Signal reproducing apparatus

Abstract: An apparatus for reproducing an information signal modulating a carrier signal recorded on a magnetic tape (1), a portion of the signal being boosted in accordance with the characteristic of the information signal is comprised of a playback device for reproducing the information signal from the magnetic tape (1), a detector (30) for detecting the levels of the portion of the signal and the rest of the signal respectively and comparing the levels with each other, and a judging circuit for judging the characteristics of the signal in accordance with the output of the detector.

Description: The present invention generally relates to reproducing apparatus and more particularly to a reproducing apparatus of, or for use with, for example, a so-called 8-mm video tape recorder (8-mm VTR) or the like. Conventional video tape recorders, such as an 8-mm video tape recorder and so on are designed so as to process either a monaural audio signal in which a carrier of the audio signal is 1.5 MHz or a stereo audio signal in which the carrier of one audio signal is 1.5 MHz and the carrier of the other audio signal is 1.7 MHz. In the 8-mm video tape recorder which records and/or reproduces the stereo audio signal, if an audio signal in the signal to be recorded is a stereo signal, then such audio signal is not processed and directly recorded on a magnetic tape, but rather is encoded into the video signal. The two audio channels are not necessariy used for a stereo signal and can instead be used for two separate audio channels, for example, to record a bilingual soundtrack. To distinguish between these two usages of the channels, if the audio signal to be recorded is a bilingual signal (e.g. Japanese, English, etc.), then the vertical blanking period, for example, of the audio signal is boosted by about 7 dB above the normal level and then recorded on the magnetic tape. A recording format for recording a television signal is represented in Figure 1 of the accompanying drawings. A lefthand channel frequency-modulated audio signal, (hereinafter simply referred to as an AFM (audio FM) signal) utilizes a carrier of 1.5 MHz, while the righthand audio signal (AFM signal) utilizes a carrier of 1.7 MHz. These left and right audio signals are FM-modulated together with a pilot signal, a chroma signal and a luminance signal and then recorded on a magnetic tape. When the thus recorded signals are reproduced from the magnetic tape, it is determined by comparing the average level of the vertical blanking periods of the reproduced AFM signal (RF signal) with the average level of the AFM signal of one field period whether the AFM signal is the bilingual signal or the stereo audio signal. If it is determined that the AFM signal is the bilingual signal on the basis of the detected result, then the bilingual signal is not subjected to the addition and subtraction processings. On the other hand, if it is determined that the AFM signal is the stereo audio signal, then the stereo audio signal, i.e., the sum and difference signals of the left and right audio signals are subjected to the addition and subtraction processings to thereby obtain left and right audio channel signals. In the above method in which the average level of the AFM signal of the vertical blanking period and the average level of the AFM signal of one field period are compared with each other, the level of the AFM signal fluctuates due to other factors (as,in practice, it will), then the level difference between the average level of the AFM signal of the vertical blanking period and the average level of the AFM signal of one field period is decreased. As a consequence, even though the level of vertical blanking period of the AFM signal is boosted, it may erroneously determine that the AFM signal is that of a stereo audio signal. Likewise, a fluctuation of the level in the opposite direction may result in an erroneous determination that the AFM signal is a bilingual signal. There is thus the disadvantage that the technique of deliberately boosting the pilot signal is not entirely satisfactory. EP-A-0411948, which is only relevant under Article 54(3) EPC, discloses various circuits for detecting the level of a difference between the carrier level of the AFM signal obtained in a specific period and that obtained other than in the specific period. EP-A-0404465, which forms part of the state of the art only under Article 54(3) EPC, discloses detecting the level of at least one AFM signal component during blanking intervals by comparing a sampled envelope detected signal with a low pass filtered signal obtained from the envelope detected signal. It is an object of the present invention to provide an improved reproducing apparatus in which the aforenoted shortcomings and disadvantages encountered with the prior art can be reduced or eliminated. According to the present invention there is provided an apparatus for reproducing an information signal frequency modulating a carrier signal recorded on a magnetic tape, a portion of said signal being selectively boosted, or not, in accordance with the intended mode of use of said information signal or the characteristics of said information signal, comprising: (a) playback means for reproducing said information signal from said magnetic tape; (b) detector means for detecting the levels of said portion of said signal and the rest of said signal respectively and comparing said levels with each other, the detector means comprising: an envelope detector circuit for envelope detecting the reproduced signal to generate a detected signal; a detection pulse generating circuit for generating a detection pulse within said portion; a delay circuit for delaying said detection pulse to generate a delayed pulse outside of said portion; a first gate cirduit and a second gate circuit for receiving said detected signal and being responsive to, respectively said delayed pulse and said detection pulse; a first capacitor for holding an output signal of said first gate circuit; a second capacitor for holding an output signal of said second gate circuit; and a comparator for comparing the gate output of the first gate circuit held by said first capacitor and the gate output of the second gate circuit held by said second capacitor; and (c) judging means for determining said intended mode or said characteristics of said signal in accordance with the output of said comparator. The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which:- Figure 1 is a frequency spectrum used to explain a tape signal format of 8-mm video tape recorder; Figure 2 (formed of Figures 2A and 2B) is a block diagram showing an embodiment of a reproducing apparatus according to the present invention; and Figures 3A through 3J and Figures 4A through 4J are timing charts, respectively, used to explain operation of the present invention. An embodiment of the reproducing apparatus according to the present invention will hereinafter be described with reference to Figure 2. Figure 2 shows in block form the circuit arrangement of the reproducing apparatus of this invention and is formed of Figures 2A and 2B drawn on two sheets so as to permit the use of a suitably large scale. In Figure 2, reference numeral 1 designates a magnetic tape on which a pilot signal for identifying the stereo signal boosted by, for example, 7 dB or the bilingual signal is recorded together with a video signal or the like. Recording signals recorded on the magnetic tape 1 are reproduced by magnetic heads 2a and 2b. Reference numeral 3 designates a switch, and the magnetic head 2a is connected to one fixed contact 3a of the switch 3, while the magnetic head 2b is connected to the other fixed contact 3b of the switch 3. A movable contact 3 of the switch 3 is connected to an input terminal of a playback amplifier 6. A switching pulse generator circuit 5 generates a switching pulse P 3 (see Figures 3C and 4C), which switches the switch 3, in response to a reference signal or the like supplied thereto from a video signal processing system (not shown) through an input terminal 4. The AFM signals (RF signals) from the magnetic heads 2a and 2b are alternately supplied to the playback amplifier 6 in response to the switching pulse P 3 . The AFM signal from the playback amplifier 6 is supplied to the video signal processing system through a terminal 31 and bandpass filters 7 and 11. The AFM signal P 1 (see Figures 3A and 4A) supplied to the bandpass filter 7 is the signal whose carrier, for example, is 1.5 MHz and this AFM signal P 1 is supplied through the bandpass filter 7 to demodulator circuit 8. On the other hand, the AFM Signal P 2 (see Figures 3B and 4B) supplied to the bandpass filter 11 is the signal whose carrier, for example, is 1.7 MHz and this AFM signal P 2 is supplied through the bandpass filter 11 to a demodulator circuit 12. Reference numeral 9 designates a switch, whose movable contact 9c is connected to the output terminal of the demodulator circuit 8. One fixed contact 9a of the switch 9 is connected to a bilingual signal output terminal 10, and the other fixed contact 9b of the switch 9 is connected to one input terminal of an adder 15 and a subtractor 16. Further, an output terminal of the adder 15 is connected to a left audio signal output terminal 17 and an output terminal of the subtractor 16 is connected to a right audio signal output terminal 18. Reference numeral 13 designates a switch, whose movable contact 13c is connected to the output terminal of the demodulator circuit 12. One fixed contact 13a of the switch 13 is connected to the other bilingual signal output terminal 14, and the other fixed contact 13b of the switch 13 is connected to the other input terminals of the adder 15 and the subtractor 16. The above switches 9 and 13 are changed in position in response to a control signal P 10 (see Figures 3J and 4J) from a detecting circuit 30 which will be described later when the reproduced signals from the demodulator circuits 8 and 12 are the bilingual signals or the stereo audio signals. Accordingly, when the reproduced signals from the demodulator circuits 8 and 12 are the bilingual signals, the movable contacts 9c and 13c of the switches 9 and 13 are connected, respectively, to the fixed contacts 9a and 13a, whereby the bilingual signals are fed to the outside through the bilingual signal output terminals 10 and 14, respectively. When the reproduced signals from the demodulator circuits 8 and 12 are the stereo audio signals, that is, sum and difference signals of the left and right audio signals, the movable contacts 9c and 13c of the switches 9 and 13 are connected, respectively, to the fixed contacts 9b and 13b. Thus, the result which results from adding the sum signal and difference signal of the left and right audio signals, that is , the left audio signal is supplied to the left audio signal output terminal 17, and the result which results from subtracting the sum signal and difference signal of the left and right audio signals, that is, the right audio signal is supplied to the right audio signal output terminal 18. The detecting circuit 30 for generating the control signal P 10 which controls the switches 9 and 13 as described above will be described below. In the detecting circuit 30, a detection pulse generating circuit 26 generates a detection pulse P 5 (see Figures 3E and 4E) in response to the switching pulse P 3 from the switching pulse generator circuit 5. The detection pulse P 5 is supplied to gate circuits 22 and 24 and is also supplied through a delay circuit 27 to a gate circuit 21. The AFM signal (RF signal) P 1 (see Figures 3A and 4A) from the bandpass filter 7 is supplied through an amplifier 19 to an envelope detector circuit 20. The detector circuit 20 envelope-detects the AFM signal P 1 to generate a detected signal P 4 (see Figures 3D and 4D), and the detected signal P 4 is supplied to the gate circuits 21 and 22. The delay circuit 27 delays the detected signal P 5 (see Figures 3E and 4E) to generate a detected pulse P 6 (see Figures 3F and 4F), and the gate circuit 21 permits the detected signal P 4 from the detecting circuit 20 to pass therethrough on the basis of the detected pulse P 6 from the delay circuit 27. An output signal from the gate circuit 21 is held by a capacitor C 1 and supplied to a comparator 23 as a gate output signal P 7 (see Figures 3G and 4G). Further, the gate circuit 22 permits the detected signal P 4 from the detector circuit 20 to pass therethrough in response to the detected pulse P 5 from the detection pulse generator circuit 26. A gate output signal P 8 (see Figures 3H and 4H) from the gate circuit 22 is supplied to a comparator 23. The comparator 23 compares the gate output signal P 7 from the gate circuit 21 and the gate output signal P 8 from the gate circuit 22 and supplies a compared result signal to the gate circuit 24. The gate circuit 24 permits the compared result signal from the comparator 23 to pass therethrough on the basis of the detected pulse P 5 from the detection pulse generator circuit 26. The gate output signal from the gate circuit 24 is held by a capacitor C 3 , inverted by an invertor 25 and fed to the above switches 9 and 13 as a detected result signal P 10 (see Figures 3J and 4J), that is, a control signal. Operation of the above detecting circuit 30 will be described with reference to the timings chart forming Figures 3A through 3J and Figures 4A through 4J. Figures 3A through 3J show the case in which the vertical blanking period of the AFM signal is boosted, that is, the bilingual signal. The AFM signal P 1 (Figure 3A) from the bandpass filter 7 is envelope-detected by the envelope detector circuit 20, and the detected signal P 4 (Figure 3D) from the detector circuit 20 is selected to be low in level, for example, 1 Volt. Upon recording, the high level portion as the pilot signal is boosted by, for example, about 7 dB as compared with the low level portion. The detected signal P 4 is gated by the gate circuits 21 and 22 during the periods in which the detected pulses P 5 and P 6 (Figures 3E and 3F) are at low "0" level, thereby being produced as the gate output signals P 7 and P 8 (Figures 3G and 3H), respectively. Then, the two gate output signals P 7 and P 8 are compared with each other by the comparator 23 and fed through the gate circuit 24 to the invertor 25 as a gate output signal P 9 . The gate output signal P 9 is inverted by the invertor 25 and supplied to the switches 9 and 13 as the detected result signal P 10 . Then, the movable contacts 9c and 13c of these switches 9 and 13 are connected to the fixed contacts 9a and 13a by the detected result signal P 10 . In consequence, the bilingual signals from the demodulator circuits 8 and 12 are supplied, respectively, to the bilingual signal output terminals 10 and 14. Figures 4A through 4J show the case in which the vertical blanking period of the AFM signal is not boosted, that is, the stereo audio signal. The AFM signal P 1 (Figure 4A) from the bandpass filter 7 is envelope-detected by the envelope detector circuit 20. The detected signal P 4 (Figure 4D) from the envelope detector circuit 20 is not boosted as the pilot signal upon recording and is therefore at low level (e.g. 1 Volt). The detected signal P 4 is gated by the gate circuits 21 and 22 during the periods in which the detected pulses P 5 and P 6 are at low "0" level (Figures 4E and 4F), thereby being generated as the gate output signals P 7 and P 8 (Figures 4G and 4H, respectively. The two gate output signals P 7 and P 8 are compared with each other by the comparator 23 and supplied through the gate circuit 24 to the invertor 25 as the gate output signal P 9 . The gate output signal P 9 is inverted by the invertor 25 and supplied to the switches 9 and 13 as the detected result signal P 10 . In consequence, the sum signal and the difference signal of the left and right audio signals from the demodulator circuits 8 and 12 are supplied, respectively, to the adder 15 and the subtractor 16. The sum signal and the difference signal of the left and right audio signals are added and subtracted, respectively, and the resultant left and right audio signals are supplied to the left audio signal output terminal 17 and the right audio signal output terminal 18, respectively. As will be clear from the above description, since the detected signal P 4 from the envelope detector circuit 20 is gated by the detected pulse P 5 whose gate period exists in the vertical blanking period of the boosted portion of the detected pulse P 4 , while the detected signal P 4 is gated by the detected pulse P 6 whose gate period exists in other period than the vertical blanking period of the boosted portion of the detected signal P 4 , the levels of the detected signal P 4 in the vertical blanking period and in the period other than the vertical blanking period can be compared. Therefore, even when the level of the AFM signal is fluctuated, either the bilingual signal or the stereo audio signal can be detected reliably. Further, since it can be determined reliably whether the AFM signal is the bilingual signal or the AFM signal regardless of the level of fluctuation of the AFM signal, the AFM signal can be reliably identified as the bilingual signal or the stereo audio signal even in the variable speed playback mode in which the level of the AFM signal is fluctuated. Since there is a tolerance in the margin to detect whether the AFM signal is the bilingual signal or the stereo audio signal, such detection can be made similarly even when a magnetic tape on which signals are recorded by another apparatus is reproduced, which can improve compatibility with other apparatus. While the AFM signal is the stereo audio signal as described above and a circuit for processing the monaural audio signal is not shown and the explanation thereof is not made, it can be easily understood that a monaural signal can be similarly processed. According to the present invention, the boost period of the signal in which the signal is boosted and inserted as the pilot signal and the period in which the signal is not boosted are compared, the presence or absence of the pilot signal is detected and the reproduced signal which reproduces the signal is processed on the detected signal. Thus, even when the level of the reproduced signal which reproduces the signal is fluctuated, the signal boosted as the pilot signal is satisfactorily detected, thereby the compatibility with other apparatus being improved. Having described the preferred embodiment of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to that precise embodiment and that various changes and modifications thereof could be effected by one skilled in the art without departing from the scope of the invention as defined in the appended claims. In particular, it will be appreciated that the invention is not limited to distinguishing between stereo and bilingual use of the audio channels, but is equally applicable to distinguishing between other different modes of use of the channels, or characteristics of information recorded in the channels.

Claims: An apparatus for reproducing an information signal frequency modulating a carrier signal recorded on a magnetic tape (1), a portion of said signal being selectively boosted, or not, in accordance with the intended mode of use of said information signal or the characteristics of said information signal, comprising: (a) playback means (2,3) for reproducing said information signal from said magnetic tape (1); (b) detector means (30) for detecting the levels of said portion of said signal and the rest of said signal respectively and comparing said levels with each other, the detector means (30) comprising: an envelope detector circuit (20) for envelope detecting the reproduced signal to generate a detected signal; a detection pulse generating circuit (26) for generating a detection pulse (P 5 ) within said portion; a delay circuit (27) for delaying said detection pulse (P 5 ) to generate a delayed pulse (P 6 ) outside of said portion; a first gate circuit (21) and a second gate circuit (22) for receiving said detected signal and being responsive to, respectively, said delayed pulse (P 6 ) and said detection pulse (P 5 ) ; a first capacitor (C 1 ) for holding an output signal (P 7 ) of said first gate circuit (21); a second capacitor (C 2 ) for holding an output signal (P 8 ) of said second gate circuit (22); and a comparator (23) for comparing the gate output (P 7 ) of the first gate circuit (21) held by said first capacitor (C 1 ) and the gate output (P 8 ) of the second gate circuit (22) held by said second capacitor (C 2 ) ; and (c) judging means for determining said intended mode or said characteristics of said signal in accordance with the output of said comparator (23). An apparatus according to claim 1, wherein said information signal is an audio signal. An apparatus according to claim 2, wherein said portion is boosted when said audio signal is a bilingual signal. An apparatus according to any one of the preceding claims, wherein said portion is a vertical blanking period. An apparatus according to any one of the preceding claims, further comprising processor means for processing said reproduced signal. An apparatus according to claim 5, wherein said processor means comprises first switching means (9), second switching means (13), an adder (15) and a subtractor (16) which are controlled by the output (P 10 ) of said judging means. An apparatus according to any one of the preceding claims, wherein said carrier signal is frequency-modulated by the, or an, audio signal. An apparatus according to claim 7, wherein said audio signal comprises first audio signal and a second audio signal modulating first and second carrier signals, respectively. An apparatus according to claim 8, wherein said first and second audio signals are recorded with a luminance signal and a chrominance signal. An apparatus according to claim 9, wherein said first and second audio signals are further recorded with a tracking signal.

Inventors: MURAOKA YOSHIRO, SHIROCHI YOSHIKI, MURAOKA, YOSHIRO, SHIROCHI, YOSHIKI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1991
Main classification: H04N5
IPC codes: G11B5, H04N9, H04N5, G11B20