Stereophonic receiver having a noise reduction control circuit

Stereophonic receiver having a noise reduction control circuit
United States Patent [191
[11] Patent_ Number:
4,574,389
Schotz
[45] 'Date of Patent:
Mar. 4, 1986
[54] STEREOPHONIC RECEIVER HAVING A
NOISE REDUCTION CONTROL CIRCUIT
[76] Inventor:
Larry Schotz, 118 Green Bay Rd.,
Thiensville, Wis. 53092
[21] Appl. No.: 514,034
[22] Filedr
Jul. 15, 1983
[51]
[52]
[58] Field of Search .................. .. 381/ 13, 4, 15, 16, 1O
References Cited
unit interconnects the output buffer of each channel to
the opposite channel, and includes a voltage sensitive
input to control the transmitted signal level. A control
channel includes a high frequency ?lter to pass the high
frequency signal of the stereophonic sound, which sig
nal is ‘recti?ed and ?ltered by suitable attack and decay
?lters. A log-conversion unit senses and converts the
U.S. PATENT DOCUMENTS
3,823,268
tude and frequency of the sound signals monitored. 3
Each sound channel is similar and includes an input
buffer and an output buffer to directly transmit the
corresponding channel signal. A variable highpass ?lter
Int. Cl.4 ............................................. .. H04H 5/00
US. Cl. ............................ .. 381/10; 381/13
[56]
control in which the sound separation is directly in
accordance with noise reduction need and the ampli
7/1974 Modafferi ....................... .. 381/13 X
4,221,928 9/1980 Franssen et al.
4,379,207 4/1983 Kubota
381/ 13 X
381/13 X
4,390,749
6/1983 Pearson .
381/13 X
4,426,727
l/ 1984 Hamada .............................. .. 381/13
?ltered signal to a C. logarithmic DC. signal, which is
directly proportional to the amplitude and frequency of
the corresponding channel stereophonic signal. The
DC. signal is connected to the input of the ?lter unit
and inversely adjusts the bandwidth thereof and
thereby inversely controls blending of the stereo sig
Primary Examiner-Gene Z. Rubinson
Assistant Examiner—W. J. Brady
Attorney, Agent, or Firm-Andrus,’ Sceales, Starke &
nals. A low limit control is connected to the input of the
Sawall
control is connected directly to the variable ?lter unit.
[57]
log-conversion unit. A high limit and mono-signal limit
ABSTRACT
A stereophonic receiver includes internal separation
22 Claims, 2 Drawing Figures
U. S. Patent
Mar. 4, 1986
4,574,389
wxww*
QQ
4,574,389
2
nals. Generally, in accordance with the teaching of the
STEREOPHONIC RECEIVER HAVING A NOISE
REDUCTION CONTROL CIRCUIT
I present invention, a monitoring means is coupled to the
BACKGROUND OF THE INVENTION
This invention relates to a stereophonic receiver hav
ing a noise reduction control circuit.
5 either factor alone may be monitored and provide some
stereo channels and the amplitude and frequency of the
stereophonic sound signals are monitored. Although
advantage both factors should be considered in the
system to produce a more effective separation control.
A separation control means includes an input means
connected to the monitoring means and an output
means coupling the two channels to control the blend
stereophonic broadcasting and receiving is widely
available throughout the world. At the present time,
stereophonic broadcasting is essentially exclusively FM
(frequency modulated). However, AM (amplitude
ing of the two signals in each channel and the control of
modulated) as well ‘ as television and Am systems in
the separation generally inversely with the amplitude
stereophonic are being developed. Stereophonic trans
mission provides two distinct signals of basically the
and frequency of the monitored signals. Thus, when
ever a high amplitude or high frequency channel signal
is received, the percentage of blending is reduced. Con
versely, as the stereophonic level is reduced, the blend
ing of the signals is increased.
The inventor recognized that at relatively high levels
of amplitude signals, the necessity for noise reduction
may be essentially eliminated. Under such conditions
the system may automatically lock out the blending
same sounds. The two stereo signals are generated from
different locations within the original sound environ
ment, and upon reproduction create a more realistic
presentation of the original sound. The signals gener
ated and transmitted differ in phase slightly so as to
permit separation at the receiver. The received signals
as transmitted are processed by both left and right chan
nels, within the receiver. The left channel transmits and
reproduces essentially only the left channel signal. The
right channel transmits and reproduces essentially the
right channel signal in the stereo receiver.
The transmitted and processed signals include a cer
tain level of noise which distorts the sound signal. Gen
erally, about 20db separates the relative noise content of
a stereo signal as compared to mono signal of the same
20
control. Conversely, with signals, generally equivalent
to monophonic or single signal reception, the necessity
for blending is removed and the separation control
system may also be locked out. Further, at a certain low
level or amplitude of stereophonic reception, the blend
ing can be locked at a ?xed level to provide appropriate
cancellation.
strength. Means have been provided to minimize the 30 More particularly, in a preferred construction, each
channel of the receiver is similarly constructed. Each
noise content of the stereo signal. A method used is the
channel
includes a conventional transmission circuit
blending of the stereo signals in the right and left chan
including an input buffer and an output buffer intercon
nels in the receivers so as to approach somewhat a
nected to directly transmit the corresponding channel
“mono” signal characteristic. A standard procedure
includes passing a small value of the stereo signal of one 35 signal. In accordance with the teaching herein, a vari
able highpass ?lter interconnects the output buffer to
channel to the opposite channel where it blends with
the opposite channel for controlled receipt of the oppo
the principle stereo signal of that channel. The out of
site channel signal from the opposite channel. The vari
phase relationship of the signals creates some cancella
able highpass ?lter unit includes an electrically respon
tion of the stereo signal content. However, the random
noise content is signi?cantly reduced by the blending of 40 sive control input, such as a voltage sensitive input, to
control the level of the transmitted signal. A control
i the signals. Cancellation is particularly required at low
channel includes a high frequency filter adapted to pass
level signals where the noise tends to mask and signifi
cantly distort the sound signals. At high amplitude or
the high frequency signal of the stereophonic sound.
The high frequency signal is recti?ed and ?ltered by
levels of sound, the noise content is essentially the same
and thus is a signi?cant lesser percentage of the total 45 suitable attack and decay ?lter means to produce a DC.
signal. A log-conversion unit may sense the amplitude
sound and creates relatively less perceptive distortion.
of the ?ltered signal and converts it to a corresponding
Thus, depending on the receiver construction, the “ste
D.C. logrithmic signal. The output of the log-conver
reo” and “mono” quieting approach each other and the
sion unit is connected to the input control of the vari
same level at about 3000 microvolts. As the result the
need for noise cancellation is correspondingly mini 50 able highpass ?lter unit. The output of the log-conver
mized.
'
sion unit is therefore a DC. signal directly proportional
to the amplitude and frequency of the corresponding
There is a need for an improved method in high qual
channel stereophonic signal. The input control is con
ity stereo systems of noise reduction which operates
nected to inversely adjust the bandwidth of the electri
without distortion, or loss of content in the signal of the
stereophonic sound received and reproduced. The pres 55 cally adjustable ?lter means, and thereby inversely con
trols the blending of the stereo signals with the ampli
ent invention is particularly directed to such a noise
tude and frequency of the channel signal. The low limit
reduction blending system which reduces the noise
content with minimum deterioration in the reproduc
control is connected conveniently to the inputs of the
tion of the stereo sound content.
SUMMARY OF THE PRESENT INVENTION
The present invention is particularly directed to an
log-conversion unit. The high limit and mono-signal
60 limit controls are conveniently connected directly into
the variable ?lter unit. The system thus provides inverse
blending with the amplitude of the stereo signal within
improved stereophonic receiver having internal separa
the desired signal range, with maximum blending at a
tion control means for modifying and controlling the
low stereo signal range and essentially no blending in
separation in the stereophonic sound directly in accor 65 the mono-signal or high amplitude stereo signal range.
dance with the need for noise reduction, thereby per
The present invention has been found to create a
mitting a more accurate reproduction of the stereo
significant improvement in the reception of the stereo
phonic sound content over the range of received sig
phonic sound with relatively simple and readily avail
3
4,574,389
able components, thereby producing a signi?cant cost
effective improvement in a stereo FM tuner.
DESCRIPTION OF THE DRAWING FIGURES
The accompanying drawing illustrates the best mode
presently contemplated by the inventor for carrying out
the invention.
when the noise factor is such as to require proper con
tinuous noise reduction at the maximum level.
In the drawing:
FIG. 1 is a block diagram of a stereo broadcasting
system; and
4
range, thereby producing the necessary stereo separa
tion for optimum noise cancellation with minimum dis
tortion of the stereophonic sound. At the opposite lim~
its, the system either looks out the blending where such
blending is not necessary and thereby permits optimum
sound transmission or conversely locks in the blending
More particularly, in the illustrated embodiment of
10 the invention, the right and left channels are corre
’ FIG. 2 is a schematic circuit of an FM tuner shown in
FIG. 1 and illustrating an embodiment of the present
invention.
DESCRIPTION OF THE ILLUSTRATED
EMBODIMENT
Referring to the drawings and particularly to FIG. 1,
a broadcast system is schematically illustrated including
a stereophonic broadcasting station 1 from which left
and right channel stereophonic sound signals 2 and 3 are
transmitted. As previously discussed the present inven
tion is generally applicable to stereophonic broadcast
ing. Because stereo receivers presently use FM tuners
the invention is described for such a system. An FM
stereo tuner 4 constructed in accordance with the teach» 25
spondingly constructed. The right channel is described,
with the corresponding elements of the left channel
identi?ed by corresponding primed number.
Referring to the right channel 6, an input buffer am
pli?er 10 is connected to the receiving signal 3. The
output of the input buffer ampli?er 10 is connected
directly to the ?rst input 11 of a two-input output buffer
ampli?er 12, the second input 13 of which is connected
to the highpass ?lter unit 8 from the opposite channel 5.
The stereo channel signal is transmitted to the output
ampli?er 12 and combined with a proportion of the
signal from the left channel 5 to produce a controlled
blended output signal. In various tuners, the input and
output buffers ampli?er may be incorporated into the
tuner and separate input/output buffers as shown in the
ing of the present invention is coupled to receive such
drawings need not necessarily be provided. The input
broadcast signals, and includes a left channel 5 for re
/output buffer ampli?ers as shown however provide a
self-contained integrated unit which can be directly
ception and reproduction of the left channel signals 2
and a right channel 6 for reception and reproduction of
the right channel signals 3. Broadcasting of such signals
" is in accordance with well-known and readily under
stood systems, and no further description thereof is
:5 given other than is necessary to understand the present
invention. Similarly, the receiver is only shown in such
detail as to explain the present invention, because the
applied to any stereo receiver.
Thus, the output of the input ampli?er 10 is also con
nected to the control channel 7 and particularly to a
frequency bandpass ?lter 14, which is set to transmit,
the stereo signal frequency. The output signal of ?lter
14 is proportional to both the amplitude and the fre
quency of the stereophonic right channel signal. Thus,
other components may be of any desired type and con
as the frequency increases, the voltage output of the
1; struction and will be readily provided by those skilled in
?lter 14 increases with a constant amplitude input. The
'the art.
increase is substantially linear with frequency. The
"The FM stereo receiver is constructed in accordance
input stereo signal to the monitor is taken immediately
with the teaching of the present invention with identical 40 after the low pass ?lter of the multiplex decoder to
.separation control units 7 and connected to control the
provide the necessary stereo signal. The input to the
output of a pair of highpass ?lter unit 8 which connect
monitor is taken after the low pass ?lter to eliminate the
the left and right channels 5 and 6. Each control unit 7
effects of high frequency switching within the tuner.
is a signal monitoring unit having an input connected to
Such switching signals may otherwise appear in the
corresponding channel and monitors both the amplitude 45 monitor as a stereo signal demanding certain blending
and the frequency of the received signals. The output of
when in fact blending and separation is not necessary or
the control unit 7 is coupled to the variable highpass
desired.
?lter unit 8 and directly controls and varies the level of
The ?ltered A.C. signal is applied to a full wave
the signal from the receiving channel to the opposite
rectifying circuit 15 to rectify the A.C. signal and estab
channel to produce a controlled blending of the two
lish a D.C. signal corresponding to the A.C. stereo
signals in each channel as the amplitude, the frequency,
phonic frequency signal. The D.C. signal level is of
or both increase.
In addition, a mono stereo signal monitor and a high
course proportional to both the change in frequency
and the change in amplitude in the illustrated embodi
limit stereo signal monitor are coupled to each channel,
the output of monitors is connected to the input of the
variable highpass unit and cooperates to lock the unit
off and whenever such conditions exist. In addition, a
ment. A multiple bank of ?lters is connected to the
recti?er 15 and includes a ?rst attack ?lter 16, a second
attack ?lter 17 and a decay ?lter 18. The ?rst attack
channels 5 and 6 . The output of this monitor is con
?lter 16 is a slow attack ?lter for slowly changing sig
nals. The second attack ?lter is a fast attack for rapidly
changing signals and the decay ?lter 18 is a decay time
nected to the input of the monitor unit 7 and is operable
?lter which is‘set to an appropriate cut off time.
to lock the control in a maximum blend state whenever
The ?ltered output signal is a D.C. signal the level of
which varies in accordance with the amplitude and
low level stereophonic signal monitor is coupled to both
the stereophonic signal level drops below a predeter
mined limit.
frequency of the stereophonic frequency content of the
signal. The D.C. signal is shown applied to a log-con
when both signals are received, the monitor unit will 65 version unit 19 to establish a D.C. log signal directly
continuously vary the bandwidth of the variable band
proportional to and varying with the amplitude and the
width highpass ?lter unit 8 and adjust the blending
frequency of the right channel sound signal 3. The D.C.
conversely with the level of the sound within such
log signal is applied as the control input 190 of the
Thus during normal stereophonic sound transmission
4,574,389
'
5
,
variable highpass ?lter 8, and thereby controls the cou
pling of the opposite channel 5 to the right channel 6
and the blend of the two signals at the summing output
6
1
Thus, generally the‘ present invention is based on a
unique circuit wherein when the high frequency con
tent iri a channel is low or essentially absent, minimum
ampli?er 12.
high frequency separation is established by operation of
The left channel 5 is identically constructed with its
left channel monitoring circuit 7 is connected from the
right channel 6 to the second input 13 of the summation
buffer ampli?er 12. The left channel 5 thus functions in
the circuit 25. If a substantial level of high frequency
content or information is present, the circuit functions
to reduce or minimize the noise content by closing the
stereo separation, with a signi?cant improvement in the
overall sound characteristic.
In the illustrated embodiment of the invention the
the same manner as that described for the right channel.
right and left channel high frequency separation is inde
Further, when the signal content level is suf?ciently
high, there is no need for the blending. A similar situa
duction.
monitoring circuitry responsive to the amplitude of the
' left channel signal. The variable highpass ?lter 8 of the
pendently varied so as to provide full stereo noise re
.
A schematic circuit illustration of the block diagram
tion occurs, if the receiver is receiving a mono-signal
rather than a stereo signal. In the illustrated embodi 15 circuit shown in FIG. 1 is given in FIG. 2 for purposes
of showing a preferred embodiment of the invention. As
ment of the invention, a cutoff circuit 20 is connected to
in FIG. 1, the right channel 6 of FIG. 2 is described‘
the input of both highpass ?lters which are held off in
with the left channel elements identi?ed with corre
response to such conditions and eliminates all blending
sponding prime numbers.
I
of the signals between the channels.
The illustrated cutoff circuit 20 includes an amplitude 20 Referring to the drawings and particularly FIG. 2,
the input buffer ampli?er 10 is shown as a known opera
sensitive switching ampli?er 21 having its input con
tional ampli?er con?guration having a negative input
nected to respond to the incoming signal. A ?rst input
connected to the signal input line in series with a cou
line 22 provides an input signal proportional to the
pling resistor 30. The output is connected to the ampli
signal strength. A second input signal line 23 provides
?er 12 in series with a coupling resistor 31. In FIG. 2,
an input, dependent-upon receipt of a monosignal or a
the input of the highpass ?lter 14 is connected directly
stereo signal. A latching or clamping circuit 24, is con
to the output of the ampli?er 10, as shown by line 32.
nected to the output of the ampli?er 21 and to the inputs
The output ampli?er 12 is also shown as a known opera
:. of both the variable highpass ?lters 8, and functions to
' ground the signal from the log converters I9 and 19’,
thereby effectively eliminating blending. If the signal
strength is above a selected level, or if a mono-signal is
_ indicated at its input line 23, the ampli?er 21 provides an
output to the clamping circuit 24 to hold the ?lters 8
effectively closed.
If the received stereo signal is very small or.weak, a
low signal monitor 25 senses such condition and is con
nected to turn the blending signal circuit 7 fully on and
tional ampli?er for raising the output voltage signal to a
level suitable for subsequent processing and driving of a
I sound reproducing unit, not shown, but which may be
of any known or desired construction.
The output of the ampli?er 10 is also connected to the
right channel signal processing monitoring circuit 7.
35
The highpass filter 14 consists of a coupling capacitor
33 and a capacitor resistor unit 34, with the common
junction 35 connected to the recti?er 15.
The recti?er 15 in FIG. 2 is shown including a pair of
converting the system into a conventional-high-blend
stereo receiver. Thus, with such weak signal noise is
suf?ciently great that the high blend is critical to proper
interconnected'ampli?ers 36 and diodes 37 for produc
ing a full wave recti?ed output signal.
A diode bias compensation circuit 38 is shown in
T -1--' sound reproduction.‘ If the monitor is allowed to oper
" ate at such a low signal level, pumping may be gener
FIG. 2. The bias compensation circuit is connected to ' I
the D.C. output of recti?er 15 and provides compensa
tion for diode loss in the signal processing. The circuit
includes a coupling transistor 39 connecting the output
ated within the signal and audibly reproduced.
The lowv level monitor 25 also includes a switching 45
ampli?er 26 having its input line 27 connected in com
of the rectifying diodes 37 to the input of a ?lter 39a for
mon with the signal strength line 22 to monitor the
removing extraneous signals from the D.C. signal and
incoming signal levels. The output of the ampli?er 26 is
thereby producing an optimum D.C. signal changing
connected to a switching circuit 28, the output of which
with the amplitude and/or the frequency of the re
is connected to the input of a full wave recti?er and 50 ceived signal. The output of ?lter 39a is impressed on
provides a signal to both channels 7, which locks the
the ?lters 16-18, inclusive.
,
system to produce maximum blending, or conditions the
The slow attack ?lter 16 is shown as a known opera
circuit to pass the high frequency content related signal.
tional ampli?er circuit having an. R-C network includ
The limit strength signal is derived from any suitable
ing a resistor 40 and a capacitor 41, connected to the
signal strength point in the receivenA convenient sig 55 output of an operational ampli?er 42 and to ground and
nal point in the receiver is the signal strength meter
with appropriate feedbacks for proper ?ltering. A
provided in the conventional FM tuner.
blocking diode 43 is connected between the ampli?er 42
Thus, in summary the stereo content of the right and
and the resistor. The decay filter 18 includes resistor 44
left channels 5 and 6 are transmitted to the respective ,
monitoring circuit where each is recti?ed, ?ltered and
converted to a D.C. voltage log signal which is im
pressed on the voltage sensitive control input of the
corresponding variable highpass ?lter 8. If there is no
high frequency content or information in a channel, the
and diode 45 connected to the capacitor 41 to de?ne a
60
discharge circuit for controlling the hold-on or decay
period of the control signal and thus the separation
control period as the stereo signal and through the con
trol signal level changes. Filters 16 and 17 are con
nected to log-conversion unit 19 by an operational am
high frequency separation of that channel issigni? 65 pli?er 45a. The fast attack ?lter 17 includes a resistor
cantly reduced. This signi?cantly improves the signal to
and capacitor network 46 to ground. A transistor 47 is
noise ratio but does not audibly affect or change the . connected to the R-C network 46. The output of transis
sound content.
tor 47 is connected to the output of ampli?er 45a and is
7
4,574,389
driven on if the signal changes rapidly to drive the
control output rapidly.
8
with the maximum level of noise reduction permitted by
blending while providing a minimum loss and distortion
The output of this ?lter circuit is connected to the
of the desired sound content. The control circuit contin
input of the log converter 19. The log converter 19 is
uously monitors the broadcast signals and in particular
again shown as a known unit including an operational
to the individual right and left channel signals as to both
ampli?er 48 and an output transistor 49 which produces
amplitude of the stereo signal and the frequency of the
a logrithimic DC. signal.
stereo signal to determine the necessity for separation
The transistor 49 is connected by a coupling resistor
and proceeds to provide appropriate separation based
49a to the input of the variable highpass ?lter unit 8 via
upon the received signals. Individual control on each
line 51.
10 channel is provided by the adjustable or ?xed factory
In the illustrated embodiment of FIG. 2, the highpass
wired resistor 58 which controls the level at which the
?lter 8 is illustrated as a transductor ampli?er 50 includ
separation control is cut off.
ing a ?lter network 51. The unit of FIG. 2 is commer
Although shown in a preferred embodiment, the
cially available as an integrated chip circuit manufac
system can of course be made with other circuits pro
tured and sold by various manufacturers such as Na
viding the desired stereo signal related separation.
tional Semiconductor Corporation under the chip num
Thus, the separation may be controlled by only moni
ber LM 13600. Further details may be readily obtained
toring the amplitude or only the frequency of the stereo
from the application information for such LM 13600
signal. The ?ltering means may be simpli?ed or modi
chip.
?ed to produce a DC. signal. Further, the individual
In the circuit of FIG. 2, the blend or separation cutoff 20 channel control may be replaced with a single control
circuit 20 is shown including a switching transistor 55
channel having its input connected in common to both
which is biased off in the absence of an appropriate
stereo input channels. Thus the system however will
input signal from the high strength line 22 or the mono
then operate at less than full separation control and will
signal line 23. The output of the transistor 55 is similarly
not produce the same high quality as the illustrated
connected to the input lines of ?lters 8 and 8' by similar 25 embodiment.
coupling diodes 56. The transistor 55 has its input termi
These and various other changes and modi?cations to
nal connected by a suitable coupling network 57, to the
the system may be provided within the present inven
signal lines 22 and 23. When the transistor 55 is biased
tion.
on, its grounds the input signals from the log converter
The present invention has been found to provide a
7' 19 by tieingv the ?lter lines effectively to a negative
signi?cant improvement in the sound quality of a stereo
voltage level.
receiver. As previously noted, the system is equally
When the signal strength is high, a turn on signal is
i '. applied to the transistor 55 causing it to conduct and
clamping the variable highpass ?lter off. The particular
effective high level at which the system operates is
made adjustable through an adjustable resistor 58 con
” nected between signal line 22 and theinput to the ampli
‘ '?er.
Resistor 58 is generally a factory adjustment for the
applicable to other forms of stereo transmission and
reception including the more recent AM systems. The
system components are standard commercially avail
able components which can be readily mass produced
and assembled to provide a low cost improvement in
stereo receivers.
Various modes of carrying out the invention are con
templated as being within the scope of the following
" particular tuner. The tuner output for any given input 40 claims and particularly pointing out and distinctly
Zjj‘may vary from tuner to tuner. The resistor 58 compen
claiming the subject matter which is regarded as the
“sates for such variation and may be a ?xed resistor hard
invention.
wired in place and of a proper resistance to provide the
I claim:
desired signal output.
1. A stereo signal receiver apparatus for receiving
The mono stereo line 23 is connected to the input 45 broadcast stereo sound signals including a left channel
network 57 of the transistor 55 by a Zener diode 59.
signal and a right channel signal and reproducing said
When a mono signal is received, the signal voltage rises
rapidly to 12 volts thereby effectively causing conduc
tion of the Zener diode and turn on of transistor 55. This
again clamps the input of the variable highpass ?lter to
a negative level and holds the ?lter off.
The low level signal circuit 25 in FIG. 2 includes the
switching ampli?er 26 having a transistor 60. The input
of transistor 60 is connected to the signal line 22 and its
output is connected to a switching transistor 61 of 55
received sound signals with noise reduction, compris
mg:
a left channel for transmitting the received left chan
nel signal,
a right channel for transmitting the received right
channel signal,
high frequency separation control means connected
to the right and left channels for blending of the
two signals in each of said channels, and
means to monitor the high frequency stereo signal
clamp unit 28. Transistor 60 is normally biased off in the
absence of a high signal. If the signal rises above 10
microvolts, the transistor 60 is turned on and effectively
grounds the input to the switching transistor 61 of the
clamp unit 28. If the signal is below 10 microvolts,
transistor 60 turns off and the supply voltage signal is
2. The apparatus of claim 1 having a limit control
transmitted as an input to the switching transistor 61.
Transistor 61 is driven on and effectively grounds the
means responsive to said received stereo signals to se
lectively lock said control means fully on in response to
input to the highpass ?lter and bypasses the signal from
content of each of said received signals and con
nected to said control means to control the separa
tion in each channel in accordance with said high
frequency signal content.
a selected sound characteristic of the received signals.
the recti?er 15 to ground.
65
3. The apparatus of claim 1 having a limit control
Thus, the illustrated circuit function as previously
means responsive to said received stereo signals to se
discussed with respect to FIG. 1 and provides a reliable
lectively lock said control means fully off in response to
and highly effective noise reduction system and circuit
a selected sound characteristic of the received signals.
4,574,389
9
-
4. The apparatus of claim 1 having a high level signal
monitor connected to monitor the level of received
stereo signal and connected to said control means to
turn said control means fully on at a ?rst level and fully
off at a second level.
10
.
high frequency separation control means having out
put means connected to the ?rst means of the right
and left channels for establishing blends of said
right and left channel sound signals, and having a
control input means connected to said second
5. The apparatus of claim 1 having a mono stereo
signal monitor connected to said control means to turn
said control means fully off in the presence of a mono
means to control the high frequency separation in
each channel in accordance with the strength of
the high frequency signal content in the corre
6. The apparatus of claim 1 wherein separation in said
14. The FM tuner apparatus of claim 13 wherein said
second means are constructed to respond to the ampli
tude of the stereo signal and to the frequency level of
signal.
right and left channels are separately controlled.
'
7. The apparatus of claim 1 wherein said monitor
means is responsive to the amplitude of said stereo sig
nal and is responsive to the frequency level of said ste
reo signal.
8. The apparatus of claim 1 wherein said monitor
means includes signal detecting means for generating a
?rst DC. voltage signal proportional to the stereo fre
quency signal in said left signal channel and, a second
DC. voltage signal proportional to the stereo fre 20
quency signal in said right signal channel,
a ?rst signal coupling means connecting said left
channel to said right channel and having a voltage
responsive input connected to said detecting means
to control the transmitting of the left channel signal 25
to the right channel in accordance with said ?rst
DC. signal, and
I
a second signal coupling means connecting said right
channel to said left channel and having a voltage
responsive input connected to said detecting means 30
to control the transmitting of the right channel
signal to the left channel in accordance with said
second DC. voltage signal.
9. The apparatus of claim 8 wherein said detecting
means includes:
a highpass ?lter set to pass only the stereo sound
frequencies in said received signals, and
a full wave recti?er means connectedsto said highpass
?lter and operable to establish said DC. signal
proportional to said stereo sound frequencies.
40
10. Theapparatus of claim 9 including a signal modi
“""""“‘fying ?lter means connected to said recti?er means to
remove extraneous signals from said DC. signal.
11. The apparatus of claim 10 including:
a log-conversion unit connected to said ?lter means 45
to establish a logrithmic DC. signal proportional
sponding channel.
the stereo signal.
-
15. The FM tuner apparatus of claim 13 wherein each
of said left and right channels include:
an input buffer ampli?er and an output summing
ampli?er connected to said corresonding input
buffer ampli?er,
each of said channels including a signal detecting
means for generating a ?rst DC. signal voltage and
a second- DC signal voltage, and includes a ?rst
coupling means connecting said left channel to said
right channel and having a voltage responsive
input connected to receive said ?rst DC. signal
voltage to control the transmitting of the left chan
nel signal to the right channel, and including a
second coupling means connecting said right chan
nel to said left channel and having a voltage re
sponsive input connected to receive said second:
DC. signal voltage to control the transmitting of
the right channel signal to the left channel.
16. The FM tuner apparatus of claim 13 wherein said
second means of each channel includes:
a highpass ?lter set to pass only the stereo sound
frequencies in said received signals,
a full wave recti?er means connected to said highpass
?lter and operable to establish a DC. signal pro
portional to said stereo sound frequencies,
_
modifying ?lter means to remove extranious signals
from said DC. signal, and
a variable highpass ?lter having a voltage sensitive
input means for adjusting the bandwidth of the
highpass ?lter and thereby the stereo signal separa
tions to produce a varying blending of the signal
inversely proportional to the strength of the high
frequency component in said sound signal.
to said stereo sound frequencies.
17. The FM tuner apparatus of claim 16 having a
12. The apparatus of claim 8 wherein said coupling
log-conversion unit connected between said ?lter
means include a variable highpass ?lter having a volt
age sensitive input means for adjusting the bandwidth of 50 means and said highpass ?lter means to establish a lo
the highpass ?lter and thereby the stereo signal separa
tion to produce a varying blending of the signal in
versely proportional to the received stereo signal.
grithmic DC. signal proportional to said-stereo sound
frequencies.
'
-
18. The FM tuner apparatus of claim 13 including
13. FM tuner apparatus for receiving a mono sound
limit control means responsive to the received signal to
signal and stereo sound signal including a left channel 55 selectively lock said separation control means fully on
sound signal and a right channel sound signal and trans
to produce a maximum blending of said signals in re
mitting said received sound signals with a noise reduc
sponse to the sound characteristic of the received signal. ‘
tion, comprising:
19. The FM tuner apparatus of claim 13 including '
limit control means responsive to the received signal to
selectively locksaid separation control means off to
produce minimum ‘blending of said signals in response .
to the sound characteristic of the received signal.
sound signal,
‘
20. The FM tuner apparatus of claim 15 including:
a right channel having ?rst means for transmitting the
a high level signal monitor connected to monitor the
received right channel sound signal and second 65
level of received stereo signal,
means to monitor the strength of the high fre
and connected to saidseparation means to turn said
quency signal content of the receivedright channel
separation means fully on at a ?rst level and fully
a left channel having ?rst means for transmitting the
received left channel sound signal and second
means to monitor the strength of the high fre
quency signal content of the received left channel
sound signal, and
off at a second level,
11
4,574,389
and operable to establish a DC. signal proportional
to said stereo sound high frequencies,
attack ?lter means connected to said recti?er to trans
mit said DC signal,
separation means to respond to said DC. voltage
signals in the presence of a stereo signal.
21. A stereo tuner apparatus for receiving stereo
decay ?lter means connected to said recti?er to hold
said DC. signal, and
a transducter ampli?er connected to said separation
control means and connected to said decay ?lter
sound signals and transmitting said received sound sig
nals with noise reduction cosprising:
means and to said attack ?lter means having a volt
age sensitive input means for adjusting the band
a left channel and a right channel each having an
width of the amplifer and thereby separately ad
justing the stereo signal separation in the left chan
nel and in the right channel to produce a varying
input buffer amplitier and an output buffer means
for transmitting the received signal and monitor
means to monitor the high frequency signal content
of the received signal,
12
a full wave recti?er connected to said highpass ?lter
a mono stereo signal monitor connected to said sepa
ration means to turn said separation means fully on
in the presence of a mono signal and to activate the
15
blending of the signal inversely proportional to the
high frequency content in the received stereo sig
nal.
a left channel signal separation control means and a
22. The apparatus of claim 21 including:
right channel signal separation control means, each
20
of said separation control means comprising,
' a highpass ?lter set to pass only the stereo sound high
a log-conversion unit connected between said trans
ducter ampli?er and said ?lter means to establish a
logrithmic DC. signal proportional to said stereo
sound frequencies.
frequencies in said received signals,
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