Inspection apparatus

An inspection apparatus utilizing a pulse compression technique is described which comprises a signal generator, a transmission/reception probe; first and second correlators and an adder. The signal generator generates a composite transmission signal consisting of signals Sap(t), Saq(t), Sbp(t) and Sbq(t) respectively based on a basic unit signal ga(t) and a sequence {p}, the signal ga(t) and a sequence {q}, a basic unit signal gb(t) and the sequence {p}, and the signal gb(t) and the sequence {q}, to the probe to transmit the composite transmission signal to a target. The first correlator performs a correlation operation of echo signals Rap(t), Raq(t), Rbp(t) and Rbq(t) corresponding to the signal Sap(t), Saq(t) and Sbq(t) by utilizing reference signals Ua(t) and Ub(t) based on the sequences to provide results Caap(t), Caaq(t), Cbbp(t) and Cbbq(t). The second correlator performs a correlation operation of the results Caap(t), Caaq(t) Cbbp(t) and Cbbq(t) by utilizing the sequences {p} and {q} to provide compressed pulses Caapp(t), Caaqq(t), Cbbpp(t) and Cbbqq(t). These pulses are summed up at the adder to provide a composite compressed pulse C having the large amplitude main lobe and small amplitude side lobes.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
The present invention relates to an inspection apparatus utilizing 
ultrasonic waves, electromagnetic waves or the like, and more 
particularly, relates to an inspection apparatus such as an ultrasonic 
non-destructive inspection apparatus utilizing a pulse compression method. 
2. Prior Art 
Conventional inspection apparatuses of the type explained above are 
disclosed, for example, in literatures A, B, C as listed below. 
Literature A: B. B. Lee and E. S. Furgason, "High-Speed Digital Golay Code 
Flaw Detection System", in proceedings of IEEE Ultrasonics Symposium, 
1981, pp 888 -891. 
Literature B: B. B. Lee and E. S. Furgason, "An Evaluation of Ultrasound 
NDE Correlation Flaw Detection Systems", IEEE Transactions on Sonics and 
Ultrasonics, Vol. SU-29, No. 6, November, 1982, pp 359-369. 
Literature C: B. B. Lee and E. S. Furgason, "High-Speed Digital Golay Code 
Flaw Detection System", Ultrasonics, July, 1983, pp 153-161. 
The construction of a prior art will now be explained by referring to FIG. 
1. 
FIG. 1 is a block diagram illustrating an inspection apparatus utilizing 
ultrasonic waves as shown in the above Literature C. The inspection 
apparatus in FIG. 1 comprises a signal source 1, a digital delay line 2 
indirectly connected to the signal source 1, a bipolar converter 3 
indirectly connected to the signal source 1 and the digital delay line 2, 
a transmitter 4 connected to the bipolar converter 3, a bipolar converter 
5 indirectly connected also to the signal source 1 and the digital delay 
line 2, an ultrasonic probe 6, an analog correlator 7 connected to the 
ultrasonic probe 6, the transmitter 4 and the bipolar converter 5, a 
display 8 connected to the analog correlator 7, and a system control unit 
9. 
It is to be noted that the ultrasonic probe 6 is submerged in a water 
vessel and a target S to be inspected is disposed at the location 
opposedly facing the ultrasonic probe 6 in the water vessel. It is also to 
be noted that the analog correlator 7 consists of a multiplier 7aconnected 
to the ultrasonic probe 6 and the bipolar converter 5 and an integrator 7b 
connected to the multiplier 7a. Furthermore, logic circuits such as NAND 
gate and the like are interposed between the signal source 1 and the 
bipolar converters 3 and 5 as well as between the digital delay line 2 and 
the bipolar converters 3 and 5. The system control unit is connected to 
the respective elements as described above to control the system. 
Operation of the prior art as shown in FIG. 1 will now be explained by 
referring to FIGS. 2 and 3. 
FIGS. 2 and 3 are waveform diagrams respectively illustrating a 
transmission signal and a compressed pulse signal provided by the 
inspection apparatus as disclosed in Literature B. 
In FIG. 2, the abscissa is illustrated in the unit of bits and if the unit 
time is regarded to correspond to the unit of bits, the unit of the 
abscissa can be taken as the time unit. In Literature B, the unit time 
corresponding to the unit bit is expressed by .delta.. Therefore, the 
pulse duration of the transmission signal in FIG. 2 is 63.times..delta.. 
This transmission signal comprises a signal having a frequency of the base 
band, and an amplitude of which has been encoded by a special sequence. 
Encoding of the amplitude will be explained later and the sequence 
utilized for encoding will be firstly explained. 
The utilized sequence is a finite length sequence which has been provided 
by taking out one cycle of the maximal length sequence (M-sequence) which 
is a cyclic sequence having a cyclic length of 63 bits. 
The M-sequence is described in detail in "Coding Theory" coauthored by 
Hiroshi Miyagawa, Yoshihiro Iwatare and Hideki Imai, published on Jun. 29, 
1979 by Shoukoudo, pp 474-499 (to be referred to a Literature D). 
The M-sequence is a cyclic sequence having an infinite length and is a 
binary sequence components of which are comprised of two elements. The two 
elements may be allocated with symbols (+) and (-), numeral values +1 and 
-1 or numeric values 1 and 0 depending on the cases. In the example shown 
in FIG. 2, a finite length sequence is provided by using one cycle of the 
M-sequence having the cyclic length of 63 bits and a infinite length. 
Encoding of the amplitude of the signal by utilizing this one cycle of 
M-sequence, or finite length sequence will next be explained. 
By providing one element of the finite length sequence with the amplitude 
+1 and the other element with the amplitude -1, the amplitude for each 
unit time .delta. is modulated with +1 by the relative value in the order 
of appearance of these two elements of the sequence. The modulated signal 
may be called an amplitude-encoded signal. 
Similarly to FIG. 2, in FIG. 3, the abscissa is indicated in terms of unit 
of bits and if a bit as a unit is regarded as the unit of time .delta., 
the unit of the abscissa may be read as the time. 
This compressed pulse signal is an example in which the transmission signal 
amplitude of which have been encoded by the finite length sequence having 
a length of 64 bits, is used. This sequence having 64 bits has been 
provided by adding one bit to the finite length sequence of having 63 bits 
which was used for generating the transmission signals as shown in FIG. 2. 
Accordingly, the pulse duration of this transmission signal is 64 
.times..delta.. The pulse duration of corresponding echo signals has also 
the nearly same length. 
However, as seen in FIG. 3, a majority of the energy of the compressed 
pulse signal is concentrated on the central part of the abscissa (time) (a 
few bit .times..delta.) in the drawing. The portion of the signal located 
on the central part of the abscissa which have been a considerable 
amplitude is called as the main lobe of the compressed pulse. The pulse 
duration of the main lobe is short. This means that the energy of the echo 
signal which has been substantially uniformly distributed over a long 
period of time similarly to the pulse duration of the transmission signal 
has been compressed substantially at one point along the time base. The 
signal portions having smaller amplitudes at the both sides of the main 
lobe are called as range side lobes of the compressed pulse. 
It is to be noted that the transmission signal as shown in FIG. 2 is 
generated from the signal source 1 through the digital delay line 2, 
bipolar converter 3 and the transmitter 4, and the ultrasonic probe 6 is 
driven by the transmission signal to emit an ultrasonic wave. 
The ultrasonic wave emitted into the water in the vessel by the ultrasonic 
probe 6 will be reflected by the target S and returned again to the 
ultrasonic probe 6. The echo signal received by the ultrasonic probe 6 
will be sent to the multiplier 7a of the analog correlator 7. 
The pulse width of the echo signal has nearly the same length as that of 
the transmission signal. More specifically, the energy of the echo signal 
has been substantially uniformly distributed over a long duration of time 
nearly corresponding to the pulse width of the transmission signal (i.e., 
nearly 63.times..delta. in the case of FIG. 2, and nearly 64.times..delta. 
in the case of FIG. 3). 
The same signal as the transmission signal as described above is sent to 
the multiplier 7a of the analog correlator 7 via the digital delay line 2 
and the bipolar converter 5. The analog correlator 7 is adapted to execute 
a correlation operation between the echo signal and the transmission 
signal. This correlation operation will cause the energy of the echo 
signal, which is substantially uniformly distributed along the time base 
for a long time duration equivalent to that of the transmission signal, to 
be compressed substantially at one point along the time base. It is to be 
noted that the pulse signal obtained through such correlation operation is 
called the compressed pulse. 
The compressed pulse provided by the analog correlator 7 is sent to the 
display 8 where it is displayed as the final result. 
The distance resolution of the conventional inspection apparatus explained 
above depends on the duration of the main lobe of the compressed pulse 
(which is referred briefly to as the pulse duration of the compressed 
pulse). The pulse duration of the compressed pulse is short as described 
above despite the pulse duration of the transmission signal being long. 
Accordingly, an equivalent resolution to that of the prior inspection 
apparatus based on a pulse echo method using a transmission signal with a 
short pulse duration will be obtained. 
On the other hand, the S/N ratio (signal vs noise ratio) becomes higher, as 
the average transmission energy of the transmission signal becomes larger, 
and the average transmission energy is larger, as the pulse duration of 
the transmission signal is larger. Accordingly, according to the 
conventional inspection apparatus, a higher S/N ratio may be obtained as 
compared to the pulse echo method using a transmission signal with a short 
pulse duration. 
As explained above, the prior inspection apparatus utilizing the finite 
length sequence is excellent in resolution and can attain a high S/N 
ratio. 
It is here to be understood that the result of the correlation operation 
between the echo signal and the transmission signal is represented by a 
new function with .tau. as a variable, expressed as the following 
equation: 
##EQU1## 
where r(t) and s(t) respectively represent the echo signal and the 
transmission signal. This new function is called as correlation function 
and represents the compressed pulse described above. It is needless to say 
that the above integration range (-.infin.-.infin.) can be limited to a 
finite range of time, if either of the echo signal r(t) or the 
transmission signal s(t) assumes to take value(s) other than zero in the 
finite time range and to take zero out of the finite time range. 
As explained above, according to the conventional inspection apparatus, the 
correlation operation between the echo signal and the transmission signal 
is executed by use of the analog correlator 7. However, since the analog 
correlator 7 consists only of the multiplier 7a and the integrator 7b, 
operation of varying the variable .tau. in the equation (1) has to be 
externally executed. In other words, the operation of delaying the 
transmission signal s(t) by .tau. will be executed by the digital delay 
line 2 and the system control unit 9 and s(t-.tau.) is input to the 
multiplier 7a. This means the following. 
Since operation of varying the variable .tau. in the relation of equation 
(1) will not be executed only in the analog correlator 7, this means that 
the analog correlator 7 is not a correlator in the strict sense of the 
correlation operation. Furthermore, a single transmission will not provide 
a wave form of a compressed pulse (correlation function). In other words, 
what is obtained by a single transmission is only the value of a 
compressed pulse with regard to a fixed certain value of the variable 
.tau.. In order to obtain the whole waveform of a compressed pulse, signal 
transmission must be repeated a number of times by changing the value of 
the variable .tau. for each transmission. Accordingly, it takes a relative 
long time until the final result of the whole waveform of the compressed 
pulse is obtained. 
Other correlators for executing strictly (exactly) the correlation 
operation as expressed by equation (1) will be explained by referring to 
FIG. 4. 
FIG. 4 is a block diagram illustrating another correlator disclosed in 
Japanese Patent Application No. 1-45316 relating to the present invention. 
In FIG. 4, a correlator 10 is constituted by the delay line 10a with output 
taps, a plurality of multipliers 10b respectively connected to the output 
taps of the delay line 10a and an adder 10c connected to these multipliers 
10b. 
The correlator 10 realizes the correlation operation by utilizing the fact 
that the equation (1) can be transformed as follows: 
##EQU2## 
provided that the transmission signal s(t) is assumed to take zero out of 
the time range from 0 to T, k and l are integers, .DELTA.t is a sampling 
interval, K is a constant, t=k.DELTA.t, .tau.=l.DELTA.t and T=k.DELTA.t. 
According to the correlator 10, .DELTA.t designates a unit time delay of 
the delay line 10a between neighboring taps and K designates the aggregate 
number of taps. When the echo signal r(t) is input to the delay line 10a, 
an output from the k-th tap (k=1, 2, . . . , K) will be multiplied by the 
multiplier 10b with a weight value s(k.DELTA.t) which has been prepared in 
advance. Subsequently, the adder 10c is caused to add outputs from all the 
multipliers 10b, whereby the result of the addition is equivalent to 
equation (2). 
According to this correlator 10, operation of changing the variable .tau. 
corresponds to inputting the echo signal r(t) to the delay line 10a in the 
sequential timing. The echo signal r(t) is naturally input from the 
ultrasonic probe 6 in the sequential timing. Accordingly, the operation of 
changing the variable .tau. is automatically executed. Namely, according 
to the correlator 10 shown in FIG. 4, the time waveform of the compressed 
pulse can be obtained by only one transmission and in real time. 
However, when the duration of the transmission signal, namely T becomes 
larger, the delay line 10a having a greater many number of taps will be 
required, and hence a greater many number of multipliers 10b will also be 
required. Further, also the adder 10c having a greater many number of 
input terminals will be required when a greater many number of the 
multipliers 10b is required. As the number of multipliers 10b and the 
number of input terminals of the adder 10c increase, the operation speed 
of the correlator 10 decreases. Moreover, the cost of such a correlator 
will be more expensive. 
Furthermore, as seen in FIG. 3, the conventional apparatus has such a 
drawback as the level of the side lobes of the compressed pulse being 
relatively high. 
Accordingly, the prior inspection apparatus as explained above takes a 
great deal of time to obtain the compressed pulse as a final result, while 
the operation speed has to be made slower if an attempt is to be made to 
realize real-time inspection by shortening the time required for obtaining 
the compressed pulse and the cost of the apparatus becomes expensive. 
There is also a problem that the level of the side lobes of the compressed 
pulse is high. 
SUMMARY OF THE INVENTION 
The present invention has been provided to solve the problems as explained 
above. Accordingly, an object of the present invention is to provide an 
inspection apparatus which is inexpensive and capable of increasing the 
operation speed. Further object of the present invention is to provide an 
inspection apparatus which is capable of obtaining a compressed pulse 
having side lobes with a low level, preferably zero, in addition to being 
inexpensive and capable of attaining a high operational speed. 
To attain these objects, an inspection apparatus according to the present 
invention is provided with the following means: 
(1) transmission signal generation means adapted to generate first and 
second basic unit signals ga(t) and gb(t) in accordance with first and 
second sequences {a} and {b}, generate a first transmission signal Sap(t) 
based on the first basic unit signal ga(t) and a third sequence {p}, 
generate a second transmission signal Saq(t) based on the first basic unit 
signal ga(t) and a fourth sequence {q}, generate a third transmission 
signal Sbp(t) based on the second basic unit signal gb(t) and the third 
sequence {p}, and generate a fourth transmission signal Sbq(b) based on 
the second basic unit signal gb(t) and the fourth sequence {q}; 
(2) transmission means adapted to transmit waves driven by the first, 
second, third and fourth transmission signals to a target; 
(3) reception means adapted to receive first, second, third and fourth 
echoes reflected from the target to provide echo signals Rap(t), Raq(t), 
Rbp(t) and Rbq(t) corresponding to the first, second, third and fourth 
transmission signals; 
(4) first correlation means adapted to process by correlation the first and 
second echo signals Rap(t) and Raq(t) by using a first reference signal 
Ua(t) generated based on the first sequence {a} and process by correlation 
the third and fourth echo signals Rbq(t) and Rbq(t) by using a second 
reference signal Ub(t) generated based on the second sequence {b}; 
(5) second correlation means adapted to process by correlation the output 
of the first correlation means corresponding to the first and third echo 
signals Rap(t) and Rbp(t) by using a third reference signal Up(t) 
generated based on the third sequence {p} and process by correlation the 
output of the first correlation means corresponding to the second and 
fourth echo signals Raq(t) and Rbq(t) by using a fourth reference signal 
Uq(t) generated based on the fourth sequence {q}; and 
(6) adder means adapted to sum up the respective outputs from the second 
correlation means.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The constitution of a first embodiment of the present invention will 
firstly be explained by referring to FIG. 5. 
As seen in FIG. 5, the first embodiment of the present invention is 
constituted with an ultrasonic probe 6 and a display 8 which are identical 
to those of the conventional apparatus as shown in FIG. 1, an 
amplitude-encoded transmission signal generator 1A, a first correlator 11 
connected to the generator 1A and the ultrasonic probe 6, a second 
correlator 12 connected to the first correlator and the generator 1A and 
an adder 15 having memory function, an input of which is connected to the 
second correlator 12 and an output of which is connected to the display 8. 
It is to be noted that the ultrasonic probe 6 is connected not only to the 
first correlator but also to the generator 1A and is contacted to a target 
S to be inspected. 
Operation of the first embodiment will be explained by referring to FIGS. 6 
through 11. 
FIGS. 6 and 7 are waveform diagrams respectively showing first and second 
basic unit signals ga(t) and gb(t) in the first embodiment. FIGS. 8 
through 11 are waveform diagrams respectively showing first through fourth 
transmission signals Sap(t), Saq(t), Sbp(t) and Sbq(t). 
The generator 1A is adapted to internally generate first through fourth 
sequences {a}, {b}, {p} and {q}, and to generate the first and second 
basic unit signals ga(t) and gb(t) as shown in FIGS. 6 and 7, which is 
defined respectively by the first and second sequences {a} and {b}. 
The generator 1A outwardly generates first through fourth transmission 
signals. The first transmission signal is provided based on the third 
sequence {b} and the first basic unit signal ga(t), the second 
transmission signal is provided based on the fourth sequence {q} and the 
first basic unit signal ga(t), the third transmission signal is provided 
based on the third sequence {p} and the second basic unit signal gb(t), 
and the fourth transmission signal is provided based on the fourth 
sequence {q} and the second basic unit signal gb(t). 
These first through fourth transmission signals are generated repeatedly 
with a constant transmission repetition period Tr and then sequentially 
sent to the ultrasonic probe 6. 
The first sequence {a} has a length (M) of 8, and is represented as 
follows: 
##EQU3## 
As shown in FIG. 6, the first basic unit signal ga(t) is generated by 
encoding amplitudes with the sequence {a}, similarly to the prior art as 
shown in FIG. 2. In FIG. 6, in order to enable the relationship between 
the first sequence {a} and the amplitude-encoding operation to be better 
understood, the symbols (+) and (-) which are the contents of the 
components of the sequence {a} are also inserted. .delta. is a fixed time 
duration. 
Similarly, the second basic unit signal gb(t) is generated by encoding 
amplitudes with the sequence {b} which has a length of 8 and is 
represented as follows: 
##EQU4## 
The first and second transmission signals Sap(t) and Saq(t) are 
respectively represented in FIGS. 8 and 9, together with the symbols (+) 
and (-) representing the contents of the components of the third and 
fourth sequences. These sequences {p} and {q} are expressed as follows: 
##EQU5## 
The first transmission signal Sap(t) is generated in accordance with the 
following relationships: 
When the component of the third sequence {p} is (+), the signal Sap(t) is 
allocated to the first basic unit signal ga(t), while the component is 
(-), the signal Sap (t) is allocated to the signal -ga(t) which is 
obtained by multiplying the signal ga(t) by -1. Accordingly, the first 
transmission signal Sap(t) are formed by arranging ga(t) and -ga(t) along 
the time base according to the order in which the symbols (+) and (-) in 
the third sequence {p} will appear. 
In a similar manner, the second transmission signal Saq(t) is generated 
using the fourth sequence {q} and the first basic unit signal ga(t). 
The third and fourth transmission signals Sbp(t) and Sbq(t) is respectively 
shown in FIGS. 10 and 11, together with the arrangements of the symbols 
(+) and (-) according to the third and fourth sequences {p} and {q}. These 
signals Sbp(t) and Sbq(t) are formed, in the similar manner to the first 
and second transmission signals Sap(t) and Saq(t), by arranging the 
signals gb(t) and -gb(t) along the time base according to the order of the 
symbols (+) and (-) in the third and fourth sequences {p} and {q}. 
The ultrasonic probe 6 is driven by the first through fourth transmission 
signals to transmit ultrasonic waves into the target, or test piece S for 
inspection, and receives the echoes reflected from objects such as flaws 
in the test piece S. It is to be understood that received electrical echo 
signals corresponding respectively to the first through fourth 
transmission signals are to be referred to as first through fourth echo 
signals and designated respectively with Rap(t), Raq(t), Rbp(t) and 
Rbq(t). 
The received first through fourth echo signals are sent to the first 
correlator 11. 
On the other hand, a first reference signal Ua(t) which sill be used for 
correlation processing of the first and second echo signals Rap(t) and 
Raq(t) is generated by the generator 1A and sent to the first correlator 
11. The first reference signal is a signal relevant to the first sequence 
{a}, for example the signal ga(t). A second reference signal Ub(t) which 
will be used for correlation processing of the third and fourth echo 
signals Rbp(t) and Rbq(t) is also generated by the generator 1A and sent 
to the first correlator 11. The second reference signal Ub(t) is the 
signal relevant to the second sequence {b}, for example gb(t). 
The first correlator 11 executes a correlation operation between the first 
echo signal Rap(t) and the first reference signal Ua(t). The result of the 
correlation operation is expressed by Caap(t) and referred to as a first 
correlation operation result. The first correlator 11 also executes 
correlation operation between the second echo signal Raq(t) and the first 
reference signal Ua(t). The result of this correlation operation is 
expressed by Caaq(t) and referred to as a second correlation operation 
result. The first correlator 11 executes correlation operation between the 
third echo signal Rbp(t) and the second reference signal Ub(t) and also 
between the fourth echo signal Rbq(t) and the second reference signal 
Ub(t). The results of these correlation operation are expressed by Cbbp(t) 
and Cbbq(t) and referred to as third and fourth correlation operation 
results respectively. 
These first through fourth correlation operation results of the first 
correlator 11 are then sent to the second correlator 12. 
On the other hand, third and fourth reference signals Up(t) and Uq(t) which 
will be utilized in the correlation processing in the second correlator 12 
are generated by the generator 1A and sent to the second correlator 12. 
The third and fourth reference signals are signals relevant to the third 
and fourth sequences {p} and {q} respectively. 
In the second correlator 12, correlation operation will be conducted 
between the first correlation operation result Caap(t) and the third 
reference signal Up(t), between the second correlation operation result 
Caaq(t) and the fourth reference signal Uq(t), between the third 
correlation operation result Cbbp(t) and the third reference signal Up(t) 
and between the fourth correlation operation result Cbbq(t) and the fourth 
reference signal Uq(t). The results of these correlation operation in the 
second correlator 12 are respectively designated by Caapp(t), Caaqq(t), 
Cbbpp(t) and Cbbqq(t) and referred to respectively as the first through 
fourth compressed pulses. 
These first through fourth compressed pulses are sent to the adder 15 and 
stored therein to sum up these compressed pulses as follows: 
EQU C=Caapp(t)+Caaqq(t)+Cbbpp(t)+Cbbqq(t) 
The result of this summing operation is referred to as a composite 
compressed pulse. 
This composite compressed pulse C is transmitted to the display 8 from the 
adder 15, and displayed in a similar manner to that of a prior art. 
The operational principle of the first embodiment as mentioned above will 
next be explained by referring to FIGS. 12 through 25. 
FIGS. 12 and 13 show waveform diagrams illustrating first and second basic 
unit compressed pulses according to first embodiment of the present 
invention, FIGS. 14 through 17 waveform diagrams respectively illustrating 
the first through fourth correlation operation results Caap(t), Caaq(t), 
Cbbp(t) and Cbbq(t), and FIGS. 18 and 19 waveform diagrams respectively 
illustrating the third and fourth reference signals Up(t) and Uq(t), FIGS. 
20 through 23 waveform diagrams respectively illustrating the first 
through fourth compressed pulses Caapp(t), Caaqq(t), Cbbpp(t) and 
Cbbqq(t). FIG. 24 is a waveform diagram illustrating the composite 
compressed pulse C, and FIG. 25 is a waveform diagram illustrating a 
composite basic unit compressed pulse Aa(t-t.sub.0)+Ab(t-t.sub.0). 
The first transmission signal Sap(t) shown in FIG. 8 is expressed by the 
following equation. 
##EQU6## 
where the symbols (+) and (-) of the component p.sub.i of the third 
sequence {p} are regarded as identical to +1 and -1 respectively and thus 
ga(t) is multiplied by p.sub.1. (Same applies to the following 
transmission signals.) 
The second transmission signal Saq(t) shown in FIG. 9 can be expressed by 
the equation which has replaced the component p.sub.i of the third 
sequence {p} by the component q.sub.i of the fourth sequence {q} at the 
right side of the equation (3). The third transmission signal Sbp(t) shown 
in FIG. 10 can be expressed by the equation which has replaced the first 
basic unit signal ga(t) by the second basic unit signal gb(t) at the right 
side of equation (3). Furthermore, the fourth transmission signal Sbq(t) 
shown in FIG. 11 can be expressed by the equation which has replaced the 
component p.sub.i of the third sequence {p} by the component q.sub.i of 
the fourth sequence {q} and the first basic unit signal ga(t) by the 
second basic unit signal gb(t) at the right side of the equation (3). It 
is to be noted that the time origin is rearranged to each generation time 
of the second through fourth transmission signals. 
The first echo signal Rap(t) is expressed by the following equation: 
##EQU7## 
In the above, t.sub.0 is a constant and h(t) signifies the inverted Fourier 
transform of the frequency of response characteristics in the signal 
propagation path from the output terminal of the generator 1A to the input 
terminal of the first correlator 11 by way of the ultrasonic probe 6, the 
reflective body of the test piece S and again the ultrasonic probe 6. That 
is, h(t) represents impulse response characteristics at the signal 
propagation path. t.sub.0 is the time required for the ultrasonic wave to 
travel to and from the reflective body in the test piece S. 
Even if Co=1 is applied, generality is not lost. Therefore, Co=1 is used in 
the following explanation. 
The second through fourth echo signals Raq(t), Rbp(t) and Rbq(t) can be 
expressed by the equations which have replaced the first transmission 
signal Sap(t) by the second through fourth transmission signal Saq(t), 
Sbp(t) and Sbq(t) at the right side of equation (4), respectively. 
The first correlation operation results Caap(t) is expressed as follows: 
##EQU8## 
If the following equation 6 is applied, 
##EQU9## 
the result Caap(t) can be expressed by the following equation, in 
accordance with equations (3) through (6): 
##EQU10## 
In equation (7), Aa(t-t.sub.0) corresponds to a compressed pulse which is 
obtained by causing the ultrasonic probe 6 to be driven by the first basic 
unit signal ga(t) to obtain an echo signal and correlation processing this 
echo signal using the first reference signal Ua(t) as the reference 
signal. This compressed pulse is referred to as a first basic unit 
compressed pulse and shown in FIG. 12. 
It is seen from equation (7) that Caap(t) is equal to what is obtained by 
displacing four of the first basic unit compressed pulses Aa(t-t.sub.0) in 
respect of time along the time base by 0, Tp, 2Tp and 3Tp, multiplying the 
displaced pulses with the components p.sub.1, p.sub.2, p.sub.3, p.sub.4 of 
the third sequence {p} and adding them together. 
The second correlation operation result Caaq(t) can be expressed by the 
equation which has replaced the first echo signal Rap(t) by the second 
echo signal Raq(t) at the right side of equation (5). This is also 
equivalent to the equation which has replaced p.sub.i by q.sub.i (i=1, 2, 
3, 4) at the right side of equation (7). 
Similarly, the third correlation operation result Cbbp(t) can be expressed 
by the equation which has replaced the first echo signal Rap(t) by the 
third echo signal Rbp(t) and also replaced the first reference signal 
Ua(t) by the second reference signal Ub(t) at the right side of equation 
(5). This Cbbp(t) is also equivalent to the equation which has replaced 
Aa(t) by Ab(t) at the right side of equation (7) if the following equation 
(8) is applied. 
##EQU11## 
It is to be noted that Ab(t-t.sub.0) corresponds to a compressed pulse 
which is obtained by causing the ultrasonic probe 6 to be driven by the 
second basic unit signal gb(t) and correlation processing an echo signal 
by utilizing the second reference signal Ub(t). This compressed pulse 
Ab(t-t.sub.0) is referred to as a second basic unit compressed pulse and 
shown in FIG. 13. 
The fourth correlation operation result Cbbq(t) can be expressed by the 
equation which has replaced the first reference signal Rap(t) by the 
fourth echo Rbq(t) and also replaced the first reference signal Ua(t) by 
the second reference signal Ub(t) at the right side of equation (5). The 
result Cbbq(t) is equivalent to the equation which has replaced Aa(t) by 
Ab(t) and also replace p.sub.i and q.sub.i (i=1, 2, 3, 4) at the right 
side of equation (7). 
The first compressed pulse Caapp(t) can be expressed by the following 
equation: 
##EQU12## 
The second compressed pulse Caaqq(t) can be expressed by the equation which 
has replaced the first correlation operation result Caap(t) by the second 
correlation operation result Caaq(t) and replaced the third reference 
signal Up(t) by the fourth reference signal Uq(t) at the right side of 
equation (9). The third compressed pulse Cbbpp(t) can be expressed by the 
equation which has replaced the first correlation operation result Caap(t) 
by the third correlation operation result Cbbp(t) at the right side of 
equation (9). The fourth compressed pulse Cbbqq(t) can be expressed by the 
equation which has replaced the first correlation operation result Caap(t) 
by the fourth correlation operation result Cbbq(t) and also replaced the 
third reference signal Up(t) by the fourth reference signal Uq(t) at the 
right side of equation (9). Accordingly, the second, third and fourth 
compressed pulses Caaqq(t), Cbbpp(t) and Cbbqq(t) are expressed as 
follows: 
##EQU13## 
The first basic unit compressed pulse Aa(t-t.sub.0) shown in FIG. 12 has 
been provided as the result of computing equation (6) by utilizing the 
signal shown in FIG. 6 as the first basic unit signal ga(t) and this first 
basic unit signal ga(t) as the first reference signal Ua(t) with h(t) 
being a delta (.delta.) function. 
The second basic unit compressed pulse Ab(t-t.sub.0) shown in FIG. 13 has 
been provided as the result of computing equation (8) by using the signal 
shown in FIG. 7 as the second basic unit signal gb(t) and this second 
basic unit signal gb(t) as the second reference signal Ub(t) with h(t) 
being a delta (.delta.) function. 
The first correlation operation result Caap(t) by the first correlator 11 
shown in FIG. 14 has been provided as the result of computing equation 7 
with the first basic unit compressed pulse Aa(t-t.sub.0) shown in FIG. 12. 
The second correlation operation result Caaq(t) by the first correlator 11 
shown in FIG. 15 has been provided as the result of the similar computing 
by using the first basic unit compressed pulse Aa(t-t.sub.0) shown in FIG. 
12. The third and fourth correlation operation results Cbbp(t) and Cbbq(t) 
shown in FIGS. 16 and 17 have been provided as the results of the similar 
computing by using the second basic unit compressed pulse Ab(t-t.sub.0) 
shown in FIG. 13. In these computations, Tp has been set to be 8.delta.. 
It is seen from FIGS. 14 through 17 that the first through fourth 
correlation operation results exhibit the energy being dispersed along the 
time base. This dispersion of the energy along the time base remains 
unchanged even if Tp is changed from 8.delta. to another time interval. 
It can be noted, however, that the first through fourth correlation 
operation results can be compressed respectively through correlation 
processing by the second correlator 2. 
In this respect, the signals shown in FIGS. 18 and 19 which have been 
generated using the third and fourth sequences {p} and {q} as the third 
and fourth reference signals respectively will now be explained. 
The signal Up(t) shown in FIG. 18 has a waveform an amplitude of which has 
been encoded by using the third sequence {p}={+, +, +, -}. For better 
understanding of the relationship between this signal and the symbols (+) 
and (-) of the third sequence {p}, these symbols are also correspondingly 
indicated in FIG. 18. 
The signal shown in FIG. 19 has a waveform an amplitude of which has been 
encoded by using the fourth sequence {q}={+, -, +, +}, and similarly to 
above the symbols (+) and (-) are also indicated in FIG. 15. 
In case that the third reference signal Up(t) shown in FIG. 18 is used, the 
first compressed pulse Caapp(t) will be as follows in accordance with 
equation (9): 
##EQU14## 
Furthermore, if the autocorrelation function of the third sequence {p} is 
expressed by .rho.pp(i), [i=0, .+-.1, .+-.2, . . . , (N-1)], the first 
compressed pulse Caapp(t) will be provided as follows in accordance with 
equations (7) and (10). 
##EQU15## 
The second compressed pulse Caaqq(t) can be expressed by the equation which 
has replaced Caap(t) by Caaq(t) and also replaced p.sub.i by q.sub.i (i=1, 
2, 3, 4) at the right side of equation (10). It is also to be noted that 
this replaced equation for Caaqq(t) is equivalent to the equation which 
has replaced the autocorrelation function .rho.pp(i) of the third sequence 
{p} by the autocorrelation function .rho.qq(i) of the fourth sequence {q} 
at the right side of equation (11). The third compressed pulse Cbbpp(t) 
can be expressed by the equation which has replaced Caap(t) by Cbbp(t) at 
the right side of the equation (10), and is equivalent to the equation 
which has replaced Aa(t) by Ab(t) at the right side of equation (11). The 
fourth compressed pulse Cbbqq(t) can be expressed by the equation which 
has replaced Caap(t) by Cbbq(t) and also replaced p.sub.i by q.sub.i (i=1, 
2, 3, 4) at the right side of equation (10), and is equivalent to the 
equation which has replaced the autocorrelation function .rho.pp(i) by the 
autocorrelation function .rho.qq(i) and also replaced Aa(t) by Ab(t) at 
the right side of equation (11). 
FIG. 20 illustrates the first compressed pulse Caapp(t) which has been 
provided by computation in accordance with equation (11). 
In FIG. 20, the pulse shown in FIG. 12 has been used as the first basic 
unit compressed pulse Aa(t-t.sub.0) and .rho.pp(0)=4, .rho.pp(1)=1, 
.rho.pp(2)=0 and .rho.pp(3)=-1 have been used as the autocorrelation 
function .rho.pp(i) of the third sequence {p}. Tp has been set to be 
8.delta.. 
FIGS. 21 through 23 respectively illustrate the second compressed pulse 
Caaqq(t), the third compressed pulse Cbbpp(t) and the fourth compressed 
pulse Cbbqq(t) which have been obtained respectively by executing similar 
computations to that of the first compressed pulse Caapp(t). As the second 
basic unit compressed pulse Ab(t-t.sub.0), the pulse shown in FIG. 13 has 
been employed. Further as the autocorrelation function .rho.qq(i) of the 
fourth sequence {q}, .rho.qq(0)=4, .rho.qq(1)=-1, .rho.qq(2)=0 and 
.rho.qq(3)=1 have been applied. Also Tp=8.delta. has been applied. 
As apparent from FIGS. 20 through 23, the majority of the signal energy in 
each of the first through fourth compressed pulses is concentrated near 
t=t.sub.0. In other words, they have large amplitudes only in the 
neighborhood of t=t.sub.0, and they have side lobes with certain small 
levels. It is observed, however, that there are still relatively large 
side lobes even at time locations where time t is considerably spaced from 
t.sub.0. 
FIG. 24 shows the composite compressed pulse C which has been provided by 
summing the first through fourth compressed pulses; 
C=Caapp(t)+Caaqq(t)+Cbbpp(t)+Cbbqq(t). In the composite compressed pulse 
C, according to the summing operation, the main lobes of the pulses 
Caapp(t), Caaqq(t), Cbbpp(t) and Cbbqq(t) are strengthened while the side 
lobes thereof are cancelled and reduced to zero level, as shown in FIG. 
24. 
Accordingly, it would have been understood that a compressed pulse, which 
includes the main lobe having a large amplitude at t=t.sub.0 and no side 
lobes, can be obtained by the embodiment of this invention mentioned 
above, and thus the value of t.sub.0 can be easily detected. 
It is further to be understood that when the first and second basic unit 
compressed pulses Aa(t-t.sub.0) and Ab(t-t.sub.0) are added together, the 
resultant pulse has a large amplitude, as the main lobe, only in the 
vicinity of t=t.sub.0, and amplitudes in the other range are zero, as 
shown in FIG. 25. This is induced from the fact that if the 
autocorrelation functions of the first and second sequence {a} and {b} are 
expressed as .rho.aa(i) and .rho.bb(i), then .rho.aa(0)=.rho.bb(0) and 
.rho.aa(i)=-.rho.bb(i) (i=1, 2, . . . , M-1) can be applied. Such 
relationships as those between Aa(t-t.sub.0) and Ab(t-t.sub.0), or {a} and 
{b} may be called complementary relationships. 
Similarly, between the third and fourth sequences {p} and {q}, 
.rho.pp(0)=.rho.qq(0) and .rho.pp(i)=-.rho.qq(i) (i=1, 2, . . . , N-1) can 
be applied. Accordingly, in the embodiment described above, the first and 
second sequences {a} and {b} constitute complementary relationships and 
the third and fourth sequences {p} and {q} also constitute complementary 
relationships. 
Another effect which can be derived by the first embodiment of the present 
invention will be explained. 
In an inspection apparatus of this sort, improvement of S/N ratio can be 
more enhanced, as the duration of a transmission pulse signal becomes 
longer, and, in order to realize a longer pulse duration of the 
transmission signal, it is necessary to utilize sequences having a longer 
sequence length. 
According to the first embodiment of the present invention, as it is seen 
from FIGS. 8 through 11, the pulse duration of the transmission signal 
depends not only on the length M of the first and second sequences {a} and 
{b} but also on the length N of the third and fourth sequences {p} and 
{q}. If the third and fourth sequences the length N of which is larger are 
employed, the pulse duration of the transmission signal can be made longer 
correspondingly. 
Those sequences which are in the complementary relationships can not exist 
at any length but exist at specific sequence lengths. According to the 
first embodiment of the present invention, since two kinds of sequence 
length M and N are used in combination, the substantial length of the 
sequence of the transmission signal is M.times.N. 
Accordingly, since the pulse duration of the transmission signal can be 
made longer, S/N ratio can be improved. Furthermore, by combining 
different lengths M and N, freedom for selection of various pulse 
durations for transmission signals can be provided. 
Other effects of the first embodiment of the present invention will further 
be explained by referring to FIGS. 26 through 28. 
FIG. 26 is a waveform diagram showing another first transmission signal 
Sap(t) when Tp=8.delta. according to the first embodiment of the present 
invention is applied. 
FIGS. 27 and 28 are block diagrams illustrating constitutions of the first 
and second correlators 11 and 12. 
The first transmission signal shown in FIG. 26 is equivalent to a signal an 
amplitude of which has been encoded by using the following sequence: 
##EQU16## 
It is to be noted that the above sequence is equivalent to the following 
sequence having a length of 32 obtained using the first and third 
sequences {a} and {p}: 
##EQU17## 
It is here to be noted that the symbols (+) and (-) are regarded as 
equivalent to +1 and -1, and thus multiplication operation is utilized. 
Next, a consideration is made on such a situation that the first 
transmission signal shown in FIG. 26 having a long pulse duration is 
employed as a transmission signal in the conventional apparatus shown in 
FIGS. 1 and 4 for the purpose of improving the S/N ratio. 
The duration T of the transmission signal is 32.delta. as seen from FIG. 
26. Accordingly, when sampling of K.sub.1 times per the unit time .delta. 
is executed and the correlator 10 shown in FIG. 4 is constituted in 
accordance with equation (2), since K=32.times.K.sub.1 is obtained, a 
delay line having 32.times.K.sub.1 number of taps as the delay line 10a, 
32.times.K.sub.1 number of multipliers as the multipliers 10b, and adder 
having 32.times.K.sub.1 number of input terminals as the adder 10c are 
required. 
The inspection apparatus according to the first embodiment of the present 
invention utilizes the first basic unit signal ga(t) as the first 
reference signal, and the duration of the first basic unit signal ga(t) is 
8.times..delta. as seen from FIG. 6. Accordingly, the first correlator 11 
is constituted as shown in FIG. 27 if equation (5) is modified like 
equation (2) and sampling of K.sub.1 times is executed for the unit time 
.delta.. 
As shown in FIG. 27, the first correlator 11 consists of a delay line 11a 
with 8.times.K.sub.1 number of output taps, 8.times.K.sub.1 number of 
multipliers 11b connected to the respective output taps of the delay line 
11a, and adder 11c having 8.times.K.sub.1 number of input terminals 
connected to the multipliers 11b. 
Consideration is next made with regard to the second correlator 12 for the 
inspection apparatus according to the first embodiment of the present 
invention. 
The second correlator 12 is acceptable if it is provided with a function to 
compute the right side of equation (10) based on the first correlation 
operation result Caap(t) by the first correlator 11. The right side of 
equation (10) comprises Caap(t).times.p.sub.1 at the time t, 
Caap(t).times.p.sub.2 at the time (t+Tp), Caap(t).times.p.sub.3 at the 
time (t+2Tp), and Caap(t).times.p.sub.4 at the time (t+3Tp), which are 
summed all together. Accordingly, supposing that sampling of K.sub.1 times 
for the unit time .delta. is executed, since Tp=8.delta., the second 
correlator 12 may be constituted as shown in FIG. 28. 
More specifically, in FIG. 28, the second correlator 12 consists of the 
delay line 12a with 24.times.K.sub.1 number of output taps, four 
multipliers 12b connected to the output taps of the delay line 12a for 
each 8.times.K.sub.1 number of output taps (corresponding to each Tp) and 
adder 12c having four input terminals connected to the multipliers. 
Now, the aggregate number of the multipliers 11b and 12b required by the 
first and second correlators 11 and 12 of the first embodiment of the 
present invention is compared to the number of the multipliers 10b 
required by the correlator 10 shown in FIG. 4 of the conventional 
inspection apparatus. According to the former, the total (8.times.K.sub.1 
+4) of the multipliers are required while 32.times.K.sub.1 number of the 
multipliers are required for the latter. Namely, a large number of the 
multipliers can be dispensed with according to the first embodiment of the 
present invention. In this way, reduction of the number of the multipliers 
contributes to higher operational speed of the apparatus and reduction in 
costs. 
Furthermore, weighting p.sub.i (i=1, 2, 3, 4) of the multipliers 12b 
required at the second correlator 12 is either .+-.1 according to the 
first embodiment as described above. If weighting is .+-.1, this means 
that the multipliers 12b are not necessary. If weighting is -1, this means 
that the multipliers 12b may be replaced by inverters. Accordingly, the 
first embodiment of the present invention is increasingly advantageous in 
terms of higher speed and lower cost. 
Similar comparison is next made with regard to the adders. The first 
embodiment of the present invention requires the adder 11c having 
8.times.K.sub.1 number of the input terminals, and the adder having four 
input terminals. On the other hand, the conventional apparatus requires 
the adder having 32.times.K.sub.1 number of the input terminals. Since, in 
general, an adder operates to add input values accumulatively, the less 
the number of input terminals is required, the more the operational speed 
can be increased and the more the cost can be reduced. Accordingly, those 
are further advantages of the first embodiment. 
For correlation processing of the second echo signal, the same correlator 
as the one shown in FIG. 27 can be used as the first correlator 11, and 
the second correlator 12 shown in FIG. 28 but which has replaced the 
weighting p.sub.i (i=1, 2, 3, 4) for the multiplier 12b by q.sub.i can be 
utilized. 
For correlation processing of the third echo signal, the correlator shown 
in FIG. 27 but which has replaced the weighting Ua(k.DELTA.t) (k=1, 2, . . 
. , 8K.sub.1) for the multiplier 11b by Ub(k.DELTA.t) can be used as the 
first correlator 11, and as the second correlator 12, the one identical to 
the one shown in FIG. 28 can be used. 
For correlation processing of the fourth echo signal, the correlator such 
as shown in FIG. 27 but which has replaced the weighting Ua(k.DELTA.t) 
(k=1, 2, . . . , 8K.sub.1) for the multiplier 11b by Ub(k.DELTA.t) can be 
used as the first correlator 11, and as the second correlator 12, such a 
correlator as identical with the one shown in FIG. 28 but which has 
replaced the weighting p.sub.i (i=1, 2, 3, 4) for the multiplier 12b by 
q.sub.i can be utilized. Or the first and second correlators may be 
independently provided for correlation processing of the second, third and 
fourth echo signals. 
With reference to the first embodiment as described above, the unit 
waveform corresponding to the respective components (.+-.) of the first 
and second sequences {a} and {b} has been described as rectangular in the 
first and second basic unit signals, as shown in FIGS. 6 and 7. However, 
even if the unit waveform is modified to such a shape rather than 
rectangular as shown in FIGS. 29(a) or 29(b), similar function and effects 
to those explained above can be attained. 
Constitution of a second embodiment of the present invention will now be 
explained by referring to FIG. 30. 
In this drawing, first and second correlators 11 and 12, adder 15, 
ultrasonic prove 6 and display 8 are identical to those in the first 
embodiment, however, amplitude-encoded transmission signal generator 1A 
does not have the same constitution and function as those of the generator 
1A in the first embodiment. That is, in the second embodiment, although 
the generator 1A is constituted to generate the first through fourth 
transmission signals Sap(t), Saq(t), Sbp(t) and Sbq(t) in the same manner 
as that of the first embodiment, but none of the first through fourth 
reference signals Ua(t), Ub(t), Up(t) and Uq(t). Instead thereof, first 
through fourth reference signal generators 13A, 13B, 14A and 14B are 
incorporated thereinto as shown in FIG. 30. 
The generators 13A and 13B are enabled by the transmission signal generator 
1A to generate first and second reference signals having waveforms similar 
to or identical with waveforms of echo signals which are obtained when the 
probe 6 is driven by the first and second basic unit signals ga(t) and 
gb(t), respectively. These generated reference signals are sent to the 
first correlator 11. 
On the other hand, the generator 14A and 14B are enabled by the generator 
1A to generate third and fourth reference signals having waveforms similar 
to or identical with those of the third and fourth reference signals Up(t) 
and Uq(t) in the first embodiment, and supplies them to the second 
correlator 12. 
The reference signals generated by the generators 13A and 13B are identical 
to or similar to the signals expressed by the right side of equation (4) 
but Sap(t) has been replaced by ga(t) and gb(t), respectively. 
Accordingly, each of the generators 13A and 13B functions as a filter 
having frequency response characteristics in both transmission and 
reception of the ultrasonic probe 6, those of the test piece S and those 
relating to the ultrasonic reflection from the reflective portion such as 
defects in the test piece S. 
Correlation processing of echo signals by utilizing reference signals 
having waveforms identical to or similar to waveforms of echo signals such 
as the first and second reference signals generated by the generators 13A 
and 13B is equivalent to signal processing causing the echo signal to pass 
through a matching filter or a quasi-matching filter which is effective in 
receiving a signal buried in noises with the maximum S/N ratio. 
Accordingly, the second embodiment of the present invention can derive an 
advantage of further improving the S/N ratio in addition to the advantages 
provided by the first embodiment which utilizes the first and second basic 
unit signals themselves as the reference signals for the first correlator 
11. 
It is to be understood that the generators 13A and 13B are acceptable if 
they have the function of generating the following reference signals. 
Firstly, in the case where a surface of bottom echo reflected by the front 
or bottom surface of the test piece S and received by the ultrasonic probe 
6 when the ultrasonic probe 6 is driven by the first basic unit signal 
ga(t) can be measured and provided with a large S/N ratio, a waveform of 
an echo signal corresponding to the front or bottom surface echo is 
measured and a signal having a wave form identical to or similar to the 
measured waveform is generated as the first reference signal by the first 
reference signal generator 13A. The second reference signal from the 
second reference signal generator 13B is similarly generated, but the 
second basic unit signal gb(t) is used instead of the first one. 
If an adequate S/N ratio cannot be attained by the echo reflected from the 
front or bottom surface of the test piece S, then another test piece 
S.sub.1 should be prepared. Then, similarly to the above, the ultrasonic 
probe 6 is driven by the first and second basic unit signals ga(t) and 
gb(t) respectively, the echoes reflected from the test piece S.sub.1 are 
received by the ultrasonic probe 6, and signals having waveforms identical 
to or similar to those of the corresponding echo signals are generated as 
the reference signals by the generators 13A and 13B. 
The generator 13A may be constituted as to generate a reference signal 
having a waveform computed in accordance with frequency response 
characteristics of a signal transmission path from the output terminal of 
the amplitude-encoded transmission signal generator 1A through the 
ultrasonic probe 6, the test piece S and again the ultrasonic probe 6 to 
the input terminal of the first correlator 11 as well as the first basic 
unit signal when the ultrasonic probe 6 is driven by the first basic unit 
signal. Similarly, the generator 13B may be constituted in considering the 
second basic unit signal. In these cases, if frequency response 
characteristics relating to reflection by the reflective body in the test 
piece S is involved in the frequency response characteristics of the 
signal transmission path, S/N ratio may further be enhanced. 
Further, if a plurality of the first and second reference signals which 
have different frequency response characteristics related to reflection 
from the reflection body in the test piece S are prepared to be generated, 
the function of discriminating the reflective bodies may be additionally 
provided as disclosed in the Japanese Patent Application No. 86383/89 
relating to the field of the present invention. 
With regard to the third and fourth reference signal generators, the 
generators 14A and 14B may be constituted to generate signals, as the 
third and fourth reference signals, amplitudes of which are slightly 
changed from .+-.1 for each time Tp. The signals enabling the first 
through fourth compressed pulses Caapp(t), Caaqq(t), Cbbpp(t) and 
Cbbqq(t), and/or the composite compressed pulse C to be provided at a high 
S/N ratio may be generated as the third and fourth reference signals. 
According to the second embodiment of the present invention, supposing that 
there are K.sub.3 number of sampling points in the time duration of the 
first or second reference signal and K.sub.2 number of sampling points in 
the time duration Tp, the first correlator 11 may be constituted, in the 
similar manners to illustration of FIG. 27, by a delay line with K.sub.3 
number of taps, K.sub.3 number of multipliers connected to the respective 
output taps of the delay line and an adder with K.sub.3 number of input 
terminals, as it is seen if equation (5) is changed like equation (2). The 
second correlator 12 may be constituted, in the similar manner to 
illustration of FIG. 28, by a delay line having (N-1).times.K.sub.2 number 
of taps, N number of multipliers connected to the output taps of the delay 
line for every K.sub.2 number of taps and an adder having N number of 
input terminals. Weightings of p.sub.i and q.sub.i (i=1, 2, 3, . . . , N) 
to N number of multipliers may be executed as .+-.1, or as explained 
above, it may be changed from .+-.1 respectively for every i so that the 
first through fourth compressed pulses and hence the composite compressed 
pulse may be obtained at a high S/N ratio. 
Constitution of a third embodiment of the present invention will now be 
explained by referring to FIG. 31. 
The third embodiment comprises the same components as those in the first 
embodiment except a phase-encoded transmission signal generator 1B is 
substituted for the amplitude-encoded transmission signal generator 1A in 
the first embodiment. 
Operation of the third embodiment will be explained by referring to FIGS. 
32 through 38. 
FIGS. 32 and 33 are waveform diagrams illustrating first and second basic 
unit signals ga(t) and gb(t) in the third embodiment. FIGS. 34(a) and (b) 
are waveform diagrams illustrating other unit waveforms constituting the 
basic unit signals. FIGS. 35 through 38 respectively show waveforms of 
first through fourth transmission signals Sap(t), Saq(t), Sbp(t) and 
Sbq(t). 
In FIG. 32, the first basic unit signals ga(t) is a signal generated using 
the same first sequence {a} as that in the first embodiment, and .delta. 
and .delta..sub.0 designate fixed times. For better understanding of 
relationships between the first sequence {a} and the first basic unit 
signal ga(t), the symbols (+) and (-) of the first sequence {a} are also 
indicated in the drawing. 
In FIG. 33, the second basic unit signal gb(t) is a signal generated by 
using the same second sequence {b} as that in the first embodiment. The 
symbols (+) and (-) of the second sequence {b} are also indicated in the 
drawing. 
In FIGS. 32 and 33, the unit waveforms correspondingly shown to the 
respective components (+) and (-) of the first or second sequences {a} and 
{b} are illustrated as sinusoidal waves. The above-mentioned unit 
waveforms may take waveforms having smooth curves or oscillation waveforms 
having non-uniform amplitudes and/or zero-crossing points as shown in FIG. 
34(a) or (b). 
It is to be noted from FIGS. 32 and 33 that when .delta.=.delta..sub.0, the 
first and second basic unit signals ga(t) and gb(t) may have waveforms 
which have been phase-encoded. Methods of phase-encoding are described in 
detail in the Japanese Patent Application No. 45316/89 to which the 
present invention relates. 
In FIG. 35, the first transmission signal Sap(t) is a signal that has been 
generated in the same steps as those in the first embodiment by using the 
same third sequence {p} as in the first embodiment and the first basic 
unit signal ga(t) shown in FIG. 32. More specifically, the first basic 
unit signal ga(t) is allocated to the symbol (+) of the third sequence 
{p}, the signal -ga(t) obtained by multiplying the first basic unit signal 
ga(t) by -1 is allocated to the symbol (-) and .+-.ga(t) are arranged 
along the time base in the appearance order of the symbol of the third 
sequence {p}. For better understanding of the relationships between the 
symbols (+) and (-) of the third sequence {p} and the signals .+-.ga(t), 
the symbols of the third sequence {p} are also indicated in the drawing. 
In FIG. 36, the second transmission signal Saq(t) is a signal which has 
been generated in the same steps of the first embodiment by using the same 
fourth sequence {q} as that employed in the first embodiment and the first 
basic unit signal ga(t) shown in FIG. 32. Similarly to FIG. 35, symbols 
(+) and (-) of the fourth sequence {q} are also indicated therein. 
In FIG. 37, the third transmission signal Sbp(t) is a signal that has been 
generated in the same steps as those of the first embodiment in accordance 
with the same third sequence {p} as that in the first embodiment and the 
second basic unit signal gb(t) shown in FIG. 33, and the symbols (+) and 
(-) corresponding to the component symbols of the third sequence {p} are 
also indicated. 
In FIG. 38, the fourth transmission signal Sbq(t) is a signal that has been 
generated in the same steps as those of the first embodiment in accordance 
with the same fourth sequence {q} as that in the first embodiment and the 
second basic unit signal gb(t) shown in FIG. 33, and the symbols (+) and 
(-) correspond to the component symbols of the fourth sequence {q}. 
According to the third embodiment, the first, second, third and fourth 
transmission signals shown in FIGS. 35 through 38 are supplied from the 
phase-encoded transmission signal generator 1B to drive the ultrasonic 
probe 6. Signal processing of echo signals is similar to that of the first 
embodiment. Namely, the first basic unit signal ga(t) shown in FIG. 32 is 
utilized as the first reference signal for the first correlator 11, the 
second basic unit signal gb(t) shown in FIG. 33 is utilized as the second 
reference signal for the correlator 11 and signals exactly same as the 
third and fourth reference signals in the first embodiment are used as the 
third and fourth reference signals for the second correlator 12, in the 
third embodiment. 
Also according to the third embodiment, the same function and advantages as 
those of the first embodiment may be provided, because equations (3) 
through (11) are applicable regardless of the waveform of the first basic 
unit signal ga(t), a composite basic unit compressed pulse, which is the 
result of summing up of the first and second basic unit compressed pulses 
Aa(t) and Ab(t) obtained by utilizing the first and second basic unit 
signals shown in FIGS. 32 and 33 in equations (6) and (8), provides a 
large amplitude only around t=t.sub.0 while the zero amplitude at the 
others, in other words, Aa(t) and Ab(t) are in the complementary 
relationship, and the third and fourth sequences {p} and {q} are in the 
complementary relationships. The complementary relationships between the 
first and second basic unit compressed pulses as described above are also 
applicable when the unit waveform shown in FIG. 34(a) or 34(b) is used, so 
that the similar effects as those of the first embodiment may also be 
obtained in these cases. 
As explained above, the third embodiment can derive similar function and 
effects to those of the first embodiment. Further, in the third 
embodiment, frequency characteristics can be made close to those which are 
composed of the frequency characteristics of the ultrasonic probe 6 both 
in transmission and reception, the frequency characteristics of the test 
piece S and the frequency characteristics of the ultrasonic reflection of 
the reflection body in the piece S, as understood from Japanese Patent 
Application Nos. 45316/89 and 86383/89 to which the present invention also 
relates. 
Accordingly, a high utilization efficiency of the signal energy can be 
expected. If the unit waveforms corresponding to the components (+) and 
(-) of the first and second sequences {a} and {b}, as shown in the period 
.delta..sub.0 of FIG. 32 or 33 or in FIG. 34(a) or 34(b), are so selected 
that it has frequency characteristics close to the composite frequency 
characteristics as described above, the utilizing efficiency of the signal 
energy may be increasingly enhanced and thus S/N ratio may be increased. 
When the first correlator 11 and the second correlator 12 are constituted 
by a delay line with taps, multipliers and an adder, the same constitution 
as that in the first embodiment can be employed. 
Constitution of a fourth embodiment of the present invention will next be 
explained by referring to FIG. 39. 
The fourth embodiment comprises exactly the same components as those of the 
second embodiment as described above except that a phase-encoded 
transmission signal generator 1B is employed instead of the 
amplitude-encoded transmission signal generator 1A of the second 
embodiment. 
The generator 1B is adapted to generate the same first and second basic 
unit signals ga(t) and gb(t) as in the third embodiment to the respective 
first and second reference signal generator 13A and 13B, where first and 
second reference signals having waveforms identical to or similar to those 
of echo signals which are obtained when the ultrasonic probe 6 is driven 
with the signals ga(t) and gb(t) in the third embodiment are generated to 
be sent to the first correlator 11. 
The third and fourth reference signal generators 14A and 14B respectively 
generate third and fourth reference signals having waveforms identical to 
or similar to those in the third embodiment to be sent to the second 
correlator 12. 
In the fourth embodiment, the first and second reference signals are 
identical to or similar to the signals which are obtained from equation 
(4) in which Sap(t) is replaced by the respective first and second basic 
unit signals ga(t) and gb(t) in the third embodiment at the right side 
thereof. Accordingly, the fourth embodiment may derive the same advantages 
of the second embodiment in addition to those of the third embodiment. 
It is also to be noted that the generators 13A and 13B in the fourth 
embodiment may be constituted in other types similar to those explained in 
connection with the second embodiment. 
Also in the fourth embodiment, the generators 14A and 14B may be 
constituted to generate signals as the third and fourth reference signals 
having amplitudes slightly varied from .+-.1 for every time Tp, similarly 
to the second embodiment. That is, the third and fourth reference signals 
having the waveforms enabling the first through fourth compressed pulses 
and/or the composite compressed pulse to be obtained at a high S/N ratio 
may be generated. 
The first correlator 11 and the second correlator 12 may be constituted in 
a similar manner to that in the second embodiment. 
Constitution of a fifth embodiment of the present invention will now be 
explained by referring to FIG. 40. 
The fifth embodiment comprises the same elements as those of the fourth 
embodiment except that an ultrasonic probe 6A for transmission and an 
ultrasonic probe 6B for reception are employed in this embodiment. 
The fifth embodiment has advantages similar to those of the fourth 
embodiment. 
It is naturally possible to apply the ultrasonic probe 6A and the 
ultrasonic probe 6B to the first, second and third embodiments of the 
present invention. 
Constitution of a sixth embodiment of the present invention will be 
explained by referring to FIG. 41. 
The sixth embodiment consists components identical to the fifth embodiment 
shown in FIG. 40 except that a third correlator 16, fifth and sixth 
reference signal generators 17A and 17B are newly incorporated in this 
sixth embodiment. 
The generators 17A and 17B are connected to receive signals from the phase 
encoded transmission signal generator 1B and the third correlator 16 is 
connected to receive signals from the second correlator 12 and the 
generators 17A and 17B and to supply its output signal to the adder 15. 
According to the sixth embodiment, the transmission signal generator 1B is 
adapted to newly generate fifth and sixth sequences {v} and {w} and also 
new first transmission signals by assuming the first through fourth 
transmission signals Sap(t), Saq(t), Sbp(t) and Sbq(t) as shown in FIGS. 
35-38 relating to the fifth embodiment respectively as new first through 
fourth basic unit signals designated as g.sub.1 (t), g.sub.2 (t), g.sub.3 
(t), g.sub.4 (t) and also by utilizing the fifth sequence {v} and the 
first basic unit signal g.sub.1 (t). The steps of generating this first 
transmission signal follow the same steps of generating the first 
transmission signal Sap(t) as applied in the fifth embodiment by utilizing 
the first basic unit signal ga(t) and the third sequence {p}. 
That is, the first basic unit signal g.sub.1 (t) is allocated to the 
component symbol (+) of the fifth sequence {v}, the signal -g.sub.1 (t) 
obtained by multiplying the first basic unit signal g.sub.1 (t) by -1 is 
applied to the component symbol (-) thereof and these signals g.sub.1 (t) 
and -g.sub.1 (t) are arranged in the appearing order of the symbols (+) 
and (-) of the fifth sequence {v}. The time duration for each symbol is 
decided to be Tpp. 
Furthermore, new second, third and fourth transmission signals are 
generated in a similar manner by utilizing respectively the sequence {v} 
and the second basic unit signal g.sub.2 (t), the sequence {v} and the 
third basic unit signal g.sub.3 (t), and the sequence {v} and the fourth 
basic unit signal g.sub.4 (t). Furthermore, fifth through eighth 
transmission signals are generated by respectively utilizing the sixth 
sequence {w} and the first basic unit signal g.sub.1 (t), the sequence {w} 
and the second basic unit signal g.sub.2 (t), the sequence {w} and the 
third basic unit signal g.sub.3 (t), and the sequence {w} and the fourth 
basic unit signal g.sub.4 (t). 
And these first through eighth transmission signals are sent to the 
ultrasonic probe 6A with a constant repetition period. 
The generators 17A and 17B are adapted to generate respective fifth and 
sixth reference signals identical to or similar to signals amplitudes of 
which have been encoded by using the fifth and sixth sequences {v} and {w} 
and transmit them to the third correlator 16. 
The correlator 16 is adapted to execute correlation processing of outputs 
from the second correlator 13, relating to the first through fourth 
transmission signals, by utilizing the fifth reference signal. It is 
further adapted to execute correlation processing of the outputs from the 
correlator 12, relating to the fifth through eighth transmission signals, 
by utilizing the sixth reference signal and transmit the results of these 
correlation processing to the adder 15. 
The adder 15 in turn stores the results from the third correlator 16 
relating to the first through eighth transmission signals, adds them to 
obtain a composite compressed pulse and transmit the composite compressed 
pulse to the display 8. 
In this case, if the fifth and sixth sequences {v} and {w} are in the 
complementary relationships, the composite compressed pulse will provide a 
zero range side lobe. 
It is further to be noted that when the third correlator 16 is constituted 
by a delay line with taps, multipliers and an adder, such a constitution 
is similar to that of the second correlator 12. It is to be understood, 
however, that if the length of each of the fifth and sixth sequences {v} 
and {w} is assumed to be L, the total number L of the multipliers should 
be provided to connect to the output taps spaced to each other 
corresponding to the time interval Tpp. The adder should also be provided 
with L number of input terminals. 
According to the sixth embodiment, the duration time of each of the 
transmission signals can be made longer than in the case of the fifth 
embodiment. In this way, the longer is the duration of the transmission 
signals, the number of the multipliers as well as the number of the input 
terminals of the adder may be relatively decreased in comparison with a 
prior art as mentioned above, so that more advantage can be provided in 
respect of the operational speed and the cost. 
Furthermore, by repeating the steps of generating the transmission signal 
in the sixth embodiment, or repeating the waveforms of the transmission 
signals to assume the repeated waveforms as a new basic unit signal and 
providing seventh, eighth, ninth, tenth, . . . reference signal generators 
and fourth, fifth, . . . correlators corresponding to the newly assumed 
basic unit signal, the duration of a new composite transmission signal S 
can further be prolonged, such that differences in the number of the 
multipliers and the number of the input terminals of the adder between 
such an inspection apparatus operable to generate the above new composite 
transmission signal and a prior art operable to generate a similar long 
duration transmission signal may be relatively increased resulting in more 
and more advantages in respect of operational speed and cost. 
The generation steps of such a composite transmission signal having a long 
duration as described above regarding the sixth embodiment can be 
naturally applied to the first through fourth embodiments. 
Various modification will next be explained. 
First will follow an explanation of a modification wherein the second 
sequence {b} is not utilized in the generator 1A in the first embodiment 
shown in FIG. 5, and thus no second reference signal Ub(t) and neither 
third nor fourth transmission signal Sbp(t), Sbq(t) is generated from the 
generator 1A. 
In this case, the composite transmission signal S from the generator 1A is 
comprised of the first and second transmission signals Sap(t) and Saq(t), 
and accordingly, the composite echo signal R includes the first and second 
echo signals Rap(t) and Raq(t) but not either the third or fourth echo 
signal Rbp(t) and Rbq(t). 
The first correlator 11 performs a correlation-operation between the first 
reference signal Ua(t) and the first and second echo signals Rap(t) and 
Raq(t) to provide the correlation results Caap(t) and Caaq(t), and the 
second correlator 12 performs the correlation-operation between the 
correlation results Caap(t) and the third reference signal Up(t), and 
between the correlation results Caaq(t) and the fourth reference signal 
Uq(t) to provide the first and second correlation results Caapp(t) and 
Caaqq(t). These results are then summed at the adder 15 to provide the 
composite compressed pulse C. 
As seen from FIGS. 20-24, since neither results Cbbpp(t) nor Cbbqq(t) are 
summed with the results Caapp(t) and Caaqq(t), the compressed pulse 
C=Caapp(t)+Caaqq(t) has side lobes of certain amplitudes which are not 
zero as shown in FIG. 42 in comparison with the pulse 
C=Caapp(t)+Caaqq(t)+Cbbpp(t)+Cbbqq(t) shown in FIG. 24. 
In view of equation (11), Caaqq(t) is similarly represented as follows: 
##EQU18## 
The third and fourth sequences are in the complementary relationships as 
described above, and thus 
EQU .rho.pp(0)=.rho.qq(0) 
EQU .rho.pp(1)=-.rho.qq(1) 
EQU .rho.pp(2)=-.rho.qq(2) 
EQU .rho.pp(3)=-.rho.qq(3) 
Accordingly, 
EQU Caapp(t)+Caaqq(t)=2.rho.pp(0)Aa(t-t.sub.0) (12) 
From equation (12), it is apparent that if the first basic unit compressed 
pulse Aa(t-t.sub.0) has substantially small side lobes, the compressed 
pulse C=Caapp(t)+Caaqq(t) has also substantially small side lobes. 
Since a Barker sequence is a well-known sequence, a autocorrelation 
function of which has small side lobes, such a Barker sequence can be 
employed as the first sequence {a} to reduce the side lobes of the pulse C 
when the second sequence {b} is not utilized. 
Further, modification which does not employ the fourth sequence {q} as well 
as the second sequence {b}. 
In this case, the transmission signal S comprises only the first 
transmission signal Sap(t), and thus the echo signal R only the first echo 
signal Rap(t). 
The correlation result at the first correlator 11 is only Caap(t) which is 
fed from the first reference signal Ua(t) and the first echo signal 
Rap(t). The correlation result, or compressed pulse C at the second 
correlator 12 is only Caapp(t) as shown in FIG. 20 which is fed from the 
result Caap(t) and the third reference signal Up(t). 
The result Caapp(t) is representable as equation (11). That is, 
##EQU19## 
Although the compressed pulse Caapp(t) has side lobes having certain 
amplitudes as shown in FIG. 20, it is obvious from the above equation that 
the amplitudes of the side lobes can be sufficiently lowered, relative to 
the main lobe at t=t.sub.0, if the autocorrelation function value 
.rho.pp(0) is sufficiently larger than the others .rho.pp(1) through 
.rho.pp(3) and also if the basic unit compressed pulse Aa(t-t.sub.0) has 
sufficiently larger amplitude at t=t.sub.0 (main lobe) and near thereto 
than the others at the other time t (side lobes). 
The latter condition can obviously be satisfied by the sequence {a} being a 
Barker sequence, as the first modification. Accordingly, if a Barker 
sequence is employed as the first sequence {a} and the autocorrelation 
function .rho.pp(i) of the third sequence {p} has a sufficiently large 
value at i=0 in comparison with the others (i.noteq.0), the second 
modification can derive advantages similar to those of the first 
embodiment. In this case, since the compressed pulse C contains only 
Caapp(t), the adder 15 in the first embodiment can be neglected as shown 
in FIG. 43. 
It is to be understood that the above modifications explained with regard 
to the first embodiment can also be applied to the second through sixth 
embodiments. 
In the respective embodiments which have been explained so far, the length 
M of the first and second sequences {a} and {b} is 8 while the length N of 
the third and fourth sequences {p} and {q} is 4. For the length M and N, 
any natural number may be applied. 
For example, supposing that the first basic unit signal is used as the 
first reference signal and there are K.sub.1 number of sampling points in 
the time period .delta., let each of the length M and N be any natural 
number including 1. 
In a case of Tp=M.delta., the first correlator 11 can be constituted in a 
similar manner to FIG. 27 by a delay line 11a having M.times.K.sub.1 
number of output taps, M.times.K.sub.1 number of multipliers 11b connected 
respectively to the output taps of the delay line 11a and an adder 11c 
having M.times.K.sub.1 number of input terminals. The second correlator 12 
can be constituted in a similar manner to FIG. 28 by a delay line 12a 
having (N-1).times.M.times.K.sub.1 number of output taps, N number of 
multipliers connected to the output taps of the delay line 12a for every 
(M.times.K.sub.1)-th taps, and an adder 12c having N number of input 
terminals. 
Compared to the above-mentioned embodiment, the correlator 10 (FIG. 4) of 
the conventional apparatus requires a delay line 10a having 
M.times.N.times.K.sub.1 number of output taps, M.times.N.times.K.sub.1 
number of multipliers 10b connected to the respective output taps and an 
adder 10c having M.times.N.times.K.sub.1 number of input terminals in 
order to attain a similar effect to that of the above-mentioned 
embodiment, and thus requires a larger number of multipliers and input 
terminal of the adder. 
Consideration is made next on the case of Tp&gt;M.delta. or Tp&lt;M.delta.. 
Supposing that there are K.sub.2 number of sampling points in the time 
period Tp in these cases, the first correlator 11 can be constituted in a 
similar manner to FIG. 27 by a delay line 11a having M.times.K.sub.1 
number of taps, M.times.K.sub.1 number of multipliers 11b connected to the 
respective taps of the delay line 11a, and an adder 11c having 
M.times.K.sub.1 number of input terminals. The second correlator 12 can be 
constituted in a similar manner to FIG. 28 by the delay line 12a having 
(N-1).times.K.sub.2 number of taps, and N number of multipliers 12b 
connected to the output taps of the delay line 12a for every K.sub.2 -th 
taps and the adder 12c having N number of the input terminals. This 
constitution can provide similar advantages to those of the embodiment as 
described above. That is, a number of the multipliers and input terminals 
of the adder is decreased in comparison with the corresponding prior art. 
It is to be noted that if M=1, the waveforms of the first and second basic 
unit signals are equivalent to unit waveforms such as a rectangular 
waveform, a quasi-rectangular waveform, a sinusoidal waveform, a waveform 
having a smooth curve or a vibration waveform. In this case, therefore, 
the first correlator 11 may be dispensed with in the first and third 
embodiments. In the second, fourth, fifth and sixth embodiments, the first 
correlator 11 may be left as a matching filter or a quasi-matching filter. 
In a case of N=1, the second correlator 12 may be dispensed with in the 
first through sixth embodiments. 
Furthermore, in general, supposing that there are K.sub.3 number of 
sampling points within the time duration of the first reference signal and 
there are K.sub.2 number of sampling points within the time duration Tp, 
the first correlator 11 may be constituted in a similar manner to FIG. 24 
by a delay line 11a having K.sub.3 number of taps, K.sub.3 number of 
multipliers 11c connected to the respective taps of the delay line 11a and 
an adder 11c having K.sub.3 number of input terminals. The second 
correlator 12 may be constituted in a similar manner to FIG. 28 by a delay 
line 12a having (N-1).times.K.sub.2 number of taps, N number of 
multipliers 12b connected to the taps of the delay line 12a for every 
K.sub.2 -th taps and the adder 12c having N number of input terminals. 
This constitution also has the similar function and advantages to those of 
the embodiment as described above. 
In the respective embodiments as described above, the impulse response h(t) 
is a delta function. The present invention is not limited to these 
embodiments and a similar function and effect to those of the embodiments 
may be expected when h(t) is an arbitrary function, for example, a 
function with waveform portions of vibrations. 
In the respective embodiments as described above, the first and second 
sequences are in the complementary relationships. The present invention is 
not limited to these embodiments and a similar function and effect to 
those of the embodiments may be expected when the first and second basic 
unit compressed pulses are in the complementary relationships. 
Further, in the respective embodiments as described above, such embodiments 
have been described in which the third and fourth sequences are in the 
complementary relationships, but the present invention is not also limited 
to them. A similar function and effect to those of the embodiments may be 
attained if the result of summation of the first and third compressed 
pulses and the results of summation of the second and fourth compressed 
pulses are in the complementary relationships under that the first and 
second basic unit compressed pulses are in the complementary 
relationships. 
Although the present invention has been explained with respect to 
application to an ultrasonic flaw detector apparatus, it can be applied to 
other apparatuses such as an ultrasonic diagnostic apparatus. 
Also, although the explanation has been made on such a situation where the 
test piece S is in contact with the ultrasonic probe 6, the test piece may 
need not be in contact with the ultrasonic probe. In this instance, 
transmission and reception of the ultrasonic wave between the ultrasonic 
probe and the test piece may be effected through such a coupling medium as 
water. 
The present invention can also be applied to an ultrasonic wave 
transmission and reception system utilizing a plurality of probe elements 
consisting an ultrasonic probe array, and to a transmission and reception 
system utilizing another wave, for instance an electromagnetic wave.