CA1211803A - A.m. stereo receiver with inverse amplitude modulator - Google Patents
A.m. stereo receiver with inverse amplitude modulatorInfo
- Publication number
- CA1211803A CA1211803A CA000482065A CA482065A CA1211803A CA 1211803 A CA1211803 A CA 1211803A CA 000482065 A CA000482065 A CA 000482065A CA 482065 A CA482065 A CA 482065A CA 1211803 A CA1211803 A CA 1211803A
- Authority
- CA
- Canada
- Prior art keywords
- signal
- supplied
- modulator
- inverse
- modulating
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Landscapes
- Stereo-Broadcasting Methods (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
The present invention provides an inverse amplitude modulator for use in a receiver to provide distortion of the quadrature modulation component of a signal received at the receiver. The inverse amplitude modulator has a modulation characteristic of the form with respect to a supplied modulating signal (B) and a supplied signal (A) which is to be inversely modulated. The modulator supplies a first signal comprising the modulating signal (B) and a DC component in the form of (1+B) and a second signal comprising the (A) signal to be modulated for logarithmically amplifying the first signal and for developing a third signal (C) comprising substantially only the AC components of the logarithmically amplified signal. The modulator has a modulation characteristic of the form A(1-.5C), for inversely modulating the second signal (A) by the third signal (C), whereby with respect to the supplied signals (A) and (B), the combination exhibits a modulation characteristic substantially of the desired form
The present invention provides an inverse amplitude modulator for use in a receiver to provide distortion of the quadrature modulation component of a signal received at the receiver. The inverse amplitude modulator has a modulation characteristic of the form with respect to a supplied modulating signal (B) and a supplied signal (A) which is to be inversely modulated. The modulator supplies a first signal comprising the modulating signal (B) and a DC component in the form of (1+B) and a second signal comprising the (A) signal to be modulated for logarithmically amplifying the first signal and for developing a third signal (C) comprising substantially only the AC components of the logarithmically amplified signal. The modulator has a modulation characteristic of the form A(1-.5C), for inversely modulating the second signal (A) by the third signal (C), whereby with respect to the supplied signals (A) and (B), the combination exhibits a modulation characteristic substantially of the desired form
Description
". f~, lZ~803
2 This invention relates to AM stereophonic
3 receivers, ~nd particularly to QM stereophonic
4 receivers for receiving signals modulated according to
5 the well known independent sideband (ISB) modulation
6 technique. However, the invention is also useful in
7 receivers for other composite amplitude and angular
8 modulated stereo signals wherein a demodulated AM
9 signal is used in conjunction with the angular modulation of such a composite signal for stereo 11 reproduction.
12 U.S. Patent No. 4,018,994 to Kahn describes 13 an AM stereophonic receiver which includes an envelope 14 detector, for detecting the amplitude modulation component of a received ISB AM stereo IF signal, and 16 circuitry for detecting the quadrature modulation 17 resulting from the phase modulation in the IF signal.
18 The particular quadrature demodulation circuitry used 19 by Kahn modifies the quadrature modulation component of the IF signal prior to quadrature detection for 21 purposes of distortion correction.
22 In AM receivers, and particularly those 23 with continuous tuning, it is often useful to have a 24 non-flat automatic gain control (AGC) characteristic _ ~2~1803 so that the signal level at the output of the envelope detector continuously increases with increasing received signal strength, rather than level off at some point. If an ideal AGC circuit is used, the output signal level increases with icreasing received signal level up to a certain level, and thereafter is flat. If no AGC is present, the output signal increases linearly as a function of the input signal level. With a non-flat AGC
circuit, the output signal level increases linearly with increasing input signal level up to a certain level, and thereafter increases at a lesser rate with respect to the input signal level. A non-flat AGC characteristic is helpful to the operator of such a receiver in that tuning the receiver is easier, because the frequency at which maximum signal strength is received (i.e., the center of the band) is more easily sensed. Non-flat AGC also minimizes interstation noise and requires a lower value of maximum gain in a receiver's IF circuitry.
If a non-flat AGC characteristic is used in connection with an AM stereo receiver of the type ~2~803 1 described in prior Kahn patent, the changing level of the 2 envelope detector output, corresponding to the carrier 3 level at the detector (which is a function of tuning and 4 received signal strength), will result in an improper ~orrection signal being supplied to the inverse modulator 6 and, therefore, improper distortion correction.
7 It is, therefore, an object of the present 8 invention to provide a receiver which is generally of the 9 distortion correcting type disclosed in the aforementioned Kahn patent, but which has a non-flat AGC
11 characteristic.
13 The present invention is applicable in a 14 receiver for receiving a signal having a carrier which has been amplitude modulated with a first modulating 16 signal and angle modulated with a second modulating 17 signal. In particular, the invention is applicable in a 18 receiver wherein there are provided first means for lg demodulating such a composite signal to obtain a first demodulated signal representative of the 21 first modulating signal and wherein the first de-22 modulated signal has a signal level dependent on received ~ ~'~
` ~21~803 ~
1 carrier level. The receiver also includes second 2 means for demodulating the composite signal to obtain 3 a second demodulated signal representative of the 4 quadrature modulation component produced by angular modulation of the carrier by the second modulating 6 signal, the second demodulating means being responsive 7 also to the first demodulated signal. In accordance 8 with the invention, there is provided an improvement 9 wherein the second demodulating means includes means responsive to the first demodulated signal for 11 deriving therefrom a modification signal which is 12 substantially independent of carrier signal level.
13 The second demodulating means also includes means 14 responsive to the modification signal for modifying the quadrature modulation component of the received 16 signal thereby to derive the second demodulated signal.
17 The first demodulating means may comprise 18 an envelope detector, in which case the first 19 demodulated signal has a magnitude multiplier term which is dependent on the received carrier level. The 21 means for deriving the modification signal may 22 therefore comprise means for normalizing the first 23 demodulated signal. The normalizing means in one case 24 may be the series combination of a logarithmic amplifier and a high pass filter. The output of the 26 logarithmic amplifier and high pass filter can - \
1 thereafter be provided to a subtractive type inverse 2 modulator for modifying the quadrature component of 3 the recei~ed signal by acting on either the composite 4 signal or the quadrature demodulated signal. In another case, the normalizing means may comprise the 6 series combination of a low pass filter and a 7 divider. In this case, the inverse modulator 8 preferably comprises a reciprocal type inverse 9 modulator. As noted previously, the invention is of particular advantage in receivers which have a 11 non-flat ~GC characteristic.
12 In accordance with another aspect of the 13 invention, there is provided apparatus for receiving 14 and demodulating a composite AM stereo signal wherein a stereo sum signal is amplitude modulated on a 16 carrier and a stereo difference signal is phase 17 modulated on the carrier. The apparatus includes 18 first means for amplitude demodulating the received 19 composite signal to derive therefrom a first demodulated signal representative of the stereo sum 21 signal and having a signal characteristic dependent on 22 the carrier signal level of the composite signal.
23 There is provided a second means which is responsive 24 to the first demodulated signal, for deriving a modification signal having signal characteristics 26 independent of the carrier signal level of the ~21~803 1 composite signal. Third means are provided which is 2 responsive to the modification signal for modifying 3 the quadrature modulation component of said composite 4 signal.
In one embodiment, the second means 6 comprises a logarithmic amplifier which is responsive 7 to the first demodulated signal and has an output 8 signal representative of the sum of a first 9 logarithmic signal, which is proportional to the logarithm of the DC component of the first demodulated 11 signal level, and a second logarithmic signal, which 12 is representative of the stereo sum signal. A high 13 pass filter is provided for removing the first 14 logarithmic signal, whereby the modification signal comprises substantially only the second logarithmic 16 signal. In this embodiment, the third means 17 preferably comprises a subtractive type inverse 18 modulator.
19 In another embodiment, the second means comprises a low pass filter for deriving the DC
21 component of the first demodulated signal and a 22 divider for dividing the first demodulated signal by 23 the DC component, whereby the divider output comprises 24 the desired modification signal. Tn thls embodiment, the third means preferably comprises a reciprocal type 26 inverse ~odulator.
. . !`
12118~3 1 In either of the above described 2 embodiments the inverse modulator may be introduced 3 either ahead of or following quadrature demodulation 4 of the composite signal, so as to modify the quadrature modulation component of the signal.
6 For a better understanding of the present 7 invention, together with other and further objects, 8 reference is made to the following description, taken 9 in conjunction with the accompanying drawings, and its scope will be pointed out in the appended claims.
.
12 Figure 1 is a graph illustrating detected 13 signal output level as a function of input signal 14 ievel for various AGC characteristics.
Figure 2 is a block diagram of a composite 16 signal receiver in accordance with the prior art.
17 Figure 3 is a block diagram of a composite 18 signal demodulating apparatus usable in a receiver 19 according to the present invention.
, Figure 4 is a block diagram of an 21 alternative configuration of a composite signal 22 demodulating apparatus usable in a receiver according 23 to the present invention.
~2~8~3 1 Figure 5 is a block diagram illustrating an 2 alternative arrangement for the embodiments shown in 3 Figs. 3 and 4.
Referring to Figure 2, which iIlustrates in 6 block diagram form a receiver according to the, 7 principles of Kahn U.S. Patent No. 4,018,994, there is 8 shown a receiver 10 for receiving and demodulating AM
9 stereo signals, and particular,ly signals of the type known as independent sideband (ISB) AM stereo 11 signals. This receiver is more fully described in the 12 aforementioned Kahn patent, but will be reviewed for 13 purposes of clarifying the nature and scope of the 14 present invention.
Receiver 10 includes an antenna 12 and RF
16 and IF circuits 14 which receive a composite 17 amplitude and phase modulated.AM stereo signal and 18 provide a composite IF signal output on lead 15 which 19 is supplied to a first demodulating means, comprising envelope detector 16, and a second demodulating means, 21 comprising the combination of inverse modulator 22 and 22 quadrature detector 26. The output of envelope 23 detector 16 consists of a first demodulated signal 24 (A+AX+) or A(l+X+) which is representative of a first modulating signal, the stereo sum signal (X+), `` lZ11803 1 `which is amplitude modulated onto the carrier at the 2 transmitter. Assuming that IF circuit 14 includes an 3 AGC circuit which provides a non-flat AGC
4 characteristic, such as that described previously herein with respect to curve 7,of Figure 1, as a 6 result, the envelope detector oupu~ includes a 7 magnitude modifier (A) which is dependent on the 8 carrier signal level of the received composite 9 signal. In accordance with the prior U.S. patent to Kahn, it is desirable to use the AC components of the 11 demodulated signal, represented by X+, for modifying 12 the quadrature modulation component of the IF signal 1~ in inverse modulator 22 prior to supplying the 14 modified IF signal to quadrature detector 26. The DC
component, represented by (1) in the expression 16 (l+X+), would normally be discarded by means of a 17 series AC coupling capacitor 9.
18 However, if the IF circuit 14 has a 19 non-flat AGC characteristic such as.curve 7 shown in Figure 1, the amplitude of the modification signal 21 supplied on lead 20 to inverse modulator 22 will vary 22 according to the carrier level at envelope detector 23 16, whic.h is a function of receiver tuning and 24 received signal strength. As a result, the modified IF signal at the output of inverse modulator 22 will 26 include the undesired magnitude modifier (A) which is 27 a function of the carrier signal level. Accordingly,
12 U.S. Patent No. 4,018,994 to Kahn describes 13 an AM stereophonic receiver which includes an envelope 14 detector, for detecting the amplitude modulation component of a received ISB AM stereo IF signal, and 16 circuitry for detecting the quadrature modulation 17 resulting from the phase modulation in the IF signal.
18 The particular quadrature demodulation circuitry used 19 by Kahn modifies the quadrature modulation component of the IF signal prior to quadrature detection for 21 purposes of distortion correction.
22 In AM receivers, and particularly those 23 with continuous tuning, it is often useful to have a 24 non-flat automatic gain control (AGC) characteristic _ ~2~1803 so that the signal level at the output of the envelope detector continuously increases with increasing received signal strength, rather than level off at some point. If an ideal AGC circuit is used, the output signal level increases with icreasing received signal level up to a certain level, and thereafter is flat. If no AGC is present, the output signal increases linearly as a function of the input signal level. With a non-flat AGC
circuit, the output signal level increases linearly with increasing input signal level up to a certain level, and thereafter increases at a lesser rate with respect to the input signal level. A non-flat AGC characteristic is helpful to the operator of such a receiver in that tuning the receiver is easier, because the frequency at which maximum signal strength is received (i.e., the center of the band) is more easily sensed. Non-flat AGC also minimizes interstation noise and requires a lower value of maximum gain in a receiver's IF circuitry.
If a non-flat AGC characteristic is used in connection with an AM stereo receiver of the type ~2~803 1 described in prior Kahn patent, the changing level of the 2 envelope detector output, corresponding to the carrier 3 level at the detector (which is a function of tuning and 4 received signal strength), will result in an improper ~orrection signal being supplied to the inverse modulator 6 and, therefore, improper distortion correction.
7 It is, therefore, an object of the present 8 invention to provide a receiver which is generally of the 9 distortion correcting type disclosed in the aforementioned Kahn patent, but which has a non-flat AGC
11 characteristic.
13 The present invention is applicable in a 14 receiver for receiving a signal having a carrier which has been amplitude modulated with a first modulating 16 signal and angle modulated with a second modulating 17 signal. In particular, the invention is applicable in a 18 receiver wherein there are provided first means for lg demodulating such a composite signal to obtain a first demodulated signal representative of the 21 first modulating signal and wherein the first de-22 modulated signal has a signal level dependent on received ~ ~'~
` ~21~803 ~
1 carrier level. The receiver also includes second 2 means for demodulating the composite signal to obtain 3 a second demodulated signal representative of the 4 quadrature modulation component produced by angular modulation of the carrier by the second modulating 6 signal, the second demodulating means being responsive 7 also to the first demodulated signal. In accordance 8 with the invention, there is provided an improvement 9 wherein the second demodulating means includes means responsive to the first demodulated signal for 11 deriving therefrom a modification signal which is 12 substantially independent of carrier signal level.
13 The second demodulating means also includes means 14 responsive to the modification signal for modifying the quadrature modulation component of the received 16 signal thereby to derive the second demodulated signal.
17 The first demodulating means may comprise 18 an envelope detector, in which case the first 19 demodulated signal has a magnitude multiplier term which is dependent on the received carrier level. The 21 means for deriving the modification signal may 22 therefore comprise means for normalizing the first 23 demodulated signal. The normalizing means in one case 24 may be the series combination of a logarithmic amplifier and a high pass filter. The output of the 26 logarithmic amplifier and high pass filter can - \
1 thereafter be provided to a subtractive type inverse 2 modulator for modifying the quadrature component of 3 the recei~ed signal by acting on either the composite 4 signal or the quadrature demodulated signal. In another case, the normalizing means may comprise the 6 series combination of a low pass filter and a 7 divider. In this case, the inverse modulator 8 preferably comprises a reciprocal type inverse 9 modulator. As noted previously, the invention is of particular advantage in receivers which have a 11 non-flat ~GC characteristic.
12 In accordance with another aspect of the 13 invention, there is provided apparatus for receiving 14 and demodulating a composite AM stereo signal wherein a stereo sum signal is amplitude modulated on a 16 carrier and a stereo difference signal is phase 17 modulated on the carrier. The apparatus includes 18 first means for amplitude demodulating the received 19 composite signal to derive therefrom a first demodulated signal representative of the stereo sum 21 signal and having a signal characteristic dependent on 22 the carrier signal level of the composite signal.
23 There is provided a second means which is responsive 24 to the first demodulated signal, for deriving a modification signal having signal characteristics 26 independent of the carrier signal level of the ~21~803 1 composite signal. Third means are provided which is 2 responsive to the modification signal for modifying 3 the quadrature modulation component of said composite 4 signal.
In one embodiment, the second means 6 comprises a logarithmic amplifier which is responsive 7 to the first demodulated signal and has an output 8 signal representative of the sum of a first 9 logarithmic signal, which is proportional to the logarithm of the DC component of the first demodulated 11 signal level, and a second logarithmic signal, which 12 is representative of the stereo sum signal. A high 13 pass filter is provided for removing the first 14 logarithmic signal, whereby the modification signal comprises substantially only the second logarithmic 16 signal. In this embodiment, the third means 17 preferably comprises a subtractive type inverse 18 modulator.
19 In another embodiment, the second means comprises a low pass filter for deriving the DC
21 component of the first demodulated signal and a 22 divider for dividing the first demodulated signal by 23 the DC component, whereby the divider output comprises 24 the desired modification signal. Tn thls embodiment, the third means preferably comprises a reciprocal type 26 inverse ~odulator.
. . !`
12118~3 1 In either of the above described 2 embodiments the inverse modulator may be introduced 3 either ahead of or following quadrature demodulation 4 of the composite signal, so as to modify the quadrature modulation component of the signal.
6 For a better understanding of the present 7 invention, together with other and further objects, 8 reference is made to the following description, taken 9 in conjunction with the accompanying drawings, and its scope will be pointed out in the appended claims.
.
12 Figure 1 is a graph illustrating detected 13 signal output level as a function of input signal 14 ievel for various AGC characteristics.
Figure 2 is a block diagram of a composite 16 signal receiver in accordance with the prior art.
17 Figure 3 is a block diagram of a composite 18 signal demodulating apparatus usable in a receiver 19 according to the present invention.
, Figure 4 is a block diagram of an 21 alternative configuration of a composite signal 22 demodulating apparatus usable in a receiver according 23 to the present invention.
~2~8~3 1 Figure 5 is a block diagram illustrating an 2 alternative arrangement for the embodiments shown in 3 Figs. 3 and 4.
Referring to Figure 2, which iIlustrates in 6 block diagram form a receiver according to the, 7 principles of Kahn U.S. Patent No. 4,018,994, there is 8 shown a receiver 10 for receiving and demodulating AM
9 stereo signals, and particular,ly signals of the type known as independent sideband (ISB) AM stereo 11 signals. This receiver is more fully described in the 12 aforementioned Kahn patent, but will be reviewed for 13 purposes of clarifying the nature and scope of the 14 present invention.
Receiver 10 includes an antenna 12 and RF
16 and IF circuits 14 which receive a composite 17 amplitude and phase modulated.AM stereo signal and 18 provide a composite IF signal output on lead 15 which 19 is supplied to a first demodulating means, comprising envelope detector 16, and a second demodulating means, 21 comprising the combination of inverse modulator 22 and 22 quadrature detector 26. The output of envelope 23 detector 16 consists of a first demodulated signal 24 (A+AX+) or A(l+X+) which is representative of a first modulating signal, the stereo sum signal (X+), `` lZ11803 1 `which is amplitude modulated onto the carrier at the 2 transmitter. Assuming that IF circuit 14 includes an 3 AGC circuit which provides a non-flat AGC
4 characteristic, such as that described previously herein with respect to curve 7,of Figure 1, as a 6 result, the envelope detector oupu~ includes a 7 magnitude modifier (A) which is dependent on the 8 carrier signal level of the received composite 9 signal. In accordance with the prior U.S. patent to Kahn, it is desirable to use the AC components of the 11 demodulated signal, represented by X+, for modifying 12 the quadrature modulation component of the IF signal 1~ in inverse modulator 22 prior to supplying the 14 modified IF signal to quadrature detector 26. The DC
component, represented by (1) in the expression 16 (l+X+), would normally be discarded by means of a 17 series AC coupling capacitor 9.
18 However, if the IF circuit 14 has a 19 non-flat AGC characteristic such as.curve 7 shown in Figure 1, the amplitude of the modification signal 21 supplied on lead 20 to inverse modulator 22 will vary 22 according to the carrier level at envelope detector 23 16, whic.h is a function of receiver tuning and 24 received signal strength. As a result, the modified IF signal at the output of inverse modulator 22 will 26 include the undesired magnitude modifier (A) which is 27 a function of the carrier signal level. Accordingly,
-10--1 the signal at the output of quadrature detector 26 has 2 improper distortion correction because of the presence 3 undesired carrier level dependent magnltude modlfier 4 (A) in the modification signal supplied to inverse modulator 22. More particularly, the inverse 6 modulation percentage may be the correct value to 7 cancel the cross modulation distortion in the X
8 signal at only one carrier level (A), since the 9 percent inverse modulation is dependent upon the lû carrier level (A).
8 signal at only one carrier level (A), since the 9 percent inverse modulation is dependent upon the lû carrier level (A).
11 The outputs of envelope detector 16 and
12 quadrature detector 26 are supplied to 90 degree phase
13 difference networks and a combining matrix, designated
14 by block 28 in Figure 2. The matrix in 28 develops.
separate left and right stereo signals L and R as 16 fully described in the aforementioned Kahn patent.
17 As previously indicated, it is an object of 18 the present invention to provide a receiver of the 19 general type shown in Figure 2 which includes distortion correction to the quadrature modulation 21 component of the received signal and wherein the 22 distortion correction is substantially independent of 23 carrier signal level at envelope detector 16. Various 24 embodiments of such receivers are described herein with reference to Figures 3, 4 and 5.
1 Figure 3 illustrates demodulating apparatus 2 29 which provides distortion correction to the 3 quadrature component of the received signal in a 4 manner which is independent of received carrier signal level. Apparatus 29 may be used in conjunction with 6 the same antenna 12 and RF and IF circuits 14 as are 7 used in the recelver 10 of Figure 2. The IF composite 8 signal on lead 15 can reflect a non-flat AGC
9 characteristic which, as previously mentioned, is desirable particularly for continuous-tuned AM
11 receivers. The first demodulating means, envelope 12 detector 16, provides an output on lead 18 to unit 28 13 which is a first demodulated signal (A+AX+) or 14 A(l+X+), having a magnitude modifier (A) which is dependent on the received carrier signal lével at the 16 detector. This first demodulated signal is supplied 17 to logarithmic amplifier 30 which provides an output 18 signal on lead 32 that is the sum of a first 19 logarithmic signal, logE (A) which is representative only of the carrier signal level dependent modifier 21 (A), and a second logarithmic signal, LogE (l+X+) 22 which is representative only of the stereo sum 23 modulating signal X+. This occurs because of the 24 known mathematical relationship log (X)(Y) = Log (X) + Log (Y). Since the carrier signal level, and 26 therefore the signal logE (A), changes relatively _12-lZ~803 `~ `
1 slowly, it may be removed by high-pass filter 34 2 having a low-frequency cut-off of 5 Hz, for example, 3 to develop an output modification signal on lead 36 4 which is representative substantially only of the stereo sum signal modulation, and is substantially 6 independent of the received carrier signal level. This 7 modification signal is shown as being applied to a 8 substractive type inverse modulator 22, which is also 9 supplied with the IF signal via lead 15. Subtractive type inverse modulator 22 has a transfer 11 characteristic of (1-.5B), for example, [where B =
12 logE (l+X+)] and modifies the IF signal to develop 13 a modified IF signal which is supplied, via lead 38, 14 to quadrature detector 26. The modification performed by substractive inverse modulator 22, using the 16 logarithmic signal representative of the stereo sum 17 modulating signal X+, provides appropriate 18 distortion correction to the quadrature modulation 19 component of the IF signal prior to quadrature detection. Quadrature detector 26 demodulates the 21 quadrature component of the modified IF signal to 22 provide an output on lead 40 which is proportional to 23 the sterço difference signal X and has proper 24 distortion correction. The stereo sum and difference represent-ative signals present on leads 18 and 4û, 26 respectively, are then phase shifted and combined in ` ~i:'`
12~1803 1 the 90 degree phase difference networks and matrix 2 unit 28 illustrated in Figure 3 to derive separate 3 left and right stereo signals.
4 The combination of the subtractive type in-verse modulation function (1-.5B) and the log function 6 B = logE (l+X+) provides the equivalent of a 7 reciprocal type inverse modulation function ~
8 When the log function has a magnitude which corresponds 9 to the natural logarithm (i.e., logE), the first three terms in the expansion of ~ and 11 of the function 1-.5 logE (l+X+) are identical.
12 Good correspondence (for example, within some close 13 tolerance like + a few percent) results over a 14 somewhat greater range of X+ values when the log function has a magnitude of .95 logE.
16 Figure 4 is a block diagram of another 17 demodulatlng apparatus 41 which provides for removal of 18 the carrier signal level dependent modifier (A) from 1~ the output of envelope detector 16, thereby to develop a suitable modification signal for use in ?.~ ~
.
12~18~;)3 l inverse modulator 22. In the demodulating apparatus 2 41 of Figure 4., the output (A+AX+) of the first 3 demodulator 16 is supplied to a low pass filter 42 4 having a high-frequency cut-off of 5 Hz, for example, whose output on lead 44 is a DC
6 component representative of the term (A).
7 The AC components (AX+) of the first demodulated 8 signal are supplied via DC blocking capacitor 9 9 and lead 46 to divider 48 which divides the (AX+) term by the DC term (A) to derive a normalized ll signal X+ which is representative of the sum stereo 12 signal and is independent of carrier signal level. This 13 signal is the modification signal which is supplied on 14 lead 50 to inverse modulator 22. Inverse modulator 22 in this case is preferably a reciprocal type inverse 16 modulator having a transfer characteristic of (1+ 5X ) 17 .for example, whereby proper distortion correction is 18 provided to the quadrature modulation component of the 19 IF signal supplied to inverse modulator 22 on lead 15.
The resulting modified IF signal is supplied, via lead 21 52, to quadrature detector 26 to derive a second 22 demodulated signal proportional to the stereo f`, Ç~
121~803 1 difference signal X and having proper distortion 2 correction. The first and second demodulated signals 3 are then phase shifted and combined in the 90 degree 4 phase difference networks and matrix unit 28 to derive separate left and right stereo signals.
6 It should be recognized that as an 7 alternative the inverse modulation operation, which in 8 the embodiments illustrated in Figs. 3 and 4 is shown 9 being performed on the IF signal prior to quadrature detection, can be performed equally as well on the 11 output signal from the quadrature detector as shown in 12 Fig. 5. In either case the desired distortion 13 correction of the quadrature modulation component of 14 the received signal is accomplished.
_16-.
separate left and right stereo signals L and R as 16 fully described in the aforementioned Kahn patent.
17 As previously indicated, it is an object of 18 the present invention to provide a receiver of the 19 general type shown in Figure 2 which includes distortion correction to the quadrature modulation 21 component of the received signal and wherein the 22 distortion correction is substantially independent of 23 carrier signal level at envelope detector 16. Various 24 embodiments of such receivers are described herein with reference to Figures 3, 4 and 5.
1 Figure 3 illustrates demodulating apparatus 2 29 which provides distortion correction to the 3 quadrature component of the received signal in a 4 manner which is independent of received carrier signal level. Apparatus 29 may be used in conjunction with 6 the same antenna 12 and RF and IF circuits 14 as are 7 used in the recelver 10 of Figure 2. The IF composite 8 signal on lead 15 can reflect a non-flat AGC
9 characteristic which, as previously mentioned, is desirable particularly for continuous-tuned AM
11 receivers. The first demodulating means, envelope 12 detector 16, provides an output on lead 18 to unit 28 13 which is a first demodulated signal (A+AX+) or 14 A(l+X+), having a magnitude modifier (A) which is dependent on the received carrier signal lével at the 16 detector. This first demodulated signal is supplied 17 to logarithmic amplifier 30 which provides an output 18 signal on lead 32 that is the sum of a first 19 logarithmic signal, logE (A) which is representative only of the carrier signal level dependent modifier 21 (A), and a second logarithmic signal, LogE (l+X+) 22 which is representative only of the stereo sum 23 modulating signal X+. This occurs because of the 24 known mathematical relationship log (X)(Y) = Log (X) + Log (Y). Since the carrier signal level, and 26 therefore the signal logE (A), changes relatively _12-lZ~803 `~ `
1 slowly, it may be removed by high-pass filter 34 2 having a low-frequency cut-off of 5 Hz, for example, 3 to develop an output modification signal on lead 36 4 which is representative substantially only of the stereo sum signal modulation, and is substantially 6 independent of the received carrier signal level. This 7 modification signal is shown as being applied to a 8 substractive type inverse modulator 22, which is also 9 supplied with the IF signal via lead 15. Subtractive type inverse modulator 22 has a transfer 11 characteristic of (1-.5B), for example, [where B =
12 logE (l+X+)] and modifies the IF signal to develop 13 a modified IF signal which is supplied, via lead 38, 14 to quadrature detector 26. The modification performed by substractive inverse modulator 22, using the 16 logarithmic signal representative of the stereo sum 17 modulating signal X+, provides appropriate 18 distortion correction to the quadrature modulation 19 component of the IF signal prior to quadrature detection. Quadrature detector 26 demodulates the 21 quadrature component of the modified IF signal to 22 provide an output on lead 40 which is proportional to 23 the sterço difference signal X and has proper 24 distortion correction. The stereo sum and difference represent-ative signals present on leads 18 and 4û, 26 respectively, are then phase shifted and combined in ` ~i:'`
12~1803 1 the 90 degree phase difference networks and matrix 2 unit 28 illustrated in Figure 3 to derive separate 3 left and right stereo signals.
4 The combination of the subtractive type in-verse modulation function (1-.5B) and the log function 6 B = logE (l+X+) provides the equivalent of a 7 reciprocal type inverse modulation function ~
8 When the log function has a magnitude which corresponds 9 to the natural logarithm (i.e., logE), the first three terms in the expansion of ~ and 11 of the function 1-.5 logE (l+X+) are identical.
12 Good correspondence (for example, within some close 13 tolerance like + a few percent) results over a 14 somewhat greater range of X+ values when the log function has a magnitude of .95 logE.
16 Figure 4 is a block diagram of another 17 demodulatlng apparatus 41 which provides for removal of 18 the carrier signal level dependent modifier (A) from 1~ the output of envelope detector 16, thereby to develop a suitable modification signal for use in ?.~ ~
.
12~18~;)3 l inverse modulator 22. In the demodulating apparatus 2 41 of Figure 4., the output (A+AX+) of the first 3 demodulator 16 is supplied to a low pass filter 42 4 having a high-frequency cut-off of 5 Hz, for example, whose output on lead 44 is a DC
6 component representative of the term (A).
7 The AC components (AX+) of the first demodulated 8 signal are supplied via DC blocking capacitor 9 9 and lead 46 to divider 48 which divides the (AX+) term by the DC term (A) to derive a normalized ll signal X+ which is representative of the sum stereo 12 signal and is independent of carrier signal level. This 13 signal is the modification signal which is supplied on 14 lead 50 to inverse modulator 22. Inverse modulator 22 in this case is preferably a reciprocal type inverse 16 modulator having a transfer characteristic of (1+ 5X ) 17 .for example, whereby proper distortion correction is 18 provided to the quadrature modulation component of the 19 IF signal supplied to inverse modulator 22 on lead 15.
The resulting modified IF signal is supplied, via lead 21 52, to quadrature detector 26 to derive a second 22 demodulated signal proportional to the stereo f`, Ç~
121~803 1 difference signal X and having proper distortion 2 correction. The first and second demodulated signals 3 are then phase shifted and combined in the 90 degree 4 phase difference networks and matrix unit 28 to derive separate left and right stereo signals.
6 It should be recognized that as an 7 alternative the inverse modulation operation, which in 8 the embodiments illustrated in Figs. 3 and 4 is shown 9 being performed on the IF signal prior to quadrature detection, can be performed equally as well on the 11 output signal from the quadrature detector as shown in 12 Fig. 5. In either case the desired distortion 13 correction of the quadrature modulation component of 14 the received signal is accomplished.
_16-.
Claims (3)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED
AS FOLLOWS:
1. An inverse amplitude modulator having a modulation characteristic of the form with respect to a supplied modulating signal (B) and a supplied signal (A) which is to be inversely modulated, comprising:
means for supplying a first signal, comprising said modulating signal (B) and a DC
component, in the form (l+B);
means for supplying a second signal comprising said signal to be modulated (A);
means for logarithmically amplifying said first signal and for developing a third signal (C) comprising substantially only the AC components of said logarithmically amplified signal;
and means, having a modulation characteristic of the form A(1-.5C), for inversely modulating said second signal (A) by said third signal (C);
whereby with respect to the supplied signals (A) and (B), said combination exhibits a modulation characteristic substantially of the desired form
means for supplying a first signal, comprising said modulating signal (B) and a DC
component, in the form (l+B);
means for supplying a second signal comprising said signal to be modulated (A);
means for logarithmically amplifying said first signal and for developing a third signal (C) comprising substantially only the AC components of said logarithmically amplified signal;
and means, having a modulation characteristic of the form A(1-.5C), for inversely modulating said second signal (A) by said third signal (C);
whereby with respect to the supplied signals (A) and (B), said combination exhibits a modulation characteristic substantially of the desired form
2. An inverse modulator as specified in claim 1 wherein said supplied signals (A) and (B) are representative of stereo difference and stereo sum signals, respectively.
3. An inverse modulator as specified in claim 2 wherein said logarithmic amplifying means comprises a log amplifier and a high pass filter connected in series.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000482065A CA1211803A (en) | 1982-03-03 | 1985-05-22 | A.m. stereo receiver with inverse amplitude modulator |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/354,332 US4479233A (en) | 1982-03-03 | 1982-03-03 | Distortion correcting AM stereo receiver with non-flat AGC |
| US06/354/332 | 1982-03-03 | ||
| CA000418871A CA1201172A (en) | 1982-03-03 | 1983-01-04 | Distortion correcting am steroe with non-flat agc |
| CA000482065A CA1211803A (en) | 1982-03-03 | 1985-05-22 | A.m. stereo receiver with inverse amplitude modulator |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000418871A Division CA1201172A (en) | 1982-03-03 | 1983-01-04 | Distortion correcting am steroe with non-flat agc |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1211803A true CA1211803A (en) | 1986-09-23 |
Family
ID=25669902
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000482065A Expired CA1211803A (en) | 1982-03-03 | 1985-05-22 | A.m. stereo receiver with inverse amplitude modulator |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1211803A (en) |
-
1985
- 1985-05-22 CA CA000482065A patent/CA1211803A/en not_active Expired
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US4602381A (en) | Adaptive expanders for FM stereophonic broadcasting system utilizing companding of difference signal | |
| US4704726A (en) | Filter arrangement for an audio companding system | |
| US4323731A (en) | Variable-angle, multiple channel amplitude modulation system | |
| EP0185729A1 (en) | Compatible am broadcast/data transmission system | |
| EP0418036A2 (en) | Audible noise reducing | |
| US5253298A (en) | Reducing audible noise in stereo receiving | |
| US4536885A (en) | Distortion correcting AM stereo receiver with non-flat AGC | |
| EP0003867B1 (en) | Am stereophonic receiver | |
| JPS633492B2 (en) | ||
| CA1211803A (en) | A.m. stereo receiver with inverse amplitude modulator | |
| US4479233A (en) | Distortion correcting AM stereo receiver with non-flat AGC | |
| US4270221A (en) | Phaselocked receiver with orderwire channel | |
| US4169968A (en) | Noise protection circuit for am stereo cosine correction factor | |
| US4220818A (en) | AM Stereo transmitter | |
| US4593402A (en) | AM stereo transmitter | |
| US4406922A (en) | Stereo broadcast system | |
| US4466116A (en) | Signal processor for AM stereophonic receiving apparatus | |
| GB2105130A (en) | Synchronous AM envelope detector | |
| US4340782A (en) | Circuit for demodulating amplitude and angle modulated broadcast signals | |
| NZ213273A (en) | Inverse amplitude modulator for distortion correcting am stereo receiver with non-flat agc | |
| US4409436A (en) | Stereo pilot signal eliminating circuit for AM stereophonic receiver | |
| US5046129A (en) | Reducing phase error in received FM multiplex signal | |
| EP0509725A2 (en) | Reducing audible noise in stereo receiving | |
| JPS6315778B2 (en) | ||
| EP1066677B1 (en) | Electronic circuits |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |