U.S. patent number 3,894,195 [Application Number 05/478,462] was granted by the patent office on 1975-07-08 for method of and apparatus for aiding hearing and the like.
Invention is credited to Karl D. Kryter.
United States Patent |
3,894,195 |
Kryter |
July 8, 1975 |
Method of and apparatus for aiding hearing and the like
Abstract
This disclosure deals with electronically aiding sensori-neural
deafness with frequency-segmented, dynamic range-compressed speech
signal processing, wherein noise vs. speech signal discrimination
is employed with an optional semi-remote microphone input, and with
an optional electronic frequency-shift processing of the signal to
prevent or reduce oscillation due to acoustic airborne and/or
vibrational feedback between the earphone(s) and the
microphone(s).
Inventors: |
Kryter; Karl D. (Los Altos,
CA) |
Family
ID: |
23900047 |
Appl.
No.: |
05/478,462 |
Filed: |
June 12, 1974 |
Current U.S.
Class: |
381/23.1;
381/317; 381/320; 381/318; 381/309 |
Current CPC
Class: |
H04R
25/453 (20130101); H04R 25/502 (20130101) |
Current International
Class: |
H04R
25/00 (20060101); H04r 025/00 () |
Field of
Search: |
;179/17FD,17R,17S,1D,1FS |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Blakeslee; Ralph D.
Attorney, Agent or Firm: Rines and Rines Shapiro and
Shapiro
Claims
What is claimed is:
1. A method of aiding hearing, that comprises, adjusting the
over-all intensity level of speech signals with substantially
linear gain over a predetermined range of intensities; applying the
adjusted-intensity signals along a plurality of frequency filtering
paths, one passing a broad band of the speech signal frequencies,
and the others passing successive adjacent narrow bands within said
broad band; reducing separately in each of the other paths, the
dynamic range of intensity levels corresponding to segments of
speech signals that vary in intensity for brief moments in the
corresponding narrow bands, as distinguished from the more steady
state segments of background noise and steady-state signals; and
combining the signals from said paths.
2. A method as claimed in claim 1 and in which the signals in each
of said paths are split and fed along a pair of further paths for
right and left ear excitation, with the signal combining step being
effectd by combining the right ear further paths and separately
combining the left ear further paths.
3. A method as claimed in claim 2 and in which independent level
adjustments are effected in each of the further paths prior to such
combining.
4. A method as claimed in claim 1 and in which the speech signals
are derived from a pair of right and left ear acoustic pick-up
regions and a further pick-up region adjustable closer to the
source of speech, and then the same are combined prior to said
over-all intensity level adjusting step.
5. Hearing aid apparatus having, in combination, microphone pick-up
means; automatic gain control means connected with the pick-up
means to adjust the overall signal intensity level of speech
signals with substantially linear gain over a predetermined range
of intensities; a plurality of filter paths connected with the
automatic gain control means and comprising a first path with broad
band filter means for the speech signal frequencies and a plurality
of further paths containing band-pass filters of successive
adjacent narrow bands within said broad band; a plurality of
speech-noise discrimination means, one connected in each of the
plurality of further paths for separately reducing in each path the
dynamic range of signal intensity levels corresponding to segments
of speech signals that vary in intensity for brief moments in the
respective narrow bands, as distinguished from the more steady
state segments of background noise and steady-state signals; and
means for combining the signals from said paths.
6. Apparatus as claimed in claim 5 and in which said combining
means comprises pairs of right and left ear paths, each pair split
from the output of the broad band filter means and the outputs of
the plurality of speech-noise discrimination means, with all right
ear paths connected together and all left ear paths connected
together.
7. Apparatus as claimed in claim 6 and in which said pairs of paths
comprise separate variable gain amplifier means and resistive
combining networks.
8. Apparatus as claimed in claim 7 and in which further variable
gain amplifier means is provided at the output of each of the
connected-together right and left ear paths, independently operable
with respect to the said separate variable gain amplifier
means.
9. Apparatus as claimed in claim 4 and in which each of said
speech-noise discrimination means comprises a pair of signal
processing paths connected to the corresponding band pass filter
means, one of said paths including gating means and attack-release
time control means for operating the gating means to apply
amplification emphasis along the other processing path for the
weaker short-duration segments of the signal relative to the strong
intensity segments of the speech signal, but without providing
added amplification to relatively low intensity background
noise.
10. Apparatus as claimed in claim 4 and in which said microphone
pick-up means comprises right and left ear microphones and a remote
microphone adjustable to regions closer to the source of speech,
with all of the microphones connected to the automatic gain control
means.
11. Hearing aid apparatus having, in combination, right and left
ear microphone pick-up means, remote microphone means adjustable to
regions closer to the source of sound, and common automatic gain
control means connected to all of said microphone means to receive
the combined inputs thereof.
12. Hearing aid apparatus as claimed in claim 5 and in which
frequency shift means is provided connected with the automatic gain
control means for shifting the frequency of signals picked up by
the microphone pick-up means and mechanical vibratory linkages
associated therewith, said frequency shift means comprising means
for modulating with one frequency and de-modulating with a second
and different frequency.
Description
The present invention relates to methods of and apparatus for
electronically aiding hearing or similar applications, being more
particularly directed to improving noise vs. speech signal
discrimination.
The most prevalent type of deafness is so-called sensori-neural
hearing loss, wherein the inner ear loses some ability to perceive
the weaker intensity portions of the speech signal and also loses
some ability to make normal discriminations among some frequency
components even though of sufficient intensity to be audible to the
person with sensori-neutral hearing loss. Usually these losses in
hearing ability are greater for the higher sound frequencies, say,
2000 Hertz) than for the lower (below, say 2000 Hertz). The
sensori-neural deafened ear, moreover, causes the perception of
sounds that are very intense as excessively loud. Distortions not
formed in the normal inner ear, which contains the sensori-neural
receptors, moreover, apparently occur in the sensori-neural
deafened ear and result in less discrimination than normal among
the various speech sounds.
There are many electronic hearing aids which provide means for
increasing the intensity of the speech signal reaching the inner
ear so that the weakened sounds are audible to the deafened ear.
These hearing aids, however, while of help to persons suffering
so-called conductive type deafness, are not very helpful to
sensori-neural deafness because of the aforementioned loss in
discrimination ability, and because of the inner-ear distortions
and excessive loudnesses that occur when sound amplification is
applied to the strong as well as weak components of the varying
intensity speech signal in order that the weaker sounds be made
audible to the sensori-neural ear. For example, a word such as
"show" contains the consonant "sh", which is much weaker than the
vowel sound "ow". A hearing aid that sufficiently amplifies all the
sounds uniformly or linearly so that the weaker "sh" component, or
"phoneme", as it is called, is audible to the sensori-neural ear,
may also make the "ow" portion of the word extraordinarily loud and
cause distortion in the inner ear, thereby tending to lessen
understanding of the speech signal. It is also important to note
that these weaker phonemes tend to have durations ranging from
about 0.01 to less than 0.5 second. It has been discovered, in
accordance with the present invention, that effective use can be
made of the relative difference in amplitude of segments of the
speech signal and the relatively short duration of the speech
segments of phonemes, particularly the less intense phonemes, to
produce the improved results herein described.
In attempts to overcome the deficiency of linear-gain hearing aids,
automatic non-linear or compression gain control systems have
sometimes been used wherein the intensity of the speech signal is
averaged for a brief period of time and this information is used
automatically to adjust the gain of the amplifier. If the level is
too low, the gain of the amplifier is increased by an amount
proportional to the degree the average input voltage (over some
specified period of time) falls below a specified level. This
process is called dynamic range compression; but it is difficult
satisfactorily to achieve with speech signals because the signal
level changes so quickly from one speech sound to another. Changing
the gain without an adequate determination of the average envelope
will cause distortion of the signal waveform and thereby degrade
its understandability. In brief, an automatic gain control system
that more or less continuously (or too frequently) modifies the
degree of gain will tend to introduce distortion and as a result
will not always make the speech signal more understandable, as
described by E. Trinder, An Attempt to Correct Speech
Discrimination Loss in Cochlear Deafness by Graded Instantaneous
Compression, Sound, Vol. 5, pp. 62-67, (1972). Conversely,
maintaining a given gain for too long a period of time will also
degrade the understandability of the speech signal because the gain
setting will be inappropriate over significant segments of the
speech phonemes wherein the level changes are very rapid.
Another shortcoming of automatic compression gain control systems
is that during periods of time when there is a pause in the input
speech signal, the gain control is progressively increased to a
maximum amount and thereby tends to make objectionable to the
hearing aid user, the normally low level, or residual, noise
present at the input of inherent in the electronics of the hearing
aid. It is noted that in the present hearing aid invention, as will
be described later, an automatic nonlinear-linear gain control (to
be labelled NLGC) device is utilized that has the ability to
discriminate to a degree between speech signals and background
noise and adjust the system gain appropriately on the basis of this
information; i.e., prevent excessive amplification to the weak
noise segments.
It might be noted that some reduction in the distortions that occur
with automatic compression gain can be reduced to some extent by
the application of independent automatic compression gain controls
to different portions of the speech spectrum; the amount, if any,
for each portion being adjusted to meet the degree and kind of
hearing loss experienced by a given ear with a sensori-neural
hearing loss. Such automatic compression gain of frequency segment
speech signals has been described, for example, by E. Villchur,
Signal Processing to Improve Speech Intelligibility in Perspective
Deafness, J. Acoust. Soc. Am. 53, 1647-1657, (1973). While this
technique does provide improvement in understanding of speech by
persons with sensori-neural deafness, it does not provide for the
discrimination between weak speech segments and weak noise segments
providing increased gain for the speech segments but not the noise
segments, as does the present invention.
It is well known that persons wearing hearing aids with
microphones, either non-directional or so-called directional
located on or near the head of the listener, have difficulty in
understandidng speech when in a conference or other situation where
several speech or other competing auditory signals reach the
listener at about the same time. This difficulty can be partly
overcome by orienting the listener's microphones, especially if
they are of the directional type, as described, for example, in
U.S. Pat. No. 3,770,911, so that they pick up the desired signal to
a greater extent that the undesired signals because of acoustical
reasons. An additional advantage, however, can be provided if the
listener were to place a microphone nearer the source of the
desired signal which would increase the intensity of this signal at
the microphone pick-up relative to that of the other signals that
are present. Under many social circumstances it would be
appropriate to accomplish this without obvious and awkward
movements on the part of the listener using a hearing aid with such
a movable microphone.
A common problem of hearing aids that are designed to provide large
amounts of signal gain for persons with unusually large amounts of
hearing loss is that some of the output of the earphones of the
hearing aid "leaks" or feeds back either by air or by mechanical
paths, to the microphone of the hearing aid. This feedback causes a
cyclic reamplification or oscillation that leads to complete
overloading of the hearing aid causing it to "squeal" and be
obnoxious and useless to the user. A procedure for reducing a
related type of oscillation, but in the different application and
requirements of public-address systems operated in a reverberant
room, has been described by M. R. Schroeder, "Improvement of
Acoustic-Feedback Stability by Frequency Shifting," J. Acoust.
Soc., 36, 1718-1724, (1964).
In this procedure, the airborne signal picked up by the microphone
is shifted, by well-known modulation techniques, either upwards or
downwards by about 5 to 10 Hz before it is presented to the
acoustic output transducers or loudspeakers of the public address
system. This shift in frequency is not sufficient significantly to
interfere with the audible quality of the signal, particularly if
the signal is speech, coming from the loudspeakers but does allow
the output signal to reach levels about 10 dB higher without
causing feedback oscillation than is possible without the
application of the frequency shift processing. This frequency
shifting process, properly critically adapted, has not heretofore
been utilized for the prevention or reduction of either the
mechanical linkage or the acoustic airborne feedback that may be
present in such hearing aids. Indeed, it is to be noted that in
earlevel hearing aids wherein the microphone and earphone are
mounted in the same case or are mechanically linked through tubes
or wires, the oscillation present in high-gain hearing aids is more
often caused by the mechanical vibrations than the airborne. It is
readily appreciated, however, that shifting the frequency coming
from the earphone will tend, to a significant degree, to prevent
the vibrations in the mechanical connection between the earphone
and microphone from progressively enlarging, that occurs when the
gain of the amplifier of the hearing aid is cyclically reapplied to
the same input/output frequency. In brief, the input signal cannot
be added to itself following amplification by the hearing aid and
feedback, as normally can cause oscillation, because the signal is
changed in frequency each time it passes through the hearing aid
system and will therefore have a waveform, of feedback, that is not
consistently in phase with the input waveform as is required,
within limits, to cause oscillation of the system.
An object of the present invention, accordingly, is to provide a
new and improved method of and apparatus for electronic hearing
aiding that shall not be so subject to the above-described
limitations and disadvantages of prior techniques, but that, to the
contrary, significantly increases noise vs. speech signal
discrimination, particularly useful for sensori-neural deafness
problems and the like.
A further object is to provide a novel hearing aid or similar
improved speech or related apparatus of more general character;
other and further objects being later discussed and more
specifically delineated in the appended claims.
In summary, the present invention provides real-time operation with
special automatic gain control signal processing for both the
overall signal and also for different parts of the speech spectrum
in ways that can be adjusted to best suit the needs of individual
sensori-neural deafened ears that suffer different degrees and
patterns, as a function of frequency, of hearing deficiencies. The
aid of the invention provides means of inserting one or more fixed
increases in linear gain to segments of the speech signal that fall
below given levels relative to the gain provided to segments that
fall above given levels. The amount of increased gains and the
given levels below which they are to be inserted may be set
separately for each of the different parts filtered from the speech
spectrum. Further, the invention will automatically discriminate
between segments of the signal that constitute speech sounds and
those segments consisting of background noise and will apply extra
gain to the speech semgents, but not to the noise segments. The
invention also provides for so-called bi-ear listening where the
treatment of the signal for each of the ears of the listener can be
somewhat different, and further provides for pick-up, if desired,
by two microphones of a stereo signal, in order to utilize the
information found in so-called phase differences between speech and
other signals as present at two microphones; one placed at the
position or pick-up region of each ear. Further, the hearing aid of
the invention provides for an optional "remote" microphone that can
be used for pick-up of signals at points at a farther-than-normal
distance from the user, i.e. closer to the sound source. Further,
the aid of the invention provides for an optional electronic
frequency-shift of the signal picked up at the microphone so that
the signal output at the earphones is at a somewhat different
frequency (about 10 Hz) than the signal picked up by the microphone
either by airborne or mechanical agitations.
The invention will now be described with reference to the
accompanying drawing,
FIG. 1 of which is a block diagram of a preferred apparatus
embodying the invention; and
FIG. 2 is a similar diagram of a suitable NLGC (non-linear-linear
gun control) apparatus for use in the system of FIG. 1.
In addition to the optional remote microphone, so-labelled, two
microphones (left and right) are indicated in FIG. 1 as the typical
pick-up sources of the signal input, although the system could
operate with even but one microphone. While the microphones may be
nondirectional, they are preferrably of the directional type, such
as those described in said U.S. Pat. No. 3,770,911. One microphone
will normally be located near or on the right ear, and one on or
near the left ear; and their two inputs are inter-connected at the
input of an automatic gain control circuit, labelled AGC No. 1. The
optional "remote" microphone may be worn strapped to the user's
wrist so that it appears as a wrist watch or bracelet and can be
placed closer to a desired signal source by movement of his hand
and arm, or it may be incapsulated in a pen or pencil type case,
not shown, that can be laid on a conference table with a
retractable cord extending to the hearing aid amplifier. The
amplifiers, batteries and associated electronics of the apparatus,
moreover, may be enclosed in a case worn in a clothes pocket of the
user or attached to his or her body or clothing in any convenient
manner.
The major purpose of the AGC circuit is to adjust the over-all
speech signal to an intensity level for the filtering and
additional automatic gain control processes to follow, such that
the subsequent system will not overload, but yet will be at a level
adequate to give proper signal transmission. Generally, at a
distance of about three feet from a talker, the weaker speech
sounds in conversational speech are at a level of about 20 dB re
0.0002 microbar, and the more intense speech sounds in a
conversational speech signal are of the order of 60 dB. A dynamic
range of about 40 dB is accordingly present in a speech signal
uttered at a constant and conversational level of effort. When the
listener is closer to the talker, furthermore, or when the talker
uses a higher than normal effort of speaking, the level of the
speech signals may go up to 100 decibels or so.
The AGC circuit is adjusted to provide a decrease in gain when the
signal envelope is above a specified level (typically 60 dB) for
approximately 0.001 seconds. Conversely, whenever the envelope
level falls below a specified level (typically 60 dB) for
approximately 0.02 seconds, the gain of the system automatically
assumes its normal state of gain and treats signals between about
20 to 60 dB input in a substantially linear fashion. The decreases
in gain effected by AGC are proportional to the degree to which the
speech envelope (averaged over about 0.001 second) exceeds the
level equivalent at that point in the system to a speech level of
about 50 dB at the input to the microphone(s). Thus, AGC adjusts
the average gain so that speech at an intensity greater than about
50 dB at the microphone(s) will generally be placed within the
optimum operation region of the filters and associated electronic
components that follow, as hereinafter described.
The signal from AGC may be fed to the frequency shifter section, if
used, shown at FS, prior to being fed to Sections 1, 2, 3, and 4 of
the hearing aid as shown in FIG. 1. Such a frequency-shifter FS, by
means of standard RF modulation and demodulation techniques, as of
the type disclosed in said Schroeder article, for example, shifts
the frequency spectrum of the signal picked up at the microphone by
about 10 Hz. Accordingly, the frequency spectrum coming from the
earphones of the hearing aid is shifted from its location on the
frequency spectrum, from the location it occupied when picked up by
the microphone, increasing by about 10 dB the tolerable level of
the level of output from the earphones that can be reached before
acoustic feedback between the earphones and microphones causes
oscillation in the hearing aid amplifiers. It is to be further
noted that this frequency shifting process will also tend
significantly to reduce the vibrations that may be set up in the
mechanical linkage between the earphone(s) and microphone(s) of the
hearing aid, said vibrations, if sufficiently strong, being a
source of causing oscillation and overload in th hearing aid.
The signal from AGC or the optional frequency shifter is fed to and
processed by Sections 1, 2, 3, and 4 of the hearing aid, as shown
in FIG. 1. Section 1 passes a broad band of frequencies and each of
sections 2, 3 and 4 contains a narrow band filter of different
adjacent bands, as later explained. Section 1 transmits the
broadband signal over the range of about 200 Hz to 7000 Hz to the
listener, with adjustment of its level made possible by means of
variable gain amplifiers 1A, 1B, LE, and RE. To this broad-band
signal are added the outputs of Sections 2, 3, and 4, which have
broadly similar functions but are individually adjustable in
several regards. The purpose of these sections is to separate or
filter the speech or other acoustic signals into relatively narrow
bands of frequencies so that the respective bands can be
amplitude-processed and gain-adjusted in ways that will enhance the
understandability of speech and other acoustic signals for persons
with sensori-neural deafness. As indicated above, in certain
regions, usually the higher frequency regions, the ear with
sensori-neural deafness will usually have a usable dynamic range of
but 10 dB or so between levels that are inaudible and levels that
overload the ear, as compared with a usable dynamic range of more
than 60 dB for the normal ear. At other frequency regions, the
dynamic range may be greater or less, depending on the particular
pattern of damage to the sensori-neural receptors in the inner ear.
The purpose of the Sections 2, 3, and 4 is to provide the means of
processing the different frequency bands of speech to the degree
and in the way best suited for the hearing characteristics of a
given ear suffering sensori-neural deafness, and to add these
specially processed frequency bands to the normal, broadband signal
being transmitted by Section 1, as shown in FIG. 1. It is to be
understood that for some sensori-neural deaf ears, fewer or more
than four such sections of signal processors will be required, or
that the bandwidths of one or more of the sections indicated may be
changed, and that the non-linear gain control processing to be
later described may be inactivated in given sections.
The description to follow of the functioning of Section 4 of FIG.
1, for example, will suffice to explain also the operation of
Sections 2 and 3, except that the frequency-bands, the amplitude
levels to which the gain is specially adjusted, and the following
gain settings may be at different values for each section.
The band-pass filter of Section 4 separates the energy in the
frequency band 2500 to 7000 Hz from the total spectrum of the
signal. The output of this band-pass filter is then passed through
a nonlinear-linear compresser gain control (NLGC). The amount of
signal compression is set to be suited to the loss in a given ear
in dynamic range of hearing ability for sounds in the frequency
band 2500 to 7000 Hz. The NLGC operates such that when the signal
is, for about 0.005 seconds, below a given level, an additional
amount of signal energy is applied to the signal energy in the
frequency band 2500 to 7000 Hz.
The output of this NLGC circuit is then further amplified in
separate split paths containing amplifiers 4A and 4B, if necessary,
to meet possible differences in sensitivity between the left and
right ears of the listener.
Sections 2 and 3 are also individually separately adjusted to
provide the degree, if any, of signal dynamic range compression
best suited for optimizing the reception and understandability of
signals, especially speech, as determined by the contributions of
the several respective different frequency bands 750-1500 Hz and
1500-2500 Hz. The outputs of these three sections are then split
into pairs and combined through resistor networks with the
broad-band signal from Section 1 for presentation to the listener,
with all the right and left ear paths of the NLGC outputs being
connected together, respectively.
It is to be understood that the specific number of filter sections
and the cut-off frequencies given in FIG. 1 are illustrative only,
and that greater or fewer sections and different cut-off
frequencies may be used in various specific applications of this
invention. Further, the use of separate output amplifiers for each
of the two ears is often not required, because both ears of a
person suffering sensori-neural deafness often have similar
characteristics.
In accordance with the present invention, the NLGC part of the
hearing aid, with its speech-noise discrimination operation, may be
of the form illustrated in FIG. 2 for operation, for example, in
Section 4 of FIG. 1. In FIG. 2, when the input signal envelope is
between 50 to 60 dB or greater, Gate 1 remains closed and these
time segments of the signal pass directly through towards the
output, so-labelled, without the emphasis or extra gain available
from amplifier B. When the signal envelope falls to a value
indicating that the input signal is below 50 dB, Gate 1 opens and
the signal in Path B (which has been amplified by amplifier B by a
given amount relative to the level in Path A) is added to the
signal present in Path A, provided that Gate 2 is also open. Gate
2, by means of the attack and release time control 2' is open when
the signal envelope is more than the illustrative 50 dB; however,
when the signal remains below 50 dB for more than 0.5 sec., Gate 2
closes, thereby preventing further gain-emphasized signal segments
coming through Gate 1 from reaching Path A. Accordingly, the extra
emphasis or amplification given to the signal by amplifier B is not
added to the signal in Path A. Amplifier B is adjustable so that
the amount of extra emphasis given to the signal, relative to its
level in Path A, can be varied to best meet the needs of different
degrees of hearing loss.
Rectifier R, amplifier 1" and attack-release time control elements
1' and 2' perform the following functions: rectifier R provides a
means of making the negative parts of the signal continuum positive
in voltage; and amplifier 1" is adjustable and provides a means for
adjusting the rectified signal continuum level reaching the
attack-release time controls 1' and 2'. Accordingly, depending on
the desires of the user during a given input signal-noise
condition, the signal continuum level can be increased or decreased
from rectifier, R, so that the attack-release controls 1' and 2',
which affect Gates 1 and 2, respectively, and which are set to
operate at specified voltages, will be activated with different
signal-continuum levels at the input to the microphones. Thus,
amplifier 1" provides a means of causing the gates to be activated
with lesser or greater input signals at the microphone as will be
advantageous to persons with different degrees of sensori-neural
deafness.
The purpose of the described double-gate action is to give the
weaker, short duration (less than 0.5 sec) segments of the signal
the extra amplification or emphasis relative to the strong
intensity segments of the speech signal; but not to give this extra
amplification to relatively low intensity background noise which is
typically more steady-state than the speech signal. This background
noise may continue at a level below, say, 50 dB for much longer
duration than 0.5 sec. and is especially objectionable to persons
using hearing aids that provide automatic gain compression that
typically increases the relative intensity of this background
noise.
It is to be understood that for some types of speech or other
signals, the attack and release times for the operation of Gates 1
and 2 may be changed for optimum results from those specified in
FIG. 2. It is also to be noted that the NLGC processing system
herein described has other possible applications beyond that in
hearing aids where it is desirable to provide relative emphasis or
de-emphasis to different segments of electronic signals that
dynamically vary in intensity in somewhat predictable ways such
that its use, while particularly adapted to the present invention,
is also applicable in other signal processing systems wherein
similar advantages are sought.
Suitable components for the circuit elements are as follows: Gates
1 and 2 may, for example, be of the Field Effect Transistor (FET)
type, described in Electronic Principles by Malvino, McGraw-Hill,
New York, 1973: attack and release time control circuits 1' and 2'
may be of the operational amplifier type with appropriate
capacitive and resistive feedback elements, as described in the
same publication. Clearly, other types of well-known circuits may
be similarly employed, and further modifications, within the spirit
and scope of the invention, will suggest themselves to those
skilled in this art.
* * * * *