Abstract:
An aural carrier signal is corrected in a common amplifier system wherein the correction serves to reduce cross-modulation distortion of the aural carrier signal caused by non-linearities of the common amplifier system. An aural corrector receives a modulated visual carrier signal comprised of a visual carrier signal modulated by a video baseband signal and a phase corrected aural carrier signal and provides therefrom a combined corrected aural carrier signal. The visual carrier signal is combined with the combined corrected aural carrier signal to provide a corrected output carrier signal.

Description:
FIELD OF THE INVENTION 
     This invention relates to the art of correctors for use in common amplification transmitters wherein aural and visual signals are commonly amplified. 
     BACKGROUND OF THE INVENTION 
     It is known to provide common amplification in television transmitters wherein the visual and aural carriers are commonly amplified. The U.S. Pat. to Ta, et al. No. 5,198,904 noted that a problem resulting from such common amplification is aural carrier distortion. The distortion occurs in both the phase and amplitude domains of the aural carrier. This is sometimes referred to as “vision to aural crosstalk” distortion and is caused by the transfer of modulation from a vision carrier to an aural carrier in a combined amplifier. This is primarily due to the non-linear characteristic of the power amplifier used in the transmitter. 
     The patent to Ta proposes to correct the unwanted distortion of the aural carrier by predistorting phase and amplitude components of the aural carrier so as to be directly opposite to the unwanted distortions caused by the common amplification in a television transmitter. To this end, Ta samples the baseband video signal prior to the signal being used to modulate a visual IF carrier signal. The sampled baseband video signal is delayed by a video delay to compensate for the delays that are introduced in an IF vision modulator to which the baseband video signal is also supplied for modulating a visual carrier signal. The delayed baseband video signal is then applied to a complementary nonlinear amplifier which provides a phase correction signal and an amplitude correction signal. These correction signals are supplied to an amplitude and phase modulator which also receives the aural carrier. The aural carrier is modulated by the amplitude and phase correction signals to provide a modified aural signal which is summed with the output of the IF vision modulator. 
     The U.S. Pat. to D. Culling No. 5,418,578 is also directed to correcting an aural carrier signal in a common amplifier system. The Culling patent discloses a system wherein the correcting serves to minimize cross-modulation distortion of the aural carrier signal caused by common amplification of aural and visual frequency signals. The correcting apparatus in Culling provides circuitry for receiving a modulated visual signal comprised of a visual carrier signal modulated by a video baseband signal and providing therefrom a detected video signal. The detected video signal is received by an aural corrector which provides therefrom a phase correction signal and an amplitude correction signal. These signals are supplied to phase and amplitude modulation circuitry for modulating an aural carrier with the phase and amplitude correction signals to provide a corrected aural signal which is opposite the cross-modulation distortion. The visual signal and the corrected aural signal are then combined. 
     It is to be noted that the Ta patent and the Culling patent provide both a phase correction signal and an amplitude correction signal which are then used to phase and amplitude modulate the aural carrier to provide a corrected aural carrier. 
     Another aural correction system is described in a paper entitled “Aural/Visual Multiplex Operation of Klystron Type UHF Television Transmitters”, The Pennsylvania State University Graduate School, report in Engineering Science by Ronald W. Zborowski, March 1981. The Zborowski paper discloses (in FIG. 12) an aural corrector which performs phase correction to eliminate phase modulation of an aural carrier but does not perform amplitude correction and does not include an amplitude modulator for amplitude modulating the aural signal with an amplitude correction signal as in the Ta and Culling patents noted above. Also, the Zborowski paper does not provide a second aural corrector for receiving a modulated visual carrier signal and a phase corrected aural carrier signal to provide therefrom a combined corrected aural carrier signal which is then combined with the visual carrier signal to provide an output signal. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an aural correction system which does not employ generation of an amplitude correction signal and does not employ an amplitude modulator for modulating the aural carrier with the amplitude correction signal. 
     In accordance with one aspect of the present invention, apparatus is provided for correcting an aural carrier signal in a common amplifier system wherein the correcting serves to reduce cross-modulation distortion of the aural carrier signal caused by non-linearities of the common amplifier system. An aural corrector receives a modulated visual carrier signal comprised of a visual carrier signal modulated by a video baseband signal and a phase corrected aural carrier signal and provides therefrom a combined corrected aural carrier signal. The modulated visual carrier signal is combined with the combined corrected aural carrier signal in such a manner to provide a corrected output signal. 
     Another aspect of the present invention provides an aural corrector for reducing or removing vision to aural crosstalk in a combined amplifier, which corrector includes apparatus for providing an original IF vision signal and an IF aural signal. The IF aural signal and an IF vision signal in phase with the original IF vision signal are combined. Also, the IF aural signal and an IF vision signal which is out of phase with the original IF vision signal are combined. A first non-linear circuit introduces non-linear characteristics to the combined in phase IF vision signal and IF aural signal such that the non-linear characteristics of the combined in phase IF vision signal and IF aural signal are the inverse of the non-linear characteristics of the combined vision and aural signal at the output of the amplifier. A second non-linear circuit introduces non-linear characteristics to the combined out of phase IF vision signal and IF aural signal such that the non-linear characteristics of the combined out of phase IF vision signal and IF aural signal are the inverse of the non-linear characteristics of the combined vision and aural signal at the output of the amplifier. A combiner serves to combine the corrected combined in phase IF vision signal and IF aural signal with the corrected combined out of phase IF vision signal and IF aural signal to produce a corrected IF aural signal such that when the original IF vision signal and the corrected IF aural signal are commonly amplified, any vision signal modulated onto the aural signal is substantially reduced. 
     In accordance with another aspect of the present invention, the aural correcting apparatus includes circuitry for receiving a modulated visual carrier signal comprised of a visual carrier signal modulated by a video baseband signal and providing therefrom a sampled video signal. A first aural corrector receives the sampled video signal and provides therefrom a phase correction signal. A phase modulator receives the aural carrier signal and this signal is modulated with the phase correction signal to provide a phase corrected aural carrier signal. The second aural corrector receives the modulated visual carrier signal and the phase corrected aural carrier signal and provides therefrom a combined corrected aural carrier signal. The visual carrier signal and the combined corrected aural carrier signal are then combined to provide an output signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects and advantages of the invention will become more readily apparent from the following description as taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic-block diagram illustration of a prior art aural corrector to be described herein; 
     FIG. 2 is a schematic-block diagram illustration of a second prior art aural corrector to be described herein; 
     FIG. 3 is a schematic-block diagram illustration of an embodiment of the invention herein; 
     FIG. 4 is a schematic-block diagram illustration of a portion of the circuit shown in FIG. 3; and 
     FIG. 5 is a schematic-block illustration of input voltage V IN  versus output voltage V OUT  and which is helpful in the description of the invention herein. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Before describing the preferred embodiment of the invention herein, reference is first made to FIG. 1 which illustrates a prior art aural carrier correction circuit based on that shown in the U.S. Pat. to Ta et al. No. 5,198,904. The character references shown in FIG. 1 herein correspond to those employed in FIG. 1 of the Ta et al. patent. 
     As shown in FIG. 1, there is provided an aural carrier correction system  11  in conjunction with a television transmitter, partly shown at  12 . The transmitter includes an IF vision modulator  19  followed by a summing circuit  21  and an IF system corrector  23 . The system  11  includes a video delay  13 , a processor  15  and an amplitude and phase modulator  17 . The IF vision modulator receives a baseband video input signal  25  which is used to modulate a visual IF carrier signal  17  to provide an IF visual signal  26 . The IF visual signal  26  is summed with a modified IF aural signal  28  at the summing circuit  21  and the combined signal  35  is then supplied to an IF system corrector  23 . The baseband video signal  25  is also used as a sample point to supply a sampled baseband video signal to the aural corrector system  11 . System  11  includes a complementary amplifier which uses the sampled baseband video signal to provide an amplitude correction signal  29  and a phase correction signal  31 . These signals amplitude and phase modulate the aural carrier in the modulator circuit  17  which supplies a modified IF aural signal  28  to the summer  21 . Since the baseband video signal  25  is delayed by the IF vision modulator  19 , a similar delay is provided by the video delay circuit  13  in the aural corrector system  11  so that there is approximate coincidence between the modified IF aural signal  28  and the visual signal  26  outputted by the IF vision modulator  19 . These two signals  26  and  28  are summed by the summer  21  and supplied to the IF system corrector circuit  23 . 
     Reference is now made to FIG. 2 which illustrates a prior art aural carrier correction circuit based on that shown in the U.S. Pat. to Culling No. 5,418,578. A baseband video input signal  99  is supplied to an IF vision modulator  100  which also receives a visual IF carrier signal  101 . The modulated visual signal is then precorrected for distortions introduced in the power amplifier in a conventional IF linearity corrector circuit  102  with the corrected visual signal being supplied to an automatic gain control circuit  104 . The automatic gain control (AGC) circuit  104  provides a constant amplitude visual signal used in the aural signal correction circuit to be described below. The visual signal obtained from the automatic gain control circuit (AGC)  104  is supplied to a filter and delay circuit  106 , the purpose of which is to equalize the processing time in processor  114 , the output of which is supplied to a summing circuit  108 . The visual signal from the automatic gain control circuit  104  is demodulated or detected by a detector  110  with the detected signal then being supplied to a processor  114  which provides both a phase correction signal  115  and an amplitude correction signal  117 . 
     The aural IF carrier input signal  119  is applied to a phase modulator  116  and then to an amplitude modulator  118  where the aural carrier is modulated by the phase correction signal  115  and the amplitude correction signal  117  with the output being supplied to an automatic gain control (AGC)  120 . The precorrected or modified carrier signal is then supplied to the summing circuit  108  with the output thereof, as in the case of the circuit of FIG. 1, being commonly amplified and transmitted. 
     Reference is now made to FIG.  3 . Portions of the circuitry of FIG. 3 are similar to that of FIG.  2  and to simplify the description herein similar elements in FIG. 3 are described with similar character references but using a prime indication. As for example, the IF vision modulator  100 ′ in FIG. 3 corresponds with the IF vision modulator  100  in FIG.  2 . 
     It is to be particularly noted that FIG. 3 does not include an amplitude modulator such as modulator  118  in FIG. 2 or an amplitude correction signal such as signal  117  in FIG.  2 . FIG. 3 does include an aural corrector which receives a detected video signal from detector  110 ′ with the detected signal being at the baseband frequency. The detected signal is supplied to the processor  114 ′, which does not generate an amplitude correction signal. Instead, processor  114 ′ provides only a phase correction signal  115 ′ to phase modulator  116 ′. The aural IF input signal  119 ′ is phase modulated at the phase modulator  116 ′ by the phase correction signal  115 ′. The output from the phase modulator  116 ′ is a phase corrected aural IF signal which is then applied through an automatic gain control circuit  120 ′ to a second aural corrector circuit to be described below. 
     Reference is now made to the right hand portion of FIG. 3 which illustrates a second aural corrector that receives the modulated visual carrier signal from the delay and filter circuit  106 ′ as well as the phase corrected aural carrier signal from the automatic gain control circuit  120 ′ and provides from these signals a combined corrected aural carrier signal. The combined corrected aural carrier signal is combined with a delayed modulated visual carrier signal and is then suitably amplified by an amplifier to provide an output signal. The second aural corrector circuit is described in greater detail below. 
     Referring to FIG. 3, an aural corrector  201  embodying the present invention comprises a vision branch  202  and an aural branch  203 . The vision branch  202  is fed with a vision IF signal which passes through a buffer amplifier  204  to an in-phase signal splitter  205 . One part of the split vision IF signal is then fed to a delay circuit  206  and thence to an in-phase signal combiner  207 . The other part of the vision IF signal is passed through a chrominance filter  208 . The vision signal comprises two main tones—a luminance signal (L) which for NTSC is centered at 45.75 MHz, and a chrominance signal (C) which is at approximately 42.17 MHz. The aural IF signal (A) is at 41.25 MHz. The chrominance filter  208  removes the chrominance tone from the vision signal leaving only the luminance signal at 45.75 MHz. 
     It is important to remove the chrominance tone which includes the color information on a separate carrier to the luminance tone as, in combination with the aural tone, the chrominance tone produces inter-modulation products at or about the same carrier frequency as the luminance tone. These inter-modulation products distort the luminance tone. 
     The vision signal then passes through a further buffer amplifier  209  to an anti-phase splitter  210 . Two signals are tapped from the output of the splitter  210 . The first signal is in-phase with the original vision signal and is passed to an in-phase signal combiner  211 . The second signal is  1800  out of phase with the original vision signal and is tapped from the splitter  210  to another in-phase signal combiner  212 . 
     The aural IF signal on the aural branch  203  passes through a buffer amplifier  213  and thence to an in-phase signal splitter  214 . The first part of the split signal from the signal splitter  214  is fed to the first signal combiner  211  and the second part of the signal from the splitter  214  is fed to the second signal combiner  212 . 
     In this manner, the output of the first combiner  211  comprises the vision IF signal without the chrominance tone in combination with the aural IF signal (i.e. =L+A) and the output of the second combiner  212  comprises a vision signal less the chrominance tone which is out of phase with the original vision signal in combination with the aural signal (i.e. =−L+A). 
     The outputs from the two combiners  211 , 212  are each fed to a non-linear circuit  215 , 216  comprising a corrector. Each of the correctors  215 , 216  operate on the same principle as known correctors and can be phase correctors, amplitude correctors or both phase and amplitude correctors. In the present example, the correctors only perform amplitude correction. 
     The correctors  215 , 216  are adjustable so as to correct each of the combined vision and aural IF signals such that the corrected combined vision and aural IF signal has the inverse of the non-linear characteristics of the combined vision and aural signal at the power amplifier so as to minimize any vision cross modulation when the corrected combined vision and aural signal is amplified. The two correctors have the same adjustment settings and therefor have the same non-linear characteristics. 
     The corrected signals are fed from the correctors  215 , 216  through respective delay branches  217 , 218  comprising a number of resistive, capacitive and inductive elements selected to impart a pre-determined delay on the first branch  217 . So that the corrected signals can be re-synchronized, the last capacitive element C 6  in the second branch  218  is adjustable so that the delay in the second branch  218  can be selected such that the corrected signals when combined in the combiner  219  downstream of the two branches  217 , 218  are re-synchronized. The re-combined corrected signal is effectively just an aural signal, the in-phase and out of phase vision signals substantially canceling one another out—except for the non-linearities introduced to the signals by the correctors  215 , 216 . The corrected “aural” signal is then passed through a buffer amplifier  220  to a further filter  221  which removes any residual luminance tone from the aural signal. The corrected aural signal passes through an automatic gain control circuit  222  and thence on to the combiner  207  which also receives the delayed vision IF signal on the vision branch  202 . 
     The delay circuit  206  in the vision branch  202  is chosen to provide a delay such that the synchronization of the vision signal and the corrected aural signal occurs at the combiner  207 . The combined vision and aural signal is output from the aural corrector from a buffer amplifier  223 . 
     Pre-correction of the signals for addressing any non-linear characteristics of the power amplifier can be carried out before input to the aural corrector  201  and it is also possible to carry out further pre-correction on the output signal from the aural corrector  201 . 
     The in phase vision IF signal from splitter  210  may be referred to herein as signal +V. The 180 degree phase shifted signal from the splitter may be referred to as signal −V. The aural IF signal from the automatic gain control circuit  120 ′ may be referred to as signal A. The in phase splitter  214  applies signal A to combiners  211  and  212 . Consequently, the output of combiner  211  is +V +A. The output of combiner  212  is −V +A. 
     The non-linear circuits  211  and  215  create products P n  and third order products P 3 . The third order products are of greater importance herein. Thus, the output of non-linear circuit  211  is: 
     
       
         + V+A +P   3   Equation 1 
       
     
     and the output of non-linear circuit  212  is: 
     
       
         − V+A+P   3   Equation 2 
       
     
     The non-linear circuits  211  and  212  are matched and their control settings are the same. Consequently, these circuits have similar non-linear transfer characteristics. These transfer characteristics are determined by the position of the control settings, to be discussed hereinafter with reference to FIGS. 4 and 5. 
     The output of combiner  219  may be referred to as the aural precorrected signal Ap and, during the combination, the +V and −V signals cancel. This leaves: 
     
       
           Ap =2 A +2 P   3   Equation 3 
       
     
     Reference is now made to FIG. 4 which illustrates the relationship between operator adjustable non-linear circuits  215  and  216 . Circuits  215  and  216  have identical non-linear transfer characteristics adjustable by an operator. This is represented in FIG. 5 by the curve therein extending through quadrants I and III. Quadrant I has two break points or onset points referred to in the drawings as onset  1  and onset  2 . These points are manually adjustable, as is typical in the art. Adjustments are made to the onset points of the non-linear transfer curve with the use of operator adjustable onset potentiometers ON- 1  and ON- 2 . The wiper arms of the potentiometers ON- 1  and ON- 2  are coupled to both circuits  215  and  216 . The amount of curvature or bending of these curves is referred to as slope. The slope at onset point  1  is operator adjusted by manipulating the wiper arm of slope potentiometer SL- 1 . Similarly the slope at onset point  2  is operator adjusted by manipulating the wiper arm on slope potentiometer SL- 2 . It is to be understood that if additional break points are desired then additional onset and slope potentiometers will be required. 
     As well as effectively removing vision to aural crosstalk by pre-correcting the aural signal when combined with respective in-phase and out of phase vision (luminance) signals, the above-mentioned system is extremely advantageous because the resultant corrected aural signal is produced not in response to the baseband video signal but uses the vestigial side band (VSB) video signal as modulated to its intermediate frequency and as filtered to produce the corrected signal. That is to say that the signal which the present invention uses to impart correction for vision to aural crosstalk is the actual signal which causes the crosstalk modulation in the downstream combined amplifier. It should be appreciated that the baseband video signal itself does not cause the vision to aural crosstalk but a modulated and filtered version thereof. Thus, the present invention provides a much truer correction of the aural signal than is provided by known aural and visual correctors. 
     The setup procedure for the aural corrector  201  illustrated in FIG. 3 contemplates that the procedure is done off line and not during transmission. The corrector is adjusted by manipulating the potentiometers ON- 1 , ON- 2 , SL- 1  and SL- 2  in FIG.  4 . In this procedure, an unmodulated aural carrier signal is added to a vision carrier signal which has been modulated by a ramp signal obtained from a video generator. The slope and onset potentiometers (FIG. 4) are adjusted to a null condition (or an OFF condition). The output of the transmitter is observed with a spectrum analyzer tuned to the unmodulated aural carrier (and not to a demodulated aural carrier). The spectrum analyzer is used in the “zero span” mode, so as to observe the aural carrier only. Any AM modulation on the aural carrier will also be visible. Since the aural input to the transmitter is unmodulated, all AM modulation visible on the aural carrier is unwanted distortion. The correction potentiometers (FIG. 4) are adjusted by the operator until the noted distortion is reduced to a minimum or acceptable level. This adjustment of the correction potentiometer is by a trial and error procedure. 
     From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.