the Doppler signal has a broad band, that the angle between the right-and-left speakers is small, and that blood flow velocity changes with time, a larger signal difference is required. The target performance of direction separation was set to be 30 dB or higher at observation

A signal processing frequency range is the range *fs* from negative-side Nyquist frequency to positive-side Nyquist frequency, where *fs* is input IQ-signal sampling frequency. We made

3. frequency characterization fs/128 to 63\*fs/128 (both direction)

A user usually sets up the Doppler range gate on a tomogram, moves it, and performs blood flow diagnosis with the Doppler ultrasound system. In searching for a small blood vessel, the Doppler audio is effective, because its response is faster than that of the spectrum image. This is because a tomogram set with the Doppler audio delays the outputs of about 20 ms, compared with the spectrum image that has a typical delay of about 40 ms. The time delay of tomogram processing is a few cycle of one frame (13.3 - 16.7 ms). In the Doppler signal processing system, it has a total processing delay of 10 ms by quadrature-detection and HPF processing, except for the Doppler processing part. Therefore, to make the tomogram and audio agree, a time delay of 3.3 - 6.7 ms is required at the Doppler signal processing part. However, because the direction separation process, which is the main factor of the Doppler signal processing part delay, requires a number of series samplings for processing, a target time delay is theoretically difficult to achieve. Therefore, the target time delay was set to be 20 ms or smaller, so that the target delay time required for the direction separation process

**3. Comparison of six kinds of Doppler audio processing** 

that were pre-existing or were newly devised.

**3.1 Design of a target specification** 

**Time-delay:** 

**Direction separation:** 

**Frequency characteristic:** 

frequency.

Since spectrum image signal processing involves 256-point FFT, an acceptable frequency (velocity) resolution is obtained. However, when the frequency resolution of the Doppler audio is unacceptable, similar to that of a small-pitch Doppler image, we set the target resolution to be *fs*/100. The frequency range is determined from sample frequency. However, the frequency resolution is proportional to the reciprocal of observation time. For example, in FFT, it is equivalent to the main robe width of the sampling function determined from observation time width and the window function.
