NZ213273A - Inverse amplitude modulator for distortion correcting am stereo receiver with non-flat agc - Google Patents

Description

£*5 Priority Date(s): .... Ki.

Complete Specification Filed: /. *. Class: Publication Date: ......A , P.O. Journal, No: ..... ........ 213273 (fhder the provisions of Regulation 23 (!) the Specification has been ante-date^' I9.<SL N.Z. No.

NEW ZEALAND Patents—Act 29 53 COKPLETE- SPECIFICATION "INVERSE AMPLITUDE MODULATOR FOR DISTORTION CORRECTING AM STEREO ^RECEIVER WITH NON-FLAT AGO" : We, HAZELTINE CORPORATION, a corporation organized and existing under the laws of the State of Delaware, United States of America, of Greenlawn, New York 11740, United States of America, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- • - J. - (followed by 1A) V'^;v:;;v:c5vv.': 'v-'' ' v- BACKGROUND OF THE INVENTION This Invention relates to AM stereophonic receivers, and particularly to AM stereophonic receivers for receiving signals modulated according to the well known independent sideband (ISB) modulation technique. However, the invention is also useful in receivers for other composite amplitude and angular modulated stereo signals wherein a demodulated AM signal is used in conjunction with the angular modulation of such a composite signal for stereo reproduction.

U.S. Patent No. 4,018,994 to Kahn describes an AM stereophonic receiver which includes an envelope detector, for detecting the amplitude modulation component of a received ISB AM stereo IF signal, and circuitry for detecting the quadrature modulation resulting from the phase modulation in the IF signal. The particular quadrature demodulation circuitry used by Kahn modifies the quadrature modulation component of the IF signal prior to quadrature detection for purposes of distortion correction.

In AM receivers, and particularly those with continuous tuning, it. is often useful to have a non-flat automatic gain control (AGC) characteristic 2 132 7 & 1 so that the signal level at the output of the envelope 2 detector continuously increases with increasing 3 received signal strength, rather than level off at 4 some point. This characteristic is illustrated in Figure 1, which is a graph of the output signal level 6 from an envelope detector as a function of received 7 signal level. If an ideal AGC circuit is used, the 8 output signal level increases with Increasing received 9 signal level up to a certain level, and thereafter is flat as illustrated by curve 6 in Figure 1. If no AGC 11 is present, the output signal level increases linearly 12 as a function of the input signal level as shown by 13 curve 8 in Figure 1. With a non-flat AGC circuit, the 14 output signal level increases linearly with increasing input signal level up to a certain level, and 16 thereafter increases at a lesser rate with respect to 17 the input signal level, as shown by curve 7 in Figure 18 1. A non-flat AGC characteristic is helpful to the 19 operator of such a receiver in that tuning the receiver is easier, because the frequency at which 21 maximum signal strength is received (i.e., the center 22 of the band) is more easily sensed. Non-flat AGC also 23 minimizes interstation noise and requires a lower 24 value of maximum gain in a receiver's IF circuitry.

If a non-flat AGC characteristic is used in 26 connection with an AM stereo receiver of the., type I*.'-' described in the prior Kahn patent, which is illustrated in Figure 2, the changing level A of the envelope detector output, corresponding to the carrier level at the detector (which is a function of tuning and received signal strength), will result in an improper correction signal being supplied to the inverse modulator 22 and, therefore, improper distortion correction. In Fig. 2, and elsewhere hereinafter, X+ represents the stereo sum signal (L+R) and X_ represents the stereo difference signal (L-R).

It is, therefore, an object of the present invention to provide a receiver which is generally of the distortion correcting type disclosed in the aforementioned Kahn patent, but which has a non-flat AGC characteristic.

SUMMARY OF THE INVENTION The present invention is applicable in a receiver for receiving a signal having a carrier which has been amplitude modulated with a first modulating signal and angle modulated with a second modulating signal. In particular, the invention is applicable in a receiver wherein there are provided first means for demodulating such a composite signal to obtain a first demodulated signal representative of the first modulating signal and wherein the first demodulated signal has a signal level dependent on received 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 modulatinq "*' 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 maonitude multiplier term © ^ 20 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 -■ -4- 213273 thereafter, be provided to a subtract!ve.type inverse modulator for modifying the quadrature component of the received signal by acting on either the composite signal or the quadrature demodulated signal. £ft another aaao/.thc normalising meano may compriac the acrica ■combination of a low paao filter and a divideri 5ft- .this—caeeT-fehe—inverse modulator preferably compriooD a rprrf parrrtca 1 typo invorco modulator. As noted previously, the invention is of particular advantage in receivers which have a non-flat AGC characteristic.

According to a broad aspect of the invention there is provided an inverse amplitude modulator having a modulation characteristic of the form ^ gg 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 (1+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 AC components of said logarithmically -5- /s ^ 4 APR 1986! amplified first signal; and means, having a modulation characteristic of the form AQ--.5C). , for inversely modulating said second signal (A) by said third signal (C); whereby with respect to the supplied signals (A) and (B), the combination exhibits a modulation charac- . A tenstic substantially of the desired form • In one embodiment the supplied signals (A) and (B) are representative of stereo difference and stereo sum signals, respectively.

In another embodiment the logarithmic amplifying means comprises a log amplifier and a high pass filter connected 213273 For a better understanding of the present invention, together with other and further objects, reference is made to the following description, taken in conjunction with the accompanying drawings, and its scope will be pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph illustrating detected signal output level as a function of input signal level for various AGC characteristics.

Figure. 2 is a block diagram of a composite signal receiver in accordance with the prior art.

Figure 3 is a block diagram of a composite signal demodulating apparatus usable in a receiver according to the present invention. 233273 Figure 4 is a block diagram illustrating an alternative arrangement for the embodiment shown in Fig . 3.

DESCRIPTION OF THE INVENTION Referring to Figure 2, which illustrates in block diagram form.a receiver according to the principles of Kahn U.S. Patent No. .4,018,994, there Is shown a receiver 10 for receiving and demodulating AM stereo signals, and particularly.signals of the type known as independent sideband (ISB) AM stereo signals. This receiver is more fully described in the aforementioned Kahn patent, but will be reviewed for purposes of clarifying the nature and scope of the present invention.

Receiver 10 includes an antenna 12 and RF and IF circuits 14 which receive a composite amplitude and phase modulated AM stereo signal and provide a composite IF signal output on lead 15 which is supplied to a first demodulating means, comprising envelope detector 16, and a second demodulating means , comprising the combination of inverse modulator 22 and quadrature detector 26. The output of envelope detector 16 consists of a first demodulated signal (A+AX+) or A(ltXt) which is representative of a first modulating signal, the stereo sum signal .(X+)/, 2 13273 which is amplitude modulated onto the carrier at the transmitter. Assuming that IF circuit 14 includes an AGC circuit which provides a non-flat AGC characteristic, such as that described previously herein with respect to curve 7 in Figure 1, as a result, the envelope detector ouput includes a magnitude modifier (A) which is dependent on the carrier signal level of the received composite signal. In accordance with the prior U.S. patent to Kahn, it is desirable to use the AC components of the demodulated signal, represented by X+, for modifying the quadrature modulation component of the IF signal in inverse modulator 22 prior to supplying the modified IF signal to quadrature detector 26. The DC component, represented by (1) in the expression (1+X+), would normally be discarded by means of a series AC coupling capacitor 9.

However, if the IF circuit 14 has a non-flat AGC characteristic such as curve 7 shown in Figure 1, the amplitude of the modification signal supplied on lead 20 to inverse modulator 22 will vary according to the carrier level at envelope detector 16, which is a function of receiver tuning and received signal strength. As a result, the modified IF signal at the output of inverse modulator 22 will include the undesired magnitude modifier (A) which is imj v 'i • ; = 3 213273 o 1 a function of the carrier signal level. Accordingly, 2 the signal at the output of quadrature detector 26 has 3 improper distortion correction because of the presence 4 undesired carrier level dependent magnitude modifier (A) in the modification signal supplied to inverse 6 modulator 22. More particularly, the inverse 7 modulation percentage may be the correct value to 8 cancel the cross modulation distortion in the X_ 9 signal at only one carrier level (A), since the percent inverse modulation is dependent upon the 11 carrier level (A). 12 The outputs of envelope detector 16 and 13 quadrature detector 26 are supplied to 90 degree phase 14 difference networks and a combining matrix, designated by block 28 in Figure 2. The matrix in 28 develops 16 separate left and right stereo signals L and R as 17 fully described in the aforementioned Kahn patent. 18 As previously indicated, it is an object of 19 the present invention to provide a receiver of the general type shown in Figure 2 which includes 21 distortion correction to the quadrature modulation 22 component of the received signal and wherein the 23 distortion correction is substantially independent of 24 carrier signal level at envelope detector 16. Various embodiments of such receivers are described herein avvi 26 with reference to Figures 3f 4 and 5-.

:•; '.T-r-v '-v-iiV: :\)- .■ ' 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 ere 7 used in the receiver 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 level 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 th.e 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, Log^ (1+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 V ■■ %:■:• ;■, ® r<'"*y te*z\ is 3 7 7 3 1 slowly, it may be removed by high-pass filter 34 2 having a low-frequency cut-off of 5 Hz, for example , j 3 to develop an output modification signal on lead 36 ¥ ;1 4 which is representative substantially only of the | 5 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 I . : ' - ■ \ 9 supplied with the IF signal via lead 15Subtract!ve i 10 type inverse modulator 22 has a transfer I ' | 11 characteristic of (1-.5B), for example, [where B= i i ; 12 log£ (l+X+)3 and modifies the IF signal to develop 1 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 stereo difference signal X_ and has proper G 24 distortion correction. The stereo sum and difference representative signals present on leads 18 and 40, 26 respectively, are then phase shifted and combined in -12- <'■' 213273 the 90 degree phase difference networks and matrix unit 28 illustrated in Figure 3 to derive separate left and right stereo signals.

The combination of the subtractive type inverse modulation function (1-.5B) and the log function B = logE (1+X+) provides the equivalent of a reciprocal type inverse modulation function i+ When the log function has a magnitude which corresponds to the natural logarithm (i.e., log^), the first three terms in the expansion of and of the function 1-.5 log^ (1+X+) are identical.

Good correspondence (for example, within some close tolerance like — a few percent) results over a somewhat greater range of X+ values when the log function has a magnitude of .95 log^.

A'tEN /- -< o // * ,, //V ■■ ■< - 13 - fir | 12MARf9S6^ 213273 It should be recognized that as an alternative the inverse modulation operation, which in the embodiment' illustrated' in"Figs. 3 is shown being performed on the IF signal prior to quadrature detection, can be performed equally as well on the output signal frpm the quadrature detector as shown in U- Fig. In either case the desired distortion correction of the quadrature modulation component of the received signal is accomplished. 213273

Claims (4)

WHAT WE CLAIM IS:

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 formf 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 AC components of said logarithmically amplified first 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), the combination exhibits a modulation characteristic A substantially of the desired form .

2. An inverse amplitude modulator as specified in claim 1 wherein said supplied signals (A) and (B) are representative of stereo difference and stereo sum signals, 213273

3. An inverse amplitude modulator as specified in claim 2 wherein said logarithmic amplifying means comprises a log amplifier and a high pass filter connected in series.

4. An inverse amplitude modulator according to claim 1 substantially as herein described with reference j- of to«the accompanying drawings. HAZELTINE CORPORATION By Their Attorneys HENRY HUGHES LIMITED