VIDEO SIGNAL REPRODUCING CIRCUIT FOR A VIDEO TAPE RECORDER

Abstract: An apparatus for reproducing from a magnetic tape (T) first and second video signals (S CV ,S SP ) which are frequency modulated on a carrier, the first video signal having the center frequency of the carrier lower than that of the second video signal is disclosed, which includes a demodulator (3) for demodulating the first and second video signals, a first low-pass filter (4) for filtering the first video signal demod­ ulated by the demodulator, the first low-pass filter having a first cut-off frequency (F₂), a second low-pass filter (5) for filtering the second video signal demodulated by the demod­ ulator (3), the second low-pass filter having a second cut-off frequency (F₁) which is lower than the first cut-off frequency (F₂), a detector (20) connected prior to the demodulator (3) for detecting whether a reproduced video signal is the first or second video signal and for generating a control signal in response thereto and a switching circuit (6) for sypplying the video signals to an output terminal in accordance with the control signal from the detector. There is also disclosed an apparatus for recording and reproducing a video signal, which includes a first modulator for frequency modulating a video signal with a first frequen­ cy carrier to produce a first video signal, a second modulator for frequency-modulating the video signal with a second frequency carrier to produce a second video signal, a recording head for recording the first and second video signals frequency-modulated by the first and second mod­ ulators on a magnetic tape, a reproducing head for reproduc­ ing the first and second video signals recorded on the magnetic tape, a detector for detecting whether the video signal reproduced from the magnetic tape is frequency-­ modulated with the first frequency carrier or with the second frequency carrier, and for generating a control signal, and a first switching circuit of selectively activating the first and second modulator in accordance with the control signal, a frequency of the carrier used in recording subsequent to reproducing being coincident with a frequency of the carrier used in the reproducing.

Description: Field of the Invention This invention relates to an apparatus for reproducing video signals recorded on a tape, especially to a video tape recorder (VTR) and in particular, is directed to a VTR suitable for use with a so-called low frequency-converted signal recording and/or reproducing system in which a chrominace subcarrier signal is low frequency-converted, recorded on a recording tape together with a frequency modulated luminance signal and reproduced therefrom. Description of the Prior Art In an apparatus for recording and/or reproducing a low frequency-converted carrier signal in which a low frequency-converted chrominance subcarrier signal S C is recorded on a recording tape together with a frequency modulated luminance signal S Y , the deviation range of the frequency-modulated frequency is occasionally made different from that of the existing one in use. Fig. 1A shows a frequency spectrum of an existing low frequency-converted color video signal S CV in a case where the deviation width of the frequency-modulated frequency is selected in a range from 3.8 MHz to 5.4 MHz and the low frequency-converted frequency fc is selected as, for example, 688 KHz (in the case of NTSC system), while Fig. 1B shows a frequency spectrum of a low frequency-converted color video signal S SP in a case where the deviation frequency is selected higher than the afore-said frequency by 1.0 MHz. When the first and second color video signals S CV and S SP which are the same in frequency-modulated deviation width but different in deviation frequency and are recorded on the same tape T in the mixed state as, for example, shown in Fig. 2, are reproduced or when a video information is reproduced not from a tape on which only the first color video signal S CV is recorded but from a magnetic tape on which only the second color video signal S SP is recorded, the reproducing system of a VTR must selectively change over to a circuit constant such as a cut-off frequency of a low-pass filter used in, for example, a frequency demodulator in accordance with the color video signals S CV and S SP to be reproduced. For such case, it is considered to form the reproducing system of the VTR as shown in Fig. 3. Fig. 3 illustrates a reproducing system 10 for only a luminance signal system. As shown in Fig. 3, a first or second color video signal S CV or S SP reproduced by a magnetic reproducing head H p from a magnetic tape T is supplied through a preamplifier 2 to a frequency demodulator 3 in which a luminance signal Y CV or Y SP is demodulated, which then is fed to a second low-pass filter 4, in which the band of the luminance signal Y CV or Y SP is restricted by a predetermined amount. For example, when the filter characteristic suitable for the first luminance signal Y CV is selected as, shown by F1 in Fig. 4, the filter characteristic suitable for the second luminance signal Y SP is selected as shown by F2 which is extended as compared with F1 into the high frequency region. A first low-pass filter 5 having a filter characteristic F1 is connected at the post stage of the second low-pass filter 4 and a switching circuit 6 is provided for selectively supplying the outputs from these low-pass filters 4 and 5 to an output terminal thereof. The switching circuit 6 is changed over such that only when the first luminance signal Y CV is demodulated, the output from the first low-pass filter 5 is delivered from the switching circuit 6. To this end, the first or second luminance signal Y CV or Y SP thus demodulated is supplied to a synchronizing signal separating circuit 7 in which each of horizontal synchronizing signal P HC and P HS is respectively separated from the first or second luminance signal Y CV and Y SP to thereby forming a gate pulse P G , by which the horizontal synchronizing pulse P HC or P HS of the first or second luminance signal Y CV or Y SP is gated. Reference numeral 8 designates a gate circuit therefor. When the input and output characteristic of the frequency demodulator 3 is selected as shown by a straight line ℓ in Fig. 5, the horizontal synchronizing pulses P HC and P HS of the frequency-demodulated first and second luminance signals Y CV and Y SP are different in DC level. This level difference is used to discriminate the first and second color video signals S CV and S SP from each other. To this end, the gate horizontal synchronizing pulses P HC and P HS from the gate circuit 8 are supplied to a level comparator 9 in which the DC levels thereof are respectively compared with a reference level so as to produce a compared control output P C of "H" (high level) for, for example, the horizontal synchronizing pulse P HC and the compared output P C of "L" (low level) for the horizontal synchronizing pulse P HS . The switching circuit 6 is controlled by the compared output control pulse P C such that when the first color video signal S CV is reproduced, the output from the first low-pass filter 5 is always delivered therefrom while when the second color video signal S SP is reproduced, the output from the second low-pass filter 4 is always delivered therefrom. When the gate pulse P G is formed from the demodulated first or second color video signal S CV or S SP , upon reproducing, if a dropout occurs, the horizontal synchronizing pulses P HC and P HS used for forming the gate pulse P G can not be synchronously, separated positively. As a result, the low-pass filters 4 and 5 cannot be changed over precisely. Further, when a noise SN as shown in Fig. 6 enters the video signal, the synchronizing separating circuit 7 is malfunctioned by the noise SN so that also in this case, the low-pass filters 4 and 5 cannot be changed over accurately. Another apparatus for reproducing from a magnetic tape a recorded video signal which is one of at least two different video signals, is described in DE-A-26 44 465. This apparatus also makes use of the horizontal synchronizing pulses. However, no level comparison is provided, but the horizontal synchronizing pulses are used as trigger pulses to start different time pulses after which different types of signals have to be expected, depending on what type of video signal is reproduced. This apparatus has the same deficiencies as the apparatus according to Fig. 3, i.e. it cannot be changed over precisely between the different types of video signals in case the horizontal synchronizing pulse is not detected correctly due to a drop-out or due to noise. Therefore the problem existed to provide an apparatus for reproducing from a magnetic tape a recorded video signal which is one of a first and second video signal, which apparatus is capable of detecting very reliably whether said first or said second video signal is reproduced. SUMMARY OF THE INVENTION An apparatus according to the present invention for reproducing from a magnetic tape a recorded video signal which is one of first and second video signals which have been frequency-modulated on first and second carriers, respectively, said first carrier having a frequency lower than that of said second carrier, comprises means for demodulating said first and second video signals; a first low-pass filter for filtering said first video signal, said first low-pass filter having a first cut-off frequency (F₂); a second low-pass filter for filtering said second video signal demodulated by said demodulating means, said second low-pass filter having a second cut-off frequency (F₁) which is lower than said first cut-off frequency (F₂); detecting means generating a detecting signal, the level of which depends on whether said first or said second video signal is reproduced, which level is compared by a level detecting means with a comparison level (V BE ) for generating a control signal, the level of which indicates whether the reproduced video signal is said first or said second video signal; and switching means for supplying either said first or said second video signal from output terminal in accordance with said control signal from said detecing means, and wherein said detecting means comprises a third filter receiving the undemodulated respective video signal for generating said detecting signal at its output, said third filter having an amplitude response which, depending on the frequency applied monotonously increases or decreases at least in the frequency range between the lower limit frequency of said first video signal and the upper limit frequency of said second video signal. The main difference of this apparatus against the prior art apparatus is that the detecting signal is not generated from the demodulated video signal, but from the still undemodulated video signal. By the third filter, a detecting signal is generated whose level indicates the level of the complete video signal, not only of its synchronizing part. therefore, a drop-out or noise does not influence the detection independently of whether said first or said second video signal is reproduced. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A and 1B illustrate frequency spectrum diagrams of color video signals useful for explaining this invention, respectively; Fig. 2 illustrates an example of a recorded pattern of a video information on a magentic tape; Fig. 3 illustrates a systematic block diagram of a reproducing system of a prior art video tape recorder useful for explaining this invention; Figs. 4 to 6 illustrate diagrams useful for explaining the operation thereof, respectively; Fig. 7 illustrates a systematic block diagram of an embodiment of the video tape recorder according to this invention; Fig. 8 illustrates a circuit diagram of a detecting circuit used in the embodiment shown in Fig. 7; Fig. 9, Figs. 10A to 10F and Figs. 11A to 11C respectively illustrate diagrams useful for explaining the operation thereof; Figs. 12A and 12B respectively illustrate diagrams useful for explaining another operation of this invention; and Fig. 13 illustrates a systematic block diagram showing another embodiment of the VTR according to this invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, an embodiment of the VTR according to this invention will hereinafter be described in detail with reference to Fig. 7 and the followings. In Fig. 7, like parts corresponding to those of Fig. 3 are marked with the same reference numerals and will not be described in detail. In Fig. 7, reference numeral 20 designates a detecting circuit which is supplied with the first or second color video signal S CV or S SP before being demodulated, in this embodiment, particularly the frequency-modulated luminance signal S YC or S YS and the detected output therefrom is supplied to the switching circuit 6 as the gate pulse P G thereof. Fig. 8 shows an example of the detecting circuit 20. Referring to Fig. 8, the first or second luminance signal S YC or S YS applied to a terminal 21 is supplied through a buffer amplifier 22 to a filter 23. As the filter 23, an LC filter having a high Q (quality)-factor is used and this filter 23 has a characteristic of a single peak (or single bottom) as shown by a curve ℓ₁ in Fig. 9. Therefore, if the filtered output from this filter 23 is supplied to an amplifier 24, the amplifier 24 produces an output with different levels on the basis of the difference of the deviation frequencies as shown in Fig. 9. In the illustrative example, the level of the first luminance signal S YC is higher than that of the second luminance signal S YS (S YC > S YS ). Fig. 9 shows the waveforms of the frequency modulated luminance signals S YC and S YS in the form of the FM demodulated output signals for convenience sake of explanation. However, in practice, the frequency modulated luminance signals S YC and S YS are high frequency signals as shown in Figs. 10A and 10D. The output from the amplifier 24 is detected by a detector circuit 25 which is formed of mainly a transistor Q1 and a diode D. When the first luminance signal S YC as shown in Fig. 10A is supplied to the detecting circuit 20, a detected output S DC obtained at the emitter of the transistor Q1 becomes as shown in Fig. 10B. This detected output S DC is divided in voltage by a resistance type voltage dividing circuit 26 and then fed to a transistor Q2 which constitutes a level detecting circuit 27. Accordingly, when the level of the divided output is higher than that of the base-emitter voltage V BE of the transistor Q2, the transistor Q2 is turned on. Consequently, the voltage dividing ratio is selected so as to turn on the transistor Q2 only when the detected output S DC based on the first luminance signal S YC is produced. Figs. 9 and 10B illustrate a level ℓ₂ which enables the transistor Q2 to turn on. The collector output of the transistor Q2 becomes as shown by a broken line in Fig. 10C. When this collector output is supplied to an integrating circuit 28, an integrated output S IC of off-delay as shown in Fig. 10C is obtained therefrom. This integrated output S IC is the output near the DC voltage. The integrated output S IC is supplied to a PNP-type transistor Q3 and compared in level with a reference level. Since the reference level is presented as (B+) - V BE (B+ is the power source voltage and V BE is the base-emitter voltage of the transistor Q3), when the level of integrated output S IC is not higher than the reference level, the transistor Q3 is turned on to make its collector output S OC become "H". In the afore-said case, the absolute value of the integrated output S IC is so small as to turn on the transistor Q3. In the case of the second luminance signal S YS ' on the contrary, the peak level of the detected output S DS is small (Fig. 10E) so that if this detected output S DS is divided in voltage, the level thereof can not exceed the base-emitter voltage V BE of the transistor Q2 and hence the collector output of the transistor Q2 holds the level of the power source voltage. As a result, since the integrated output S IC is equal to the power source voltage B+, the transistor Q3 is not turned on and hence the collector output S OS thereof is at "L". Accordingly in the case that an input signal is the first color video signal S CV , the level detected output (the collector output of the transistor Q3) is "H", while in the case that an input signal is the second color video signal S SP , the level detected output becomes "L". The level detected outputs S OC and S OS are supplied to a D terminal of a D-type flip-flop circuit 29. The D-type flip-flop circuit 29 is controlled in its latching an input to its D terminal by an output or trigger pulse P T from a control circuit 30. The control circuit 30 for producing the trigger pulse P T consists of a pair of NOR gates 31 and 32 and a pair of AND gates 33 and 34, in which a mode pulse P PB that is obtained in the playback mode of the apparatus, is supplied to a terminal 35. This mode pulse P PB is supplied through the NOR gate 31 to the AND gate 34 and the mode pulse P PB is at "L" in the playback mode. A switching pulse P SW for use in selectively switching the magnetic head is supplied to a terminal 36 of the control circuit 30 and this switching pulse P SW is differentiated by a differentiating circuit 38 and then supplied through the AND gate 33 to the AND gate 34. The switching pulse P SW is used to alternately select the odd field F O and the even field F E as shown in Figs. 11A and 11B so as to form a successive reproduced signal. A mode pulse P IN which becomes at "H" in the insert edition mode is supplied to a terminal 37 of the control circuit 30. This mode pulse P IN is supplied through the NOR gate 32 to the AND gate 33. Accordingly, in the playback mode, the level of the mode pulse P PB is "L" and that of the mode pulse P IN is "L" so that only when the switching pulse P SW rises, the outputs of the AND gates 33 and 34 both become "H" in level thereby producing the trigger pulse P T . As a result, the input signal is latched in the D terminal of the D type flip-flop circuit 29 at a timing when the trigger pulse P T is produced. As described above, in the playback mode, one level detected output S O is latched in the flip-flop circuit 29 at every rotation of the magnetic head and the level detected output S O of "H" or "L" level thus latched is used as a gate pulser or control pulser (the detected output) PG. In consequence, when the recording tape T as shown in Fig. 2 is reproduced, the first color video signal S CV is reproduced firstly so that the level detected output S O of level "H" at that time is supplied to the switching circuit 6 as the gate pulse P G and hence the first low-pass filter 5 is selected. When the second color video signal S SP comes to be reproduced, the second detected output S DS is continuously obtained immediately after the playback of the first odd field F O so that the level detected output S O becomes "L". As a result, as the next timing for changing over the magnetic head, the level detected output S O of level "L" is latched in the D-type flip-flop circuit 29 so that the gate pulse P G is changed from "H" to "L" in level, thereby operating the switching circuit 6 so as to select the second low-pass filter 4. When the detecting circuit 20 is constructed as described above, the horizontal synchronizing pulse after being demodulated is not used to form the gate pulse P G so that the switching circuit can stably be operated regardless of the dropout and the noise. According to this invention, the gate pulse or control pulse P G is formed on the basis of the color video signal before being demodulated so that even if the FM carrier is temporarily dropped due to the dropout, the detected outputs S DC and S DS are not changed so much. If the filter 23 is designed to have a high Q-factor, the FM carrier itself is very small in amount as shown in Fig. 10A or 10D, so that the detected outputs S DC and S DS are hardly changed substantially. Thus, it can be avoided that the switching operation of the switching circuit will be made unstable by the dropout. Further, since the noise can be removed by the integrating circuit 28, the switching circuit is free of the influence of the noise. In addition, in the embodiment shown in Fig. 8, the control circuit 30 is provided such that the level detected output S O is once latched in the flip-flop circuit 29 at every frame so that in association with the above-mentioned circuit arrangement, it is possible to reduce a probability of influence being exerted by the dropout and the noise very much. Since the control circuit 30 is supplied with the mode pulse P IN which is obtained in the insert mode edition, when this invention is applied to a VTR capable of the insert mode edition, the trigger pulse P T is not obtained in the insert mode and hence the D-type flip-flop circuit 29 latches therein the preceding data. In insert edition mode, instead of the first color video signal S CV , the second color video signal S SP is occasionally inserted into a predetermined interval of the tape T on which the first color video signal S CV , for example, is recorded (see Fig. 12A). If the second color video signal S SP having the different deviation frequency is inserted as mentioned above, by the demodulation characteristic as shown in Fig. 5, the demodulated output is fluctuated much at the insertion portion (see Fig. 12B). Thus, the picture is disturbed at the above-mentioned inserted portion. Such edition must be avoided. Therefore, in such insert edition mode, it is desired to inhibit the edition from being carried out under the state of the second color video signal S SP . To this end, in the reproducing system 10, when the mode pulse P IN becomes level "H", even if the switching pulse P SW is supplied to the control circuit 30, the AND gate 33 is inhibited from producing the output and the AND circuit 34 is inhibited from sending the trigger pulse P T , thus the data latching operation of the D-type flip-flop circuit 29 is inhibited. Thus, even if the level detected output S O becomes level "L", the D-type flip-flop circuit 29 is inhibited from latching therein the level detected output, thereby the state of the switching circuit 6 being not changed. While the state of the switching circuit 6 is held, the recording system is desired to change the deviation frequency of a frequency modulator provided therein into such one for use in the first color video signal S CV . Accordingly, the recording system is constructed as shown in Fig. 13. In Fig. 13, reference numeral 40 designates a recording system, in which a luminance signal Y is frequency-modulated by a frequency modulator 42 and then mixed with a low frequency-converted chrominance subcarrier signal (not shown), which then is supplied through a recording amplifier 43 to a magnetic recording head H R . In the insertion edition mode, the VTR is changed into the mode in which a color video signal that should be recorded can directly be monitored not through the tape recording-reproducing system, namely, a so-called E-E mode. For this reason, the reproducing system 10 is provided with a mode change-over switch 44 and the output from the pre-amplifier 2 and the output from the recording-amplifier 43, in this embodiment, are automatically changed by this mode change-over switch 44. The frequency modulator 42 is provided with a control switch 45 so as to change the frequency modulated deviation frequency and this control switch 45 is controlled automatically by the gate pulse P G which is obtained in the insert edition mode, Here, when the reproduced information before an insert start point is the first color video signal S CV , the movable contact of the control switch 45 is compulsorily changed in position to the contact a by the gate pulse P G as shown in Fig. 13, so that the first color video signal S CV is recorded after an insert start point, even if the second color video signal S SP is to be inserted. Thus, the luminance signal Y is frequency-modulate by the deviation frequency shown in Fig. 1A. Thus, in insert edition mode, the first color video signal S CV is always recorded on the recording tape. When the first color video signal S CV is to be inserted into the tape at its portion in which the second color video signal S SP is recorded as the recorded information, the reverse operation. is carried out. Also in this case, the control switch 45 is changed automatically to forcibly establish the insert edition mode of the second color video signal S SP . As set forth above, according to this invention, since the gate pulse P G is formed on the basis of the frequency-modulated luminance signal S Y , even when the dropout occurs and the noise is mixed thereto, the switching operation becomes stable and hence the switching operation of the circuit constants can be carried out accurately. The above description is given on the preferred embodiments of the invention, but it will be apparent that many modifications and variations could be effected by one skilled in the art without departing from the spirits or scope of the novel concepts of the invention, so that the scope of the invention should be determined by the appended claims only.

Claims: Apparatus for reproducing from a magnetic tape (T) a recorded video signal which is one of first and second video signals (S CV , S SP ) which have been frequency-modulated on first and second carriers, respectively, said first carrier having a frequency lower than that of said second carrier, said apparatus comprising: - means (3) for demodulating said first and second video signals; - a first low-pass filter (5) for filtering said first video signal , said first low-pass filter having a first cut-off frequency (F₂); - a second low-pass filter (4) for filtering said second video signal demodulated by said demodulating means (3), said second low-pass filter having a second cut-off frequency (F₁) which is lower than said first cut-off frequency (F₂); - detecting means (20) generating a detecting signal (S YC ,S YS ), the level of which depends on whether said first or said second video signal is reproduced, which level is compared by a level detecting means (27) with a comparison level (V BE ) for generating a control signal (P G ), the level of which indicates whether the reproduced video signal is said first or said second video signal; - and switching means (6) for supplying either said first or said second video signal to an output terminal in accordance with said control signal from said detecting means, characterized in that - said detecting means (20) comprises a third filter (23) receiving the undemodulated respective video signal for generating said detecting signal at its output, said third filter having an amplitude response which, depending on the frequency applied, monotonously increases or decreases at least in the frequency range between the lower limit frequency of said first video signal and the upper limit frequency of said second video signal. Apparatus according to claim 1 , characterized in that said amplitude response of said third filter means (23) represents a characteristic having a single peak. Apparatus according to claim 1 , characterized in that said amplitude response of said third filter means (23) represents a characteristic having a single bottom.

Inventors: SATO, ICHITARO C/O SONY CORPORATION
Applicants: SONY CORPORATION
Application year: 1985
Main classification: H04N9
IPC codes: H04N9, G11B20, G11B5, H04N5