Method and apparatus for detecting the mode of a frequency-modulated signal

Abstract: To detect whether an input FM signal, such as a reproduced video signal, which includes particular frequency components representing predetermined information, was recorded in a first or second mode, with the modes having different frequency bands, a first output signal is produced when a particular frequency component in one of the frequency bands is detected (23, 25) and a second output signal is produced when a different frequency component, approximately equal to the difference between twice a predetermined frequency included in the other of the frequency bands and the particular frequency in the one frequency band, is detected (24, 26). A mode identifying signal indicative of the frequency band occupied by the FM signal is produced (27) in response to the first and second output signals.

Description: This invention relates to detecting the mode of a frequency modulated (FM) signal, and, more particularly, to apparatus for detecting whether a reproduced FM signal was recorded in a first mode occupying a first frequency band or a second mode occupying a second, different frequency band; to video signal reproducing apparatus for recovering a luminance signal that is frequency modulated in either a first mode or a second mode; and to methods of detecting whether an input FM signal occupies a first frequency band or a second frequency band. In a typical video tape recorder (VTR), a composite video signal is normally recorded with a chrominance component whose original frequency is converted to a relatively low frequency region, and with a luminance component that is frequency modulated. Video signal levels in the luminance component are thus represented by a range of frequency modulations. For example, a maximum, or peak white video level may be represented by a maximum frequency, and the amplitude of the usual horizontal synchronizing signal, known as the sync tip level, is represented by a minimum frequency. The remaining video picture information is represented by frequencies within this range. Improvements in horizontal resolution and picture quality can be achieved by broadening the range of minimum to maximum frequencies that the FM luminance signal may occupy. Accordingly, it has been proposed to use a higher carrier frequency for FM luminance signal recording and to enlarge the frequency range, or deviation, used to represent video picture information in the luminance component. This can best be appreciated by recognizing that in a typical recording operation in the so-called 8mm format, the carrier frequency of the FM luminance signal varies from a low of 4.2 MHz to represent the sync tip level to a high of 5.4 MHz to represent the white peak level. Improvement in picture quality is expected if this FM range is broadened and shifted such that the sync tip level is represented by, for example, 5.7 MHz and the white peak level is represented by, for example, 7.7 MHz. This latter recording range of the FM luminance signal is referred to herein as the "high band" mode of recording, whereas the aforementioned range of 4.2 MHz to 5.4 MHz is referred to as the "low band" mode of recording. Some VTRs may have the capability of recording a video signal (specifically, the FM luminance signal) in either the high band or the low band. Of course, whichever band is selected for recording is also selected for playback. However, it may turn out that a video tape has been recorded on another VTR; and a user may not be readily aware of the particular recording mode that was used. Nevertheless, for proper video signal reproduction, the VTR used to reproduce the previously recorded signals must be matched to the VTR which recorded those signals, and thus it is desirable to provide some means for selecting a high band or a low band reproducing mode. For example, the operating characteristics of the usual FM demodulator, low-pass filter, playback amplifier, and de-emphasis circuit should be selected to accommodate either a high band or a low band FM luminance signal. While a manual mode change-over switch might satisfy this objective, it is often preferred to provide mode selection automatically and not rely upon accurate selection by a user of the VTR. Automatic band selection is particularly advantageous when a video tape has several programmes recorded thereon, some having been recorded in the low band mode and others having been recorded in the high band mode. One proposal for automatically detecting whether the reproduced FM luminance signal was recorded in the high band mode or in the low band mode comprises a band-pass filter tuned to a particular frequency component normally included in a low band FM luminance signal. For example, the filter may be tuned to the particular frequency which represents the sync tip level (for example, a frequency approximately equal to 4.2 MHz) if the FM luminance signal is recorded in the low band mode. If this particular frequency is detected, a mode identifying signal indicative of the low band mode is produced. Conversely, if this particular frequency is not detected, as when the FM luminance signal was recorded in the high band mode, the mode identifying signal indicates this. Another automatic mode detecting circuit which has been proposed includes two band-pass filters; one tuned to the frequency included in the low band mode which represents the sync tip level and the other tuned to the frequency in the high band mode which represents the sync tip level. For example, the low band sync tip frequency may be approximately equal to 4.2 MHz and the high band sync tip frequency may be approximately equal to 5.7 MHz. A low or high band indication is produced depending on which filter produces an output signal. To improve reliability, the outputs of these filters are compared with each other, resulting in a mode identifying signal of, for example, high or low amplitude depending upon which filter output signal is greater. These proposals suffer from erroneous mode detection which may be caused by side band components passed by the low band filter. For example, if the FM luminance signal is recorded in the high band mode, the lower side band component of that signal representing video signal information may be of a frequency sufficiently close to the frequency to which the low band filter is tuned as to be passed by that filter. As a result, the low band filter produces an output signal when, in fact, the FM luminance signal occupies the high band. Accordingly, the mode detecting circuit produces an erroneous indication that the FM luminance signal had been recorded in the low band mode when, in fact, it occupies the high band. This erroneous indication may be produced even when the outputs of the low band and high band filters are compared. In that case, the low band filter output derived from the lower side band of the FM luminance signal may exhibit a greater amplitude than the high band output which represents the sync tip level. In an attempt to prevent such erroneous mode identification, it has been further suggested that the FM luminance signal be supplied to the mode detecting circuit only during the interval that the synchronizing signal is present. According to this further suggestion, a gate circuit is enabled by a gate pulse which coincides in time with the synchronizing signal interval. Hence, the gate circuit is opened only when the synchronizing signal is present and, thus the lower side band component of other useful video information (that is, non-synchronizing information) is inhibited from reaching the low band filter when the FM luminance signal occupies the high band. However, the proposed use of a gate circuit suffers from the disadvantage of relying upon the demodulated FM luminance signal to provide the synchronizing signal from which the gate pulse is derived. If the FM demodulator or other FM processing circuits are not properly adjusted to match the particular mode in which the FM luminance signal had been recorded, the FM signal will not be demodulated properly and, thus, a correct gate pulse will not be produced. This means that if the recording mode has not yet been determined, or if its initial selection is incorrect, proper FM demodulation may not be achieved and, thus, the input gate circuit might not operate properly. According to one aspect of the present invention there is provided apparatus for detecting whether a reproduced FM signal was recorded in a first mode or a second mode, the first and second modes having different frequency bands with respectively different carrier frequencies, and the FM signal containing particular frequency components in the respective frequency bands to represent predetermined information, said apparatus comprising: first detecting means for producing a first output signal when a particular frequency component in one of said frequency bands is detected; second detecting means for producing a second output signal when a different frequency component approximately equal to the difference between twice a predetermined frequency included in the other of said frequency bands and the particular frequency in said one frequency band is detected; input means for supplying the reproduced FM signal to said first and second detecting means; and signal sensing means for comparing said first and second output signals for producing a mode identifying signal indicative of the mode in which the reproduced FM signal was recorded. According to a second aspect of the present invention there is also provided video signal reproducing apparatus for recovering a luminance signal that is frequency modulated in either a first mode or a second mode, the first and second modes having different frequency bands with respectively different carrier frequencies, said apparatus comprising: playback means for playing back a composite video signal including chrominance and FM luminance signals from a record medium; separating means coupled to said playback means for separating the composite video signal into a chrominance signal and an FM luminance signal; chrominance processing means for receiving and processing the separated chrominance signal; adjustable FM demodulating means for receiving and demodulating the separated FM luminance signal; de-emphasis means coupled to said FM demodulation means for de-emphasizing the demodulated luminance signal; and mode detecting means for detecting whether the FM luminance signal was recorded in the first or second mode, said mode detecting means comprising: first detecting means for producing a first output signal when a particular frequency component normally included in one of said frequency bands is detected in the FM luminance signal; second detecting means for producing a second output signal when a different frequency component approximately equal to the difference between twice a predetermined frequency normally included in the other of said frequency bands and the particular frequency in said one frequency band is detected in the FM luminance signal; means for supplying the FM luminance signal to said first and second detecting means; signal sensing means for comparing said first and second output signals for producing a mode identifying signal indicative of the mode in which the luminance signal was frequency modulated; and means for applying said mode identifying signal to said FM demodulating means to adjust the operation thereof in accordance with the frequency band occupied by the FM luminance signal. According to a third aspect of the present invention there is also provided a method of detecting whether an input FM signal, which includes predetermined information, occupies a first frequency band or a second frequency band, said predetermined information being represented by respective particular frequency components in the first and second frequency bands, said method comprising the steps of: detecting in said FM signal a particular frequency component in said first frequency band to produce a first output signal; detecting in said FM signal a different frequency component approximately equal to the difference between twice a predetermined frequency included in the second frequency band and the particular frequency component in said first frequency band to produce a second output signal; and producing a frequency band indicating signal in response to a comparison of said first and second output signals. Preferred features of the invention are set forth in the accompanying subsidiary claims. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figures 1 to 3 are block diagrams of respective previously proposed mode detecting circuits; Figures 4A and 4B are frequency spectra diagrams representing the spectra used to record low and high frequency band FM luminance signals; Figure 5 is a block diagram of the reproducing circuits used in a VTR in which the present invention finds application; Figure 6 is a block diagram of one embodiment of the present invention; Figures 7 and 8 illustrate the frequency characteristics of the band-pass filters used in the embodiment; Figures 9A to 9F and 10A to 10F illustrate frequency spectra of carriers that are frequency modulated with signals of different frequencies; Figure 11 is a frequency spectrum diagram; Figure 12 is a block diagram of another embodiment of the present invention; and Figure 13 is a schematic diagram of the embodiment of Figure 6. Previous proposals for high band and low band detecting circuits of the type discussed above will now be further described. One example of a prior proposal is illustrated in Figure 1. This circuit is intended to detect the presence of a particular frequency component in an FM signal when that signal occupies a selected frequency band. For example, and for the case of an FM luminance signal that may be recorded in a high or low frequency band, the circuit is intended to detect when the FM luminance signal contains a frequency component representing the sync tip level, when that FM luminance component is recorded in the low frequency band. As illustrated, the FM luminance signal YFM is supplied to a band-pass filter 101 tuned to the frequency fL, where fL represents the sync tip level. The circuit proceeds on the basis that when the FM luminance signal occupies its high frequency band, the lowest frequency component included therein exceeds fL and, thus, no output signal is produced by the band-pass filter 101. A signal detector 102 detects the output of the band-pass filter 101 and may be thought of as a smoothing, buffer circuit. During normal operation, an output signal is produced by the detector 102 substantially only when the frequency representing the sync tip level is received; and this frequency is normally received only when the FM luminance signal occupies its low frequency band. The output signal produced by the detector 102 is compared with a threshold level by a comparator 103. If this output signal exceeds the threshold level, a low or negative voltage level is supplied to an output terminal 104 by the comparator 103. Thus, in the mode detector circuit of Figure 1, a mode identifying signal of high or positive voltage level is produced at the output terminal 104 when the FM luminance signal supplied to an input terminal 100 occupies a high frequency band; and a mode identifying signal of low or negative voltage is produced when the FM luminance signal occupies its low band. Figure 2 illustrates another example of a previously proposed mode detecting circuit. This circuit includes parallel-connected low band and high band detectors supplied in common with an FM luminance signal YFM. The low band detector comprises a band-pass filter 111 and a signal detector 113, similar to the band pass filter 101 and the detector 102 of Figure 1. Thus, the detector 113 produces an output signal when the filter 111 detects the frequency component normally used to represent the sync tip level when the FM luminance signal occupies its low frequency band. A band-pass filter 112 is tuned to a frequency fH normally used to represent the sync tip level when the FM luminance signal YFM occupies its high frequency band. A signal detector 114 may be similar to the detector 113 and, thus, produces an output signal when the FM luminance signal includes the high band sync tip frequency fH. A comparator 115 compares the outputs produced by the detectors 113 and 114 to apply a mode identifying signal to an output terminal 116. It is expected that the output signal produced by the detector 113 will exceed that produced by the detector 114 only when the FM luminance signal contains the frequency component fL, that is, only when the FM luminance signal occupies its low band. Conversely, it is expected that the output signal produced by the detector 114 will exceed that produced by the detector 113 only when the FM luminance signal includes the frequency component fH; and it is expected that this occurs only when the FM luminance signal occupies its high band. Thus, the mode identifying signal is a low or negative voltage when the signal YFM is in its low band, and a high or positive voltage when the signal YFM is in its high band. For the application wherein the FM luminance signal YFM is reproduced from a video tape, the low band mode by which that signal is recorded typically occupies the frequency band from 4.2 MHz to 5.4 MHz, and fL = 4.2 MHz. For high band recording, the FM luminance signal is recorded in the range 5.7 MHz to 7.7 MHz, and fH = 5.7 MHz. When the mode detecting circuits shown in Figures 1 and 2 are used, it may turn out that the lower side band of the FM luminance signal YFM recorded in the high band mode nevertheless may be sufficiently close to the frequency fL as to be passed by the band-pass filter 101 or the band-pass filter 111. The possibility that this frequency in the lower side band component may be passed will result in an erroneous indication by the circuit of Figure 1 that the FM luminance signal is recorded in its low band when, in fact, it was recorded in its high band mode. A similar erroneous indication will be provided by the circuit of Figure 2 if, at the time that the lower side band is passed by the band-pass filter 111, the upper side band includes no frequency components which can be passed by the band-pass filter 112. Also, in this circuit, it may turn out that, while the lower side band component is passed by the band-pass filter 111, the level of the carrier frequency then passed by the band-pass filter 112 is less than this lower side band component. As a result, the comparator 115 erroneously produces a low or negative voltage indicating that the FM luminance signal YFM was recorded in its low band mode. To avoid this erroneous detection, a modification to the circuit shown in Figure 1 has been proposed; and this modification is shown in Figure 3. It will be appreciated that a similar modification may be made to the circuit of Figure 2. In Figure 3, a gate circuit 106 is supplied via a terminal 107 with a gate pulse GP which opens the gate circuit 106 only during the interval when a horizontal synchronizing signal is expected in the FM luminance signal YFM. By using the gate circuit 106, the band-pass filter 101 is supplied with a signal only at those times at which the sync tip frequency is expected. Lower side band components of the FM luminance signal recorded in the high band mode will thus not reach the band-pass filter 101. However, the gate pulse GP is derived from the synchronizing signal demodulated from the FM luminance signal; and if the demodulator is not matched to the band in which the FM luminance signal has been recorded, such recovery of the synchronizing signal cannot be easily attained. Therefore, if the demodulator is initially adjusted so as to be matched to a high band mode of recording, but the actual FM luminance signal YFM was recorded in the low band mode, the demodulator will not operate properly and, thus, the low band FM luminance signal will not be demodulated correctly. Thus, the gate pulse GP will not be produced. As a consequence, the comparator 103 will produce a mode identifying signal erroneously indicating that the reproduced FM luminance signal was recorded in the high band mode. Figures 4A and 4B illustrate the frequency spectra of a recorded composite video signal. The luminance component is frequency modulated to occupy either a relatively low band (Figure 4A) or a high band (Figure 4B). The low band occupies the range of 4.2 MHz to 5.4 MHz, where 4.2 MHz represents the sync tip level and 5.4 MHz represents the white peak level of the video signal. When the FM luminance signal YFM is recorded in its high band mode, 5.7 MHz represents the sync tip level and 7.7 MHz represents the white peak level. In both the high band and low band recording modes, the chrominance signal C is recorded with a carrier frequency of 743 KHz and, thus, is frequency converted to a lower frequency range than it occupies in the conventional NTSC video signal. The audio signal associated with the video program is frequency modulated; and as shown in Figures 4A and 4B, the frequency modulated audio signal AFM has a centre frequency of about 1.5 MHz. A pilot signal P is also recorded in the range 102 KHz to 164 KHz, and this pilot signal P is used for ATF tracking during signal reproducing operations. A block diagram of the video signal reproducing apparatus provided in a typical VTR, such as an 8mm VTR, is shown in Figure 5. One or more rotatable magnetic pick-up heads 2 scan oblique tracks across a moving magnetic tape 1 to reproduce the video signal previously recorded in those tracks. A reproduced compositive video signal is supplied to a regenerative amplifier 3 whose characteristics are selectable so as to be matched to the high or low recording band which had been used to record the FM luminance signal YFM. A signal separator comprising a band-pass filter 4 and a high-pass filter 5 are coupled to the output of the playback circuit and are supplied with the reproduced composite video signal amplified by the amplifier 3. The band-pass filter 4 separates the chrominance component from the composite video signal whose spectrum is shown in Figure 4A or 4B, and the separated chrominance signal C is supplied to a chrominance signal processing circuit 6. The construction and operation of the circuit 6 form no part of the present invention per se, and it will not be further described. Suffice it to say that the chrominance signal is restored to its original frequency range and supplied to a chrominance output 7. The high-pass filter 5 separates the FM luminance signal YFM from the composite video signal; and this separate luminance signal is applied to a luminance processing channel comprising an adjustable FM demodulator 8, a de-emphasis circuit 9 and a picture quality adjusting circuit 10. The separated FM luminance signal YFM is also applied to a mode detector 12 for detecting whether the luminance signal had been recorded in the low band mode or in the high band mode. One form of the mode detector 12 is illustrated in Figure 6, and will be described below. The mode detector 12 produces a mode identifying signal indicative of the low or high band mode in which the FM luminance signal YFM was recorded. Advantageously, the mode identifying signal is supplied to the amplifier 3, to the demodulator 8, to the de-emphasis circuit 9 and to the picture quality adjusting circuit 10 for use by these circuits to adjust the operating characteristics thereof so as to match the low or high band in which the FM luminance signal YFM was recorded. For example, the operating characteristics of the demodulator 8 are adjusted such that it operates to demodulate FM signals in the low band range of 4.2 MHz to 5.4 MHz, or to demodulate FM signals in the high band range of 5.7 MHz to 7.7 MHz. It is appreciated that the sensitivity of the demodulator 8 thus can be shifted between low and high ranges, thus providing a more accurate recovery of the luminance signal. A broad-band demodulator capable of operating throughout both frequency ranges, that is, from 4.2 MHz to 7.7 MHz may thus not be needed. Likewise, the mode identifying signal produced by the mode detector 12 may match the de-emphasis circuit 9 to the particular frequency range in which the FM luminance signal YFM was recorded. A similar matching of the operating characteristics of the amplifier 3 and the picture quality adjusting circuit 10 likewise are attained in response to the mode identifying signal. Consequently, an accurately reproduced luminance component is provided at a luminance output 11, regardless of whether the reproduced FM luminance signal YFM was recorded in the low band or high band. A display 14 is provided to display the recording mode in which the FM luminance signal had been recorded. The display 14 responds to the mode identifying signal produced by the mode detector 12, thereby providing an indication, such as a visual indication, of whether the FM luminance signal had been recorded in the low band or high band. A system controller 13 is coupled to the display 14 and receives the mode identifying signal to drive the display 14. In one form the system controller 13 comprises a microprocessor and, as will be described below, functions to inhibit the display 14 when an accurate mode identification cannot be made. For example, in the event that the apparatus shown in Figure 5 is used with a blank magnetic tape (or a blank portion of the tape 1), FM luminance signals are not reproduced and a decision cannot be made as to the frequency band occupied by the FM luminance signal. It is preferred, then, simply to inhibit the operation of the display 14 rather than provide a possibly erroneous or ambiguous mode display. Turning now to Figure 6, there is illustrated one embodiment of the present invention. In this embodiment, first and second detecting circuits are provided to detect different frequency components normally included in the low and high frequency bands of the FM luminance signal YFM. The first detecting circuit comprises a band-pass filter 23 tuned to the frequency f₁ which represents the sync tip level in one frequency band and a level detection circuit 25 coupled to the filter 23. The other detecting circuit comprises a band-pass filter 24 tuned to a different frequency and a level detection circuit 26 coupled thereto. The different frequency to which the filter 24 is tuned is equal to (2f₂-f₁), where f₂ is approximately equal to a predetermined frequency included in the other frequency band and f₁ is the sync tip frequency included in the aforementioned first frequency band. As a numerical example, f₁ is the frequency representing the sync tip level when the luminance signal YFM is recorded in the low band mode, and the frequency f₂ is representative of a predetermined level of the luminance signal when the luminance signal YFM is recorded in the high band mode. This predetermined level is the approximate average level of video picture information and, in one embodiment, comprises the 50% white level. With reference to the frequency spectra illustrated in Figures 4A and 4B, as a numerical example, f₁ = 4.2 MHz and f₂ = 7 MHz. In an alternative embodiment the band-pass filter 23 may be tuned to the frequency representing the sync tip level when the FM luminance signal YFM is recorded in the high band mode, and f₂ may be equal to the frequency representing the 50% white level when YFM is recorded in the low band mode. As a numerical example of this alternative embodiment, f₁ = 5.7 MHz and f₂ may be in the range of 4.8 MHz to 5.0 MHz. A limiter circuit 22 is coupled to an input terminal 21 to receive the FM luminance signal YFM, remove amplitude variations therein and supply an amplitude-limited frequency modulated signal in common to the filters 23 and 24. A signal sensor is coupled to the output of the level detection circuits 25 and 26 and supplies a mode identifying signal to an output terminal 29. The signal sensor comprises a comparator 27 having, in one embodiment, an inverting input coupled to the level detection circuit 25 and a non-inverting input coupled to the level detection circuit 26. For a purpose to be described below, a level adjusting circuit is provided to adjust the level of the output signal produced by the level detection circuit 26 relative to that of the output signal produced by the level detection circuit 25. This level adjusting circuit comprises a dc offset circuit which, in the illustrated embodiment, imparts a dc offset of positive level to the output signal produced by the level detection circuit 26. This dc offset circuit comprise a source 30 of dc voltage and an adder 28 for adding this dc level to the output of the level detection circuit 26. If desired, a dc offset circuit may be used to impart a negative dc level to the output of the level detection circuit 25. As yet another embodiment, the level detection circuits 25 and 26 both may be provided with dc bias voltages, one of which is greater than the other, thereby providing the aforementioned dc offset to the output signals produced thereby. Preferably, the filter characteristic of the filter 23 exhibits a relatively narrow band such as illustrated by the frequency spectrum of Figure 7. Although the centre frequency of the filter 23 has been described as equal to the frequency representing the sync tip level in the low (or alternatively, high) frequency band, this frequency may be approximately equal to that which represents the pedestal level in the low (or high) band. The filter 23 provides good sensitivity to the particular frequency component which it detects. Figure 8 illustrates the frequency characteristics of the filter 24. As shown, it is preferred that the filter 24 has a wide pass-band relative to that of the filter 23. For the example wherein it is assumed that the frequency f₂ is approximately equal to the frequency representing a 50% white level when the FM luminance signal YFM is recorded in the high band mode, the filter 24 is tuned to (2f₂-f₁) = (2 x 7 - 4.2) = 9.8 MHz. Alternatively, if the filter 23, having the aforementioned narrow pass band, exhibits a centre frequency of f₁ = 5.7 MHz (that is, the frequency representing the sync tip level when the FM luminance signal is recorded in the high band mode), the filter 24, having the wider pass band, is tuned to the frequency (2 x 4.8 - 5.7) = 3.9 MHz. For the purpose of the following discussion, it is assumed that the filter 23 is tuned to the sync tip frequency when the luminance signal YFM is recorded in the low band mode and the filter 24 is tuned to the frequency equal to the difference between twice the 50% white level frequency (when YFM is recorded in the high band mode) and the low band sync tip frequency. That is, it is assumed, for one numerical example, that f₁ = 4.2 MHz, f₂ = 7 MHz and (2f₂-f₁) = 9.8 MHz. With the foregoing assumption, in operation, when the FM luminance signal YFM is recorded in the low band mode, the filter 23 passes a signal corresponding to the sync tip level, and the level detection circuit 25 produces an output signal when the passed signal exceeds a predetermined threshold level. Since the frequency spectrum of the FM luminance signal is substantially less than the pass band of the filter 24 (shown in Figure 8), the filter 24 does not pass a signal to the level detection circuit 26. Hence, when the FM luminance signal is recorded in the low band mode, the level detection circuit 26 does not produce an output signal. The dc offset added to the output signal produced by the level detection circuit 26 is selected to be less than the level of the output signal produced by the level detection circuit 25 when the low band sync tip frequency is detected. Accordingly, the level of the signal supplied to the inverting input of the comparator 27 is greater than the level of the signal supplied to the non-inverting input thereof. Accordingly, a mode identifying signal MDET of low or negative voltage is produced by the comparator 27 to indicate that the reproduced FM luminance signal YFM had been recorded in its low band mode. Conversely, if the FM luminance signal had been recorded in the high band mode, the low band sync tip frequency is not present and, thus, the filter 23 does not pass a signal to the level detection circuit 25; but the dc offset added to the output of the level detection circuit 26 is such that the comparator 27 produces a positive, or high voltage level, whereby the mode identifying signal MDET indicates that signal YFM was recorded in the high band mode. Assuming that the input signal YFM was recorded in the high band mode, it may turn out that the lower side band component of that modulated signal includes a frequency equal or close to the sync tip frequency f₁. As a result, this frequency included in the lower side band component may be passed by the filter 23 to the level detection circuit 25, resulting in an output signal applied thereby to the comparator 27. But, the centre frequency and pass band width of the filter 24 is such that, at this time, the upper side band component of signal YFM includes a frequency component within the pass band of the filter 24. Accordingly, this frequency component is passed and detected by the level detection circuit 26. By reason of the dc offset added thereto, the output signal produced by the level detection circuit 26 is adjusted so as to exceed the level of the output signal produced by the level detection circuit 25. Consequently, the comparator 27 produces a positive, or higher, voltage level identifying signal indicating that the reproduced FM luminance signal had been recorded in the higher frequency band. A comparison of the side bands present when the FM luminance signal is recorded in low and high frequency band will now be described with reference to Figures 9 and 10. Figures 9A to 9F illustrate the frequency spectra produced when the carrier of the FM luminance signal is modulated in the lower frequency band by a sine wave of 0.5 MHz, 1 MHz, 2 MHz, 2.5 MHz and 3 MHz, respectively. Figures 9A to 9F also illustrate the presence of the 4.2 MHz sync tip frequency component. It will be seen that, notwithstanding the modulation of the low band carrier frequency with the sine wave component, a detectable sync tip frequency of 4.2 MHz is present. Thus, the sync tip frequency component is passed by the filter 23, detected by the level detection circuit 25 and used by the comparator 27 to indicate the low band mode. Figures 10A to 10F illustrate the spectra of the carrier frequency in the high frequency band that is frequency modulated with a sine wave of 0.5 MHz, 1 MHz, 2 MHz, 2.5 MHz and 3 MHz, respectively. Notwithstanding the fact that the carrier frequency of the high frequency band is separated substantially from the 4.2 MHz frequency to which the filter 23 is tuned, it is seen that, in some instances, the lower side band component of the frequency modulated carrier contains a frequency component equal or close to this 4.2 MHz frequency. More particularly, when the carrier frequency is frequency modulated with a sine wave of 1 MHz, 1.5 MHz or 3 MHz, the lower side band component of the frequency modulated signal includes a frequency component equal or close to the centre frequency f₁ of the filter 23. Such lower side band component is identified as component LSB1 in the frequency spectra. It also will be observed that, for the sine wave frequencies which produce the lower side band component LSB1, an upper side band component USB1 also is produced. This upper side band component includes a frequency component equal or close to the centre frequency (2f₂-f₁ = 9.8 MHz) of the filter 24. Thus, although the filter 23 may pass the lower side band component LSB1 when the carrier frequency of the high band signal YFM is modulated with certain information frequencies, the filter 24 will concurrently pass the upper side band component USB1 when the carrier frequency is modulated with those same information frequencies. From the foregoing, it is appreciated that, although the lower side band component of the high band FM luminance signal may be passed by the filter 23 for some information frequencies, those same information frequencies will result in an upper side band component that is concurrently passed by the filter 24. Hence, the level detection circuits 25 and 26 will both produced output signals for the modulation of the high band FM carrier by these information frequencies. Whereas this phenomenon would result in an erroneous indication by the detector circuits shown in Figures 1 and 2, the embodiment shown in Figure 6 will indicate correctly that the FM luminance signal had been recorded in its high band mode. This is because the dc offset level added to the output signal produced by the level detection circuit 26 results in a level adjusted signal that exceeds the output signal produced by the level detection circuit 25. The comparator 27 thus produces a positive, or high voltage output signal MDET indicating that the FM luminance signal had been recorded in its high band mode. The centre frequencies of the filters 23 and 24 are selected such that not only is the sync tip frequency passed by the filter 23, but the only side band component of the high band FM luminance signal which is also passed by the filter 23 is accompanied by the other side band component of that FM signal. That is, the filters 23 and 24 are selected to pass symmetrical upper and lower side bands centred about the high band carrier for the FM luminance signal YFM. As shown in Figure 11, assuming the carrier frequency of the high band signal YFM to be f₂ (that is, assuming the carrier frequency to represent the average level of video picture information), then the lower side band component f₂-a is accompanied by an upper side band component f₂+a, the lower side band component f₂-2a is accompanied by an upper side band component f₂+2a, the lower side band component f₂-3a is accompanied by an upper side band component f₂+3a, and so on. Thus, when one of these lower side band components is of a frequency sufficient to be passed by the filter 23, its symmetrical upper side band component is passed concurrently by the filter 24. Therefore, output signals of approximately the same magnitude are produced by the level detection circuits 25 and 26; and the dc offset added to the output of the level detection circuit 26 results in a mode identifying signal MDET indicating that the FM luminance signal had been recorded in its high band mode. The converse of the aforedescribed operation does not occur. That is, assuming that the FM luminance signal had been recorded in its low band mode, a frequency component included therein that is passed by the filter 23 (that is, the sync tip frequency) is not accompanied by an upper side band component having a frequency that is passed by the filter 24. In an alternative embodiment of the circuit shown in Figure 6, the centre frequency of the filter 23 is equal to the sync tip frequency 5.7 MHz of the high band mode; and the centre frequency of the filter 24 is equal to (2 x 4.8 - 5.7) = 3.9 MHz. Now, when the FM luminance signal is recorded in its high band mode, the sync tip frequency component is passed by the filter 23, while the filter 24 passes no frequency components. Thus, the output signal supplied to the comparator 27 by the level detection circuit 25 exceeds the level adjusted output signal supplied to the comparator by the level detection circuit 26 and the adder 28. In this embodiment, the comparator 27 produces a negative or low voltage indicating that the signal YFM had been recorded in the high band. When the FM luminance signal is recorded in the low band, a lower side band component may include an information frequency that is passed by the filter 24. Concurrently, this modulation of the low band carrier may result in an upper side band component having a frequency passed by the filter 23. The signal levels of the output signals produced by the level detection circuits 25 and 26 may be approximately equal to each other; but the dc offset added to the output signal from the level detection circuit 26 results in the comparator 27 producing a mode identifying signal indicating that the reproduced FM luminance signal had been recorded in its low band mode. From the foregoing description, it is recognized that four combinations of output signals may be produced by the level detection circuits 25 and 26: A. An output signal is produced concurrently by both the level detection circuits 25 and 26. B. An output signal is produced by the level detection circuit 25 concurrently with no output signal produced by the level detection circuit 26. C. An output signal is produced by the level detection circuit 26 concurrently with no output signal produced by the level detection circuit 25. D. An output signal is produced by either level detection circuit 25 or 26. A. Let it be assumed that output signals are produced concurrently by the level detection circuits 25 and 26. This can occur only if a frequency included in the lower side band component of the FM luminance signal is passed by the filter 23 while an upper side band component of the FM luminance signal is passed by the filter 24. Thus, the comparator 27 produces the mode identifying signal MDET indicating that the FM luminance signal had been recorded in its high band mode. B. The level detection circuit 25 can produce an output signal in the absence of an output signal produced by the level detection circuit 26 only if the FM luminance signal contains a frequency that passes through the filter 23 only. This condition is satisfied only when the FM luminance signal is recorded in the low band mode so as to contain the sync tip frequency. Thus, the comparator 27 produces the mode identifying signal MDET indicating that the signal YFM had been recorded in its low band mode. C. The actual occurence of an output signal from the level detection circuit 26 in the absence of an output signal from the level detection circuit 25 may occur rarely, it at all. It is recalled from Figures 10B, 10C and 10F, an upper side band component passed by the filter 24 when the FM luminance signal is recorded in its high band mode is accompanied by a lower side band component passed by the filter 23. However, even although these symmetrical side bands may be present, the influence of noise and the differences in the filtering characteristics of the filters 23 and 24 may sufficiently attenuate the signal passed by the filter 23 such that the level detection circuit 25 produces an output signal of very low, or negligible, level. In this case, then, the comparator 27 is provided with a signal at is non-inverting input which exceeds the signal provided at its inverting input, thereby producing the mode identifying signal MDET indicating that the signal YFM had been recorded in its high band mode. D. The absence of output signals from the level detection circuits 25 and 26 generally will occur if the lower and upper side band components of the FM luminance signal are outside the pass bands of both the filter 23 and the filter 24. Since the signal YFM will include the sync tip frequency passed by the filter 23 if the FM luminance signal had been recorded in its low band mode, the condition that neither filter 23 nor 24 passes signals is satisfied if the FM luminance signal is recorded in its high band mode. Accordingly, for this case, the dc offset added to the output of the level detection circuit 26 results in a signal of higher level applied to the non-inverting input of the comparator 27 than is supplied to the inverting input thereof. Therefore, the comparator produces the mode identifying signal MDET indicating that the FM luminance signal had been recorded in its high band mode. From the foregoing, it is appreciated that the signal sensor coupled to the outputs of the level detection circuits 25 and 26 produces a mode identifying signal indicating that the FM luminance signal had been recorded in its low band mode only if an output signal is produced by the level detection circuit 25 concurrently with the absence of an output signal produced by the level detection circuit 26. At all other times, the comparator 27 produces a mode identifying signal indicating that the FM luminance signal is recorded in its high band mode. The foregoing discussion has assumed that the centre frequency of the filter 23 corresponds to the sync tip frequency when the FM luminance signal is recorded in its low band mode. It will be appreciated that an analogous operation will be carried out if the centre frequency of the filter 23 corresponds to the sync tip frequency of the FM luminance signal recorded in the high band mode. In this alternative embodiment, the mode identifying signal MDET indicates that the signal YFM had been recorded in its high band mode when an output signal is produced by the level detection circuit 25 concurrently with the absence of an output signal produced by the level detection circuit 26. For all other conditions of the level detection circuit output signals, the mode identifying signal MDET indicates that the FM luminance signal had been recorded in the low band mode. In using the video signal playback apparatus shown in Figure 5 with the mode detector 12 (shown in Figure 6), a portion (or, possibly, all) of the tape 1 may be "blank". That is, as the tape 1 is advanced, no signals may be reproduced therefrom because no signals had been recorded. In the absence of a reproduced FM luminance signal, no signals are passed by the filters 23 and 24 and, thus, no (or low level) output signals are produced by the level detection circuits 25 and 26. From the foregeoing discussion, it is recalled that the comparator 27 produces a positive, or high voltage mode identifying signal MDET when no output signals are produced by the level detection circuits 25 and 26. Thus, in the absence of reproduced video signals, the mode detector 12 may provide an erroneous indication that, not only is an FM luminance signal present but it was recorded in the high band mode. It is preferred that this possibly erroneous indication be not displayed on the display 14. Although an incorrect display will not affect the proper operation of, for example, the amplifier 3, the demodulator 8, the de-emphasis circuit 9 or the picture quality adjusting circuit 10, it is preferred not to provide the user with an erroneous mode detection indication when, in fact, an FM luminance signal is not recovered from the tape 1. It is, of course, appreciated that, even if the circuits included in the luminance signal recovery channel of the reproducing apparatus of Figure 5 are set, initially, to be compatible with, for example, the high frequency band, upon subsequent detection that the reproduced FM luminance signal is, in fact, recorded in the low band mode, the operating characteristics of those circuits will be changed over promptly so as to be matched to that low band mode. Figure 12 is a block diagram of another embodiment of the present invention comprising a mode detector 12, substantially identical to the mode detector 12 shown in Figure 6, supplemented with a display inhibit circuit to inhibit the system controller 13 from driving the display 14 with the mode identifying signal in the event that the luminance signal YFM is not reproduced from the tape 1. This display inhibit circuit comprises a level detector 81 coupled to the input terminal 21 to receive and detect the level of the luminance signal YFM. The detected signal level is compared with a dc reference level by a comparator 82. If the detected signal level of the luminance signal YFM is less than the dc reference level, the comparator 82 supplies an inhibit signal to the system controller 13, thereby inhibiting the display 14 from displaying an incorrect indication of the mode in which the luminance signal YFM was recorded. Conversely, the comparator 82 enables the system controller 13 to drive the display 14 with the mode identifying signal when the detected level of the luminance signal YFM exceeds this reference level. As an alternative, the system controller 13 may respond to a low or negative voltage produced by the comparator 82 when the detected level of the luminance signal YFM is less than the reference level so as to provide a display indicating that the luminance signal recording mode cannot be properly determined. Such a display may be an alphanumeric display, or a suitable, predetermined visual indicator may be energized to provide this indication. Turning now to Figure 13, one example of the construction of the mode detector 12 now will be described in conjunction with a schematic diagram thereof. For convenience, the individual circuits shown in block diagram form in Figure 6 are identified in Figure 13. The limiter 22 includes transistors 31 and 32 whose emitters are connected by an RC circuit formed of a resistor 33 in parallel with a resistor 34. The base of the transistor 31 receives the reproduced luminance signal YFM and is biased by a voltage divider formed of resistors 39 and 40 connected in series between a source 38 of operating potential and earth 37. Similarly, bias resistors 41 and 42 are connected in series between the source 38 and earth 37 to apply a bias voltage to the base of the transistor 32. A bias capacitor 43 also is connected to the base of the transistor 32. The signal output of the transistor 32 is derived from its collector which is coupled to an emitter-follower transistor 45, the output of which provides an amplitude-limited FM luminance signal. A capacitor 46 connected in series with parallel, oppositely-poled limiting diodes 47 and 48 is connected to the base of the transistor 45. The band-pass filter 23 includes a PNP transistor 51 whose base is coupled to the output of the limiter 22. The filter 23 includes an LC circuit formed of parallel-connected inductance 54 and capacitance 55, the LC circuit being coupled to the collector of the transistor 51. The output of this LC circuit is derived from the collector of the transistor 51 and is capacitively coupled by way of a capacitor 56 to the level detection circuit 25. The band-pass filter 24 is of similar construction to the filter 23 and includes a PNP transistor 52 whose base is coupled to the output of the limiter 22. An LC circuit formed of parallel-connected inductance 59 and capacitance 60 is coupled to the collector of the transistor 52. The output of the filter 24 is derived from the collector of the transistor 52, and is capacitively coupled by a capacitor 61 to the level detection circuit 26. The level detection circuit 25 includes an emitter-follower transistor 57 whose base is biased to a dc level DCA by a voltage divider or bias circuit formed of resistors 63 and 64 connected in series across the power supply terminals. The transistor 75 is rendered conductive if the signal supplied to its base exceeds its base-emitter bias voltage. A smoothing capacitor 66 is connected in parallel with an emitter resistor 65. The level detection circuit 26 is similarly constructed and includes an emitter-follower resistor 62 whose base is supplied with a dc bias voltage DCB produced by a voltage divider bias circuit formed of resistors 67 and 68 connected in series across the power supply terminals. Here too, the transistor 62 is rendered conductive when the voltage supplied to its base exceeds its base-emitter bias voltage. A smoothing capacitor 70 is connected in parallel with an emitter resistor 69. Preferably, the dc levels DCA and DCB are not equal. This inequality provides a dc offset to the outputs of the level detection circuits 25 and 26. In the preferred embodiment described herein, dc level DCB exceeds dc level DCA such that the difference therebetween (DCB - DCA) constitutes the aforementioned dc offset effectively added to the output signal produced by the level detection circuit 26. Thus, in the schematic diagram of Figure 13, (DCB - DCA) may be thought of as constituting the dc voltage source 30 of Figure 6. Comparator 27 comprises differentially connected PNP transistors 71 and 72. The emitters of the transistors 71 and 72 are connected in common via a resistor 73 to the source 38. The base of the transistor 71 is coupled to the emitter of the transistor 57, thus receiving the output signal produced by the level detection circuit 25. Similarly, the base of the transistor 72 is coupled to the emitter of the transistor 62 thus receiving the output signal produced by the level detection circuit 26. The output of the comparator 27 is derived from the collector of the transistor 72, and is a function of the difference between the signals supplied to the bases of the transistors 71 and 72, respectively. A simple transistor amplifier comprising a transistor 75 is coupled to the output of the comparator 27, that is, to the collector of the transistor 72, and the mode detector output is derived from the collector of the transistor 75. The operation of the mode detector 12 has been discussed in detail with reference to Figure 6. The dc voltages DCA and DCB are superimposed onto the bases of the transistors 57 and 62 of the level detection circuits 25 and 26, respectively. As mentioned above, these dc levels result in the dc offset applied to the level detection circuits 25 and 26, and contribute to the threshold levels associated with the level detection circuits 25 and 26. Assuming that the LC circuit of the filter 23 is tuned to the sync tip frequency when the signal Y FM is recorded in the low band mode (f₁ = 4.2 MHz) and assuming that the LC circuit of the filter 24 is tuned to the frequency (2f₂-f₁ = 7 MHz), if the FM luminance signal is recorded in the low band mode, the magnitude of the signal passed by the filter 23 exceeds that of the signal passed by the filter 24. Accordingly, even with the dc offset (DCB - DCA) provided across the bases of the transistors 57 and 62, the magnitude of the voltage applied to the base of the transistor 71 exceeds the magnitude of the voltage applied to the base of the transistor 72. Consequently, the transistor 72 is rendered conductive to apply a voltage sufficient to turn on the transistor 75. Hence, a relatively low voltage is produced at an output terminal 77, thereby indicating that the luminance signal YFM is recorded in the low band mode. If the outputs of the filters 23 and 24 are of approximately the same magnitude, as would occur when the lower side band component of the FM luminance signal recorded in the high band mode is accompanied by an upper side band component thereof, these respective side band components may be passed by the filters 23 and 24. The fact that the dc level DCB is greater than the dc level DCA means that the emitter voltage at the transistor 62 exceeds the emitter voltage at the transistor 57. Hence, the transistor 71 is rendered conductive, the transistor 72 is turned off, and a relatively low voltage is applied to the base of the transistor 75. The transistor 75 thus does not conduct and a relatively high voltage is produced at the terminal 77 to indicate that the FM luminance signal had been recorded in the high band mode. It is appreciated that the foregoing operation is applicable to the condition wherein signals are not passed by both the filter 23 and the filter 24, thus resulting in substantially zero signals supplied from the filters 23 and 24 to the transistors 57 and 62, respectively. Once again, since the dc level DCB exceeds the dc level DCA, the voltage supplied from the transistor 62 to the base of the transistor 72 exceeds the voltage supplied from the emitter of the transistor 57 to the base of the transistor 71. Thus, as before, transistor 72 is rendered non-conductive to turn off the transistor 75, resulting in a relatively high voltage at the terminal 77. Of course, if a frequency component is passed by the filter 24 while no signal is passed by the filter 23, the emitter voltage of the transistor 62 exceeds the emitter voltage of the transistor 57, thus rendering the transistor 72 non-conductive to turn off the transistor 75 and produce a relatively high voltage at the terminal 77. Accordingly, the mode detector 12 indicates that the input FM luminance signal was recorded in the high band mode. While the present invention has been particularly shown and described with reference to preferred embodiments, it will be readily appreciated that various changes and modifications may be made. For example, the numerical examples are simply illustrative. The present invention is equally applicable to a low band frequency range from 4.4 MHz to 5.6 MHz, where the sync tip level is represented by an FM luminance signal of 4.4 MHz and the white peak level of a video signal is represented by the FM luminance component of 5.6 MHz. This frequency range is used for low band recording in the 1/2 inch VTR format which may also be operated in the high band mode of recording wherein the FM luminance signal ranges from 6.8MHz to 8.6 MHz. Other frequency ranges typically used in other VTR formats also may be accommodated, such as the low band frequency range of 3.4 MHz to 4.4 MHz and the high band frequency range of 5.4 MHz to 7.0 MHz. In all of these formats, proper detection of the high band recording mode is achieved even if a lower side band component thereof is passed by the filter 23 because an accompanying upper side band component is passed concurrently by the filter 24. Also, embodiments of the present invention can be used to detect the high band/low band mode occupied by an FM signal other than an FM luminance signal. If the FM signal contains particular frequency components representing predetermined information, the detection of such frequency components is used to indicate the band occupied by the FM signal.

Claims: Apparatus for detecting whether a reproduced FM signal was recorded in a first mode or a second mode, the first and second modes having different frequency bands with respectively different carrier frequencies, and the FM signal containing particular frequency components in the respective frequency bands to represent predetermined information, said apparatus comprising: first detecting means (23, 25) for producing a first output signal when a particular frequency component in one of said frequency bands is detected; second detecting means (24, 26) for producing a second output signal when a different frequency component approximately equal to the difference between twice a predetermined frequency included in the other of said frequency bands and the particular frequency in said one frequency band is detected; input means (22) for supplying the reproduced FM signal to said first and second detecting means (23, 25; 24, 26); and signal sensing means (27) for comparing said first and second output signals for producing a mode identifying signal indicative of the mode in which the reproduced FM signal was recorded. Apparatus according to claim 1 wherein the reproduced FM signal is a video signal. Apparatus according to claim 1 or claim 2 wherein the predetermined information represented by particular frequency components comprises a synchronizing signal whose amplitude is represented by a first particular frequency in said one frequency band and a second particular frequency in said other frequency band. Apparatus according to claim 2 wherein said predetermined frequency included in the other of said frequency bands is representative of a predetermined level of said video signal. Apparatus according to claim 4 wherein said predetermined level comprises an approximate average level of video picture information. Apparatus according to claim 5 wherein said approximate average level of video picture information comprises a 50% white level. Apparatus according to any one of the preceding claims wherein said first and second detecting means (23, 25; 24, 26) include first and second band-pass filter means (23, 24) respectively, each tuned to a different centre frequency. Apparatus according to claim 7 wherein the first band-pass filter means (23) is tuned to a frequency f₁ approximately equal to the frequency representing a synchronizing signal amplitude level in said one frequency band; and the second band-pass filter means (24) is tuned to a frequency (2f₂-f₁), where f₂ is equal to the frequency representing an approximately average level of video picture information in said other frequency band. Apparatus according to claim 8 wherein said one frequency band is lower than said other frequency band; and wherein a lower side band component included in said other frequency band may be passed by said first band-pass filter means (23) and a corresponding upper side band component may be passed by said second band-pass filter means (24). Apparatus according to claim 8 wherein said one frequency band is higher than said other frequency band; and wherein a lower side band component included in said other frequency band may be passed by said second band-pass filter means (24) and a corresponding upper side band component may be passed by said first band-pass filter means (23). Apparatus according to claim 8 wherein each of said first and second detecting means (23, 25; 24, 26) further includes a signal level detector (25, 26) coupled to the respective band-pass filter means (23, 24) to produce an output signal if the signal passed by the bandpass filter means (23, 24) is at least equal to a predetermined level; and wherein said signal sensing means (27) comprises level adjusting means (28) for increasing the level of the output signal produced by one of said signal level detectors (26) relative to the level of the output signal produced by the other signal level detector (25), and comparator means (27) for comparing the level adjusted output signals to produce said mode identifying signal. Apparatus according to claim 11 wherein said level adjusting means (28) comprises dc offset means for providing a dc offset to the output signal produced by one of said level detectors (26). Apparatus according to claim 12 wherein said dc offset is a positive value and is added to the output signal produced by the level detector (26) coupled to said second band-pass filter means (24). Apparatus according to claim 13 wherein said comparator means (27) produces a mode identifying signal representing that the FM video signal was recorded in said second mode when the level adjusted output signal supplied from the level detector (26) coupled to said second band-pass filter means (24) exceeds the output signal supplied from the level detector (25) coupled to said first band-pass filter means (23). Apparatus according to any one of the preceding claims further comprising display means (14) coupled to said signal sensing means (27) and responsive to said mode identifying signal to display an indication of the mode in which the reproduced FM signal was recorded. Apparatus according to claim 15 further comprising inhibit means (81, 82, 13) for inhibiting the display means (14) from displaying a mode indication in the absence of an FM signal at said input means. Video signal reproducing apparatus for recovering a luminance signal that is frequency modulated in either a first mode or a second mode, the first and second modes having different frequency bands with respectively different carrier frequencies, said apparatus comprising: playback means (2) for playing back a composite video signal including chrominance and FM luminance signals from a record medium (1); separating means (4, 5) coupled to said playback means (2) for separating the composite video signal into a chrominance signal and an FM luminance signal; chrominance processing means (6) for receiving and processing the separated chrominance signal; adjustable FM demodulating means (8) for receiving and demodulating the separated FM luminance signal; de-emphasis means (9) coupled to said FM demodulation means (8) for de-emphasizing the demodulated luminance signal; and mode detecting means (12) for detecting whether the FM luminance signal was recorded in the first or second mode, said mode detecting means (12) comprising: first detecting means (23, 25) for producing a first output signal when a particular frequency component normally included in one of said frequency bands is detected in the FM luminance signal; second detecting means (24, 26) for producing a second output signal when a different frequency component approximately equal to the difference between twice a predetermined frequency normally included in the other of said frequency bands and the particular frequency in said one frequency band is detected in the FM luminance signal; means (22) for supplying the FM luminance signal to said first and second detecting means; signal sensing means (27) for comparing said first and second output signals for producing a mode identifying signal indicative of the mode in which the luminance signal was frequency modulated; and means (12) for applying said mode identifying signal to said FM demodulating means (8) to adjust the operation thereof in accordance with the frequency band occupied by the FM luminance signal. Apparatus according to claim 17 wherein said mode detecting means (12) further comprises means (12) for applying said mode identifying signal to said de-emphasis means (9) to adjust the operation thereof in accordance with the frequency band occupied by the FM luminance signal. Apparatus according to claim 17 or claim 18 wherein said mode detecting means (12) further comprises display means (14) coupled to said signal sensing means (27) for displaying the mode in which the FM luminance signal was recorded as indicated by said mode identifying signal. Apparatus according to claim 17 wherein said first and second detecting means (23, 25; 24, 26) include first and second filter means (23, 24), respectively, tuned to first and second different frequencies, such that a frequency component included in one of the lower or upper side bands of said other frequency band may be passed by said first filter means (23) and a corresponding frequency component included in the other side band may be passed by said second filter means (24). Apparatus according to claim 20 wherein said signal sensing means (27) comprises means responsive to the presence of a first output signal from said first filter means (23) concurrently with the absence of a second output signal from said second filter means (24) to indicate that said FM luminance signal was recorded in said one frequency band; and means responsive to the presence of a second output signal from said second filter means (24) regardless of whether a first output signal from said first filter means (23) also is present, to indicate that said FM luminance signal was recorded in said other frequency band. Apparatus according to claim 20 wherein said FM luminance component contains synchronizing signal information; wherein the first frequency to which said first filter means (23) is tuned is approximately equal to the frequency component in the FM luminance signal representing the amplitude of the synchronizing signal when said FM luminance signal is recorded in said one frequency band; and wherein the second frequency to which said second filter means (24) is tuned is approximately equal to the difference between twice the frequency component in the FM luminance signal representing an average level of video picture information when said FM luminance signal is recorded in said other frequency band and said first frequency. A method of detecting whether an input FM signal, which includes predetermined information, occupies a first frequency band or a second frequency band, said predetermined information being represented by respective particular frequency components in the first and second frequency bands, said method comprising the steps of: detecting in said FM signal a particular frequency component in said first frequency band to produce a first output signal; detecting in said FM signal a different frequency component approximately equal to the difference between twice a predetermined frequency included in the second frequency band and the particular frequency component in said first frequency band to produce a second output signal; and producing a frequency band indicating signal in response to a comparison of said first and second output signals. A method according to claim 23 wherein said step of producing a frequency band indicating signal comprises producing a first band indicating signal when said first output signal is produced concurrently with the absence of said second output signal, and producing a second band indicating signal when said second output signal is produced regardless of whether said first output signal also is present. A method according to claim 24 wherein said first frequency band is lower than said second frequency band; and wherein a lower side band component included in said second frequency band may be detected as said particular frequency component in said first frequency band and a corresponding upper side band component may be detected as the different frequency component. A method according to claim 24 wherein said first frequency band is higher than said second frequency band; and wherein a lower side band component included in said second frequency band may be detected as the different frequency component and a corresponding upper side band component may be detected as said particular frequency component in said first frequency band. A method according to claim 24 wherein the step of producing a frequency band indicating signal further comprises level adjusting the first and second output signals by increasing the level of one of said first and second output signals relative to the other, and comparing the level adjusted output signals to produce said band indicating signal. A method according to claim 27 wherein the step of level adjusting comprises adding a dc offset of positive value to the second output signal. A method according to claim 28 wherein the second band indicating signal is produced to indicate that the FM signal occupies the second frequency band when the level adjusted second output signal exceeds the first output signal.

Inventors: ENOKI HAJIME C O PATENT DIVISI, ENOKI, HAJIME C/O PATENT DIVISION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1989
Main classification: H04N9
IPC codes: H04N5, G11B20, G11B5, H04N9