U.S. patent number 3,825,697 [Application Number 05/383,243] was granted by the patent office on 1974-07-23 for phase-lock-loop fm-stereo decoder including stereophonic/monophonic blend system for reducing audio distortion.
This patent grant is currently assigned to General Motors Corporation. Invention is credited to Thomas V. Cornell, Wayne A. Smith, Lester Wilkinson.
United States Patent |
3,825,697 |
Cornell , et al. |
July 23, 1974 |
PHASE-LOCK-LOOP FM-STEREO DECODER INCLUDING STEREOPHONIC/MONOPHONIC
BLEND SYSTEM FOR REDUCING AUDIO DISTORTION
Abstract
An FM-stereo decoder includes a phase-lock-loop for providing an
error voltage and an audio decoder having L and R output channels
upon which L and R audio signals are developed, respectively. In
response to an initial predetermined increase in the error voltage
due to the initiation of distorted reception, the separation
between the L and R output channels is substantially
instantaneously decreased to less than full channel separation
thereby to rapidly shift toward full monophonic operation for
reducing the discernible audio distortion resulting from the
distorted reception. Further, in response to a subsequent
predetermined decrease in the error voltage due to the termination
of distorted reception, the separation between the L and R output
channels is relatively gradually increased to full channel
separation thereby to slowly shift to full stereophonic operation
for reducing the discernible audio disruption resulting from the
shift to full stereophonic operation. Preferably, the separation
between the L and R output channels of the audio decoder is varied
during distorted reception from full channel separation to an
intermediate channel separation which is greater than zero channel
separation in proportion to the frequency of the L and R audio
signals thereby to provide blended stereophonic/monophonic
operation for reducing the discernible audio disruption resulting
from the rather sudden shift toward full monophonic operation and
for maximizing the amount of stereophonic operation over the lower
frequencies of the L and R audio signals while minimizing the
discernible audio distortion resulting from the higher frequencies
of the L and R audio signals.
Inventors: |
Cornell; Thomas V. (Kokomo,
IN), Smith; Wayne A. (Russiaville, IN), Wilkinson;
Lester (Kokomo, IN) |
Assignee: |
General Motors Corporation
(Detroit, MI)
|
Family
ID: |
23512297 |
Appl.
No.: |
05/383,243 |
Filed: |
July 27, 1973 |
Current U.S.
Class: |
381/4 |
Current CPC
Class: |
H03D
1/2236 (20130101) |
Current International
Class: |
H03D
1/00 (20060101); H03D 1/22 (20060101); H04h
005/00 () |
Field of
Search: |
;179/15BT
;329/122,50,123 ;325/305,400,427,420,472 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: D'Amico; Thomas
Attorney, Agent or Firm: Jagodzinski; T. G.
Claims
What is claimed is:
1. In an FM-stereo decoder for demodulating a standard composite
stereophonic signal including an L + R signal, an L - R modulated
38 KHz suppressed subcarrier signal, and a 19 KHz pilot signal; the
combination comprising: means including a phase-lock-loop for
generating a 38 KHz demodulating signal in phase synchronization
with the 19 KHz pilot signal, the phase-lock-loop including a phase
detector for producing an error voltage having an amplitude defined
as a function of the phase difference between the 19 KHz pilot
signal and the 38 KHz demodulating signal such that the amplitude
of the error signal increases in response to shifts in the phase of
the 19 KHz pilot signal caused by distorted reception; means
including an audio decoder having L and R output channels for
demodulating the L + R signal and the L - R modulated 38 KHz
suppressed subcarrier signal in response to the 38 KHz demodulating
signal to provide an L audio signal on the L output channel and an
R audio signal on the R output channel; a coupling network
including an electronic switch connected across the L and R output
channels of the audio decoder for establishing the separation
between the L and R output channels at substantially full channel
separation when the electronic switch is turned off and for
establishing the separation between the L and R output channels at
less than full channel separation when the electronic switch is
turned on; and a level detector responsive to an initial
predetermined increase in the amplitude of the error voltage due to
the initiation of distorted reception to substantially
instantaneously turn on the electronic switch thereby to rapidly
shift toward full monophonic operation for reducing the discernible
audio distortion resulting from the distorted reception, the level
detector including a timing network responsive to a subsequent
predetermined decrease in the amplitude of the error voltage due to
the termination of distorted reception to relatively gradually turn
off the electronic switch thereby to slowly shift to full
stereophonic operation for reducing the discernible audio
disruption resulting from the shift back to full stereophonic
operation.
2. In an FM-stereo decoder for demodulating a standard composite
stereophonic signal including an L + R signal, an L - R modulated
38 KHz suppressed subcarrier signal, and a 19 KHz pilot signal; the
combination comprising: means including a phase-lock-loop for
generating a 38 KHz demodulating signal in phase synchronization
with the 19 KHz pilot signal, the phase-lock-loop including a phase
detector for producing an error voltage having an amplitude defined
as a function of the phase difference between the 19 KHz pilot
signal and the 38 KHz demodulating signal such that the amplitude
of the error signal increases in response to shifts in the phase of
the 19 KHz pilot signal caused by distorted reception; means
including an audio decoder having L and R output channels for
demodulating the L + R signal and the L - R modulated 38 KHz
suppressed subcarrier signal in response to the 38 KHz demodulating
signal to provide an L audio signal on the L output channel and an
R audio signal on the R output channel; a coupling network
including an electronic switch connected across the L and R output
channels of the audio decoder for establishing the separation
between the L and R output channels at substantially full channel
separation when the electronic switch is turned off and for
establishing the separation between the L and R output channels at
less than full channel separation when the electronic switch is
turned on; and a level detector responsive to an initial
predetermined increase in the amplitude of the error voltage due to
the initiation of distorted reception to substantially
instantaneously turn on the electronic switch thereby to rapidly
shift toward full monophonic operation for reducing discernible
audio distortion resulting from the distorted reception, the level
detector including a timing network responsive to a subsequent
predetermined decrease in the amplitude of the error voltage due to
the termination of distorted reception to relatively gradually turn
off the electronic switch thereby to slowly shift to full
stereophonic operation for reducing the discernible audio
disruption resulting from the shift to full stereophonic operation;
the coupling network including a resistive element for establishing
a minimum separation between the L and R output channels of the
audio decoder when the electronic switch is turned on at an
intermediate channel separation defined below full channel
separation and above zero channel separation thereby to provide
blended stereophonic/monophonic operation for reducing the
discernible audio distortion resulting from the sudden shift toward
full monophonic operation from full stereophonic operation.
3. In an FM-stereo decoder for demodulating a standard composite
stereophonic signal including an L + R signal, an L - R modulated
38 KHz suppressed subcarrier signal, and a 19 KHz pilot signal; the
combination comprising: means including a phase-lock-loop for
generating a 38 KHz demodulating signal in phase synchronization
with the 19 KHz pilot signal, the phase-lock-loop including a phase
detector portion for producing an error voltage having an amplitude
defined as a function of the phase difference between the 19 KHz
pilot signal and the 38 KHz demodulating signal such that the
amplitude of the error signal increases in response to shifts in
the phase of the 19 KHz pilot signal caused by distorted reception;
means including an audio decoder having L and R output channels for
demodulating the L + R signal and the L - R modulated 38 KHz
suppressed subcarrier signal in response to the 38 KHz demodulating
signal to provide an L audio signal on the L output channel and an
R audio signal on the R output channel; a coupling network
including an electronic switch connected across the L and R output
channels of the audio decoder for establishing substantially full
channel separation when the electronic switch is turned off thereby
to provide full stereophonic operation and for establishing less
than full channel separation when the electronic switch is turned
on thereby to provide less than full stereophonic operation; and a
level detector responsive to an initial predetermined increase in
the amplitude of the error voltage due to the initiation of
distorted reception to substantially instantaneously turn on the
electronic switch thereby to rapidly shift toward full monophonic
operation for reducing the audio distortion resulting from the
distorted reception, the level detector including a timing network
responsive to a subsequent predetermined decrease in the amplitude
of the error voltage due to the termination of distorted reception
to relatively gradually turn off the electronic switch thereby to
slowly shift to full stereophonic operation for reducing the audio
disruption resulting from the shift to full stereophonic operation;
the coupling network further including a reactive element for
varying the separation between the L and R output channels of the
audio decoder from full channel separation to less than full
channel separation as a direct function of the frequency of the L
and R audio signals thereby to provide blended
stereophonic/monophonic operation for maximizing the stereophonic
reproduction of the lower frequencies of the L and R audio signals
while minimizing the audio distortion resulting from the higher
frequencies of the L and R audio signals.
4. In an FM-stereo decoder for demodulating a standard composite
stereophonic signal including an L + R signal, an L - R modulated
38 KHz suppressed subcarrier signal, and a 19 KHz pilot signal; the
combination comprising: means including a phase-lock-loop for
generating a 38 KHz demodulating signal in phase synchronization
with the 19 KHz pilot signal, the phase-lock-loop including a phase
detector for producing an error voltage having an amplitude defined
as a function of the phase difference between the 19 KHz pilot
signal and the 38 KHz demodulating signal such that the amplitude
of the error signal increases in response to shifts in the phase of
the 19 KHz pilot signal caused by distorted reception; means
including an audio decoder having L and R output channels for
demodulating the L + R signal and the L - R modulated 38 KHz
suppressed subcarrier signal in response to the 38 KHz demodulating
signal to provide an L audio signal on the L output channel and an
R audio signal on the R output channel; a coupling network
including an electronic switch and at least one resistive element
connected across the L and R output channels of the audio decoder
for establishing full channel separation when the electronic switch
is turned off thereby to provide full stereophonic operation and
for establishing less than full channel separation when the
electronic switch is turned on thereby to provide less than full
stereophonic operation; and a level detector responsive to an
initial predetermined increase in the amplitude of the error
voltage due to the initiation of distorted reception to
substantially instantaneously turn on the electronic switch thereby
to rapidly shift toward full monophonic operation for reducing the
audio distortion resulting from the distorted reception, the level
detector including a timing network responsive to a subsequent
predetermined decrease in the amplitude of the error voltage due to
the termination of distorted reception to relatively gradually turn
off the electronic switch thereby to slowly shift to full
stereophonic operation for reducing audio disruption resulting from
the shift to full stereophonic operation; the coupling network
including a resistive element for establishing a minimum separation
between the L and R output channels of the audio decoder when the
electronic switch is turned on at an intermediate channel
separation defined below ful channel separation and above zero
channel separation by the resistance of the resistive element
thereby to provide blended stereophonic/monophonic operation for
reducing the audio distortion resulting from the sudden shift
toward full monophonic operation, the coupling network also
including a capacitive element for varying the separation between
the L and R output channels of the audio decoder when the
electronic switch is turned on from full channel separation to the
intermediate channel separation as determined by the reactance of
the capacitor thereby to provide blended stereophonic/monophonic
operation for maximizing stereophonic operation at the lower
frequencies of the L and R audio signals and for maximizing
monophonic operation at the higher frequencies of the L and R audio
signals so as to maximize the total stereophonic effect and
minimize the overall audio distortion.
Description
This invention relates to an FM-stereo decoder for demodulating a
standard composite stereophonic signal including an L + R signal,
an L - R modulated 38 KHz suppressed subcarrier signal, and a 19
KHz pilot signal.
In FM-stereo reception, the composite stereophonic signal,
including the 19 KHz pilot signal, is erratically phase shifted by
multipath reception as well as various forms of "overload"
reception. Overload reception occurs when a modulation product of
one or more strong undesired stations interferes with the reception
of a desired station. With either multipath reception or overload
reception, the result is corresponding audio distortion of a nature
which is readily discernible by a listener. Since audio distortion
is inherently less perceptible during monophonic operation than
during stereophonic operation, it is desirable to switch from full
stereophonic operation toward full monophonic operation in response
to the occurrence of distorted reception, whether multipath
reception or some form of overload reception.
In a known FM-stereo decoder, a phase-lock-loop generates a local
38 KHz demodulating signal in phase synchronization with the 19 KHz
pilot signal. Further, an audio decoder demodulates the L + R
signal and the L - R modulated 38 KHz suppressed subcarrier signal
in response to the local 38 KHz demodulating signal to provide an L
audio signal on an L output channel and an R audio signal on an R
output channel. Ordinarily, the phase-lock-loop includes a phase
detector which provides an error voltage having an amplitude which
is proportional to the phase difference between the received 19 KHz
pilot signal and the local 38 KHz demodulating signal.
According to one aspect of the invention, the separation between
the L and R output channels of the audio decoder is substantially
instantaneously defined at less than full channel separation in
response to a predetermined increase in the amplitude of the
phase-lock-loop error voltage due to the initiation of distorted
reception, whether multipath reception or a form of overload
reception. This "fast attack" feature of the invention is desirable
in order to shift from full stereophonic operation toward full
monophonic operation in time to reduce the discernible audio
distortion resulting from the distorted reception.
In another aspect of the invention, the separation between the L
and R output channels of the audio decoder is relatively gradually
restored to full channel separation in response to a predetermined
decrease in the amplitude of the phase-lock-loop error voltage due
to the termination of distorted reception. This "slow retreat"
feature of the invention is desirable in order to reduce the
discernible audio disruption resulting from the return to full
stereophonic operation following the excursion toward full
monophonic operation.
In an embodiment of the preceding aspects of the invention, a
coupling network including an electronic switch is connected across
the L and R output channels of the audio decoder for establishing
full channel separation when the electronic switch is turned off
and for establishing less than full channel separation when the
electronic switch is turned on. Further, a level detector is
responsive to an initial increase in the amplitude of the error
voltage due to the initiation of distorted reception to
substantially instantaneously turn on the electronic switch. The
level detector includes a timing network which is responsive to a
subsequent decrease in the amplitude of the error voltage due to
the termination of distorted reception to relatively gradually turn
off the electronic switch. Preferably, the electronic switch is a
field-effect transistor.
As contemplated by another aspect of the invention, the minimum
separation between the L and R output channels of the audio decoder
during distorted reception is defined at an intermediate channel
separation located between full channel separation and zero channel
separation. This feature of the invention is desirable in order to
reduce the discernible audio disruption resulting from the sudden
shift from full stereophonic operation toward full monophonic
operation.
In an embodiment of the preceding aspect of the invention, the
coupling network includes at least one resistive element connected
with the electronic switch for defining the intermediate channel
separation in proportion to the resistance of the resistive element
when the electronic switch is turned on. When the separation
between the L and R output channels of the audio decoder is at the
intermediate channel separation, blended stereophonic/monophonic
operation is provided.
In another aspect of the invention, the separation between the L
and R output channels of the audio decoder is varied during
distorted reception from full channel separation to less than full
channel separation in direct proportion to the frequency of the L
and R audio signals such that channel separation decreases with
increases in audio frequency. Since the greatest portion of the L
and R audio signals is composed of lower frequency components while
the major portion of the audio distortion of the L and R audio
signals results from higher frequency components, this feature of
the invention is desirable in order to maximize total stereophonic
operation and yet minimize overall audio distortion.
In an embodiment of the preceding aspect of the invention, the
coupling network includes at least one reactive element connected
with the electronic switch for defining the separation between the
L and R output channels of the audio decoder in proportion to the
reactance of the reactive element as defined in proportion to the
frequency of the L and R audio signals when the electronic switch
is turned on. Preferably, the reactive element is a capacitor.
These and other aspects and advantages of the invention may be best
understood by reference to the following detailed description of
the preferred embodiments when taken in conjunction with the
accompanying drawings.
In the drawings:
FIG. 1 is a block diagram of an FM-stereo decoder including an
inventive stereophonic/monophonic blend system incorporating one
embodiment of a unique coupling network.
FIG. 2 is a graphic diagram of the operating characteristic of the
embodiment of the coupling network illustrated in FIG. 1.
FIG. 3 is a schematic diagram of an embodiment of a level detector
incorporated within the inventive stereophonic/monophonic blend
system illustrated in FIG. 1.
FIGS. 4, 6, and 8 are schematic diagrams of other embodiments of
the coupling network incorporated within the inventive
stereophonic/monophonic blend system illustrated in FIG. 1.
FIGS. 5, 7, and 9 are graphic diagrams of the operating
characteristics of the various embodiments of the coupling network
illustrated in FIGS. 4, 6, and 8.
Referring to FIG. 1, an FM-stereo radio receiver includes an FM
detector 10 for developing a standard composite stereo signal
including a 19 KHz pilot signal, an L + R signal and an L - R
modulated 38 KHz suppressed subcarrier signal at a junction 12. An
FM-stereo decoder 14 includes a phase-lock-loop 16 and an audio
decoder 18. The phase-lock-loop 16 is connected to the junction 12
for developing a 38 KHz demodulating signal or subcarrier signal at
a junction 20 in phase synchronization with the 19 KHz pilot
signal. The audio decoder 18 is connected to the junctions 12 and
20 for demodulating the L + R signal and the L - R modulated 38 KHz
signal in response to the 38 KHz demodulating signal to provide an
L audio signal on an L output channel 22 and an R audio signal on
an R output channel 24. A pair of audio amplifiers 26 and 28 are
connected to the L and R output channels 22 and 24, respectively,
for amplifying and applying the L and R audio signals to L and R
stereo speakers 30 and 32, respectively.
More specifically, the phase-lock-loop 16 includes a voltage
controlled oscillator 34 for producing a 76 KHz reference signal
having a phase defined in proportion to the amplitude of a
differential error voltage appearing between a pair of junctions 36
and 38. A first frequency divider 40 is connected to the oscillator
34 for dividing the 76 KHz reference signal by a factor of (2) to
provide the 38 KHz demodulating signal at the junction 20 in phase
with the 76 KHz reference signal. A second frequency divider 42 is
connected to the junction 20 for dividing the 38 KHz demodulating
signal by a factor of (2) to provide a 19 KHz reference signal in
quadrature to the 38 KHz demodulating signal. A phase detector 44
is connected to the junction 12 and to the second frequency divider
42 for developing the differential error voltage between the
junctions 36 and 38 such that the amplitude of the error voltage is
defined in proportion to the difference between the phase of the 19
KHz pilot signal and the phase of the 19 KHz reference signal as
determined by the phase of the 38 KHz demodulating signal.
The FM-stereo decoder 14 may be provided by a Fairchild uA758
FM-stereo decoder which is commercially available as a monolithic
integrated circuit from Fairchild Semiconductor Corporation. A more
detailed description of the uA758 FM-stereo decoder may be obtained
by reference to "The uA758, A Phase Locked Loop FM Stereo Multiplex
Decoder" by Larry Blaser and Bill Cocke, as published in
application Note 319, June, 1972, Fairchild Semiconductor
Corporation. A similar FM-stereo decoder is described in "A
Monolithic Phase-Lock-Loop Stereo Decoder," by Michael J. Gay, as
published in I.E.E.E. Transactions on Broadcast and TV Receivers,
November, 1971.
In the presence of distorted reception, whether "multipath
reception" or a form of "overload reception," the 19 KHz signal as
well as the rest of the composite stereophonic signal is severely
phase distorted. Multipath reception occurs when two identical
signals emanating from the same transmitting antenna arrive at the
same receiving antenna out of phase after traveling over different
propagation paths. Ordinarily, one of the signals is received
directly from the transmitting antenna while the other of the
signals is received indirectly after reflection from an object
disposed between the transmitting and receiving antennas. At the
receiving antenna, the direct and reflected signals vectorily add
to form a resultant signal which is phase distorted. Overload
distortion may take a number of forms, but in general, it may be
characterized as occurring when at least two unwanted signals mix
together to provide a beat signal having a frequency component
which vectorily adds with a wanted signal to form a resultant
signal which is phase distorted. In either multipath reception or
overload reception, the ultimate phase distortion of the composite
stereophonic signal is manifested by a corresponding amplitude
distortion of the L and R audio signals which, in turn, produces a
derivative audible distortion from the stereo speakers 30 and
32.
It is well known that the signal-to-noise ratio of monophonic
operation is inherently greater than the signal-to-noise ratio of
stereophonic operation. In other words, given the same amount of
phase distortion of an FM signal, the resulting audio distortion is
less discernible to a listener during monophonic operation than
during stereophonic operation. Therefore, it is desirable to shift
from full stereophonic operation toward full monophonic operation
during distorted reception. Further, as the phase of the 19 KHz
pilot signal is initially shifted in response to the onset of
distorted reception, the amplitude of the error voltage appearing
between the junctions 36 and 38 of the phase-lock-loop 16 increases
sharply. Seizing upon the foregoing facts and circumstances, the
present invention provides a stereophonic/monophonic blend system
46 for reducing the separation between the L and R output channels
22 and 24 of the audio decoder 18 in response to an increase in the
amplitude of the error voltage indicative of distorted reception
thereby to shift from full stereophonic operation toward full
monophonic operation so as to reduce the discernible audio
distortion resulting from the distorted reception.
Referring again to FIG. 1, the stereophonic/monophonic blend system
46 includes a DC amplifier 48, a level detector 50, and a coupling
stage 52. The DC amplifier 48 is responsive to the error voltage
appearing between junctions 36 and 38 to provide an amplified
replica of the error voltage between a pair of lines 54 and 56.
Conveniently, the DC amplifier 48 may be provided by a Westinghouse
WC-115-P differential amplifier which is commercially available as
a monolithic integrated circuit from Westinghouse Electric
Corporation. Alternately, where the FM-stereo decoder 14 is
provided by a Fairchild uA758 FM-stereo decoder, the DC amplifier
48 may be provided by a DC amplifier which is internal to the
voltage controlled oscillator 34 of the uA758 integrated
circuit.
The coupling stage 52 is connected across the L and R output
channels 22 and 24 of the audio decoder 18. Preferably, the
coupling stage 52 includes an electronic switch 58 provided by a
field-effect transistor having gate, source and drain electrodes.
Further, an impedance device 60 is connected between the source
electrode of the transistor 58 and the L output channel 22 of the
audio decoder 18. The drain electrode of the transistor 52 is
connected directly to the R output channel 24 of the audio decoder
18. Of course, since the field-effect transistor 52 is a bilateral
device, the source and drain electrodes are interchangeable.
Further, at this point, it is to be understood that the
field-effect transistor 58 may be replaced by any suitable
electronic switch such as a junction bipolar transistor and that
the impedance device 60 may or may not be present within the
coupling network 52 as will become more apparent hereinafter.
The level detector 50 is connected between the gate electrode of
the field-effect transistor 58 and the lines 54 and 56 emanating
from the DC amplifier 48. When the amplitude of the error voltage
appearing between the lines 54 and 56 increases in a manner
indicative of the presence of distorted reception, the level
detector 50 applies a bias voltage to the gate electrode of the
field-effect transistor 58 to render the transistor 58 fully
conductive. Conversely, when the amplitude of the error voltage
appearing between the lines 54 and 56 decreases in a manner
indicative of the absence of distorted reception, the level
detector 50 withdraws the bias voltage from the gate electrode of
the field-effect transistor 58 to render the transistor 58 fully
nonconductive.
With the transistor 58 normally turned off, the separation between
the L and R output channels 22 and 24 of the audio decoder 18 is
defined at substantially full channel separation thereby to provide
full stereophonic operation. Alternately, when the transistor 58 is
turned on in response to distorted reception, the separation
between the L and R output channels 22 and 24 of the audio decoder
18 is defined at less than full channel separation thereby to
provide less than full stereophonic operation for reducing the
discernible audio distortion resulting from the distorted
reception. If the impedance device 60 is absent from the coupling
state 52, the separation between the L and R output channels 22 and
24 is defined at substantially zero channel separation when the
transistor 58 is turned on. FIG. 2 illustrates the operating
characteristic (i.e. a graph of channel separation versus audio
frequency) of the coupling stage 52 where the impedance device 60
is absent and the transistor 58 turned on.
In order to shift from full stereophonic operation toward full
monophonic operation in time to reduce discernible distortion
resulting from distorted reception, it is necessary that the level
detector 50 have a "fast attack" capability; that is, the level
detector 50 must be responsive to an initial increase in the
amplitude of the error voltage due to the initiation of distorted
reception to substantially instantaneously turn on the transistor
58 to decrease the separation between the L and R output channels
22 and 24 of the audio decoder 18 thereby to rapidly shift to less
than full stereophonic operation. On the other hand, in order to
minimize the discernible audio disruption resulting from the
transition to full stereophonic operation following the excursion
toward full monophonic operation, it is required that the level
detector 50 have a "slow retreat" capability; that is the level
detector 50 must relatively gradually turn off the transistor 58 to
increase the separation between the L and R output channels 22 and
24 of the audio decoder 18 thereby to slowly restore full
stereophonic operation. FIG. 3 illustrates an embodiment of the
level detector 50 which possesses the desired "fast attack" and
"slow retreat" features, and which is suitable for fabrication
within a monolithic integrated circuit.
Referring to FIG. 3, the level detector 50 includes a high
potential line 62 and a low potential line 64 across which a direct
current supply voltage is applied. Preferably, the low potential
line 64 is grounded. Where the FM-stereo decoder 14 is provided by
a Fairchild uA758-stereo decoder, the DC supply voltage may be
derived from a power supply internal to the uA758 integrated
circuit. In fact, the level detector 50 may be fabricated upon the
same semiconductor chip as the uA758 FM-stereo decoder.
The level detector 50 includes a level sensor 66, a timing network
68 and a buffer stage 70. The level sensor 66 includes a
differential voltage switch provided by a pair of voltage switching
transistors 72 and 74 of the NPN junction type and a complementary
current switch provided by a pair of current switching transistors
76 and 78 of the PNP and NPN junction type, respectively. The base
electrodes of the transistors 72 and 74 are connected directly to
the lines 54 and 56, respectively, emanating from the DC amplifier
48. The emitter electrodes of the transistors 72 and 74 are
connected together at a junction 80. The collector electrode of the
transistor 72 is connected directly to a junction 82. The collector
electrode of the transistor 74 is connected directly to the high
potential line 62. A bias diode 84 is connected between the high
potential line 62 and the junction 82. The base electrode of the
transistor 76 is connected directly to the junction 82. The emitter
electrode of the transistor 76 is connected directly to the high
potential line 62. The collector electrode of the transistor 76 is
connected directly to a junction 92. A bias resistor 86 is
connected between the junction 80 and a junction 88. A bias diode
90 is connected between the junction 88 and the low potential line
64. The base electrode of the transistor 78 is connected directly
to the junction 88. The emitter electrode of the transistor 78 is
connected directly to the low potential line 64. The collector
electrode of the transistor 76 is connected directly to the
junction 92.
The timing network 68 includes a timing capacitor 94 across which a
timing voltage V.sub.t is developed. The timing capacitor 94 is
connected between a junction 96 and the low potential line 64. A
Darlington switch is provided by a pair of transistors 98 and 100
of the NPN junction type. Specifically, the base electrode of the
transistor 98 is connected directly to the junction 92. The
collector electrode of the transistor 98 and the collector
electrode of the transistor 100 are connected together to the
junction 96. The emitter electrode of the transistor 98 is
connected directly to the base electrode of the transistor 100. The
emitter electrode of the transistor 100 is connected directly to
the low potential line 64. A pair of bias resistors 102 and 104 are
connected in series between the high potential line 68 and the
junction 96 such that a junction 106 is defined between the
resistors 102 and 104.
The buffer stage 70 includes a buffer switch formed by a pair of
transistors 108 and 110 of the PNP and NPN junction type,
respectively. The base electrode of the transistor 108 is connected
directly to the junction 106. The emitter electrode of the
transistor 108 and the collector electrode of the transistor 110
are connected together to the high potential line 62. The collector
electrode of the transistor 108 is connected directly to the base
electrode of the transistor 110. The emitter electrode of the
transistor 110 is connected directly to a junction 112. A bias
resistor 114 is connected between the junction 112 and the low
potential line 64. Further, the gate electrode of the field-effect
transistor 58 in the coupling network 52 is connected directly to
the junction 112.
In the level sensor 66, the base-emitter junction area of the
transistor 74 is greater than the base-emitter junction area of the
transistor 72 by a factor of (3). Assuming for the moment, that the
amplitude of the error voltage defined between lines 54 and 56 is
substantially zero (i.e., the base electrode of the transistor 72
is essentially shorted to the base electrode of the transistor 74),
a current I.sub.1 flows through the transistor 72 and the diode 84
and a current I.sub.2 approximately equal to (3) I.sub.1 flows
through the transistor 74. In addition, a current I.sub.3
approximately equal to the summation of the currents I.sub.2 and
I.sub.2 or (4) I.sub.1 flows through the resistor 86 and the diode
90. The anode-cathode junction area of the diode 90 is greater than
the base-emitter junction area of the transistor 78 by a factor of
(2). Thus, since the current I.sub.3 flowing through the diode 90
is approximately equal to (4) I.sub.1, a current I.sub.4
approximately equal to (2) I.sub.1 flows through the transistor 78
out of the junction 92. The anode-cathode junction area of the
diode 84 is the same as the base-emitter junction area of the
transistor 76. Hence, a current I.sub.5, which is approximately
equal to the current I.sub.1 flowing through the diode 84, flows
through the transistor 76 into the junction 92.
When the amplitude of the error voltage appearing between lines 54
and 56 is at a predetermined magnitude indicative of distorted
reception, whether multipath reception or a form of overload
reception, the current I.sub.1 flowing through the transistor 72 is
approximately equal to the current I.sub.2 flowing through the
transistor 74. As a result, the current I.sub.3 flowing through the
diode 90 is equal to (2) I.sub.1 and the current I.sub.4 flowing
through the transistor 78 is equal to I.sub.1. Likewise, the
current I.sub.5 flowing through the transistor 76 is equal to
I.sub.1. Therefore, the current I.sub.5 flowing into the junction
92 out of transistor 76 is approximately equal to the current
I.sub.4 flowing out of junction 92 into the transistor 78. As the
amplitude of the error voltage appearing between the lines 54 and
56 exceeds the predetermined magnitude indicative of distorted
reception, the current I.sub.5 flowing through the transistor 76
into the junction 92 exceeds the current I.sub.4 flowing through
the transistor 78 out of the junction 92. Conversely, as the
amplitude of the error voltage appearing between the lines 54 and
56 decreases below the predetermined magnitude indicative of
distorted reception, the current I.sub.4 flowing through the
transistor 78 out of the junction 92 exceeds the current I.sub.5
flowing through the transistor 78 into the junction 92.
When the current I.sub.4 flowing out of the junction 92 exceeds the
current I.sub.5 flowing into the junction 92, the Darlington switch
formed by the transistors 98 and 100 is rendered fully
nonconductive. With the transistors 98 and 100 turned off, the
capacitor 94 is maintained fully charged through the resistors 102
and 104 so that the amplitude of the timing voltage V.sub.t
developed across the capacitor 94 is at a maximum voltage level.
With the timing voltage V.sub.t at a maximum voltage level, the
potential at the junction 106 is held at a relatively high voltage
level by the voltage divider action of the resistors 102 and 104.
With the junction 106 at a relatively high voltage level, the
buffer switch formed by the transistors 108 and 110 is rendered
fully nonconductive. With the transistors 108 and 110 turned off,
bias voltage is withheld from across the resistor 114 at the
junction 112 so that the field-effect transistor 58 is rendered
fully nonconductive. With the transistor 58 turned off, the
separation between the L and R output channels 22 and 24 of the
stereo decoder 18 is defined at substantially full channel
separation thereby to provide full stereophonic operation.
As the current I.sub.5 flowing through the junction 92 exceeds the
current I.sub.4 flowing out of the junction 92 in response to an
initial increase in the amplitude of the error voltage appearing
between lines 54 and 56 above the predetermined magnitude
indicative of distorted reception, the Darlington switch formed by
the transistors 98 and 100 is rendered fully conductive. With the
transistors 98 and 100 turned on, the timing capacitor 94 is
substantially instantaneously discharged to rapidly decrease the
amplitude of the timing voltage V.sub.t to a minimum voltage level
defined just above the voltage level on the low potential line 64
by an amount equal to the saturation voltage drop of the
transistors 98 and 100. With the timing voltage V.sub.t at a
minimum voltage level, the potential at the junction 106 is dropped
to a relatively low voltage level defined by the voltage divider
action of the resistors 102 and 104. With the junction 106 at a
relatively low voltage level, the buffer switch formed by the
transistors 108 and 110 is rendered fully conductive. With the
transistors 108 and 110 turned on, a bias voltage is applied across
the resistor 114 at the junction 112 to substantially
instantaneously render the transistor 58 fully conductive. As the
transistors 58 is turned on, the separation between the L and R
output channels 22 and 24 of the audio decoder 18 is substantially
instantaneously decreased to less than full channel separation
thereby to shift toward full monophonic operation so as to reduce
the audio distortion resulting from the distorted reception.
When the current I.sub.4 flowing out of the junction 92 exceeds the
current I.sub.5 flowing into the junction 92 in response to a
subsequent decrease in the amplitude of the error voltage appearing
between lines 54 and 56 below the predetermined magnitude
indicative of distorted reception, the Darlington switch formed by
the transistors 98 and 100 is rendered fully nonconductive. With
the transistors 98 and 100 turned off, the timing capacitor 94
charges through the resistors 102 and 104 to relatively gradually
increase the amplitude of the timing voltage V.sub.t from the
minimum voltage level back to the maximum voltage level in
accordance with the RC time constant provided by the resistance of
the resistors 102 and 104 and the capacitance of the capacitor 94.
As the amplitude of the timing voltage V.sub.t increases, the
potential at the junction 106 likewise increases to slowly render
the buffer switch formed by the transistors 108 and 110 fully
nonconductive. As the transistors 108 and 110 are turned off, the
bias voltage applied across the resistor 114 at the junction 112 is
withdrawn to relatively gradually turn off the field-effect
transistor 58. As the transistor 58 turns off, the separation
between the L and R output channels 22 of the audio decoder 18 is
relatively gradually increased back to full channel separation
thereby to slowly shift to full stereophonic operation so as to
reduce the audio distortion resulting from the transition from less
than full stereophonic operation back to full stereophonic
operation.
It will now be appreciated that the substantially instantaneous
decrease in the separation between the L and R output channels 22
and 24 of the audio decoder 18 in response to the initiation of
distorted reception, can itself be a source of perceptible audio
distortion. In other words, the rather sudden shift from full
stereophonic operation toward full monophonic operation may cause
discernible audio disruption, particularly where the shift is from
full stereophonic operation all the way to full monophonic
operation. FIG. 4 illustrates an embodiment of the impedance device
60 for reducing the discernible audio disruption resulting fromm
the shift to less than full stereophonic operation from full
stereophonic operation.
Referring to FIG. 4, the impedance device 60 comprises a resistor
116 which is connected in series with the field-effect transistor
58 between the L and R output channels 22 and 24 of the audio
decoder 18. When the transistor 58 is turned on, the separation
between the L and R output channels 22 and 24 is established at an
intermediate channel separation defined below full channel
separation and above zero channel separation. Actually, the
intermediate channel separation is defined below full channel
separation in inverse proportion to the resistance of the resistor
116, or alternately, is defined above zero channel separation in
direct proportion to the resistance of the resistor 116. Thus, the
resistor 116 provides blended stereophonic/monophonic operation for
reducing the discernible audio disruption resulting from the rapid
shift toward full monophonic operation from full stereophonic
operation. FIG. 5 illustrates the operating characteristic of the
coupling network 52 where the impedance device 60 includes the
resistor 116 and the transistor 58 is turned on.
Further, it is well known that the major portion of the L and R
audio signals is composed of lower frequency components while most
of the audio distortion in the L and R audio signals results from
the higher frequency components. FIG. 6 illustrates an embodiment
of the impedance device 60 for maximizing the amount of
stereophonic operation over the lower frequencies of the L and R
audio signals thereby to enhance the total stereophonic effect
while maximizing the amount of monophonic operation over the high
frequencies of the L and R audio signals thereby to reduce the
resulting audio distortion.
Referring to FIG. 6, the impedance device 60 comprises a reactive
element provided by a capacitor 118 which is connected in series
with the field-effect transistor 58 between the L and R output
channels 22 and 24 of the audio decoder 18. When the transistor 58
is turned on, the separation between the L and R output channels 22
and 24 is variably defined between full channel separation and zero
channel separation in direct proportion to the frequency of the L
and R audio signals. More specifically, the separation between the
L and R output channels 22 and 24 is variably established in
inverse proportion to the reactance of the capacitor 118 which, in
turn, is determined in inverse proportion to the frequency of the L
and R audio signals.
At the lower frequencies of the L and R audio signals, the
reactance of the capacitor 118 is relatively high for increasing
the separation between the L and R output channels 22 and 24
thereby to maximize the amount of stereophonic operation. On the
other hand, at the higher frequencies of the L and R audio signals,
the reactance of the capacitor 118 is relatively low for decreasing
the separation between the L and R output channels 22 and 24
thereby to maximize the amount of monophonic operation.
Accordingly, the capacitor 116 provides blended
stereophonic/monophonic operation for maximizing total stereophonic
effect while minimizing overall audio distortion. FIG. 7
illustrates the operating characteristic of the coupling network 52
where the impedance device 60 comprises the capacitor 118 and where
the transistor 58 is turned on.
FIG. 8 illustrates an embodiment of the impedance device 60 which
is a combination of the embodiments shown in FIGS. 4 and 6 for
providing all of the advantages of the embodiments shown in FIGS. 4
and 6. Referring to FIG. 8, the impedance device 60 comprises the
series connection of the resistor 116 and the capacitor 118 with
the field-effect transistor 58 across the L and R output channels
22 and 24 of the audio decoder 18. When the transistor 58 is turned
on, a minimum separation between the L and R output channels 22 and
24 is established at an intermediate channel separation located
below full channel separation and above zero channel separation as
determined by the resistance of the resistor 116. Further, when the
transistor 58 is turned on, the separation between the L and R
output channels 22 and 24 is variably defined from full channel
separation to the intermediate channel separation in direct
proportion to the frequency of the L and R audio signals as
determined by the reactance of the capacitor 118. FIG. 9
illustrates the operating characteristic of the coupling network 52
where the impedance device 60 comprises the series combination of
the resistor 116 and the capacitor 118 and where the transistor 58
is turned on.
It will now be appreciated that the illustrated embodiments of the
invention are shown for demonstrative purposes only and that
various alterations and modifications may be made to these
embodiments without departing from the spirit and scope of the
invention.
* * * * *