an embodiment of the present invention will be described hereinbelow with reference to fig1 . fig1 shows an example of a recording system in the case where the invention was applied to a helical scanning type vtr in which a frequency modulated audio signal ( fm audio signal ) is frequency multiplexed to a luminance signal which was frequency modulated ( fm luminance signal ) and a chrominance signal which was low band converted ( low band converted chrominance signal ) and recorded onto magnetic tape by the same magnetic head . the luminance signal which was input from an input terminal 1 is supplied to an automatic gain controller ( agc ) 3 , by which an amplitude level is corrected . after that , a high frequency component is emphasized by an emphasis circuit 5 . after an output signal of the emphasis circuit 5 was frequency modulated by a frequency modulator 7 , unnecessary band components are eliminated by a high pass filter ( hpf ) 9 , so that the fm luminance signal is obtained . on the other hand , a chrominance signal which was input from an input terminal 2 is supplied to a band pass filter ( bpf ) 4 and a comb - shaped filter 6 and a chrominance signal band component is extracted by the filters 4 and 6 . after that , a chrominance signal level is regulated by an automatic color level controller ( acc ) 8 . an output chrominance signal of the acc 8 is frequency converted into a low frequency band signal by a frequency converting circuit 10 . unnecessary band components are eliminated by a low pass filter ( lpf ) 11 , so that the low band converted chrominance signal is obtained . on the other hand , the audio signals are respectively input from input terminals 23 and 24 and their levels are adjusted by agcs 25 and 29 and , after that , they are input to a matrix circuit 86 . in the case where the input signals are stereophonic audio signals , the matrix circuit 86 forms a sum signal ( l + r signal ) of the right side signal ( r signal ) which is input from the input terminal 23 and the left side signal ( l signal ) which is input from the input terminal 24 and a difference signal ( l - r signal ). in the case where the input signals are bilingual audio signals , a main audio signal which is input from the input terminal 23 and a sub audio signal which is input from the input terminal 24 are directly output from the matrix circuit 86 . the operation of the matrix circuit 86 is controlled by a stereo / bilingual switching signal which is input from an input terminal 20 . an output signal of the matrix circuit 86 is transmitted through compressing circuits 26 and 30 each for compressing a dynamic range into 1 / 2 . after that , the signals are frequency modulated by frequency modulators 27 and 31 at carrier center frequencies of 1 . 5 mhz and 1 . 7 mhz , respectively , and unnecessary band components are eliminated by lpfs 28 and 32 , so that fm audio signals of the carrier center frequencies of 1 . 5 mhz and 1 . 7 mhz are obtained . the reason why the dynamic ranges are compressed is because the generation of noises due to a crosstalk signal from the adjacent track is effectively prevented by increasing a frequency modulation ratio . after the fm audio signals which had been output from the lpfs 28 and 32 were added by an adding circuit 33 , either one of the signal whose amplitude level was doubled by passing through a 6 db amplifier 34 and the signal which is not transmitted through the 6 db amplifier 34 is selected by a switch 35 . the selected signal is output to an adder 12 through a buffer amplifier 36 . the adder 12 adds ( frequency multiplexes ) the fm luminance signal , low band converted chrominance signal , and fm audio signal . the added signal passes through recording amplifier 13 and is sequentially recorded onto a magnetic tape 14 by magnetic heads 16a and 16b attached to a cylinder 15 . a control signal generating circuit 22 generates a control signal from the stereo / bilingual switching signal which is input from the input terminal 20 and a head switching signal of the rotary heads 16a and 16b which is input from an input terminal 37 . the switching state of the switch 35 is controlled by the control signal . by the above control , the signal which passed through the 6 db amplifier 34 is output for a period of time which is almost equal to the vertical sync signal period of the luminance signal which is simultaneously recorded ( on the ( h ) side of the switch 35 ). the signal which is not transmitted through the 6 - db amplifier 34 is output for the other periods ( on the ( l ) side of the switch 35 ). a practical example of the control signal generating circuit 22 is shown in fig2 . in fig2 since there is a predetermined phase relation between the head switching signal which is input from the input terminal 37 and the vertical sync signal is shown in fig3 ( a ) and 3 ( b ), both edges of the head switching signal are detected by an edge detecting circuit 87 and pulses corresponding to the vertical sync period are generated from a monostable multivibrator 88 as shown in fig3 ( c ). the pulses generated are output from an output terminal 90 to the switch 35 through an and circuit 89 when the stereo / bilingual switching signal which is input from the input terminal 20 indicates the bilingual , that is , when the input signal of the input terminal 20 is set to the high level . thus , as shown in fig3 ( d ), when the bilingual signals are recorded , the switch 35 is set to the ( h ) side for the period of time corresponding to the vertical sync signal and the amplitude level of an output signal of the switch 35 or the fm audio signal is doubled . therefore , in the case of recording the bilingual audio signals , the amplitude level of the fm audio signal is doubled and the recording level is raised only for the recording period of time of the vertical sync signal of the luminance signal which is simultaneously recorded . generally , in the case where the fm luminance signal , low band converted chrominance signal , and fm audio signal are frequency multiplexed or overlap recorded ( the fm audio signal is first recorded and the fm luminance signal and the low band converted chrominance signal are subsequently overlappingly recorded on the fm audio signal ), unless the recording level of the fm audio signal is set to a predetermined level or less , a beat or various noises appear on the reproduced video signal , so that the picture quality deteriorates . however , as in the present invention , for the vertical sync signal period of time , an interference by the fm audio signal does not occur on the television screen . thus , the discrimination information to discriminate between the stereo / bilingual signals can be replaced to the high and low recording levels of the fm audio signal . in the embodiment of fig1 the pulses corresponding to the vertical sync signal period of time have been generated ( fig3 ( c )) on the basis of the head switching signal . however , it is also possible to directly separate the vertical sync signal from the sync signal of the luminance signal to be recorded and to use the separated vertical sync signal . on the other hand , in fig1 although the recording level of the fm audio signal has been increased by 6 db for the vertical sync signal period of time of all of the fields , the recording level can be also increased for the vertical sync signal periods of every other field or every two other fields instead of all of the fields . an example of the reproducing system will now be described with reference to fig4 . the signals reproduced from the magnetic tape 14 by the rotary magnetic heads 16a and 16b attached to the cylinder 15 are constructed to a series of reproduced signals by a preamplifier 38 on the basis of the head switching signal which is input from the input terminal 37 . after that , the reproduced fm luminance signal band is extracted by a bpf 39 and the reproduced low band converted chrominance signal is extracted by an lpf 47 . the reproduction level fluctuation of the extracted reproduced fm luminance signal is corrected by an fmagc 40 and , after that , the fm luminance signal is frequency demodulated into the reproduced luminance signal by a limiter 41 , a frequency demodulator 42 , and an lpf 43 . then , the noises are eliminated by a deemphasis circuit 45 and a noise clip 46 and the resultant signal is output to an adder 52 . after the reproduced signal level fluctuation of the extracted reproduced low band converted chrominance signal was corrected by an acc 48 , the signal is converted from the low band converted chrominance signal to the original high band chrominance signal by a frequency converting circuit 49 and a bpf 50 . after that , an interference signal from the adjacent track and noise components are eliminated by a comb - shaped filter 51 and the resultant signal is output to the adder 52 . the adder 52 adds the reproduced luminance signal and the reproduced chrominance signal and outputs as a reproduced color signal from an output terminal 53 . on the other hand , from an output signal of the preamplifier 38 , the fm audio signal of 1 . 5 mhz and the fm audio signal of 1 . 7 mhz are extracted by a bpf 56 of 1 . 5 mhz and a bpf 63 of 1 . 7 mhz , respectively . the reproduction level fluctuations of the extracted fm audio signals are corrected by fmagcs 57 and 64 , respectively . after that , the corrected fm audio signals are frequency demodulated by limiters 58 and 65 , frequency demodulators 59 and 66 , and lpfs 60 and 67 , respectively , so that demodulated audio signals are derived . in case the demodulated audio signals include the noise due to discontinuous phase of the reproduced fm waves which are generated upon switching of the rotary heads and the noises by the signal dropout , the signals are compensated by holding the values just before the noises are generated for the noise generating period of time by means of previous value holding circuits 61 and 68 . the previous value holding circuits 61 and 68 are controlled by an output signal of a pulse width expanding circuit 79 . the amplitude level of the reproduced fm audio signal is detected by an envelope detecting circuit 74 . a signal dropout is detected by a dropout detecting circuit ( d . o . detecting circuit ) 75 from an output signal of the envelope detecting circuit 74 . the pulse width expanding circuit 79 generates a control signal on the basis of an output signal of the d . o . detecting circuit 75 and the signal obtained by detecting both edges by an edge detecting circuit 80 from the head switching signal which is input from the input terminal 37 . output signals of the previous value holding circuits 61 and 68 are expanded by expanding circuits 62 and 69 each for returning the dynamic range which was compressed upon recording to the original level . after that , the expanded output signals are input to a matrix circuit 84 . the matrix circuit 84 is controlled by an output signal of a level discriminating circuit 82 to discriminate whether the audio signals recorded are the stereophonic audio signals or bilingual audio signals . an output signal of the matrix circuit 84 is input to switches ( sw ) 70 and 71 to switch the output and is output from output terminals 72 and 73 . the switches 70 and 71 are connected to the ( a ) side when the stereophonic audio signals and the main audio signal are output . the switches 70 and 71 are connected to the ( b ) side when the sub audio signal is output . fig5 shows a practical example of the matrix circuit 84 . when the stereophonic audio signals are reproduced , the addition and subtraction are executed by an adder 122 and a subtracter 89 with respect to a demodulated sum signal ( l + r signal ) and a demodulated difference signal ( l - r signal ) which were input from input terminals 92 and 93 , thereby obtaining an l signal and an r signal . the l and r signals are output from output terminals 94 and 95 through the st side of each of switches 124 and 91 . the switches 124 and 91 are controlled by an output signal of the level discriminating circuit 82 which is input from an input terminal 96 . the switches 90 and 91 are connected to the st side in the case of the stereophonic audio signals and are connected to the bl side in the case of the bilingual audio signals . on the other hand , when the bilingual audio signals are reproduced , the demodulated main audio signal and the demodulated sub audio signal which are input from the input terminals 92 and 93 are directly output from the output terminals 94 and 95 through the bl side of each of the switches 90 and 91 . a method of discriminating whether the recorded audio signals are the stereophonic audio signals or the bilingual audio signals will now be described . as mentioned above , in the case of recording the bilingual audio signals in the recording mode , the recording level of the fm audio signal has been increased by 6 db ( doubled ) for the vertical sync signal period of the luminance signal . therefore , the content of the recorded audio signals can be discriminated by detecting the amplitude level for the vertical sync signal period of the reproduced fm audio signal . that is , in fig4 when the bilingual audio signals are reproduced , the output signal of the bpf 56 of 1 . 5 mhz is as shown in fig6 ( a ) and an output signal of the envelope detecting circuit 74 to detect the amplitude level of the reproduced fm audio signal is as shown in fig6 ( b ). to detect the reproduced fm audio signal amplitude level for the vertical sync signal period , on the basis of the head switching signal ( fig6 ( c )) which is input from the input terminal 37 , two kinds of control signals shown in fig6 ( d ) and 6 ( e ) are generated from a control signal generating circuit 85 . on the basis of the control signals ( fig6 ( d ) and 6 ( e )), the signal level shown in fig6 ( b ) is held by sample and hold circuits 76 and 77 . the signal levels held in the sample and hold circuits 76 and 77 are discriminated by the level discriminating circuit 82 , thereby discriminating whether the recorded audio signals are the stereophonic audio signals or the bilingual audio signals . the result of the discrimination by the level discriminating circuit 82 , that is , the content of the stereo / bilingual audio signals is displayed by a display circuit 83 and a signal indicative of the result is used as a control signal of the matrix circuit 84 . in a variable speed reproducing mode such as what is called a search mode , a still mode , or the like , the level discriminating circuit 82 cannot execute the level discrimination . therefore , the output signal is held on the basis of a variable speed reproducing mode signal which is input from an input terminal 81 . that is , the output signal before the start of the variable speed reproducing mode is output for a period of time of the variable speed reproducing mode . fig7 shows a practical example of the simple and hold circuits 76 and 77 , level discriminating circuit 82 , and control signal generating circuit 85 . after both edges of the head switching signal which had been input from the input terminal 37 were detected by an edge detecting circuit 97 , pulse signals shown in fig6 ( d ) and 6 ( e ) are output from q and q terminals of a monostable multivibrator 98 on the basis of both of the detected edges . an output signal of the envelope detecting circuit 74 is input from an input terminal 113 and is sampled and held by the sample and hold circuits 76 and 77 comprising switches 99 and 101 and capacitors 100 and 102 on the basis of an output signal of the monostable multivibrator 98 . the levels of the signals held by the sample and hold circuits 76 and 77 are compared by comparators 103 and 104 . after that , the signals are transmitted through switches ( sw ) 105 and 106 and are input to a set - reset type flip - flop circuit 107 . the results of the level comparison of the comparators 103 and 104 are stored into the flip - flop 107 and output from an output terminal 117 . ( in the case of the bilingual audio signals , the output level of the output terminal 117 is set to the high level .) for the variable speed reproducing mode period of time , the switches 105 and 106 are opened by the variable speed reproducing mode signal which is input from the input terminal 81 , thereby preventing that the output results of the comparators 103 and 104 are input to the flip - flop 107 . an embodiment of the other reproducing system is shown in fig8 . in fig8 the parts and components having the same functions as those in fig4 are designated by the same reference numerals and , therefore , only the portions different from fig4 will now be described . first , fig8 differs from fig4 with respect to a point that an envelope detecting circuit 123 to detect a signal dropout and the envelope detecting circuit 74 to discriminate the content of the recorded audio signals are separated . as a circuit to detect the signal dropout , there is used the envelope detecting circuit 123 which receives the fm audio signal on the high frequency side which is easily influenced by a spacing loss or the like , that is , the reproduced fm audio signal of a carrier center frequency of 1 . 7 mhz . on the other hand , as a circuit to discriminate the content of the recorded audio signals , there is used the envelope detecting circuit 74 which receives the reproduced fm audio signal of the carrier center frequency of 1 . 5 mhz on the low frequency side in which the reproduced fm audio signal level is hardly influenced by the spacing loss or the like . on the other hand , a control signal generating circuit 121 to generate control signals of the sample and hold circuits 76 and 77 operates on the basis of the vertical sync signal which was separated from the sync signal of the reproduced luminance signal by a vertical sync signal separating circuit 21 . on the other hand , in the control signal generating circuit 121 , in order to prevent the erroneous operation in the discrimination of the content of the recorded audio signals due to an amplitude level fluctuation ( for instance , the amplitude level near the vertical sync signal decreases ) of the reproduced fm audio signal by a curved track in the compatible reproduction or the like , as shown in fig9 ( d ) and 9 ( e ), sampling and holding ranges of the output signal ( fig9 ( b )) of the envelope detecting circuit 74 for the vertical sync signal period and just after the vertical sync signal period are set to the portion of the same period τ 0 . fig9 ( a ) shows a reproduced fm audio signal of a tape on which the bilingual audio signals were recorded . fig9 ( c ) shows an output signal of the vertical sync signal separating circuit 21 . by limiting the amplitude level detecting range of the reproduced fm audio signal , the erroneous discrimination which is caused due to the compatible reproduction and other reproduction amplitude fluctuation can be prevented . fig1 shows a practical example of the control signal generating circuit . the trailing edge of the output signal ( fig9 ( c )) of the vertical sync signal separating circuit 21 which was input from an input terminal 108 is used as a trigger and a monostable multivibrator 118 operates for the same period as the vertical sync signal period shown in fig9 ( c ). an output signal ( fig9 ( e )) of the monostable multivibrator 118 and the vertical sync signal ( fig9 ( c )) are output from output terminals 109 and 110 as sample and hold control signals of the sample and hold circuits 76 and 77 . on the other hand , in fig8 of the above embodiment , as a control signal generating circuit 121 , there has been shown an example in which a sample and hold control signal ( fig9 ( e )) for the same period as the vertical sync signal period is generated just after the vertical sync signal . however , as shown in fig9 ( b ), the envelope detecting circuit 74 has a time constant and the amplitude level of the reproduced fm audio signal follows a change , so that a predetermined time delay occurs . therefore , as shown in fig1 , it is also possible to construct such that the sample and hold control signal ( fig1 ( e )) which is generated after the vertical sync signal period is delayed by only the time ( τ 1 in fig1 ( b )) corresponding to the time delay of the envelope detecting circuit 74 . fig1 shows a practical arrangement example of the control signal generating circuit 121 to generate operation pulses shown in fig1 . the trailing edge of the vertical sync signal ( fig1 ( c )) which is input from the input terminal 108 is used as a trigger and pulses having the time width τ 1 are generated by a monostable multivibrator 119 . the trailing edge of the pulses is used as a trigger and the sample and hold pulses having the pulse width τ 0 are generated and output from the output terminal 109 .