Phased array ultrasonic beam forming using oversampled A/D converters

A beam former in a PASS ultrasonic imaging system includes a set of sigma-delta modulators which operate to separately digitize the received echo signal from each transducer element. The oversampled one-bit digital representations of each echo signal are delayed as required for beam steering and focusing, and are summed together. A decimator filter reduces the sample rate of the digitized receive beam prior to display of the image resulting from the receive beam.

BACKGROUND OF THE INVENTION 
This invention relates to coherent imaging systems using vibratory energy, 
such as ultrasound and, in particular, to an ultrasound imaging system in 
which a focused and steered beam is produced by delaying the signals 
produced by the separate elements of a transducer array. 
There are a number of modes in which vibratory energy, such as ultrasound, 
can be used to produce images of objects. The ultrasound transmitter may 
be placed on one side of the object and the sound transmitted through the 
object to the ultrasound receiver placed on the other side ("transmission 
mode"). With transmission mode methods, an image may be produced in which 
the brightness of each pixel is a function of the amplitude of the 
ultrasound that reaches the receiver ("attenuation" mode), or the 
brightness of each pixel is a function of the time required for the sound 
to reach the receiver ("time-of-flight" or "speed of sound" mode). In the 
alternative, the receiver may be positioned on the same side of the object 
as the transmitter and an image may be produced in which the brightness of 
each pixel is a function of the amplitude or time-of-flight of the 
ultrasound reflected from the object back to the receiver ("reflection", 
"backscatter" or "echo" mode). The present invention relates to a 
backscatter method for producing ultrasound images. 
There are a number of well known backscatter methods for acquiring 
ultrasound data. In the so-called "A-scan" method, an ultrasound pulse is 
directed into the object by the transducer and the amplitude of the 
reflected sound is recorded over a period of time. The amplitude of the 
echo signal is proportional to the scattering strength of the refractors 
in the object and the time delay is proportional to the range of the 
refractors from the transducer. In the so-called "B-scan" method, the 
transducer transmits a series of ultrasonic pulses as it is scanned across 
the object along a single axis of motion. The resulting echo signals are 
recorded as with the A-scan method and either their amplitude or time 
delay is used to modulate the brightness of pixels on a display. With the 
B-scan method, enough data are acquired from which an image of the 
refractors can be reconstructed. 
In the so-called C-scan method, the transducer is scanned across a plane 
above the object and only the echoes reflecting from the focal depth of 
the transducer are recorded. The sweep of the electron beam of a CRT 
display is synchronized to the scanning of the transducer so that the x 
and y coordinates of the transducer correspond to the x and y coordinates 
of the image. 
Ultrasonic transducers for medical applications are constructed from one or 
more piezoelectric elements sandwiched between a pair of electrodes. Such 
piezoelectric elements are typically constructed of lead zirconate 
titanate (PZT), polyvinylidene difluoride (PVDF), or PZT ceramic/polymer 
composite. The electrodes are connected to a voltage source, and when a 
voltage is applied, the piezoelectric elements change in size at a 
frequency corresponding to that of the applied voltage. When a voltage 
pulse is applied, the piezoelectric element emits an ultrasonic wave into 
the media to which it is coupled at the frequencies contained in the 
excitation pulse. Conversely, when an ultrasonic wave strikes the 
piezoelectric element, the element produces a corresponding voltage across 
its electrodes. Typically, the front of the element is covered with an 
acoustic matching layer that improves the coupling with the media in which 
the ultrasonic waves propagate. In addition, a backing material is 
disposed to the rear of the piezoelectric element to absorb ultrasonic 
waves that emerge from the back side of the element so that they do not 
interfere. A number of such ultrasonic transducer constructions are 
disclosed in U.S. Pat. Nos. 4,217,684; 4,425,525; 4,441,503; 4,470,305 and 
4,569,231, all of which are assigned to the instant assignee. 
When used for ultrasound imaging, the transducer typically has a number of 
piezoelectric elements arranged in an array and driven with separate 
voltages (apodizing). By controlling the time delay (or phase) and 
amplitude of the applied voltages, the ultrasonic waves produced by the 
piezoelectric elements (transmission mode) combine to produce a net 
ultrasonic wave focused at a selected point. By controlling the time delay 
and amplitude of the applied voltages, this focal point can be moved or 
"steered," in a plane to scan the subject. 
The same principles apply when the transducer is employed to receive the 
reflected sound (receiver mode). That is, the voltages produced at the 
transducer elements in the array are summed together such that the net 
signal is indicative of the sound reflected from a single focal point in 
the subject. As with the transmission mode, this focused reception of the 
ultrasonic energy is achieved by imparting separate time delay (and/or 
phase shifts) and gains to the signal from each transducer array element. 
This form of ultrasonic imaging is referred to as "phased array sector 
scanning", or "PASS". Such a scan is comprised of a series of measurements 
in which the steered ultrasonic wave is transmitted, the system switches 
to receive mode after a short time interval, and the reflected ultrasonic 
wave is received and stored. Typically, the transmission and reception are 
steered in the same direction (.theta.) during each measurement to acquire 
data from a series of points along a scan line. The receiver is 
dynamically focused at a succession of ranges (R) along the scan line as 
the reflected ultrasonic waves are received. The time required to conduct 
the entire scan is a function of the time required to make each 
measurement and the number of measurements required to cover the entire 
region of interest at the desired resolution and signal-to-noise ratio. 
For example, a total of 128 scan lines may be acquired over a 90 degree 
sector, with each scan line being steered in increments of 0.70.degree.. A 
number of such ultrasonic imaging systems are disclosed in U.S. Pat. Nos. 
4,155,258; 4,155,260; 4,154,113; 4,155,259; 4,180,790; 4,470,303; 
4,662,223; 4,669,314; 4,809,184; 4,796,236; 4,839,652 and 4,983,970, all 
of which are assigned to the instant assignee. 
When forming a steered and focused beam from the ultrasonic echo signals 
received by each transducer element in the array, the signals produced by 
each transducer element must be delayed by a precise amount in order to 
compensate for the time of flight differences between the elements on the 
array. The accuracy with which these delays can be controlled impacts the 
quality of the final coherent sum of all the array element signals and, 
therefore, the quality of the resulting image. In medical ultrasonic 
imaging systems, a delay resolution of 1/32 of the wavelength of the 
transmitted ultrasonic frequency will provide the needed beam quality. For 
example, if a 5 MHz ultrasonic carrier frequency is employed, a delay 
resolution of 6.25 nanoseconds (1/32 (5 MHz)) is required. 
In prior systems such as those described in the above-cited patents, such 
delay resolution in the beam forming circuitry of the ultrasonic receiver 
requires a large amount of hardware and consumes a large amount of power. 
The most recent designs sample each transducer element signal using highly 
accurate A/D converters which produce multi-bit digital numbers that 
indicate the in-phase and quadrature components of the sampled signal. 
These multi-bit numbers are delayed by separate circuitry, including 
first-in/first-out, or FIFO, memories, decimators, and phase rotators, 
before being summed with the separately delayed, multi-bit, in-phase and 
quadrature signals from each of the other transducer elements. This is a 
considerable amount of hardware for a conventional 128 element transducer 
array, and is an enormous amount of hardware when a two-dimensional 
transducer array having 512 elements is considered. 
SUMMARY OF THE INVENTION 
The present invention relates to an improved beam forming section of the 
receiver in an ultrasonic imaging system. More particularly, the beam 
forming section of the invention includes a plurality of receive channels, 
one for each separate transducer array element, and each receive channel 
includes an oversampled analog-to-digital (A/D) converter which receives 
the echo signal produced by its associated transducer array element and 
generates a corresponding digital signal at a sample rate in excess of the 
Nyquist criterion, means for delaying each digital signal by an amount 
necessary to steer a resulting receive beam in the desired direction, 
means for summing the digital signals to produce the digital receive beam, 
and decimation means for reducing the sample rate of the digital receive 
beam. 
A general object of the invention is to simplify the beam forming circuitry 
in an ultrasonic system without reducing image quality. To provide the 
necessary delay resolution the sample rate of the A/D converter in each 
receive channel far exceeds the rate necessary to satisfy the Nyquist 
criterion. More specifically, the Nyquist criterion requires a sample rate 
of twice the bandwidth of the modulated ultrasonic signal, whereas the 
oversampled A/D converter in the preferred embodiment of the invention 
samples at thirty-two times the carrier frequency. As a result, the 
oversampled stream of digital signals can be delayed in increments of the 
sample period with simple shift registers, and the desired delay 
resolution of .lambda./32 is easily obtained (where .lambda. represents 
the carrier wavelength). 
Another object of the invention is to provide a relatively simple and 
easily fabricated A/D converter for the beam former in an ultrasonic 
system. The oversampled A/D converter may be a sigma-delta modulator which 
produces a single-bit digital output signal at a very high sample rate. 
Such sigma-delta modulators are simple in construction and are easily 
implemented as part of an integrated circuit. The decimator serves as the 
demodulator of the single-bit digital signal. 
A more specific object of the invention is to reduce the number and 
complexity of the delay circuits and decimators needed by the beam forming 
portion of an ultrasonic system. Rather than providing separate delays and 
separate decimators in each receive channel, the delay circuits can be 
partially shared with other receive channels and a single decimator may be 
employed after the separately delayed digital signals are summed together.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring particularly to FIG. 1, an ultrasonic imaging system includes a 
transducer array 11 comprised of a plurality of separately driven elements 
12 which each produce a burst of vibratory energy, such as ultrasonic 
energy, when energized by a pulse produced by a transmitter 13. The 
vibratory energy reflected back to transducer array 11 from the subject 
under study is converted to an electrical signal by each transducer 
element 12 and applied separately to a receiver 14 through a set of 
switches 15. Transmitter 13, receiver 14 and switches 15 are operated 
under control of a digital controller 16 responsive to commands by a human 
operator. A complete scan is performed by acquiring a series of echoes in 
which switches 15 are set to their transmit positions, transmitter 13 is 
gated on momentarily to energize each transducer element 12, switches 15 
are then set to their receive positions, and the subsequent echo signals 
produced by each transducer element 12 are applied to receiver 14. The 
separate echo signals from each transducer element 12 are combined in 
receiver 14 to produce a single echo signal which is employed to produce a 
line in an image on a display system 17. 
Transmitter 13 drives transducer array 11 such that the vibratory energy 
produced, e.g., ultrasonic energy, is directed, or steered, in a beam. A 
B-scan can therefore be performed by moving this beam through a set of 
angles from point-to-point rather than physically moving transducer array 
11. To accomplish this, transmitter 13 imparts a time delay (T.sub.k) to 
the respective pulses 20 that are applied to successive transducer 
elements 12. If the time delay is zero (T.sub.k =0), all the transducer 
elements 12 are energized simultaneously and the resulting ultrasonic beam 
is directed along an axis 21 normal to the transducer face and originating 
from the center of transducer array 11. As the time delay (T.sub.k) is 
increased, as illustrated in FIG. 1, the ultrasonic beam is directed 
downward from central axis 21 by an angle .theta.. The relationship 
between the time delay increment T.sub.k added successively to each 
k.sup.th signal from one end of the transducer array (k=1) to the other 
end (k=N) is given by the following relationship: 
EQU T.sub.k =-(k-(N-1)/2) d sin .theta./c+(k-(N-1)/2).sup.2 d.sup.2 cos.sup.2 
.theta./2R.sub.T c+T.sub.0 (1) 
where 
d=equal spacing between centers of adjacent transducer elements 12, 
c=velocity of sound in the object under study. 
R.sub.T =range at which transmit beam is to be focused. 
T.sub.0 =delay offset which insures that all calculated values (T.sub.k) 
are positive values. 
The first term in this expression steers the beam in the desired angle 
.theta., and the second is employed when the transmitted beam is to be 
focused at a fixed range. A sector scan is performed by progressively 
changing the time delays T.sub.k in successive excitations. The angle 
.theta. is thus changed in increments to steer the transmitted beam in a 
succession of directions. When the direction of the beam is above central 
axis 21, the timing of pulses 20 is reversed, but the formula of equation 
(1) still applies. 
Referring still to FIG. 1, the echo signals produced by each burst of 
ultrasonic energy emanate from reflecting objects located at successive 
positions along the ultrasonic beam. These are sensed separately by each 
segment 12 of transducer array 11 and a sample of the magnitude of the 
echo signal at a particular point in time represents the amount of 
reflection occurring at a specific range (R). Due to differences in the 
propagation paths between a focal point P and each transducer element 12, 
however, these echo signals will not occur simultaneously and their 
amplitudes will not be equal. The function of the receiver 14 is to 
amplify and demodulate these separate echo signals, impart the proper time 
delay to each and sum them together to provide a single echo signal which 
accurately indicates the total ultrasonic energy reflected from each focal 
point P located at range R along the ultrasonic beam oriented at the angle 
.theta.. 
To simultaneously sum the electrical signals produced by the echoes from 
each transducer element 12, time delays are introduced into each separate 
transducer element channel of receiver 14. In the case of linear array 11, 
the delay introduced in each channel may be divided into two components, 
one component is referred to as a beam steering time delay, and the other 
component is referred to as a beam focusing time delay. The beam steering 
and beam focusing time delays for reception are precisely the same delays 
(T.sub.k) as the transmission delays described above. However, the 
focusing time delay component introduced into each receiver channel is 
continuously changing during reception of the echo to provide dynamic 
focusing of the received beam at the range R from which the echo signal 
emanates. This dynamic focusing delay component is as follows: 
EQU T.sub.k =(k-(N-1)/2).sup.2 d.sup.2 cos.sup.2 .theta./2Rc (2) 
where 
R=the range of the focal point P from the center of the array 11; 
c=the velocity of sound in the object under study; and 
T.sub.k =the desired time delay associated with the echo signal from the 
k.sup.th element to coherently sum it with the other echo signals. 
Under direction of digital controller 16, receiver 14 provides delays 
during the scan such that steering of receiver 14 tracks with the 
direction of the beam steered by transmitter 13 and it samples the echo 
signals at a succession of ranges (R) and provides the proper delays to 
dynamically focus at points P along the beam. Thus each emission of an 
ultrasonic pulse results in acquisition of a series of data points which 
represent the amount of reflected sound from a corresponding series of 
points P located along the ultrasonic beam. 
Display system 17 receives the series of data points produced by receiver 
14 and converts the data to a form producing the desired image. For 
example, if an A-scan is desired, the magnitude of the series of data 
points is merely graphed as a function of time. If a B-scan is desired, 
each data point in the series is used to control brightness of a pixel in 
the image, and a scan comprised of a series of measurements at successive 
steering angles (.theta.) is performed to provide the data necessary for 
display. 
Referring to FIG. 2 in conjunction with FIGS. 1, 2A and 2B, transmitter 13 
includes a set of channel pulse code memories which are indicated 
collectively as memories 50. In the preferred embodiment there are 128 
separate transducer elements 12, and therefore, there are 128 separate 
channel pulse code memories 50. Each pulse code memory 50 is typically a 
1-bit by 512-bit memory which stores a bit pattern 51, shown in FIG. 2A, 
that determines the frequency of ultrasonic pulse 52, shown in FIG. 2B, 
that is to be produced. In the preferred embodiment, the bit pattern of 
FIG. 2A is read out of each pulse code memory 50 by a 40 MHz master clock 
and applied to a driver 53 which amplifies the signal to a power level 
suitable for driving transducer 11. For the example shown in FIG. 2A, the 
bit pattern is a sequence of four "1" bits alternated with four "0" bits 
to produce a 5 MHz ultrasonic pulse 52. The transducer elements 11 (FIG. 
1) to which these ultrasonic pulses 52 are applied respond by producing 
ultrasonic energy. If all 512 bits are used, a pulse of bandwidth as 
narrow as 40 kHz centered on the carrier frequency (i.e. 5 MHz in the 
example) will be emitted. 
As indicated above, to steer the transmitted beam of ultrasonic energy in 
the desired direction (.theta.), pulses 52 for each of the N channels, 
such as shown in FIG. 2B, must be delayed by the proper amount. These 
delays are provided by a transmit control 54 which receives four control 
signals (START, MASTER CLOCK, R.sub.T and .theta.) from digital controller 
16 (FIG. 1). Using the input control signal .theta., the fixed transmit 
focus R.sub.T, and the above equation (1), transmit control 54 calculates 
the delay increment T.sub.k required between successive transmit channels. 
When the START control signal is received, transmit control 54 gates one 
of four possible phases of the 40 MHz MASTER CLOCK signal through to the 
first transmit channel 50. At each successive delay time interval 
(T.sub.k) thereafter, one of the phases of the 40 MHz MASTER CLOCK signal 
is gated through to the next channel pulse code memory 50 until all N=128 
channels are producing their ultrasonic pulses 52 (FIG. 2B). Each transmit 
channel 50 is reset after its entire bit pattern 51, such as shown in FIG. 
2A, has been transmitted and transmitter 13 then waits for the next 
.theta. and next START control signals from digital controller 16. As 
indicated above, in the preferred embodiment of the invention a complete 
B-scan is comprised of 128 ultrasonic pulses steered in .DELTA..theta. 
increments of 0.70.degree. through a 90.degree. sector centered about the 
central axis 21 (FIG. 1) of transducer 11. 
For a detailed description of the transmitter 13, reference is made to U.S. 
Pat. No. 5,014,712 issued on May 14, 1991 and entitled "Coded Excitation 
For Transmission Dynamic Focusing of Vibratory Energy Beam", incorporated 
herein by reference. 
Referring particularly to FIG. 3 in conjunction with FIG. 1, receiver 14 is 
comprised of two sections: a time-gain control section 100, and a receive 
beam forming section 101. Time-gain control section 100 includes an 
amplifier 105 for each of the N=128 receiver channels and a time-gain 
control circuit 106. The input of each amplifier 105 is connected to a 
respective one of transducer elements 12 to receive and amplify the echo 
signal which it receives. The amount of amplification provided by the 
amplifiers 105 is controlled through a control line 107 that is driven by 
the time-gain control circuit 106. As is well known in the art, as the 
range of the echo signal increases, its amplitude is diminished. As a 
result, unless the echo signal emanating from more distant reflectors is 
amplified more than the echo signal from nearby reflectors, the brightness 
of the image diminishes rapidly as a function of range (R). This 
amplification is controlled by the operator who manually sets eight 
(typically) TGC linear potentiometers 108 to values which provide a 
relatively uniform brightness over the entire range of the sector scan. 
The time interval over which the echo signal is acquired determines the 
range from which it emanates, and this time interval is divided into eight 
segments by TGC control circuit 106. The settings of the eight 
potentiometers are employed to set the gain of amplifiers 105 during each 
of the eight respective time intervals so that the echo signal is 
amplified in ever increasing amounts over the acquisition time interval. 
The receive beam forming section 101 of receiver 14 includes N=128 separate 
receiver channels 110. As will be explained in more detail below, each 
receiver channel 110 receives the analog echo signal from one of the TGC 
amplifiers 105 at an input 111, and it produces a stream of digitized 
output values on a bus 112. Each of these output values represents a 
sample of the echo signal envelope at a specific range (R). These samples 
have been delayed in the manner described above such that when they are 
summed at summing point 114 with the output samples from each of the other 
receiver channels 110, they indicate the magnitude of the echo signal 
reflected from a point P located at range R on the steered beam (.theta.). 
These beam samples are 8-bit binary numbers which are provided at an 
output 121. In the preferred embodiment, each echo signal is sampled at 
equal intervals of about 150 micrometers over the entire range of the scan 
line (typically 40 to 200 millimeters). 
Referring particularly to FIGS. 1 and 4, the stream of 8-bit binary numbers 
generated by receiver 14 at its output 121 is applied to the input of 
display system 17. This "scan data" is stored in a memory 150 as an array, 
with the rows of the scan data array 150 corresponding with the respective 
beam angles (.theta.) that are acquired, and the columns of scan data 
array 150 corresponding with the respective ranges (R) at which samples 
are acquired along each beam. The R and .theta. control signals 151 and 
152 from receiver 14 indicate where each input value is to be stored in 
array 150, and a memory control circuit 153 writes that value to the 
proper memory location in array 150. The scan can be continuously repeated 
and the flow of values from receiver 14 will continuously update scan data 
array 150. 
Referring still to FIG. 4, the scan data in array 150 are read by a digital 
scan converter 154 and converted to a form producing the desired image. If 
a conventional B-scan image is being produced, for example, the sample 
values M(R,.theta.) stored in scan data array 150 are converted to sample 
values M(x,y) which indicate magnitudes at pixel locations (x,y) in the 
image. Such a polar coordinate to Cartesian coordinate conversion of the 
ultrasonic image data is described, for example, in an article by Steven 
C. Leavitt et al in Hewlett-Packard Journal, October, 1983, pp. 30-33, 
entitled "A Scan Conversion Algorithm for Displaying Ultrasound Images". 
Regardless of the particular conversion made by digital scan converter 154, 
the resulting image data are written to a memory 155 which stores a 
two-dimensional array of converted scan data. A memory control 156 
provides dual port access to the memory 155 such that the digital scan 
converter 154 can continuously update the values therein with fresh data 
while a display processor 157 reads the updated data. The display 
processor 157 is responsive to operator commands received from a control 
panel 158 to perform conventional image processing functions on the 
converted scan data in memory 155. For example, the range of brightness 
levels indicated by the converted scan data in memory 155 may far exceed 
the brightness range of display device 160. Indeed, the brightness 
resolution of the converted scan data in memory 155 may far exceed the 
brightness resolution of the human eye, and manually operable controls are 
typically provided which enable the operator to select a window of 
brightness values over which maximum image contrast is to be achieved. The 
display processor reads the converted scan data from memory 155, provides 
the desired image enhancement, and writes the enhanced brightness values 
to a display memory 161. 
Display memory 161 is shared with a display controller circuit 162 through 
a memory control circuit 163, and the brightness values therein are mapped 
to control brightness of the corresponding pixels in display 160. Display 
controller 162 is a commercially available integrated circuit which is 
designed to operate the particular type of display 160 used. For example, 
display 160 may be a CRT (cathode ray tube), in which case display 
controller 162 is a CRT controller chip which provides the required sync 
pulses for the horizontal and vertical sweep circuits and maps the display 
data to the CRT at the appropriate time during the sweep. 
It should be apparent to those skilled in the art that display system 17 
may take one of many forms depending on the capability and flexibility of 
the particular ultrasound system. In the preferred embodiment described 
above, programmed microprocessors are employed to implement the digital 
scan converter and display processor functions, and the resulting display 
system is, therefore, very flexible and powerful. 
As indicated above with reference to FIG. 3, beam forming section 101 of 
receiver 14 is comprised of a set of receiver channels 110--one for each 
element 12 of transducer 11. Referring particularly to FIG. 5, the beam 
former is responsive to a 160 MHz master clock (T.sub.s), a range signal 
(R) and a beam steering angle signal (.theta.) from digital controller 16 
(FIG. 1) in order to perform the digital beam forming functions. The 
analog input signal from each transducer element 12 is applied to the 
input 111 of a corresponding sigma-delta (.SIGMA..DELTA.) modulator 201 
and is converted thereby to a one-bit digital signal at its output 202. 
Such modulators, as is well known in the art, are characterized by 
simplicity of construction and operation when compared to conventional 
multi-bit analog-to-digital converters. The sigma-delta modulator samples 
the analog echo signal at the 160 MHz rate of the clock T.sub.s, which is 
thirty-two times the 5 MHz carrier frequency of the ultrasound echo signal 
and sixteen times the Nyquist sample rate normally applicable to multi-bit 
A/D converters. 
Oversampling of the analog echo signal accomplishes two objectives. First, 
the digital output signal of each modulator 201 is a single-bit, which is 
easily processed, and second, each bit supplied by a modulator 201 
represents a sample of the echo signal over a very small increment of time 
(6.25 nanoseconds). The resulting single-bit digitized signal can, 
therefore, be delayed in very small increments of time in delay registers 
203 to provide a high resolution means for delaying the echo signals from 
each transducer element 12 (FIG. 1). 
Delay registers 203 are coupled together in a chain and the single-bit 
signal from each transducer element 12 produced by its corresponding 
modulator 201 is inserted into this chain by respective summing circuits 
210. Each delay register 203 is a conventional variable-delay shift 
register 80 stages in length and the digital signals are shifted through 
their stages by a 160 MHz master clock signal T.sub.s. The desired time 
delay for each register 203 is determined by the location of the shift 
register stage from which the digital echo signal is produced. For 
example, if the delay is calculated to be 125 nanoseconds, then the echo 
signal is shifted through the first 125/6.25=20 stages of the delay 
register 203. It can be appreciated, therefore, that the desired high 
resolution time delay is achieved with a very simple and easily 
constructed device. 
The beam former summing circuits 210 are binary adders which arithmetically 
sum each single-bit digital signal from its associated sigma-delta 
modulator 201 with the multi-bit binary number representing the number of 
"1s" produced by the "upstream" receiver channels during the clock period 
T.sub.s. The delay registers 203 are coupled between successive summing 
circuits 210 and are separately controlled by a receive channel control 
212 to provide the proper time delay. The total delay imposed on any 
receive channel echo signal, therefore, is the sum of all the delays in 
those of delay registers 203 which are "downstream" of the point at which 
the echo signal is added to the chain. The time delay provided by each 
register 203 is equal to the delay as determined by equation (1) for the 
associated transducer element number, minus all the delays imposed by the 
"downstream" delay registers 203. The widths of delay registers 203 
increase as one moves "downstream", with the number of width bits for each 
register equal to log.sub.2 of the total number of receive channels summed 
together at its input. For example, the width of the last delay register 
203 before the decimator filter 213 is: 
EQU WIDTH (W)=log.sub.2 (N) 
where N is the total number of array elements 12. This width formula is 
also applicable to summing circuits 210 and decimator filter 213. 
The direction in which echo signal data flows through the chain of summing 
circuits 210 and delay registers 203 is determined by the steering angle 
.theta. and is controlled by receive channel control 212. As the steering 
angle .theta. is increased in one direction from central axis 21 (FIG. 1), 
the delay on receive channel 1 is greater than the delay on receive 
channel N and data flows downward through the chain. On the other hand, as 
the steering angle .theta. is increased in the opposite direction from 
central axis 21, the delay on receive channel N is greater and the data 
flows upward through the chain. The summing circuits are of the type in 
which at least one input and one output are interchangeable with each 
other. The outputs of delay registers 203 at each end of the chain are 
coupled to input channels of a multiplexer 211, and receive channel 
control 212 selects which delay register output signal will be applied to 
a decimator filter 213. 
Decimator filter 213 is a multi-bit low pass filter which filters out the 
high frequency quantization noise introduced by sigma-delta modulators 
201. In addition, a frequency decimation is simultaneously performed to 
sample down the 160 MHz oversampled frequency to a more conventional 5 MHz 
sample rate. While a conventional equiripple finite impulse response 
("FIR") decimation filter such as that described by E. Dijkstra et al. in 
"A Design Methodology For Decimation Filters In Sigma-Delta A/D 
Converters", ISCAS 87, pp. 479-482 (1987) may be used, a one-stage Comb 
decimation filter such as that described by E. Dijkstra et al. in "On The 
Use of Modulo Arithmetic Comb Filters In Sigma-Delta Modulators", IEEE, 
pp. 2001-2004 (1988) is preferred. Both of these Dijkstra et al. papers 
are herein incorporated by reference. Decimator filter 213 produces 16-bit 
samples S(R,.theta.) of steered and focused receive beam data at a 5 MHz 
sample rate which are provided through bus 121 to display 17 (FIG. 1). 
Sigma-delta modulators are well known in the art of communications and 
there are many different circuits which may be employed successfully in 
the present invention. See, for example, D. B. Ribner et al. U.S. Pat. No. 
5,065,157, issued Nov. 12, 1991 and assigned to the instant assignee. The 
Ribner et al. patent is herein incorporated by reference. The preferred 
circuit, shown in FIG. 6, is a second order sigma-delta modulator, which 
includes two serially connected integrators 220 and 221 which receive the 
analog input signal at input 111. The analog input signal is applied to 
the non-inverting input of each integrator 220 and 221, while a feedback 
signal on line 222 is applied to the inverting input of each integrator 
220 and 221. The output of the second integrator 221 is applied to the 
input of a comparator circuit 223 which produces either a logic high 
voltage or a logic low voltage at its output. This logic level signal is 
applied to the D input of a D-type flip-flop 224 which is clocked by the 
master clock signal T.sub.s every 6.25 nanoseconds. 
As the echo signal applied to input 111 rises in value the output signals 
of integrators 220 and 221 follow. When this voltage increase exceeds a 
reference value established at the negative input 225 of comparator 223, 
the comparator output switches to a logic high voltage. On the next clock 
pulse T.sub.s, therefore, flip-flop 224 is set and produces a logic high 
voltage at the delta-sigma modulator output 226 and on feedback line 222. 
The logic high voltage on feedback line 222 supplies a current to each 
integrator 220 and 221 which precisely offsets the rise in input voltage 
and drives the integrator output signals back to zero. Unless the input 
voltage continues to rise during the next clock period, therefore, the 
comparator output will be at a logic low voltage when the next clock 
signal T.sub.s is applied to the flip-flop 224, and a logic low, or "0" 
will be produced at output 226. The signal at output 226 of the 
delta-sigma modulator is thus a stream of "1"s and "0"s with each "1" 
representing an incremental increase in the echo signal and each "0" 
representing an incremental decrease in the echo signal. 
While only certain preferred features of the invention have been 
illustrated and described herein, many modifications and changes will 
occur to those skilled in the art. It is, therefore, to be understood that 
the appended claims are intended to cover all such modifications and 
changes as fall within the true spirit of the invention.