Note: Descriptions are shown in the official language in which they were submitted.
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SO 1373
BACKGROUND OF TIIE INVENTION
Field of the Invention
This invention relates generally to information
signal recording and reproducing apparatus and, more
particularly, is directed to a noise reduction circuit
for reducing noise generally accompanying a reproduced
information signal in an information signal recording and
reproducing apparatus.
Description of the Prior Art
Noise reduction circuits for reducing noise and
distortion which accompany a reproduced information signal
are well-known in the art. Such noise reduction circuits
are designed to increase the dynamic range of the signal
that can be recorded and reproduced from a recording medium
such as a magnetic tape. In a typical noise reduction
circuit, an encoder is provided for those signals which are
to be recorded, and a complementary decoder is provided for
those signals which are reproduced. The encoder generally
includes a level compression circuit and a high frequency
pre-emphasis circuit, wherein higher frequency components of
an information signal to be recorded are emphasized with
the emphasis level being inversely related to the information
signal level. The decoder generally includes a level expansion
circuit and a high frequency de-emphasis circuit to perform
a complementary operation on the information signals which
are reproduced.
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For example, in the Dolby noise reduction system, a
low level input signal is amplified with a substantially constant
gain until that input signal reaches a predetermined level.
Thereafter, the amplification of that input signal is
reduced until yet another, higher level is reached, whereupon
amplification with substantially constant gain is carried
out once again. In addition to such amplification of the
input signal prior to recording, an emphasis circuit is used
in order to pre-emphasize the high frequency components of
the input signal. This operation generally is referred to
as signal compression. After the input signal has been
suitably compressed, it is recorded. A complementary
signal expansion process is carried out when the afore-
mentioned signal is reproduced, that is, the pre-emphasized
high frequency components are de-emphasized, and the de-
emphasized signal is amplified with a gain less than unity.
This gain is substantially constant over a predetermined
range of relatively low signal levels, and when the reproduced
signal exceeds a predetermined level, the gain is increased
until a still high`er level is reached.
The aforementioned Dolby ~ noise reduction system
is of a relatively simple construction and has been used
extensively in home entertainment systems, such as magnetic
tape recorders and/or reproducers and the like. However,
although the Dolby system results in some improvement in
the dynamic range of the tape recorder and/or reproducer,
this improvement generally is limited to be of the order of
about 10 dB; and this improvement is apparent primarily in
the frequency region which exceeds 1 KHz. Furthermore, the
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aforementioned changes in the gain of the level compression
and level expansion amplifiers are non-linear, and because
of this non-linearity in the gains, level matching between
the encoding and decoding processes often is difficult. Thus,
some distortion may be apparent for those signals having
intermediate signal levels.
Another noise reduction system is the so-called
DBX ~ system, which is described in U.S. Patent No.
3,789,143. One advantage of the DBX system over the afore-
mentioned Dolby system is that the gains of the amplifiers
which carry out the signal compression and signal expansion
operations, that is, the signal compression and signal
expansion ratios, are substantially constant, regardless of
the signal level of the input information signal. For
example, prior to recording the information signal on a magnetic
tape or the like, the input information signal is compressed
with a constant compression ratio k. When the compression
signal is subsequently reproduced, the reproduced signal
is expanded with a~constant ratio l/k, that is, with an
expansion ratio which is the reciprocal of the compression
ratio. Since constant compression and expansion ratios are
used throughout the entire signal level range, the non-linearity
found in the Dolby ~ system is avoided and level matching
between recorded and reproduced signals is easily attained.
Moreover, in the DBX system, the apparent improvement in
the dynamic range of the tape recorder and/or reproducer is
of the order of about 40 dB and desirable noise reduction is
achieved over substantially the entire audio frequency range
of 20 Hz to 20 ~Hz.
1 1581~2
However, the particular compression and expansion
characteristics of the aforementioned no:ise reduction systems
generally are obtained primarily for constant input signal
levels, that is, signal levels which do not undergo abrupt
transients. Stated otherwise, the advantages attained by
these noise reduction systems are a function primarily of
the static characteristics thereof. However, difficulties
are presented in the dynamic transient characteristics of
such systems. For example, if an information signal to be
recorded exhibits a relatively low signal level, the gain, or
compression ratio, of the encoder amplifier may be relatively
high. Now, if this information signal undergoes an abrupt
increase in its signal level, that is, it undergoes a large
positive transient, the gain of the amplifier, or compression
ratio, will not be reduced as rapidly as the rate at which
the signal level increases. Hence, although the gain, or
compression ratio, should be reduced when processing the
high level information signal, in actuality it remains at
its prior high level. Consequently, the strong transient
is amplified with relatively large gain, thereby resulting in
a compressed signal that exhibits "overshoot", that is, the
level of the compressed signal is far too large. This high
level signal, when recorded, results in saturation of the
magnetic medium, thereby causing distortion in the signal
which is recorded and in the information which ultimately
is reproduced therefrom.
Another disadvantage of the aforementioned noise
reduction systems is that they may be subject to so-called
noise modulation. With noise modulation, noise components
are varied as a function of input signal level variations.
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l l5~172
Such changes in the noise components, or noise modulation, is
hlghly perceptible and is quite distracting when it accompanies
a reproduced audio signal. This phenomenon is pronounced
when the frequency components of the input`signal are
noticeably different Erom the noise frequency component.
For example, if the information signal is an audio signal
representing the sound of a piano, noise modulation is heard
separately and distinctly, and is not masked even if the
volume level of the information signal is increased.
One proposal for reducing noise modulation in a
noise reduction circuit is described in U.S. Patent No.
4,162,462. In this proposal, the high frequency components
of the information signal are pre-emphasized prior to
recording when the information signal exhibits low and
medium signal levels, and relatively little pre-emphasis
is provided when the information signal exhibits higher
levels. When the information signal processed in the
foregoing manner is reproduced, the high frequency components
are subjected to relatively high de-emphasis when the
reproduced signal exhibits low and medium signal levels,
and these high frequency components are subjected to
relatively low de-emphasis when the reproduced signal is at
a higher level. Although this proposal reduces the undesired
effects of noise modulation, saturation of the magnetic
record medium due to overshoot in the compressed signal
nevertheless is present. .
In order to overcome the aforenoted disadvantage
presented by overshoot, it also has been proposed to increase
the speed of response of the level compression circuitry.
However, if the response speed is increased, an improvement
1 7 2
in eliminating overshoot i5 accompanied by deterioration in
the noise modulation characteristic. Another proposal in
preventing overshoot isdescribed in copending Application
Serial No. 352,484 filed May 22, 1980. As wi11 be
apparent from the ensuing description, the present invention
is an improvement over the noise reduction circuit described
in this copending application.
Another proposal of a noise reduction circuit which
minimizes overshoot contemplates the use of a plurality of
substantially similar noise reduction circuits connected in
parallel. Each noise reduction circuit is intended to
operate over a selected portion of thé frequency spectrum
of the input information signal. The outputs of these
individual noise reduction circuits are combined, or mixed,
resulting in an overall level compressed information signal
suitable for recording. However, the use of a plurality of
parallel connected noise reduction circuits is relatively
complex and expensive. For example, if _ such noise
reduction circuits are used, the overall cost of the noise
reduction system is n times the cost of a noise re2uction
system in which only a single noise reduction circuit is used.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, it is a principle object of this
invention to provide an improved circuit for noise reduction
that avoids the above-described difficulties encountered with
the prior art.
More particularly, it is an object of this invention
to provide a circuit for noise reduction that can be used in
an information signal recording and/or reproducing system.
1 1 5 ~ 2
It is another object of this invention to provide
a circuit for noise reduction -that can be used in an encoder
to level compress an information signal prior to the recording
thereof, and also can be used in a decoder to level expand
the reproduced signal, thereby expanding the apparent dynamic
range of the recording and reproducing system by a factor of
the order of about 20 to 30 dB.
It is still another object of this invention to
provide a circuit for noise reduction which produces variable
pre-emphasis and de-emphasis without any external manual
adjustment thereof.
It is yet another object of this invention to provide
a level ~expansion circuit for noise reduction, which
circuit can be used with a coring circuit without adversely affecting
circuit operation, thereby preventing transient saturation
of the magnetic recording medium due to overshoot.
It is a further object of this invention to provide
a level compression circuit the transfer characteristic of
which is more frequency-sensitive for low level input signals
than for high level input signals, thereby providing greater
pre-emphasis for input signals having relatively low levels.
It is a still further object of this invention to
provide an improved level expansion circuit the transfer
characteristic of which is more frequency-sensitive for low
level input signals than for high level input signals, such
that greater de-emphasis is obtained for relatively low level
signals which are reproduced from a record medium.
It is a yet further object of this invention to
provide an improved compression/expansion circuit which can
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1 1~8;~2
be switchably connected so as to provide a level compression
function when used with a signal recorder and to provide
a level expansion function when used with a signal reproducer.
It is another object of this invention to provide a
circuit for noise reduction that is simple in construction
and inexpensive.
In accordance with an aspect of this invention, a
circuit for noise reduction includes a first signal path
including variable gain amplifier means supplied with a
subtraction signal for amplifying the subtraction signal with
controllable gain, and means for providing substantial de-
emphasis of the high frequency components of the amplified
subtraction signal from the variable gain amplifier means relative
to the low frequency components thereof to produce an output signal;
a second signal path for providing, at most, relatively minor
de-emphasis of the high frequency components of the output
signal relative to the low frequency components thereof;
subtracting means for subtracting the output of the second signal
path from an input information signal and for producing the
subtraction signal~in response thereto; and means for controlling
the gain of the variable gain amp.lifier means to exhibit higher
gain when the signal level of the input information signal is
relatively high and to exhibit lower gain when said signal level
is relatively low.
The above, and other, objects, features and
advantages of the present invention will become readily
apparent from the ensuing detailed description of the
illustrative embodiments of the invention which is to be
read in connection with the accompanying drawings.
1 1 ~ 2
BRIEF DESCRIPTION OF THE DRAWINGS
Figs. 1 and 2 are graphical representations of
the compression/expansion characteristics of two prior art
noise reduction circuits;
Fig. 3 is a block diagram OI a decoder circuit
according to a basic embodiment of the present invention;
Fig. 4 is a block diagram of a more detailed,
practical embodiment of the decoder circuit of Fig. 3;
Fig. 5 is a graphical representation of the level-
frequency characteristic of the decoder circuit of Fig. 4
for various input levels;
Fig. 6 i5 a graphical representation of the input-
output characteristic of the decoder circuit of Fig. 4 for
various frequencies;
Fig. 7 is a block-circuit wiring diagram~illustrating
one embodiment of a circuit construction of the decoder
circuit of Fig. 4; and
Fig. 8 is a block-circuit wiring diagram showing
the use of the decoder circuit of Fig. 4 as an encoder or
decoder in a noise. reduction system.
,:
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings in detail, and initially
to Fig. 1 thereof, there is shown a graphical representation
of the compression/expansion characteristic of the afore-
mentioned Dolby ~ ,noise reduction system in which the input
and output signal levels are in units of decibels. Curve R
in Fig. 1 represents the level-compression,input-output
characteristic and curve P represents the corresponding level-
:
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r
expansion, input-output characteristic for the Dolby ~ noise
reduction system. It is seen that, for input signals
having a relatively low level, substantially uniform gain
greater than unity is provided until an intermediate level
is reached, for example, between -30 dB and O dB, whereupon
the linearity of the level-compression, input-output
characteristic no longer holds true. It should be appreciated
that it is this non-linear feature which makes level matching
between the decoding and encoding processes difficult, as
previously discussed. The dot-dash curve in Fig. 1
represents the so-called "flat bass" in which input and
output signal levels are constant both for level compression
and level expansion.
Fig. 2 is a graphical representation of the level-
compression, input-output characteristic R and the level-
expansion, input-output characteristic P of the aforementioned
DBX noise reduction system. It should be appreciated that,
in this system, the compression and expansion ratios are
substantially constant throughout the entire input signal
level range. Here too, the dot-dash curve represents the
"flat bass" response. However, as previously mentioned, the
Dolby~ and DBX systems suffer from disadvantages in their
~ynamic transient characteristics which are overcome by the
present invention.
Referring now to Fig. 3, there is shown a noise
reduction circuit 10 according to a basic embodiment of this
invention for reducing noise in a reproduced information
signal. As shown therein, noise reduction circuit 10
includes an input terminal 1 supplied with a reproduced
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information signal, for example, an audio signal reproduced
from a magnetic tape and, which in turn, supplies the
reprodueed information signal to an adding input of a
subtracting circuit 2. A first signal path includes a
variable gain amplifier 3 supplied with the output signal
from subtracting circuit 2 and having its gain controlled
by the reproduced information signal from input terminal 1
after being rectified and smoothed (not shown in Fig. 3).
The output of variable gain amplifier 3 is coupled to a low
pass filter 4, also in the first signal path, in which high
frequency components of the signal supplied thereto are
de-emphasi-zed. The resultant signal from low pass filter 4
is then supplied to an output terminal 5 and to a subtracting
input of subtracting circuit 2 through a second signal path
which is shown to include a Iow pass filter 6.
Variable gain amplifier 3 is adapted to amplify
the output signal from subtracting circuit 2 by a variable
gain. In particular, the gain G of variable gain amplifier
3 is determined by a gain control voltage V which is derived
from the information signal at input terminal 1 after being
reetified and smoo`thed, as previously mentioned. The gain
G of variable gain amplifier 3 is designed to increase with
nereases in the gain control voltage Vc, for example, in
KV
aecordanee with the relationship G = K Vc or G = e c, where
K is a eonstant. In this manner, the gain of amplifier 3 is
direetly related to the level of the information signal from
input terminal 1 such that the gain thereof is relatively
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;; `` 115~1 12
h1gh when the level of the information signal from input terminal
1 is relatively high and, conversely, the gain thereof is
relatively low when the level of the information signal from
input terminal 1 is relatively low. Thus, variable gain
amplifier 3 attains level expansion of the signals supplied
thereto.
Low pass filter 4, which is also included in the
first signal path, is adapted to provide substantial de-
emphasis of the high frequency components included in the
output signal from variable gain amplifier 3. As stated
otherwise, low pass filter 4 effectively provides a substantial
pre-emphasis to the low frequency components of the signal
supplied thereto with respect to the high frequency components.
For example, the high frequency components are preferably
de-emphasized relative~to the low frequency components by a
factor of the order of about 20 dB.
Low pass filter 6 which constitutes the second signal
path provides relatively minor de-emphasis of the high frequency
components of the output signal from low pass filter 4 relative
to the low frequency components. For example, the high frequency
components may be reduced, or attenuated, relative to the low
frequency components by a factor of the order of about 6 dB.
By connecting low pass filter 6 to the output of low pass-filter 4,
it has been found that a smoother overall de-emphasis characteristic
can be obtained over that of the aforementioned copending Application
Serial No. 352,484 and the circuitry can be simplified.
Alternatively, low pass filter 6 may function merely as an
attenuator so as to provide uniform de-emphasis for the entire
frequency range, that is, for both high and low frequency components.
~owever, low pass filter 6 preferably provides a minor de-emphasis
characteristic to the high frequency components of the
signal supplied thereto. In such case, since the output of
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:'
low pass filter 6 is subtracted from the information signal
from input terminal 1 in subtracting circuit 2, this latter
circuit effectively provides a minor pre-emphasis characteristic
to the high frequency components of the information signal.
It should therefore be appreciated that the
input-output characteristic of noise reduction circuit 10
depends on which of the first and second signal paths is
dominant. More particularly, it is to be remembered that
the input-output characteristic of subtracting circuit 2
results in a slight or minor pre-emphasis of the high
frequency components of the signal supplied to variable
gain amplifier 3. For example, the level of the high frequency
components may be only a few dB greater than the level of the
low frequency components. When the level of the information
signal supplied to input terminal 1 is of a low level, the
gain of variable gain amplifier 3 is relatively low. This,
of course, does not result in much change between the levels
of the high and low frequency components so that the high
frequency components of the amplified signal supplied to low
pass filter 4 have~a level only a few dB greater than that
of the low frequency components. Since low pass filter 4
provides a substantial de-emphasis of the high frequency
components of the signal supplied to it, for example, of the
order of abou-t 20 dB, as previously discussed, the output
signal from low pass filter 4, which is supplied to output
terminal 5, has its high frequency components substantially
de-emphasized relative to the low frequency components thereof.
In other words, the first signal path comprised of variable
gain amplifier 3 and low pass filter 4 provides a dominant
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action and the affect of the second signal path constituted
by low pass filter 6 is minimal on the signal supplied to
output terminal 5.
This substantial de-emphasis of the high
frequency components of the siynal supplied to output
terminal 5 decreases with increasing signal level of the
information signal at input terminal 1. Thus, when the
level of the information signal supplied to input terminal
1 is high, the gain of variable gain amplifier 3 is also
high. Since the signal supplied to variable gain amplifier
3 has its high frequency components slightly pre-emphasized
relative to its low frequency components, amplification by
variable gain amplifier 3 results in a substantial pre-
emphasis of the high frequency components relative to the
low frequency components, for example, greater than 20 dB.
In other words, variable gain amplifier 3 provides an expansion
characteristic to the signal supplied thereto. Thus, when low
pass filter 4 provides a substantial high frequency de-emphasis
characteristic to the signal supplied thereto, for example,
of the order of about 20 dB, the resultant signal supplied
to output terminal 5 has a slight or minor high frequency
pre-emphasis characteristic. It should therefore be
appreciated that, for high level information signals, the
second signal path plays an important or dominant role in
determining the output characteristic of the signal supplied
to output terminal 5.
Referring now to Fig. 4, there is shown a practical
embodiment of noise reduction circuit 10, with elements
corresponding to those described above with reference to the
' 2
circuit of Fig. 3 being identified by the same reference
numerals. As shown therein, variable gain amplifier 3 is
comprised of a voltage controlled amplifier (VCA) 31 and
a summing circuit 33 supplied with the output from voltage
controlled amplifier 32 at one input thereof and supplied
with the input signal to voltage controlled amplifier 32
at another input thereof through a resistor 32. The
combination of VCA 31 and summing circuit 33 provides a
gain to the signal supplied thereto which is dependent on
the level of the information signal at input terminal 1.
In other words, low level signals supplied thereto are
subjected to a low gain and high level signals supplied
thereto are subjected to a high gain.
Noise reduction circuit 10 in Fig. 4 is further
provided with a gain control circuit 7 for controlling
the gain of VCA 31 and is comprised of a weighting circuit
71 coupled to input terminal 1 and a rectifier and
smoothing circuit 72 for rectifying and smoothing the output
of weighting circuit 71 and supplying the same to VCA
32 as a gain control voltage therefor. Weighting circuit
71 exhibits a high pass filter characteristic which, in one
example, may be directly opposite to the high frequency
de-emphasis characteristic of low pass filter 4. In other
words, weighting circuit 71, in such case, provides a pre-
emphasis characteristic of the high frequency components of
the signal supplied thereto relative to the low frequency
componehts. Alternatively, it may be possible to connect gain
control circuit 7 to the input cf ~'CA 31 rather than input terminal
1, although in such case, changes may have to be made with respect
to weighting circuit 71.
Further, noise reduction circuit 10 in Fig. 4
includes a coring or anti-limiting circuit 34 between
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subtracting circult 2 and VCA 31. When the level of the
signal supplied to anti-limiting circuit 34 is of a low
level, anti-limiting circuit 34 virtually has no effect on
the signal. However, when such signal level is high, that
is, exceeds a predetermined level, the signal supplied
thereto is expanded, resulting in a further expansion of
the signal supplied to variable gain amplifier 3. Anti-
limiting circuit 34, as will be discussed hereinafter in
greater detail, is provided for use in decoder or noise
reduction circuit 10 and in a complementary`encoder circuit
to prevent, or limit, transient overshoot of the signal to
be recorded on the magnetic tape and which results in
distortion of the signal due to saturation of the magnetic
tape.
As before, low pass filter 4 is adapted to provide
substantial de-emphasis to the high frequency components,
relative to the low frequency components, included in the
signal supplied thereto from summing circuit 33. Also,
low pass filter 6, as previously discussed, is adapted to
provide relatively minor de-emphasis to the high frequency
components of the signal supplied thereto relative to the
low frequency components.
Fig. 5 is a graphical representation of the level
expansion characteristics of noise reduction circuit 10
of Figs. 3 and 4. The abscissa in Fig. 5 represents the
frequency of the information signal at input terminal 1
and the ordinate represents the output signal level at output
terminal 5 in terms of decibels. Each curve in Fig. 5
represents a particular signal level. It is seen that,
when the level of the information signal is relatively low,
the high frequency de-emphasis characteristic of the first
signal path is dominant in determining the characteristic
of the output signal. In other words, for such low level
signal, the gain of VCA 31 is also relatively low so that
the slightly pre-emphasized high frequency signals supplied
thereto remain with their high frequency components slightly
pre-emphasized. However, since low pass filter 4 provides
a substantial high frequency de-emphasis to the signals
supplied thereto, such low level signals have theirhlghfrequency
eomponents substantially attenuated. This is shown
more particularly by the lower three eurves in Fig. 5,
eorresponding to input levels Vin of the information signal
of -30, -40 and -50 dB, respectively. It should be
appreeiated that anti-limiting circuit 34 has substantially
no effect on low level signals supplied thereto and therefore
does not substantially affect the output-frequency
charaeteristie of sueh signals.
When the level of the information signal
supplied to input terminal 1 is high, subtraeting eireuit
2 supplies a high level signal with its high frequency
eomponents slightly pre-emphasized over the low frequeney
eomponents to anti-limiting eireuit 34. As previously
diseussed, for high level signals, anti-limiting cireuit 34
aets as an expansion eireuit so as to expand the signal
supplied thereto. In other words, the difference in level
between the high frequency and low frequency components is
further increased. The expanded signal is then supplied to
VCA 31. Since the gain of VCA 31 is controlled by control
eircuit 7 to be high at such time, the expanded signal from
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anti-limiting circuit 34 is further expanded by variable
gain amplifier 3 to provide a greatly expanded signal, the
high frequency components of which are a-t a much greater
level than the low frequency components. This greatly
emphasized signal is supplied to low pass filter 4 which
substantially de-emphasizes the high frequency components.
For high input levels Vin of the information signal equal to
0 and 10 dB, the expansion by anti-limiting circuit 34 and
variable gain amplifier 3 generally results in a signal
the high frequency components of which are emphasized with
respect to the low frequency components by greater than 20 dB.
Since low pass filter 4 substantially de-emphasizes the
high frequency components of the signal supplied thereto,
but only on the order of about 20 dB, the signal supplied
to output terminal 5, as shown in Fig. 5, has its high
frequency components slightly pre-emphasized with respect to
the low frequency components thereof. The dividing line
between pre-emphasis and de-emphasis of the high frequency
components of the signal supplied to output terminal 5
is shown in Fig. 5 to correspond to an input signal level
Vin of -10 dB. In other words, for input levels Vin less
than -10 dB, the first signal path, and in particular, low
pass filter 4 thereof, has a dominant effect on the signal
supplied to output terminal 5. However, for high levels
of the information signal, that is, for Vin greater than
-10 dB, the affect of low pass filter 4 is greatly diminished
so that the slight pre-emphasis which results from low
pass filter 6, and which is greatly amplified or expanded
by variable gain amplifier 3, provides a dominant effect on
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the signal supplied to output terminal 5.
The level expansion characteristics of noise
reduction circuit 10 for signals of frequencies 100 Hz, 1 KHz
and 10 KHz are illustrated by the respective curves shown
in Fig. 6. It should be appreciated from this figure that
level expansion is attained over a wider range, and by a
greater degree, for high frequency components (e.g. 10 KHz)
than for low frequency components (e.g. 100 Hz and 1 KHz).
The dot-dash line shown in Fig. 6 represents the usual
flat bass response.
It will be seen from the foregoing that the noise
reduction circuit in accordance with the present invention
provides variable de-emphasis, that is, different de-emphasis
curves are obtained for different levels of the input signal.
Because of this variable de-emphasis, substantially higher
de-emphasis is provided over the high frequency range when the
input signal level is relatively low for reducing the effects
of noise modulation which are more apparent during such time.
A substantially flat de-emphasis characteristic, or a slight
high frequency `emphasis characteristic, is obtained when
the input signal level is relatively high. This is preferred
because, if the input signal level is relatively high, it
may be recorded on a magnetic medium without the need of any
pre-emphasis thereon.
The invention described in the foregoing embodiments
is of a relatively simple construction and, is therefore,
inexpensive. Nevertheless, this invention permits variable
de-emphasis to be obtained, as described above, without requiring
any external or manual adjustment. By providing large de-
emphasis over a high frequency range when the input signal
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7 2
level is low, the aforementioned phenomenon of noise modulation
is substantially reduced and, in many cases, may be effectively
eliminated. Furthermore, the present invention readily
enables an anti-limiting circuit 34, which as will be
hereinafter discussed, can be used as a limiter circuit
in a corresponding encoder section to prevent transient
saturation of the magnetic recording medium due to overshoot
caused by an abrupt, or sudden, increase in signal level which,
heretofore, could not be compensated quickly enough.
One embodiment of a circuit wiring diagram for
noise reduction circuit 10 in Fig. 4 will now be described
with reference to the illustration set out in Fig. 7, in which
elements corresponding to those described above with reference
to Fig. 4 are identified by the same reference numerals.
As shown therein, subtracting circuit 2 is comprised of two
adding resistors 2I and 22 and an operational amplifier 24.
In particular, one end of resistor 21 is connected to input
terminal 1 and the other end thereof is connected to an end
o resistor 22. The other end of resistor 22 is connected
to the output of operational amplifier 24 which acts as an
inverter to invert the output of-the second signal path so
as to produce the required subtraction signal.
The connection point between resistors 21 and 22
is connected to the inverting input of an operational
amplifier 23 which supplies the subtraction signal to the
first signal path and, in particular, to the inverting input
of an operational amplifier 35. Also included in the
first signal path is a negative feedback resistor 36 which
is connected between the output and input of amplifier 35.
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1158172
It should be appreclated by those of ordinary skill
in the art that the gain of an amplifier circuit formed of
an operational amplifier is a function of the feedback
impedance, that is, the impedance connected between the
out~put and input of the operational amplifier, divided by
the input impedance, that is, the impedance connected to the
input thereof. The gain of the amplifier circuit thus
may be adjusted by varying either the feedback impedance or
the input impedance. In the embodiment shown in Fig. 7,
the feedback impedance is fixed by feedback resistor 36,
while the input impedance, which will hereinafter be
discussed, is adjustable in response to a control signal
supplied thereto so as to vary the gain of variable gain
amplifier 3 constituted by operational amplifier 35.
In particular, the input impedance to operational
amplifier 35 is controlled by three impedance paths connected
in parallel between the output of operational amplifier 23
and the inverting input of operational amplifier 35. The
first impedance path is constituted by a resistor 37 having
a fixed resistance value; the second impedance path is
constituted by a variable resistance element 38; and the
third impedance path is constituted by anti-limi~ing circuit
34 connected in series with a resistor having a fixed
resistance value. In particular, variable resistance
element 38 hàs its resistance value changed in accordance
with the control signal from control circuit 7. As one
example thereof, variable resistance element 38 may be, for
example, a photo-responsive element, such as a CdS photo-
conductive cell, a photo-resistor, or the like, exhibiting
an impedance or resistance that is variable as a function of
the intensity of light impingent thereon. For example, the
photo-responsive element may be light coupled to a light
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emitting diode, or other light-emitting source, capable
of transmitting light to the photo-responsive element as
a function of a control voltage supplied thereto. This
control voltage is produced by control circuit 7 comprised
of weighting circuit 71 and rectifying and smoothing circuit
72. As the control voltage increases, the intensity of
the light emitted by the light-emitting source likewise
increases so as to reduce the resistance or impedance of
the light-responsive element, thereby increasing the gain of
variable gain amplifier 3. Conversely, as the control voltage
decreases, the intensity of the light emitted by the light-
emitting source likewise decreases so as to increase the
impedance of the light-responsive element, thereby decreasing
the gain of variable gain amplifier 3.
As an alternative to the aforementioned photo-
responsive element, variable resistance element 38 may be
comprised of a field effect transistor (FET), bi-polar junction
transistor, or the like, the impedance of which is controllable
in response to the control voltage supplied thereto by control
circuit 7. Thus, as the impedance of the FET or transistor
varies, the gain of the variable gain amplifier also varies.
In the embodiment shown in Fig. 7, anti-limiting
circuit 34 is illustrated as being comprised of a pair of
parallel-connected, oppositely-poled diode circuits connected
in series with a fixed resistance in the third input impedance
path. In this example, each diode circuit is formed of two
diodes in series. It should be appreciated that anti-limiter
circuit 34 acts as a coring circuit. In other words, if
each of the diodes has a cut-in voltage level Vbe equal
to 0.7 volts (for silicon diodes), then the third impedance
path conducts only when the level of the input level supplied
thereto is greater than 1.4 volts or less than -1.4 volts,
that is, only for positive or negative high level signals.
Thus, when the input signal level is low, the third impedance
path is effectively eliminated from the circuit so that the
impedance for the signal supplied to operational amplifier
35 is determined by the parallel combination of resistor
37 and variable resistance element 38. However, once the
input level exceeds the diode cut-in voltage, the illustrated
resistor in the third impedance path is connected in the
input impedance circuit and is thus disposed in parallel
with resistor 37 and variable resistance element 38,
thereby reducing the effective input impedance so as to
increase the gain of the amplifier. In other words, anti
limiter circuit 34 acts to expand the input information signal
for high levels thereof.
Low pass filter 4 is constructed by connecting a
high pass filter between the input and output of operational
amplifier 35. In particular, low pass filter 4 is comprised
of a resistor 41 and a capacitor 42 connected in a series
path across the input and output of operational amplifier 35
in parallel with negative feedback resistor 36.
The output of operational amplifier 35 is coupled to
output terminal 5 and is also coupled to the second signal
path comprised of low pass filter 6. In particular, low
pass filter 6 is formed of a pair of series-connected
resistors 61 and 62 connected between the output of
operational amplifier 35 and the inverting input of inverting
operational amplifier 24, and a capacitor 63 connected
between ground and the connection point between resistors
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61 and 62. It should therefore be appreciated that the
output of operational amplifier 35 supplied to low pass
filter 6 is inverted and added to the incoming information
signal at the connection point between resistors 21 and 22.
Control circui-t 7, which is coupled to input
terminal 1 and which is comprised of weighting circuit 71
and rectifying and smoothing circuit 72 produces the afore-
mentioned control voltage which is used to adjust the
resistance of variable resistance element 38 and thereby
adjust the gain of variable gain amplifier 3. Weighting
circuit 71 is comprised of a high pass filter formed by a
first series circuit including a capacitor 73 and resistor
74 and a second series circuit connected in parallel with the
first series circuit and including a capacitor 75 and
resistor 76. The output of these parallel-connected series
circuits is connected to an amplifier77 which, preferably, is
an operational amplifier exhibiting negative gain and having
a feedback resistor, as illustrated. It will be appreciated
that weighting circuit 71 exhibits a high pass filter
characteristic which is substantially equal to that of the
high pass filter circuit connected between the input and
output of operational amplifier 35 and which constitutes
low pass filter 4. The output of amplifier 77 is coupled to
rectifying and smoothing circuit 72, the latter being
comprised of, for example, a diode coupled to a capacitive
filter. Rectifying and smoothing circuit 72 produces a
DC control signal that is a function of the level of the
high frequency components passed by weighting circuit 71.
Thus, as previously discussed, the noise reduction
circuit in Fig. 7 is adapted to provide a variable de-emphasis
115~1'72
fu~ction whlch provides varying degrees of de-emphasis in
accordance with the level of the input signal. In this
manner, noise reduction circuit 10 of Fig. 7 provides
substantial high frequency de-emphasis for low level input
signals to effectively eliminate noise modulation as well as
permitting expansion of the dynamic range while preventing
overshoot for high level input signals. Thus, the level
expanion characteristics illustrated in Fig. 5 are obtained
by the circuit illustrated in Fig. 7.
In the embodiments of the present invention thus
far described, the noise reduction circuit has been used
as a level expansion circuit in a decoder for information
signals which have been magnetically recorded.- A level
compression circuit in an encoder should be provided with
level compression characteristics which are complementary to
the characteristics shown in Fig. 5. In this manner, the
noise reduction circuit of Fig. 4 can be used in such an
encoder, as illus-trated in Fig. 8. More particularly, noise
reduction circuit 10 is connected in the negative feedback
path of an operational amplifier 210, this operational amplifier
having a non-inverting input coupled to an input terminal 201
to receive an input signal to be recorded, and an inverting
input coupled to the output terminal 5 of noise reduction
circuit 10 of Fig. 4. The output of amplifier 210 is coupled
to input terminal 1 of noise reduction circuit 10.
Desirably, noise reduction circuit 10 is selectively .
disposed for operation either as an encoder or a decoder in a
circuit 200. To this effect, amplifier 210 is provided with
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115~
a switching element 211, schematlcally illustrated as a
me~hanical switch, havlng two swltching conditlons. When
swltch 211 engages contact _, nolse reductlon clrcult 10
is connected as a negative feedback circuit from the output
to the inverting input of amplifier 210, as described
above. When switch 211 is connected to contact _, a
feedback resistor 212 ls connected between the output and
inverting input of amplifier 210, thus establishing the
gain of the amplifier, and the output of amplifier 210 is
further connected to supply amplified information slgnals
to noise reduction circuit 10. Thus, when switch 211 is
connected to contact e, the illustrated circuit 200 functlons
asan enccder to produce level compressedlnformatlon slgnals
at output terminal 202. However, when swltch 211 ls coupled
to contact _, circuit 200 functions as a decoder to produce
level expanded slgnals at output terminal 5. As illustrated,
output terminal 5 is coupled to another output termlnal 203
which, ln turn, may be coupled to a magnetic recordlng transducer.
It is appreclated that, by using noise reductlon
circuit 10 in two switchable modes, the same circuit can
be used as an encoder and as a decoder, thus providing desirable
conservation of parts. In typical recording/reproducer apparatus,
such as in an audio tape recorder, information signals are
not recorded and reproduced concurrently. Thus, rather than
providing separate encoding and decodlng clrcultry, lt ls
advantageous to utlllze the same noise reductlon clrcult 10
for the separately performed encodlng and decodlng operatlons.
Moreoever, by uslng the same nolse reductlon clrcult ln both
modes of operation, there is no difficulty in matching the
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1 7 2
characteristics of the encoder and decoder.
The characteristics of noise reduction circuit 10
have been described in detall hereinabove and, in the interest
of brevity, this description is not repeated. It is
appreciated, therefore, that when switch 211 engages fixed
contact d, circuit 200 operates in substantially the same
manner as discussed in detail hereinabove with respect to the
embodiments of Figs. 4 and 7, that is, the input information
signal is amplified by amplifier 210 and suitably level-
expanded, with variable de-emphasis, by circuit 10.
When switch 211 engages contact e, the transfer
characteristics of circuit 10 are used as the negative
feedback gain B of circuit 200. If the open loop gain
of amplifier 210 is represented as A, then the overall gain,
or transfer characteristic, of circuit 200 is equal to 1 + AB
This, of course, is the gain of an amplifier having negative
feedback. Now, if the product AB is sufficiently large, that
is, AB ~ 1, then the gain, or transfer characteristic, of
circuit 200, when disposed in its encoding configuration, merely
is equal to l/B. Thus, when circuit 10 is connected as a
negative feedback circuit to amplifier 210, the overall
characteristics of circuit 200 are converse, or
complementary to, the decoder transfer characteristic B.
Hence, it is appreciated that, when circuit 10 is used as an
encoder, a level compressed, pre-emphasized signal which is
complementary to the decoder characteristic is produced for
recording on the record medium.
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I 1 ~81~2
Having described specific preferred embodiments of
the invention with reference to the accompanying drawings,
it is to be understood that the invention is not limited
to those precise embodiments, and that various changes
and modifications may be effected therein by one skilled in
the art without departing from the scope or spirit of the
invention as defined in the appended claims.
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