Image Processing Reference
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method based on the IQ-signals. The Hilbert transform, complex FIR filter, phase-shift,
complex IIR filter, FFT/IFFT and modulation/demodulation systems also indicated that the
direction separation system of the Doppler audio does not allow a baseline-shift. Among
these systems, the Hilbert transform and phase-shift systems enable direction separation by
addition and subtraction between signals with a 180-degree phase-difference. Since an input
IQ-signal has a 90-degree phase difference, these systems give a phase-difference of 90
degree between channels with a filter. Since the phase-difference of an IQ-signal stops being
90 degree when sampling frequency is doubled as a countermeasure, in the Hilbert
transform and phase-shift systems, which make the phase-difference between channels a
simple 90 degree, direction separation is difficult. Moreover, the complex FIR filter system
involves the same pre-processing step as that in the complex IIR filter system, and anti-alias
processing becomes possible. However, since the length of a FIR coefficient sequence
doubles, the operation load increases. On the other hand, the FFT/IFFT system can reduce
the operation load by diverting the FFT output of spectrum Doppler imaging processing.
When the FFT output is diverted, the returning anti-alias processing can be performed only
by inverse-FFT and shift-addition. The modulation/demodulation and the complex IIR filter
systems mainly involve the multiplication of modulation/demodulation and IIR filter
processing. Thus, their calculation processing is easy, and the increase in calculation load by
anti-aliasing processing is small. As mentioned above, from the viewpoints of calculation
load reduction and anti-alias processing feasibility, we chose and examined the following
three systems: the modulation/demodulation, the FFT/IFFT, and the complex IIR systems.
When evaluating these systems, we showed the same target performance required as that of
the Doppler ultrasound system in Table 4. The items 1 to 4 (time-delay, direction separation,
frequency characteristic, frequency resolution) are same as table 1.
item
target
1. time-delay
bellow 20ms ( fs =4KHz)
2. direction separation
above 30dB
- fs /128 to -127* fs /128, fs /128 to 127* fs /128
flat as possible
4. frequency resolution fs /100
5. baseline-shift range - fs /2 to + fs /2 (-0.5 to 0.5)
Table 4. Target specification of Doppler audio processing.
3. frequency characterization
baseline-shift
-0.5
-0.25
0
0.25
0.5
FB : band-width of forward
4/8
3/8
2/8
1/8
0
FBC : center freq. of forward
4/16
3/16
2/16
1/16
0
RB : band-width of reverse
0
1/8
2/8
3/8
4/8
RBC : center freq. of reverse
0
-1/16
-2/16
-3/16
-4/16
Notes: Baseline shift, FB , FBC , RB and RBC are normalized by fs .
Table 5. Frequency shift and bandwidth table of baseline-shift
Baseline-shift range:
The baseline-shift range is considered to be -0.5*fs to +0.5*fs to enable range expansion on
the positive and negative sides to twice the Nyquist frequency range. The ranges of both
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