Abstract:
Video signal processing apparatus for processing video signals in which pre-emphasis of high frequency components of signals to be recorded is effected, and de-emphasis of high frequency components of reproduced signals is carried out. The pre-emphasis circuitry and de-emphasis circuitry is of the so called forward and backward type for weighting video signals inputted at past and future time frames.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to video signal processing systems, and more particularly to a video signal processing system for application in a signal processing system which carries out a pre-emphasis in a recording system which records a video signal onto a recording medium and carries out a de-emphasis in a reproducing system which reproduces the recorded signal from the recording medium. The signal processing system according to the present invention can suppress the overshoot level of an output signal of a pre-emphasis circuit to a relatively low level, and it is unnecessary to greatly clip the output signal level of the pre-emphasis circuit. 
     Generally in a video signal processing system which carries out a pre-emphasis with respect to the signal in the recording system and carries out a de-emphasis in the reproducing system, the input video signal which is to be recorded is first supplied to a pre-emphasis circuit in the recording system in order to emphasize the high frequency component of the input video signal. The output signal of the pre-emphasis circuit is supplied to a clipping circuit wherein the signal level is clipped so that the level at the tip end of the overshoot in the signal does not exceed a predetermined level. The output signal of the clipping circuit is supplied to a frequency modulating circuit wherein a carrier is frequency modulated by this signal. The output frequency modulated video signal of the frequency modulating circuit, is recorded onto a magnetic tape by a magnetic head. In the reproducing system, the signal which is reproduced from the magnetic tape by the magnetic head, is supplied to a frequency demodulating circuit wherein the reproduced signal is demodulated. The demodulated output of the frequency demodulating circuit is supplied to a de-emphasis circuit wherein the high frequency component which was emphasized in the pre-emphasis circuit in the recording system is de-emphasized or attenuated, to restore the video signal into its original form. The output signal of the de-emphasis circuit is obtained as the reproduced video signal. 
     As is well known, the signal-to-noise (S/N) ratio becomes deteriorated with respect to the higher frequency components of the frequency modulated video signal by recording and reproducing of the signal. Thus, the S/N ratio in the high frequencies can be improved, by emphasizing the level of the high frequency component in the pre-emphasis circuit as described above, and increasing the modulation factor with respect to the high frequencies. 
     The conventional pre-emphasis circuit was designed to attenuate the low frequency component with respect to the high frequency component of the input video signal, by use of a filter comprising a capacitor and resistors. On the other hand, the conventional de-emphasis circuit was designed to suppress the high frequency component of the input video signal more than the low frequency component, by use of a filter comprising a capacitor and resistors. These conventional pre-emphasis and de-emphasis circuits each using the filter comprising the capacitor and resistors, can be considered as a circuit which performs weighting with respect to an information which was obtained prior to a present information and adds this weighted information to the present information. In the present specification, a type of circuit which performs weighting with respect to the information which was obtained prior to the present information and adds this weighted information to the present information in this manner, will be referred to as a &#34;forward type&#34; circuit. 
     As a forward type filter other than the above filters comprising the capacitor and resistors, there was a forward type transversal filter comprising a plurality of delay circuits, a plurality of coefficient multipliers, and an adder. 
     However, according to the conventional pre-emphasis circuit employing any of the above filters, a large overshoot is introduced at the rising and falling portions of the pre-emphasized signal waveform if a square wave signal is subjected to the pre-emphasis. When a large overshoot which exceeds a certain level exists in the pre-emphasized signal, a so-called inversion phenomenon occurs if a frequency modulated video signal which is obtained by frequency modulating a carrier by this pre-emphasized signal is recorded onto and reproduced from the magnetic tape. The inversion phenomenon is the phenomenon in which the black and white at parts of the reproduced picture which correspond to the overshoots become inverted. Accordingly, in order to prevent this inversion phenomenon from occurring, a clipping circuit was provided between the pre-emphasis circuit and the frequency modulating circuit in the conventional circuit. This clipping circuit clipped the signal level so that the level at the tip end of the overshoot in the signal does not exceed a predetermined level. 
     However, if the clipping circuit greatly clips the overshoot of the pre-emphasized signal so as to prevent the inversion phenomenon from occurring, the picture quality of the reproduced picture deteriorates at parts which correspond to the overshoots which have been greatly clipped. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a novel and useful video signal processing system in which the above described problems have been eliminated. 
     Another and more specific object of the present invention is to provide a video signal processing system in which the recording system comprises a series connection of a forward type pre-emphasis circuit which performs weighting with respect to a past input information which was obtained prior to a present information and adds this weighted information to the present output information, and a pre-emphasis circuit of a type (hereinafter referred to as a &#34;backward type&#34;) which performs weighting with respect to a future input information which will be obtained subsequent to an output time when an output information is obtained and adds this weighted information to the output information, and the reproducing system comprises a series connection of a forward type de-emphasis circuit, and a backward type de-emphasis circuit which performs weighting with respect to a future input information which will be obtained subsequent to an output time when an output information is obtained and adds this weighted information to the output information. According to the system of the present invention, the pre-emphasis circuit and the de-emphasis circuit each comprise a series connection of forward type and backward type circuits. Thus, the overshoot level of the output video signal of the pre-emphasis circuit can be suppressed to a low level compared to the conventional circuit, without deteriorating the improving effect on the S/N ratio of the video signal in the high frequencies which is obtained by the pre-emphasis and the de-emphasis. For this reason, it is unlikely that the inversion phenomenon will occur. Furthermore, there is no deterioration in the picture quality caused by the clipping carried out in a clipping circuit which clips the level of the output video signal of the pre-emphasis circuit to a level under a predetermined level, because there is no need to greatly clip the level of the video signal. 
     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are systematic block diagrams respectively showing an embodiment of a recording system and a reproducing system of a video signal processing system according to the present invention; 
     FIG. 2 is a systematic circuit diagram showing an embodiment of a backward type pre-emphasis circuit shown in FIG. 1A; 
     FIG. 3 is a systematic circuit diagram showing an example of a forward type pre-emphasis circuit shown in FIG. 1A; 
     FIG. 4 is a systematic circuit diagram showing an embodiment of a backward type de-emphasis circuit shown in FIG. 1B; 
     FIG. 5 is a systematic circuit diagram showing an example of a forward type de-emphasis circuit shown in FIG. 1B; 
     FIGS. 6(A) through 6(E) are graphs for explaining the waveforms of the input and output signals of the pre-emphasis circuit; 
     FIGS. 7(A) through 7(E) are graphs for explaining the waveforms of the input and output signals of the de-emphasis circuit; 
     FIG. 8 is a concrete systematic circuit diagram showing a modification of the backward type pre-emphasis circuit shown in FIG. 2; 
     FIG. 9 is a concrete circuit diagram showing an embodiment of a coefficient multiplier within the circuit system shown in FIG. 8; 
     FIG. 10 is a concrete systematic circuit diagram showing the backward type de-emphasis circuit shown in FIG. 3; 
     FIG. 11 is a systematic circuit diagram showing a pre-emphasis circuit in which backward type and forward type pre-emphasis circuits are combined; 
     FIG. 12 is a systematic circuit diagram showing a de-emphasis circuit in which backward type and forward type de-emphasis circuits are combined; 
     FIG. 13 is a systematic circuit diagram showing another embodiment of a pre-emphasis circuit; and 
     FIG. 14 is a systematic circuit diagram showing another embodiment of a de-emphasis circuit. 
    
    
     DETAILED DESCRIPTION 
     An embodiment of a video signal recording system in the video signal processing circuit according to the present invention is shown in FIG. 1A, and an embodiment of a reproducing system is shown in FIG. 1B. First, in FIG. 1A, a video signal which is to be recorded, is supplied to a pre-emphasis circuit 12 which constitutes an essential part of the system according to the present invention, through an input terminal 11. The video signal supplied to the pre-emphasis circuit 12, is successively supplied to a backward type pre-emphasis circuit 13 and a forward type pre-emphasis circuit 14 which are connected in series and constitute the pre-emphasis circuit 12. The video signal which is emphasized of its high frequency component, is supplied to a frequency modulator 15 wherein the signal frequency modulates a carrier. An output frequency modulated video signal of the frequency modulator 15, is recorded onto a magnetic tape 17 by a magnetic head 16. 
     In FIG. 1B, the signal which has been recorded onto the magnetic tape 17 in the above described manner, is reproduced by the magnetic head 16. The reproduced signal from the magnetic head 16 is supplied to a frequency demodulator 21 wherein the signal is demodulated. The demodulated video signal is supplied to a de-emphasis circuit 22 which constitutes an essential part of the present invention. The demodulated signal supplied to the de-emphasis circuit 22, is successively supplied to a backward type de-emphasis circuit 23 and a forward type de-emphasis circuit 24 which are connected in series and constitute the de-emphasis circuit 22. The video signal which is suppressed of its high frequency component and restored to its original form, is obtained as a reproduced video signal through an output terminal 25. 
     An embodiment of the backward type pre-emphasis circuit 13 of the pre-emphasis circuit 12 shown in FIG. 1A, is shown in FIG. 2. Delay circuits 32 1 , 32 2 , . . . , 32 n-1 , and 32 n  respectively having delay times τ n , τ n-1 , . . . , τ 2 , and τ 1 , are connected in series between an input terminal 31 which is applied with the video signal from the input terminal 11, and an adder 35. Inputs to these delay circuits 32 1  through 32 n  are respectively supplied to differentiators 33 1 , 33 2 , . . . , and 33 n . An output of the delay circuit 33 n  is supplied to a differentiator 33 n+1 . Output signals of the differentiators 33 1  through 33 n+1  are each supplied to the adder 35, through coefficient multipliers 34 1 , 34 2 , . . . , 34 n , and 34 n+1 . The coefficient multipliers 341 through 34 n+1  respectively multiply coefficients -a n , -a n-1 , . . . , -a 2 , -a 1 , and -a 0  to the output signals of the differentiators 33 1  through 33 n+1 , and supplies the multiplied signals to the adder 35. The adder 35 adds these multiplied signals, and the output of the adder 35 which is obtained through an output terminal 36 is supplied to the forward type pre-emphasis circuit 14. The values of the delay times of the delay circuits 32, through 32 n  may be the same. However, in the present embodiment, the delay times are selected such that a relation τ n  &gt;τ n-1  &gt; . . . &gt;τ.sub. 2 &gt;τ 1  stands. Moreover, the coefficients of the coefficient multipliers 34 1  through 34 n+1  are selected such that a relation a 0  &gt;a 1  &gt; . . . &gt;a n-1  &gt;a n  stands. 
     In the backward type pre-emphasis circuit 13 which is designed as described heretofore, the high frequency component of the video signal is emphasized relative to the low frequency component in the differentiators 33 1  through 33 n+1 . Further, a predetermined pre-emphasis characteristic is obtained by setting each of the coefficients of the coefficient multipliers 34 1  through 34 n+1  to predetermined values and performing weighting. It will be assumed that the &#34;present&#34; is an output time when the video signal is obtained from the delay circuit 32 n  after being delayed by a delay time (τ n  +τ n-1  +. . . +τ 2  +τ 1 ). Then, the video signals which will be obtained at times (τ 1  +τ 2  . . . τ n-1  +τ n ), . . . , (τ 1  +τ 2 ), and τ 1  subsequent to the present, that is, in the future with respect to the present, are respectively obtained from the input side of the delay circuit 32 1  and the output sides of the delay circuits 32 1  through 32 n-1 . 
     When a signal having a waveform shown in FIG. 6(A) is applied to the input terminal 31 of the backward type pre-emphasis circuit 13, a signal having a waveform in which the high frequency component is emphasized as shown in FIG. 6(D) is obtained through the output terminal 36. 
     The signal obtained through the output terminal 36, is applied to an input terminal 41 of an embodiment of the forward type pre-emphasis circuit 14 shown in FIG. 3. The present embodiment of the forward type pre-emphasis circuit 14 is designed as a transversal filter. 
     The video signal which is applied to the input terminal 41, is directly supplied to an adder 45, and is also supplied to a differentiator 42 wherein the high frequency component of the video signal is emphasized relative to the low frequency component. The output of the differentiator 42 is successively passed through n delay circuits 43 1  through 43 n  which respectively have delay times τ 1 , τ 2 , . . . , and τ n , and is delayed by a delay time (τ 1  +τ 2  + . . . +τ n ). The output of the delay circuit 43 n  is supplied to a coefficient multiplier 44 n+1  wherein a coefficient a n  is multiplied to the signal. In addition, each of the outputs of the differentiator 42 and the delay circuits 43 1  through 43 n-1 , are respectively supplied to coefficient multipliers 44 1  through 44 n-1 . These coefficient multipliers 44 1  through 44 n-1  respectively multiply coefficients a 0  through a n-1  to the respective inputs, and each of the outputs of these coefficient multipliers 44 1  through 44 n-1  are supplied to the adder 45. The adder 45 adds the signals supplied thereto, and produces an output through an output terminal 46. 
     In the forward type pre-emphasis circuit 14 which is designed as described heretofore, the adder 45 adds to the present video signal which is obtained through the input terminal 41, signals which are obtained by weighting each of the differentiated video signals at times τ 1 , (τ 1  +τ 2 ), . . . , (τ 1  +τ 2  + . . . +τ n ) prior to the present, that is, in the past with respect to the present. Thus, a signal which is given a pre-emphasis characteristic, is obtained from the forward type pre-emphasis circuit 14. 
     The forward type pre-emphasis circuit 14 utilizes the present and past input information, in order to obtain a signal which is given a present pre-emphasis characteristic. Hence, the forward type pre-emphasis circuit 14 carries out a forward type signal processing which is in accordance with the time flow. 
     When the signal having the waveform shown in FIG. 6(A) is applied to the input terminal 41, a signal having a waveform in which the high frequency component is emphasized as shown in FIG. 6(C) is obtained through the output terminal 46. 
     Each of the delay times τ n  through τ 1  of the delay circuits 32 1  through 32 n  within the backward type pre-emphasis circuit 13 shown in FIG. 2, are arranged in a reverse sequence to each of the delay times τ 1  through τ n  of the delay circuits 32 1  through 43 n  within the forward type pre-emphasis circuit 14 shown in FIG. 3. Moreover, each of the weighting coefficients -a n  through -a 0  of the coefficient multipliers 34 1  through 34 n+1  within the backward type pre-emphasis circuit 13 are arranged in a reverse sequence to each of the weighting coefficients a 0  through a n  of the coefficient multipliers 44 1  through 44 n+1  within the forward type pre-emphasis circuit 14, and the polarity of each of the weighting coefficients of the coefficient multipliers 34 1  through 34 n+1  are opposite to the polarity of each of the weighting coefficients of the coefficient multipliers 44 1  through 44 n+1 . Accordingly, with respect to the same input signal waveform, the output signal waveform of the backward type pre-emphasis circuit 13 and the output signal waveform of the forward type pre-emphasis circuit 14 are symmetrical with respect to the time base, as may be seen by comparing FIGS. 6(D) and 6(C). The S/N ratio improvement factors of the backward type pre-emphasis circuit 13 and the forward type pre-emphasis circuit 14, are set to be essentially the same. 
     Conventionally, a pre-emphasis circuit was constituted solely from a backward type pre-emphasis circuit or a forward type pre-emphasis circuit. For this reason, with respect to an input signal waveform shown in FIG. 6(A), an output signal waveform of the conventional pre-emphasis circuit became as shown in FIG. 6(B). If the degree of pre-emphasis is increased in this conventional pre-emphasis circuit in order to obtain a desired S/N ratio improvement factor, the overshoots in the signal waveform shown in FIG. 6(B) become greater. However, when the overshoots are large, the previously described inversion phenomenon occurs. The pre-emphasized signal was thus passed through a clipping circuit so as to prevent the occurrence of this inversion phenomenon. However, if the overshoots are clipped, the picture quality becomes deteriorated. 
     On the other hand, according to the system of the present invention, the backward type pre-emphasis circuit 13 and the forward type pre-emphasis circuit 14 are connected in series. Hence, according to this arrangement, the degree of pre-emphasis which is required to obtain thc desired S/N ratio improvement factor, may be small compared to the degree of pre-emphasis required in the conventional pre-emphasis circuit constituted solely from a forward type pre-emphasis circuit, for example. In addition, with respect to the same input signal waveform, the positions where the overshoots occur in the output signals of the backward type and forward type pre-emphasis circuits 13 and 14 differ as may be seen from FIGS. 6(D) and 6(C). Accordingly, when the signal having the waveform shown in FIG. 6(A) is applied to the input terminal 31, the output signal waveform of the forward type pre-emphasis circuit 14 which is supplied to the frequency modulator 15, assumes a waveform shown in FIG. 6(E). As may be seen from FIG. 6(E), it is possible to reduce the heights of the overshoots compared to the waveform shown in FIG. 6(B). Therefore, according to the system of the present invention, the inversion phenomenon is unlikely to occur. Moreover, it is unnecessary to clip the overshoots, or, even if the clipping is required, the clipping which is required is exceedingly small compared to the clipping which was required in the conventional pre-emphasis circuit. The picture quality will not become deteriorated by such an exceedingly small clipping. 
     Next, description will be given with respect to the construction of the de-emphasis circuit 22. The demodulated video signal which is obtained from the frequency demodulator 21, is successively supplied to the backward type de-emphasis circuit 23 and the forward type de-emphasis circuit 24. An embodiment of the backward type de-emphasis circuit 23 is shown in FIG. 4, and an embodiment of the forward type de-emphasis circuit 24 is shown in FIG. 5. The de-emphasis circuit 22 gives the signal de-emphasis characteristic which is complementary to the pre-emphasis characteristic given in the pre-emphasis circuit 12. The signal which is given this de-emphasis characteristic, is obtained through the output terminal 25. 
     The backward type de-emphasis circuit 23 shown in FIG. 4 comprises n delay circuits 52 1  through 52 n , coefficient multipliers 53 1  through 53 n+1 , and an adder 54. The delay circuits 52 1  through 52 n  respectively have delay times T n  through T 1 , and are connected in series. The delay circuits 52 1  through 52 n  respectively delay the input demodulated video signal which is applied to an input terminal 51. The coefficient multiplier 53 1  multiplies a weighting coefficient b n  to the input demodulated video signal. The coefficient multipliers 53 2  through 53 n+1  respectively multiply weighting coefficients b n-1  through b 0  to the respective outputs of the delay circuits 52 1  through 52 n . The outputs of the coefficient multipliers 53 1  through 53 n+1  are added in the adder 54, and the added output is produced through an output terminal 55. 
     It will be assumed that a reference time is a time when the output signal of the coefficient multiplier 53 n+1  is obtained. Then, the backward type de-emphasis circuit 23 is a circuit which performs weighting with respect to video signals at times T 1 , (T 1  +T 2 ), . . . , and (T 1  +T 2  + . . . +T n ) subsequent to this reference time, that is, in the future with respect to the reference time, and adds the weighted video signals with the video signal at the reference time which is obtained from the coefficient multiplier 53 n+1 . Accordingly, as in the case of the backward type pre-emphasis circuit 13 described before, the backward type de-emphasis circuit 23 utilizes the future information which will be obtained subsequent to the present, and carries out a signal processing which is counter to the time flow. Thus, when a signal having a waveform shown in FIG. 7(A) is applied to the input terminal 51, a signal having a waveform shown in FIG. 7(D) is obtained through the output terminal 55 of the de-enphasis circuit 23 shown in FIG. 4. On the other hand, if the same signal shown in FIG. 7(A) is applied to a conventional de-emphasis circuit constituted solely from a forward type de-emphasis circuit, the output signal will assume a waveform shown in FIG. 7(B). The coefficients of the coefficient multipliers 53 1  through 53 n+1  are selected such that a relation b n  &lt;b n-1  &lt; . . . &lt;b 1  &lt;b 0  stands. Moreover, the values of the delay times of the delay circuits 52 1  -52 n  may be the same. However, in the present embodiment, the delay times are selected such that a relation T 1  &lt;T 2  &lt; . . . &lt;T n-1  &lt;T n  stands. 
     The forward type de-emphasis circuit 24 shown in FIG. 5 is designed as a transversal filter, and comprises n delay circuits 62 1  through 62 n , coefficient multipliers 63 1  through 63 n+1 , and an adder 64. The n delay circuits 62 1  through 62 n  respectively have delay times T 1  through T n , and are connected in series. The delay circuits 62 1  through 62 n  respectively delay the input signal which is applied to an input terminal 61. The coefficient multiplier 63 1  multiplies a weighting coefficient b 0  to the input signal. The coefficient multipliers 63 2  through 63 n+1  respectively multiply weighting coefficients b 1  through b n  to the respective outputs of the delay circuits 62 1  through 62 n . The outputs of the coefficient multipliers 63 1  through 63 n+1  are added in the adder 64, and the added output is produced through an output terminal 65. As in the case of the forward type pre-emphasis circuit 14 described before in conjunction with FIG. 3, the forward type de-emphasis circuit 24 utilizes the present and past input information in order to obtain the present output waveform, and carries out a signal processing which is in accordance with the time flow. 
     Each of the delay times T 1  through T n  of the delay circuits 62 1  through 62 n  within the forward type de-emphasis circuit 24, are arranged in a reverse sequence to each of the delay times T n  through T 1  of the delay circuits 52 1  through 53 n  within the backward type de-emphasis circuit 23. Moreover, each of the weighting coefficients b 0  through b n  of the coefficient multipliers 64 1  through 64 n+1  within the forward type de-emphasis circuit 24 are arranged in a reverse sequence to each of the weighting coefficients b n  through b 0  of the coefficient multipliers 54 1  through 54 n+1  within the backward type de-emphasis circuit 23. Accordingly, with respect to the same input signal waveform, the output signal waveform of the forward type de-emphasis circuit 24 and the output signal waveform of the backward type de-emphasis circuit 23 are symmetrical with respect to the time base. As a result, when the signal having the waveform shown in FIG. 7(A) is applied to the input terminal 61, a signal having a waveform shown in FIG. 7(C) which is complementary to the waveform shown in FIG. 7(D), is obtained through the output terminal 65. Thus, when the square wave signal shown in FIG. 7(A) is applied to the de-emphasis circuit 22 comprising the backward type de-emphasis circuit 23 and the forward type de-emphasis circuit 24, a de-emphasized signal having a waveform shown in FIG. 7(E) is produced from the de-emphasis circuit 22. As may be seen from FIG. 7(E), the high frequency component of the signal is suppressed. Therefore, the signal which is pre-emphasized as shown in FIG. 6(E) in the recording system, is de-emphasized in the de-emphasis circuits 23 and 24 and restored to the square wave signal shown in FIG. 6(A) or FIG. 7(A). 
     In the backward type pre-emphasis circuit 13 shown in FIG. 2, the differentiators 33 1  through 33 n+1  may be omitted. A concrete circuit diagram of the backward type pre-emphasis circuit 13 in which the differentiators are omitted, is shown in FIG. 8. In FIG. 8, those parts which are the same as those corresponding parts in FIG. 2 are designated by the same reference numerals. The outputs of the coefficient multipliers 34 1  through 34 n+1  are supplied to an inverting input terminal of an operational amplifier 71 which constitutes the adder 35. The output of the delay circuit 32 n  is supplied to the coefficient multiplier 34 n+1 , and is also supplied to a non-inverting input terminal of the operational amplifier 71 through a delay circuit 32 n+1 . The output of the operational amplifier 71 is connected to its inverting input terminal through a resistor. The output signal of the operational amplifier 71 is obtained through the output terminal 36 as the output signal of the backward type pre-emphasis circuit 13. In this embodiment, each of the delay circuits 32 1  -32 n+1  has the same delay time τ (for example, 0.13 μsec). 
     A concrete circuit diagram of the coefficient multiplier 34 1  is shown in FIG. 9. The coefficient multiplier 34 1  comprises transistors Q1 and Q2, resistors R1 through R6, and a capacitor C1 which are connected as shown in FIG. 9. The other coefficient multipliers 34 2  through 34 n+1  are constructed similarly. The coefficient multipliers 34 1  through 34 n+1  are set to have different coefficients, by appropriately selecting the resistance of the variable resistor R2. 
     A concrete circuit diagram of the backward type de-emphasis circuit 23 shown in FIG. 4, is shown in FIG. 10. In FIG. 10, those parts which are the same as those corresponding parts in FIG. 4 are designed by the same reference numerals. The output signals of the coefficient multipliers 53 1  through 53 n+1  are supplied to a non-inverting input terminal of an operational amplifier 75 which constitutes the adder 54. The output of the operational amplifier 75 is connected to its inverting input terminal through a resistor. This inverting input terminal of the operational amplifier 75 is grounded through another resistor and a capacitor which are connected in series. The output signal of the operational amplifier 75 is obtained through the output terminal 55 as the output signal of the backward type de-emphasis circuit 23. In the present embodiment, each of the delay circuits 52 1  through 52 n  has the same delay time T. This delay time T may be the same as the delay time τ in the pre-emphasis circuit. The coefficient multipliers 53 1  through 53 n+1  may also be constructed as shown in FIG. 9. 
     In the embodiment described previously, the pre-emphasis circuit comprises the backward type pre-emphasis circuit 13 and the forward type pre-emphasis circuit 14 which are independent, and connected in series. Similarly, the de-emphasis circuit comprises the backward type de-emphasis circuit 23 and the forward type de-emphasis circuit 24 which are independent, and connected in series. However, the backward type and forward type circuits may be integrally connected as shown in FIGS. 11 and 12. 
     FIG. 11 shows a pre-emphasis circuit 12A as another embodiment of the pre-emphasis circuit 12. In FIG. 11, those parts which are the same as those corresponding parts in FIGS. 2 and 3 are designed by the same reference numerals, and their description will be omitted. According to the present embodiment, the pre-emphasis circuit corresponds to a circuit in which the backward type pre-emphasis circuit 13 shown in FIG. 2 and the forward type pre-emphasis circuit 14 shown in FIG. 3 are integrally connected. The output of the delay circuit 32 n  is supplied to the differentiator 33 n+1 , and is also supplied to an adder 81 and the differentiator 42. The adder 81 is commonly used as the adders 35 and 45 shown in FIGS. 2 and 3. The outputs of the coefficient multipliers 34 1  through 34 n+1 , the delay circuit 32 n , and the coefficient multipliers 44 1  through 44 n+1  are respectively supplied to the adder 81 wherein these outputs are added. The pre-emphasized output signal of the adder 81 is obtained through the output terminal 46. Accordingly, when the signal having the waveform shown in FIG. 6(A) is supplied to the input terminal 31, the signal having the waveform shown in FIG. 6(E) is obtained through the output terminal 46. In the present embodiment, the differentiators 33 1  through 33 n+1  may also be omitted. 
     FIG. 12 shows a de-emphasis circuit 22A as another embodiment of the de-emphasis circuit 22. In FIG. 12, those parts which are the same as those corresponding parts in FIGS. 4 and 5 are designed by the same reference numerals, and their description will be omitted. According to the present embodiment, the de-emphasis circuit corresponds to a circuit in which the backward type de-emphasis circuit 23 shown in FIG. 4 and the forward type de-emphasis circuit 24 shown in FIG. 5 are integrally connected. The output of the delay circuit 52 n  is supplied to an adder 92 through a coefficient multiplier 91 which multiplies a coefficient b 0 . The output of the delay circuit 52 n  is also supplied to the delay circuit 62 1 . The coefficient multiplier 91 is commonly used as the coefficient multipliers 53 n+1  and 63 1  shown in FIGS. 4 and 5. In addition, the adder 92 is commonly used as the adders 54 and 64 shown in FIGS. 4 and 5. The outputs of the coefficient multipliers 53 1  through 53 n , 91, and 63 2  through 63 n+1  are respectively supplied to the adder 92 wherein these outputs are added. The de-emphasized output signal of the adder 92 is obtained through the output terminal 65. Accordingly, when the signal having the waveform shown in FIG. 7(A) is supplied to the input terminal 51, the signal having the waveform shown in FIG. 7(E) is obtained through the output terminal 65. When the signal having the waveform shown in FIG. 6(E) is supplied to the input terminal 51, the signal having the waveform shown in FIG. 6(A) (or FIG. 7(A)) is obtained through the output terminal 65. 
     In each of the embodiments described heretofore, the forward type pre-emphasis circuit 14 is connected in series at a stage subsequent to the backward type pre-emphasis circuit 13. However, the connection may be in reverse, that is, the forward type pre-emphasis circuit 14 may be connected in series at a stage prior to the backward type pre-emphasis circuit 13. Similarly, the forward type de-emphasis circuit 24 may be connected in series at a stage prior to the backward type de-emphasis circuit 23. In other words, the backward type and forward type circuits may be connected in series in either sequence. 
     Further, in the embodiments described before, the forward type pre-emphasis circuit 14 and the forward type de-emphasis circuit 24 are each designed as a transversal filter. However, the circuits 14 and 24 are not limited to the transversal filter, and other conventional circuits may be utilized. For example, the forward type pre-emphasis circuit 14 may be designed as a highpass filter comprising a capacitor and resistors, and the forward type de-emphasis circuit 24 may be designed as a lag-lead filter comprising a capacitor and resistors. Moreover, memories may be used instead of these filters, to obtain similar characteristics. 
     In addition, the backward type pre-emphasis circuit 14 and the backward type de-emphasis circuit 23 may respectively be designed in the form of an adapter. In this case, such an adapter is connected to a recording and reproducing apparatus which already has a built-in forward type pre-emphasis circuit and a forward type de-emphasis circuit. 
     FIG. 13 shows another embodiment of the pre-emphasis circuit. In FIG. 13, a forward type pre-emphasis circuit 102 comprises a capacitor C11 and resistors R11 and R12, and is of a known construction. The recording video signal from the input terminal 11 in FIG. 1A is applied to an input terminal 101. The input terminal 31 of the backward type pre-emphasis circuit 13 is connected to an output terminal 103. 
     FIG. 14 shows another embodiment of the de-emphasis circuit. In FIG. 14, a forward type de-emphasis circuit 112 comprises a capacitor C12 and a resistor R13, and is of a known construction. The output signal of the frequency demodulator 21 shown in FIG. 1B is supplied to an input terminal 111. The input terminal 51 of the backward type de-emphasis circuit 23 is connected to an output terminal 113. 
     According to the pre-emphasis circuit shown in FIG. 13 and the de-emphasis circuit shown in FIG. 14, it is also possible to similarly obtain the same desired pre-emphasis characteristic and de-emphasis characteristic, as in the embodiments described previously. 
     Further, the present invention is not limited to these embodiments, and various variations and modifications may be made without departing from the scope of the present invention.