Patent Publication Number: US-7593065-B2

Title: Method and circuit for channel filtering of analog or digitally modulated TV signals

Description:
PRIORITY INFORMATION 
     This patent application claims priority from German patent application 10 2004 054 893.5 filed Nov. 12, 2004, which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The invention relates to the field of signal processing of received television signals, and in particular to channel filtering of analog or digitally modulated television signals. 
     Television signals are transmitted on various frequency channels. A tuner in a television receiver selects a channel based on the setting provided by the operator, then amplifies this channel. In most television receivers, the selected television signal is subsequently converted to a first intermediate frequency (IF) of about 40 MHz. The intermediate frequency signal is typically bandpass filtered by a surface wave filter. In many implementations of television receivers, the TV signal converted to the first intermediate frequency is then converted to a second intermediate frequency after subsequent, generally analog, signal processing. Such a conversion is described, for example, in DE 198 25 455 C2. For purposes of clear differentiation, this second intermediate frequency will be abbreviated to 2 nd  IF. The second intermediate frequency, 2 nd  IF, is selected such that the video carrier of the lower side channel is set at zero frequency. As a result, the side channel video carrier can be easily filtered out by an analog high-pass filter. Since unwanted mixing products of the second conversion must also be suppressed, and subsequent analog-to-digital conversion requires an anti-aliasing low-pass filter, this filter is also designed as a bandpass filter. The resultant signal, also called the 2 nd  IF signal, is then fed to another, preferably digital, signal processor. 
     Modem tuner units, so-called silicon tuners, are also able to convert the channel directly to the low IF, or use as the IF a significantly higher frequency (e.g., 1.2 GHz), and then convert to the low IF. 
     A multiplicity of prior art techniques effect channel filtering of an analog or digitally modulated TV signal of a channel (selected from a tuner) having a video carrier signal converted to an intermediate frequency (IF or 2 nd  IF), video signal(s), generally two sound carrier signals, and sound signal(s). Although these techniques have in principle proven successful, they all require special transmission-standard-dependent prefilters. Otherwise, generally not all interfering signals from adjacent channels—specifically, higher or lower side channels—are effectively suppressed or filtered out if only one prefilter is to be used. In addition, with these methods the prefilters have completely different amplitude responses relative to the prefilters required for digital television signals. 
     There is a need for improved channel filtering of analog or digitally modulated TV signals converted to an intermediate frequency, which suppresses interference more effectively, and uses the same standard-independent prefiltering for analog and digitally modulated television signals. 
     SUMMARY OF THE INVENTION 
     An analog or digitally modulated TV signal comprises a video signal, a sound signal, and video carrier signal with a video carrier frequency. The TV signal is converted to an intermediate frequency forming an intermediate frequency signal. To suppress interference signals of adjacent TV channels, and to separate video and sound information, the intermediate frequency signal is demodulated with a signal having the video carrier frequency into an in-phase signal, and is demodulated with a signal, having the video carrier frequency and shifted in phase by a phase angle of π/2 relative to the signal, into a quadrature signal—for example, by a so-called I/Q demodulator. The in-phase signal is filtered by a first Hilbert filter of a Hilbert filter pair and having an even symmetrical impulse response in order to obtain a Hilbert-filtered in-phase signal. The quadrature signal is filtered by a second Hilbert filter of the Hilbert filter pair having an uneven symmetrical impulse response in order to obtain a Hilbert-filtered quadrature signal. The Hilbert-filtered quadrature signal is added to the Hilbert-filtered in-phase signal in order to obtain a video output signal. The Hilbert-filtered quadrature signal is subtracted from the Hilbert-filtered in-phase signal to obtain a sound output signal. Splitting of the video signal from the sound signal is combined with the filtering to achieve improved filtering in the respective split frequency ranges for video and sound. 
     The system of the present invention may filter a TV signal that has been converted to any given intermediate frequency. In addition, it is fundamentally irrelevant whether the filtering is implemented by analog technology or digitally, although digital filtering often has advantages in terms of realizing the desired filter characteristic. 
     In the case of digital filtering, it is particularly advantageous if the intermediate frequency signal subject to the above-described I/Q demodulation, Hilbert filtering, and subsequent summation or subtraction is generated by converting a video carrier from a lower side channel to zero frequency since this produces the lowest possible signal frequencies, and as a result, the digital processing can be implemented as slowly as possible. Expressed differently, this means when the above-indicated 2 nd  IF signal or low, IF signal is split into a video signal and/or sound signal. 
     In order to obtain on the input side an essentially interference-free intermediate frequency signal (in particular, 2 nd  IF signal), and in order to keep away as many interference signals as possible from the critical analog-to-digital conversion, the system may include bandpass filtering of the intermediate frequency signal before the I/Q demodulation. A system according to the invention may include one or more bandpass filters on the input side of the I/Q demodulator. 
     Although in principle one filter may be provided for each transmission standard to suppress unwanted interference/signals, it has proven more useful if the bandpass filtering suppresses at least and primarily residual interference from the lower side channel. Any residual interferences remaining from the upper sideband after bandpass filtering can be tolerated up to a certain level. This approach has the advantage that a comparatively simple (and thus inexpensive) filtering provides good results. 
     Since the lower adjacent video carrier may be converted to frequency zero, even lower-frequency components can be folded back into the useful band (image frequency of conversion). Therefore, a system according to the present invention should contain a device that prevents this. Bandpass filtering before the conversion, or of a complex conversion, may be included in order to suppress the image frequency of conversion. It is especially useful if this arrangement also suppresses the video carrier of the lower side channel. An analog high-pass filter may be included after conversion to the 2 nd  IF, the cut-off frequency of this filter being above the video carrier of the lower side channel and below the useful channel. 
     A digital bandpass filter may be provided such that it has a passband that extends between a frequency greater than the frequency to which the video carrier of the lower side channel has been converted (frequency zero in the case of the 2 nd  IF signal) and a frequency smaller than half the sampling frequency. 
     To provide additional interference signal suppression, the in-phase signal and/or quadrature signal may be low-pass-filtered prior to Hilbert filtering. The low-pass filters may be configured such that by low-pass-filtering the in-phase signal and/or low-pass-filtering the quadrature signal, unwanted mixing products are suppressed around the second harmonic of the video carrier from the I/Q demodulation. 
     It has proven to be advantageous if the low-pass filter(s) for the in-phase signal or quadrature signal has (have) a transfer function with a passband that extends up to a frequency corresponding to a channel bandwidth of the TV signal (which, for example, for the PAL standard is around 7 or 8 MHz, and for the NTSC standard is around 6 MHz). 
     Since the mixing products are located within a frequency distance, corresponding to the channel width, from the frequency of the second harmonic of the video carrier frequency (corresponding to double the video carrier frequency), in an especially advantageous embodiment of the invention one or both of the low-pass filters for the in-phase signal or quadrature signal have a zero value at a frequency that corresponds to double the video carrier frequency minus the channel bandwidth. 
     The Hilbert filters of the Hilbert filter pair may be configured such that interference products from an upper side channel are suppressed by the Hilbert filtering. Alternatively or in addition, during Hilbert filtering the phase relationship, in particular, the sign, of video and sound signal components in the in-phase signal and the quadrature signal, are synchronized such that the separation into the video output signal and into the sound output signal results from the sum of the Hilbert-filtered in-phase signal or quadrature signal, or from the difference taken between the in-phase signal and the Hilbert-filtered quadrature signal. 
     The transfer function of the Hilbert filter may have a passband, for example, for the in-phase signal, which extends up to an upper frequency that approximately corresponds to the channel bandwidth. Similarly, the transfer function of the Hilbert filter may have a passband for the quadrature signal that extends up to an upper frequency that approximately corresponds to the channel bandwidth. 
     The Hilbert filter for the quadrature signal also has, for example, a transfer function with a passband that extends up to a lower frequency corresponding to the vestigial sideband width. Vestigial sideband width is the width of the sideband which is partially suppressed during transmission of the television signal. Within the vestigial sideband, no useful signal is produced within the Q branch since the upper and lower sidebands cancel each other out. 
     The Hilbert filter for the quadrature signal may be configured so that it has a transfer function between a frequency that corresponds to a bandwidth of the video signal (e.g., the bandwidth of the signal is 5 MHz according to the PAL standard, and 4.2 MHz according to the NTSC standard) and a frequency that corresponds to a channel bandwidth (see above: 7 or 8 MHz according to the PAL standard and 6 MHz according to the NTSC standard), which transfer function has exactly the reverse sign relative to the transfer function between the above-indicated residual frequency and a frequency that corresponds to the bandwidth of the video signal. 
     In order to suppress the above-mentioned interference signals, the transfer function of the Hilbert filter of the Hilbert filter pair preferably also has the same zero values as the above-described low-pass filters that are optionally on the input side of the Hilbert filters. In particular, for example, the transfer function of the Hilbert filter for the in-phase signal has a zero value at a frequency that corresponds to double the video carrier frequency minus the channel bandwidth. The Hilbert filter for the quadrature signal also has, in a preferred embodiment, alternatively or in addition, a zero value at a frequency that corresponds to double the video carrier frequency minus the channel bandwidth. To separate the video signal and sound signal, the Hilbert filter for the quadrature signal may include a transfer function that corresponds to a bandwidth of the video signal. 
     Although substantially interference-free output signals for sound and video are generated by the above-indicated filter arrangements or methods, the sound output signal can still contain residual disturbance signals. These can be suppressed if the sound output signal obtained by subtracting the Hilbert-filtered quadrature signal from the Hilbert-filtered in-phase signal using a high-pass filter following the subtractor is once again high-pass-filtered so as to obtain a filtered sound output signal. 
     In particular, the high-pass filter is preferably designed to suppress by high-pass filtering the residual interference (in particular, sound signal interference) from an upper side channel. 
     These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustration of a system for channel-filtering of analog or digitally modulated TV signals; 
         FIG. 2  illustrates a spectrum of an IF signal (prior art) converted to a first intermediate frequency; 
         FIG. 3  illustrates a spectrum of a 2 nd  IF signal (prior art) converted to a second intermediate frequency (2 nd  IF, zero IF); 
         FIGS. 4A and 4B  show an I/Q demodulation of the spectral lines, selected as an example, of a 2 nd  IF signal, a) 2 nd  IF input signal, b) I and Q components of the 2 nd  IF signal after I/Q demodulation; 
         FIGS. 5A and 5B  illustrates Hilbert filtering of a spectral line, selected as an example, of an I component and a Q component using a Hilbert filter pair with Hilbert filters in the I and Q signal paths, a) I and Q components, and b) I and Q components after Hilbert filtering; 
         FIG. 6  illustrates a transfer function of the digital bandpass filter, connected on the input side of the I/Q demodulator, from the circuit of  FIG. 1 ; 
         FIG. 7  illustrates plots of a transfer function of the low-pass filters, connected on the input side of the Hilbert filter pair, from the circuit of  FIG. 1 , where the low-pass filters attenuate the unwanted mixing products generated during the I/Q demodulation around the second harmonic of the video carrier; 
         FIGS. 8A and 8B  illustrate plots of transfer functions for the Hilbert filter pair of the circuit of  FIG. 1 , where the Hilbert filter pair attenuate disturbance products from the upper side channel, and correctly to set the sign of the video and sound signals for the separation; 
         FIG. 9  illustrates a spectrum, selected as an example, of an intermediate frequency signal input to the circuit of  FIG. 1 ; 
         FIG. 10  illustrates the spectrum of the intermediate frequency signal fed to the input of the circuit of  FIG. 1  after bandpass filtering on the input side—after bandpass filtering, the residual interference of the lower-side-channel video carrier is suppressed; 
         FIG. 11  illustrates spectra of the in-phase and quadrature signals after demodulation by the I/Q demodulator of FIG.  1 .—after I/Q demodulation, the in-phase and quadrature signals are mixed to zero frequency—the use of mixing during the I/Q demodulation creates harmonics (not shown here) of the fundamental spectrum, the harmonics being suppressed by the low-pass filters of  FIG. 1 ; 
         FIG. 12  illustrates the spectrum of the Hilbert-filtered in-phase signal; 
         FIG. 13  illustrates the spectrum of the Hilbert-filtered quadrature signal—the Q-signal spectrum is rotated in phase by the phase angle π for higher frequencies; 
         FIG. 14  illustrates the spectrum of the video output signal; 
         FIG. 15  illustrates the spectrum of the sound output signal; and 
         FIG. 16  illustrates the spectrum of the sound output signal after high-pass filtering to attenuate residual interference of the sound signals from the upper side channel is suppressed. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The starting point of the following considerations is a spectrum of a TV signal that has been selected from a tuner as specified by the operator, subsequently amplified, then converted to a first intermediate frequency. A frequency spectrum  130  of this type is shown in  FIG. 2 .  FIG. 2  is a plot of the amplitude of the IF signal s ZF  as a function of the frequency f. 
     Essentially, three characteristic frequency ranges are found in the figure which are identified by reference numbers  125 - 127 . The frequency range identified by reference number  125  represents the useful signal channel for the video and sound of the selected channel. 
     The useful signal channel  125  has (as do the lower and upper side channels) two characteristic subranges, specifically, a subrange for low frequencies, the so-called sound signal range  125   a , and the subrange for higher frequencies, the so-called video signal range  125   b . The spectral lines of the first and second sound carriers for the two sound carrier frequencies f TT1  and f TT2  are plotted within the sound signal range  125   a . This range is generally identified as an intrinsic sound carrier range  123 . 
     The video signal range  125   b  comprises the double-sideband range of the video amplitude modulation  121  with the video carrier BT shown in  FIG. 2  at video carrier frequency f BT , and the single-sideband range  120 . 
     Since the side channels have essentially the same subranges, the corresponding signal lines in  FIG. 2  are also plotted for the sound carrier TT* of the upper side channel  127  at the sound carrier frequencies f TT1 * und f TT2 *, as is the spectral line of the video carrier BT* of the lower side channel  126  at a frequency f BT *. 
     For the sake of completeness, the adjacent sound carrier range of the upper side channel  127  and the adjacent video carrier range of the lower channel  126  are identified by reference numbers  122  and  124  in  FIG. 2 . 
     As already described above, the IF signal spectrum is converted to a second IF signal s IF . The 2 nd  IF signal s IF  is characterized by the fact that the video carrier BT* of the lower side channel  126  is now at zero frequency: f* BT =0. The total 2 nd  IF signal spectrum, that is, the 2 nd  IF signal s IF  plotted as a function of the frequency f, is shown in  FIG. 3  and identified by reference number  131 . To simplify orientation, the actual useful signal channel, as well as the lower and upper side channels, are identified by primed reference symbols  125 ′,  126 ′, and  127 ′. 
     Also plotted are the spectral lines, indicated above as characteristic, for the video carrier BT* of the lower side channel  126  at a frequency f* BT =0, the two intrinsic sound carriers TT at frequencies f TT1  and f TT2 , the intrinsic video carrier BT at a frequency f BT , and the two sound carriers TT 1 *, TT 2 * of upper side channel  127 ′ at frequencies f* TT1  or f* TT2 . 
     The channel width B is determined by the distance between the adjacent video carriers BT, BT*. The bandwidth of video signal BB is determined by the transmission standard. 
     As explained above, prior art signal processing of the television signal selected by the tuner is subject to too much interference to provide television reception of optimum video and sound quality. The problems include residual interference BT*, TT* from the lower and upper side channels  126 ,  127  that need to be suppressed. In addition, the video and sound signals that are synchronously demodulated with the video carrier BT at a video carrier frequency f BT  need to be outputted separately. 
       FIG. 1  illustrates a circuit  1000  for effect channel filtering of analog or digitally modulated TV signals. The circuit  1000  comprises a prefilter such as a bandpass filter (BPF)  1001 , an I/Q demodulator  3 , a Hilbert filter pair with individual filters HF-I, HF-Q, an arithmetic unit in the form of an adder  21  and a subtractor  22  to separate the video and sound signals, and a post-filter in the form of a high-pass filter HPF  1010 . 
     The circuit for conversion to the 2 nd  IF frequency and corresponding prefilter are also illustrated. In particular, a multiplier  150  is shown which generates conversion to the 2 nd  IF. Connected on the input side of multiplier  150  is a bandpass filter  151  which suppresses the image frequency of the conversion. In addition, a bandpass filter  153  and an analog-to-digital converter (ADC)  152  are connected on the output side of multiplier  150 . 
     The bandpass filter  153  functions as an anti-aliasing filter that suppresses the video carrier of the lower side channel prior to the analog-to-digital conversion. In addition, another high-pass filter not shown can be provided that filters out the video carrier of the lower side channel in order to simplify analog-to-digital conversion. 
     The bandpass filter BPF  1001  (which can comprise multiple analog and/or digital filters) with the filter characteristic shown in  FIG. 6  receives the intermediate frequency signal on a line  8 . The output of the bandpass filter  1001  is provided on a line  9  to the I/Q demodulator  3 . 
     Referring to  FIG. 6 , the bandpass filter BPF  1001 , which in this embodiment is digital, has a passband  135  that extends between a frequency f greater than a frequency  133  (f BT =0) to which the video carrier BT* of the lower side channel  126 ′ has been converted, and a frequency  134  less than half the sampling frequency f s/2 ist . In addition, the bandpass filter BPF  1001  ( FIG. 1 ) has a zero value  133  at frequency f BT *=0 to which the video carrier BT* of the lower side channel  126  has been converted, and a zero value  134  at half the sampling frequency f s /2. 
     Referring again to  FIG. 1 , in the I/Q demodulator  3 , an input signal on the line  9  is input to a first mixer  1  and a second mixer  2 . The first mixer  1  provides an in-phase signal on a line  6  to a first low-pass filter TPF-I.  FIG. 7  illustrates a plot of the characteristic of the low pass filter TPF-I. The second mixer  2  provides a quadrature signal output on a line  7  to a second low-pass filter TPF-Q which also has a filter characteristic as shown in  FIG. 7 . 
     Referring to  FIG. 7 , the low-pass filters TPF-I, TPF-Q have the same filter characteristic. The low-pass filter TPF-I for the in-phase signal I and the low-pass filter TPF-Q for the quadrature signal Q have a zero value  139  at a frequency f=2f BT −B, which corresponds to double the video carrier frequency 2f BT  minus the channel bandwidth B. Passband  136  extends from a frequency f=0 up to a frequency f=B that corresponds to a channel bandwidth B of the TV signal. 
     The first low-pass filter TPF-I provides a filtered output signal on line  13  to a first Hilbert filter HF-I. The second low-pass filter TPF-Q provides an output signal on line  14  to a second Hilbert filter HF-Q. The filter characteristics of the Hilbert filters HF-I and HF-Q are illustrated in  FIGS. 8A and 8B , respectively. 
     Referring to  FIG. 8A , the Hilbert filter HF-I has a passband  137  that extends from zero frequency f=0 up to an upper frequency f=B corresponding to the channel bandwidth B. The Hilbert filter HF-I for the in-phase signal I has a zero value  139  at a frequency f=2f BT −B that corresponds to double the video carrier frequency 2f BT  minus the channel bandwidth. Referring to  FIG. 8B , the Hilbert filter HF-Q for the quadrature signal Q has a passband  138   a ,  138   b  that extends by an additional frequency f=f R  corresponding to the residual frequency f R  up to an upper frequency f=B that corresponds to the channel bandwidth B. 
     Referring to  FIG. 8B , while the transfer function H Q =A Q  between residual frequency f R  and a frequency corresponding to a bandwidth BB of the video signal has positive sign, the transfer function in the remaining passband  138   b  is inverted. The Hilbert filter HF_Q for the quadrature signal Q has a zero value  139  at a frequency f=2f BT −B that corresponds to double the video carrier frequency 2f BT  minus channel bandwidth B and has a zero value  139  at a frequency f=BB corresponding to a bandwidth BB of the video signal. 
     Referring again to  FIG. 1 , the first Hilbert filter HF-I provides an in-phase output signal on line  4 . Similarly, the second Hilbert filter HF-Q provides an output signal on line  5 . An adder  21  sums the signals on the lines  4  and  5 , and provides a video output signal on line  23  to a video output A Video . 
     Subtractor  22  computes the difference between the signals on the lines  4  and  5  and provides a sound signal output on a line  24  to a high-pass filter  1010  having the filter characteristic illustrated in  FIG. 16 . The high-pass filter  1010  provides a sound output signal on line  25  to a sound output A Sound . 
     To enhance understanding of the circuit, the following explains the functionalities of the I/Q demodulator  3  and the Hilbert filter pair with Hilbert filters HF-I and HF-Q on the basis of simple frequency spectra. 
     I/Q Demodulator 
     It is assumed that a cosinusoidal input signal is input on the line  9  to the I/Q demodulator  3  and the signal is processed along in-phase and quadrature signal paths, which provide outputs A 3,I  or A 3,Q , respectively. 
     Each component of a modulated input signal that can be described by the following equation as a function of time t:
 
 s   IF ( t )= A ·cos(2π( f   BT   +f ) t )
 
with amplitude A and on frequency f+f BT  is shifted in frequency in the I/Q demodulator  3 , is split with a rotation to the I and Q outputs, and is multiplied according to the position relative to carrier frequency f BT  with plus or minus signs, and is low-pass-filtered with filter characteristics H TPF-I , H TPF-Q  according to the following equations:
 
     
       
         
           
             
               
                 
                   
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       FIGS. 4A and 4B  show an example of the spectral lines of an input signal s IF  at frequency f 2  which is smaller than video carrier frequency f BT  of video carrier BT, and of an input signal s IF  at a frequency f 1  which is greater than video carrier frequency f BT  ( FIG. 4   a )), as well as their conversion to corresponding in-phase components I and quadrature components Q ( FIG. 4   b )). 
     Hilbert Filter Pair 
     A Hilbert filter pair contains two filters HF-I and HF-Q with the following properties in the frequency range:
         1. Both filters have a linear phase transition; and;   2. The phase transitions of the two filters HF-I, HF-Q differ by 90°.       

     In a digital implementation of the Hilbert filter pair, the even-symmetrical impulse response of the I-filter can be described by the equation:
 
 h   I ( L−n )= h   I ( n )
 
while the uneven-symmetrical impulse response can described by the equation:
 
 h   Q ( L−n )=− h   Q ( n )
 
where L is the filter length (in sampling values) and n is the number of the given filter section.
 
     The transfer functions H I , H Q  of the Hilbert filter HF-I sampling at the sampling frequency fs, and of the Hilbert filter HF-Q sampling at sampling frequency f s  in the frequency range are produced by the following equations: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     A sinusoidal input signal with a signal amplitude A and frequency f x , as in  FIG. 5A , is transformed upon filtering by the in-phase Hilbert filter HF-I into a sinusoidal signal of signal amplitude A*A 1 (f x ), and upon filtering by the quadrature Hilbert filter HF-Q is transformed into a signal shifted 90° in phase of amplitude A*A R (f x ) (see  FIG. 5B ). 
     If a 2 nd  IF signal having the signal spectrum shown in  FIG. 3  undergoes filtering by the circuit illustrated in  FIG. 1 , then modified signals are applied at the inputs or outputs of the individual circuit elements of  FIG. 1 , which signals are explained based on  FIGS. 9-14  using various spectral lines selected as examples. 
     As an example, the spectral line of video carrier BT* of the lower side channel, the intrinsic sound carrier TT, a video signal B 2  selected as an example in single sideband  120 , a video signal B 1  selected as an example in double sideband  121 , the intrinsic video carrier BT, and the sound carrier TT* of the upper side channel  127  are viewed as they change upon passage through the filter. The individual spectral lines are illustrated in  FIG. 9  for simple reference and to elucidate the position of these spectral lines. 
     The 2 nd  IF signal S IF  is provided on the line  8  to the bandpass filter  1001 . The bandpass filter  1001  with a zero value at frequency f=0 and at half the sampling frequency f s/2  has an essentially constant amplitude transition between its two zero values, and concomitantly thereto a linear phase transition. The bandpass filter is thus designed to filter out the residual interference from the lower side channel, in particular, residual interference through video carrier BT* of the lower side channel. In concrete terms, all of the above-mentioned spectral lines of  FIG. 9  are passed through essentially in unchanged form—except for the video carrier signal BT* at f BT * =0.  FIG. 10  illustrates the characteristic spectral lines of  FIG. 9  which remain after bandpass filtering. 
     In the I/Q demodulator  3 , each component of the bandpass-filtered signal spectrum S BPF  is shifted in frequency, split with rotation to the I and Q outlets A 3,I , A 3,Q , and multiplied according to the position relative to carrier frequency f BT  with the plus or minus sign, then low-pass-filtered. The in-phase signal I now encompasses video carrier BT at frequency f=0, at a distance from video signal frequency f 1  video signal B 1  from the second sideband, at half amplitude the sound carrier signals TT* from the upper side channel, at half amplitude the video signal B 2  from the single sideband at video frequency f 2 , and at half amplitude the intrinsic sound carrier signals TT. Quadrature signal Q encompasses at half amplitude the video signal from B 2  from the single sideband at frequency f 2 , and the two intrinsic sound carrier signals TT at half amplitude. In addition, the residual interference of sound carrier TT* from the upper side channel is shifted by 180° in phase, and is thus also provided at half amplitude. The mixing products produced by mixing with the intrinsic video carrier frequency f BT  around the second harmonic of video carrier BT were suppressed using low-pass filters TPF-I, TPF-Q. 
     The in-phase signal I is fed on the line  13  to the Hilbert filter HF-I. Since the amplitude response of the Hilbert filter HF-I is essentially constant within the range of interest (see  FIG. 8A ), the in-phase signal I is passed through essentially unchanged and is applied in the form of Hilbert-filtered signal I H  at the output of the Hilbert filter HF-I. The spectrum of the selected spectral lines is found in  FIG. 12 . 
     The quadrature signal Q is fed to the Hilbert filter HF-Q. As is evident from  FIG. 8B , spectral lines at a frequency below residual frequency f R  are suppressed. Spectral lines between residual frequency f R  and a frequency corresponding to the bandwidth of video signal BB are passed through unchanged. Signal lines between this frequency BB and a frequency corresponding to channel bandwidth B are inverted. 
       FIG. 13  shows the spectrum for the selected characteristic lines of the Q H  signal. Specifically, as shown in  FIG. 13  no video carrier signal BT of the lower side channel is present in the Q branch, and no video signal B 1  is present in the Q branch of the double sideband range (vestigial sideband range). Video signal B 2  from the single sideband is found at a distance from frequency f 2 . As before, this signal is present at only half the signal amplitude. Sound signal TT is inverted and has half the signal amplitude. Also inverted and at half the signal amplitude is the sound carrier signal of the upper side channel. 
     Hilbert-filtered in-phase signal I H  and Hilbert-filtered quadrature signal Q H  are summed by the adder  21  and output on the line  23  at output A Video  as video signal S Video .  FIG. 14  shows the selected characteristic lines of the signal spectrum after addition of the Hilbert-filtered in-phase signal and the Hilbert-filtered quadrature signal Q H . At the output A Video , only the lines of the video signal are present here, that is, video carrier BT at zero frequency, the video signal of the double sideband B 1  at frequency f 1 , and video signal B 2  of the single sideband at frequency at frequency f 2 . 
     The subtractor  22  generates the difference between the Hilbert-filtered in-phase signal and the Hilbert-filtered quadrature signal Q H .  FIG. 15  shows the selected characteristic lines for the input signal spectrum of  FIG. 9  after subtraction of the Hilbert-filtered quadrature signal Q H  from the Hilbert filter in-phase signal I H . Both the sound carrier of the intrinsic channel and the sound carrier of the upper side channel are seen. In order to eliminate the latter, the signal S Sound ′ output from the subtractor  22  on the line  24  is input to the high-pass filter  1010  in which the residual interference from the upper side channel, that is, sound carrier signal TT* of the upper side channel is suppressed. The high-pass-filtered difference signal between the Hilbert-filtered in-phase signal I H  and the Hilbert-filtered quadrature signal Q H  are output as signal S Sound . Ideally, what remains as output signal S Sound  at sound signal output A Sound  is only the two sound carrier signals which are manifested by the two sound carrier signals TT. 
     Although the present invention has been illustrated and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.