IL26583A - Methods and apparatus for the magnetic recording of colour television signals - Google Patents

Description

C O H E N Z E D E K & S P I S B A C H R EG D. PATENT ATTO R N EYS 24, LEVONTIN STR.. P. O. B. 1169 T E L - A V I V 15188/66 SPECIFICATION IMPROVEMENTS TO BETHOBS AND APPARATUSES FOR THE MAGNETIC RECORD IN G OF COLOUR TELEVISION SIGN LS COMP GNIE FR NC IS E DE TELEVISION, a French Socie t * Anoayme, of 1, rue d'Anjou έ Asnieres (Hauts de Seine), France , O HEREBY DECLARE the nature of this invention and in what manner the same is to be performed to be particularly described and ascertained in and by the following statement: The present invention has for its object an improvement in the methods and apparatuses for recording the picture information contained in the composite video signal of the sequential-simultaneous system with memory, as operating with a frequency-modulated subcarrier.

In this system, the composite video signal comprises a luminance signal, and a subcarrier, which is alternately frequency-modulated by two colour signals alternating at the line frequency. The spectrum of the frequency-modulated subcarrier is located in the upper region of the frequency bandwidth of the composite video signal, and, generally, superimposed onto luminance components.

Preferably, within a system for protecting the colour information from noise, the frequency-modulated subcarrier, is, before it is added to the luminance signal, passed through a filter, referred to as a coding filter, whose gain increases strongly on either side of a predetermined frequency of the subcarrier channel.

Passing the subcarrier through the coder filter impresses to the subcarrier an additional amplitude and phase modulation. In the receiver, the subcarrier is applied to a filter, referred to as a decoding filter, compensating for this amplitude and phase distorsion, and, in particular, restoring a constant amplitude for the suboarrier.

The difficulties, as concerns the magnetic recording of a video signal are mainly the following : a) the shape of the frequency-response characteristic of the writing and reading device, which is such that the necessary correction for amplitude distortion can practically be performed only in a frequency range comprising a limited number of octaves, for example b) non-linearity of the writing and reading device; c) the high speed of the motion of the magnetic heads relatively to the writing medium which is required for the recording of components of higher frequencies; d) to a lesser degree, the small response of the recording apparatus to very low frequency components.

In order to remedy to drawback a), b) and d), it is knoan to record the video signal, which it is desired to store, under the form of an auxiliarly carrier wave frequency-modulated by the signal. But this increases difficulty c),and thus requires a delicate and expensive mechanical device for obtaining high and stable speeds for the motion of the magnetic heads relatively to the writing medium.

On the other hand, in the case of colour television, the presence of the modulated suboarrier, due the non-linearity of the apparatus, leads to defects, which result in moire effects in the picture reproduced by means of the recorded signals, if the eorreotions for non-linearity are not effected with the greatest precision^ Another known method for recording a signal comprising low frequency components, which method is generally used for recording audio-frequency signals, consists in adding to this signal a bias sinusoidal voltage. Experience shows that this method remedies in a large measure to difficulties b) and d), but not to difficulty a).

The present invention takes advantage of the fact, that in the colour television system which is considered here, the colour information is carried by the frequency-modulation of the subcarx-ier, whose spectrum, on the other hand, does not reach the lower frequencies. it is, for the colour information to "be preserved as well as this is the case for the whole of the video signal in the known art using frequency modulation.

In what follows, there will be considered, by way of a non-limitative example, an input video signal comprising a 5.5 Hc/s luminance signal and a colour subcarrier C superimposed thereon in a colour channel extending from 3.5 to 4.8 Mc/s.

Fig. 1 in an explanatory figure showing the frequency-response curves a(f) of two filters by means of which this video signal may be separated into two complementary signals A and B , the latter including the whole of the channel of the subcarrier C , and the former extending over a frequency band from 0 to an upper frequency F , Tihichj in Fig, 1, is of the order of magnitude of 2,5 Mc/s.

It has been said hereinabove that the modulated subcarrier C could be recorded without transformation. The same practically applies for the luminance components included in signal B , whose frequencies are comparatively high. However, the storage of signal A belongs to the technique of a video signal comprising lower frequency components.

One solution would consist in writing signal B , as such, on one track, and signal A» under the form of a frequency-modulated irave, an another track. But this solution is not suitable for a simplified apparatus.

Another solution would be to effect the frequency modulation of a carrier wave by signal A in such a way that the frequency band of the FM wave would be above that of signal B . But then, the above-mentioned difficulty c) is met with.

I The simplified magnetic recording method according to the invention allows, at the cost, if necessary, of a loss of definition of the luminance signal, the recording of the luminance and colour information contained in . the input video signal on a single track, and in a frequency band which does not exceed that of the i t signal this results in great simplicity of the apparatus for carrying this method into effect. In this method, two signals L and H, respectively approximating two complementary signals A and B such as those of ig.l, are distinguished in the input video signal.

According to the invention, there is provided u .method for the magnetic recording, on a single track, of the luminance and colour information contained in an input video signal comprising a luminance signal and a colour subcarrier C, frequency modulated by the colour information, and superimposed on the luminance signal in the upper region of the spectrum thereof, or occupying a frequency b ndv/idt above that of the luminance signal, wherein two signals L r.d H are distinguished in the input video signal, the frequency bandwidth of signals L and H not- encroaching upon each other, signal K inducing at least the modulated subcarrier C, and signal L at lease cue. lover portion of the spectrum of the luminance signal up to a frequency of the order of at least 1,5 Mc/s, and wherein the input signal of the magnetic writing device comprises a signal L', derived xrc:.

L and signal H itself, signal L* being such that itj requency bajad is included in that of the input video signal and that its prese ce does not result in the supcrimposition of any components of ooiuo ajnpa>p¾anoo to signal H, at least in the subcarrier frequency bandwith ; signal L' being formed either by a wave frequency-modulated by signal and suitably filtered, or by signal L itself, attenuated relative to signal H, so that the subcarrier, included in signal H, should play for signal L the part of a bias sinusoidal voltage.

The invention has also for its object apparatuses for carrying out the aforesaid method.

The invention will bo better understood with the aid of the following description and the. drawings relating to it, in which : Fig. 1 is the above-mentioned explanatory diagram ; Fig. 2 is the block diagram of an apparatus in accordance with the invention, using a signal L1 built up by a wave frequency-modulated by signal L ; Figs. 3 and are the block diagrams of two embodiments of an apparatus using a signal L' obtained through attenuating signal L relatively to signal H.

In Figure 2, which illustrates a first e bodir.snt of the apparatus in accordance with the invention, is shown at 50 /input of the apparatus, to which is applied the input video signal.

The input 50 feeds an amplifier 1, the output of which feeds in parallel a high-pass (or band-pass) filter 3 supplying signal H and a low-pass filter 2 supplying signal L.

Signal L supplied by filter 2 is applied to the :;. d latior input of frequency modulating means 4* comprising an output filter, and supplying signal L' built up by a wave frequency-modulated by signal L.

It may be recalled that in principle the useful spectrum of a frequency-modulated wave is generally considered as comprising the frequency swing interval (interval covered by the instantaneous frequencies) and two side bands.

It is sufficient, for a satisfactory restitutitr. of the signal, that one of the two side bands should have ^ sufficient width, the minimal width of this side band depeiidia^ upon the bandwidth of the modulating signal on the one hend, an upon the modulation index, on the other hand ; the other side band may then be very narrow.

In the present case, one is led to usin a wide, lower side band and a very narrow upper side band.

A possible, choice of signals L, H and L1 w ll indicated for the numerical data given hereinabove.

The frequency band of signal L extends iron 0 2 i-c/s and that of signal H from 3>5 to 5*5 Mc/s.

As concerns signal L', a frequency band froci 0,2.Mc/s to 3,3 Mc/s will then be used, this, on the one hand avoid..;; the recording of very low frequencies, and, on the other hand, allowing a safety, interval between signal L1 and signal K. This frequency band from 0,2 to 3*3 Mc/s of signal L1 will include wide lower side band from 0,2 to 2,5 Mc/s, a swing from 2,5 z 3.2 Mc/s and a reduced, vestigial upper side band from 3,2 to 3.3 Mc/s.

The frequency swing extends from 2.5 to 3.2 Mc/s, which allows a lower marginal band of 2.3 Hc/s, sufficient for the modulation index used, and loaves only a very narrow upper marginal band.

In order to obtain more easily a satisfactory linearity of the frequency modulation, and to eliminate the modulating signal from the spectrum of the modulated signal, it is preferred to use a known artifice, consisting in effecting the frequency modulation in a higher frequency band, for example with a frequency swing extending from 52i5 to 53.2 Mc/s, the modulated wave thus obtained being thereafter translated to the lower frequency band through mixing in with a sinusoidal wave of 50 Mc/s.

Signal H supplied by filter 3 and signal L* supplied by the frequency modulator 4 are applied to the two inputs of an adder 5 supplying the signal to be recorded.

To this end, the output of the adder 5 is connected to the input 7 of the writing and reading apparatus 6 , comprising the magnetic heads and their associated circuits, and restoring at its output 9» on reading, the signal previously applied to input 7 i this apparatus 6 can be of any suitable type.

The output 9 of the apparatus 6 is connected to the input of an amplifier 10.

In the figure the writing-reading switching devices have not been shown, Amplifier 10 feeds two filters 12 and 13 in parallel.

The pass-band of filter 13 is the same as that of filter 3· Filter 12 is a low-pass filter whose pass-band covers the useful spectrum, defined earlier, of the auxiliarly frequency-modulated carrier forming the signal L' .

The output of filter 12 is connected to the input of a frequency demodulator 14 supplying signal L · An adder 15 has its two inputs connected to the outputs of filter 13 and of the frequency demodulator 14 respectively. The output 60 of the adder 15 provides the output video signal of the apparatus.

It will be seen readilly that, in the example considered here, the reproduced luminance signal, while being usable, has suffered a loss of definition.

It is possible to give to signal L a wider frequency band, but at the cost of a reduction of the modulation index. Another solution consists, after signal L1 has been demodulated, to complete signal L , before it is added to signal H , by the crispening method. This method, as is known, consists in adding to the signal of which it is desired to increase the apparent definition, components of higher frequencies, essentially obtained through differentiation of this signal.

If the subcarrier included in the input video signal was submitted to a filtering in a coding filter before it was added to the luminance signal, the operation of the apparatus of Pig. 2 is improved if the subcarrier is brought back to a constant amplitude by means of a decoding filter, which may be combined with filter 3 } a coding filter is then inserted in the output circuit, this coding filter being advantageously combined with filter 13.

Figure 3 shows another embodiment of the apparatus according to the invention, in which elements 50, 1, 5, 6, 7, 9, 10, 15 and 60, identified by the same numbers as in Figure 1 , have the same significance.

Amplifier 1 feeds, in parallel two filters 22 and 23, respectively supplying signals L and H , the first signal covering, for example, a band from 0 to 2 MHz and the second one the band of the subcarrier C, i.e. 3.5 to 4.8 Mc/a.

The output of filter 23, supplying signal H , is connected to one of the inputs of the adder 5, the other input of which is connected to the output of filter 22, providing signal L , through an attenuator 24.

The remainder of the input circuit (circuit preceding the writing and reading device is unchanged from Figure 1.

In the output circuit (following the writing and reading device) the two channels connecting amplifier 10 to the two inputs of the adder 1 , are made up as follows : One channel comprises a low-pass filter 32, with the same pass-band as filter 22, and the other a filter 33, with the same pass-band as filter 23, followed by an attenuator 34.

The purpose of attenuator 24 is to reduce the level of signal L with respect to the f^quency-modulated subcarrier included in signal H sufficiently to enable this subcarrier to act, with respect to signals, as the auxiliary high frequency oscillation, which it is known may be added to a signal, as recalled above, to facilitate the magnetic recording thereof.

Attenuator 34 is used to restore the correct ratio "between the values of signals L and H.

It should he pointed out that in standard practice the auxiliary high frequency oscillation is a pure sinusoidal oscillation of constant amplitude. It is easy to see that the frequency modulation of the auboGxrier does not adversely affects the part -which it is to play in this respect. However, amplitude modulation would introduce distortion.

As the coding filter imparts an amplitude modulation to the subcarrier, it is necessary, if the subcarrier of the input signal was passed through a coding filter, to bring hack the amplitude of the subcarrier to a constant value by using in the input circuit a decoding filter, which can for example be inserted between filter 23 and the adder 5, or combined in a single filter with filter 23. A coding filter will then be inserted in the channel containing attenuator 34 and filter 33, or combined with the latter.

It is of course understood that attenuator 24 could be replaced by an amplifier inserted, for example, between filter 23 and adder 5, and attenuator 34 by an amplifier inserted, for example, between filter 32 and adder 15.

On the other hand, if the level of adjustment of attenuators 24 and 34 is fixed, these attenuators could be combined with the filter(s) of their respective channels.

Hero again, a crispening circuit may be used, it being inserted between amplifier 34 and adder 15.

Figure 4 illustrates a variation of the device shorn in Figure 3» in which the group of elements 22, 23, 24 and 5 has been replaced by a filter 40 of suitable characteristic , in other words whoso gain is approximately that of filter 23 for the frequency band of signal H and that of filter 22 multiplied by the attenuation of attenuator 24 for the frequency band of signal L » Similarly, elements 33» 32, 34 and 15 of Figure 3 have been replaced by a single filter 41 , with opposite characteristic to that of filter 40 in the band of signals L and H .

For this apparatus to operate as that of Fig, 3» filters 40 and 41 should be in principle band rejection filters for the frequency band separating the bands of signals L and H , But it will appear immediately that it is sufficient that one of the two filters,preferably filter 41, should fulfill this condition.

Of course, each of the two filters 40 and 41 may be a composite filter, including two or more simple filters in series.

If the suboarrier of the input video signal was applied through a coding filter, a decoding and a coding filter will be again used. Those filters may be combined with filters 40 and 41 respectively, It should be pointed out, and this remarks applies for the circuits of Figs 2, 3 and 4> that the decoding and coding filter of the known art are defined only for the channel of the subcarrier C · If the band of signal H exceeds that of signal C, the characteristics of the decoding and coding filters are not critical for those portions of the band of signal H which are outside the band of subcarrier 0 , but of course those characteristics should preferably compensate for It should be understood that any filter referred to may bo built up by tiro or more filters in series.

The colour subcarrier is generally suppressed during the transmission of the sync signals. In this case, the apparatuses of Pigs 3 ad 4 should be associated with a device for regenerating the sync signals, which device may be of any known type used for black and white television.

As in every apparatus for the magnetic recording of a video signal, the d.c. component is restored by conventional means at the output of the apparatus.

Of course the invention is not limited to the embodiments described, which were shown by way of example.

Claims (10)

26583/1 WHAT IS CLAB-ED, IS

1. A method for the magnetic recording, on a single track, of the Ivuttinance and colour information contained in an input composite video signal comprising a luminance signal. nd a colour subcarrier C, frequency modulated by the colour information, and superimposed on ! the luminance signal in the upper region of the spectrum thereof, or occupying a frequency bandwith above that of the luminance signal, wherein two signals L and H are distinguished in the input video signal, the frequency bands of signals L and H not encroaching upon each other, signal H including at least the modulated subcarrier C, and signal L at least the lower portion of the spectrum of the luminance signal up to a frequency of the order of at least 1, 5 c/s, and wherein the input signal of a magnetic writing device comprises a signal L1, derived from signal L, ar.d signal H itself, signal L' being such that its frequency band is included in that of the input composite video signal and that its presence does not result in the superimposition of any components of oonoidor-ab e ■amplitude to signal H, at least in the subcarrier frequency bandwidth ; signal L1 being formed. either by a wave frequency-modulated by signal L, and suitably filtered, or by signal L itself, attenuated relatively to signal H, so that the subcarrier, included in signal K, should play for signal L the part of a bias sinusoidal voltage.

2. An apparatus for carrying out the method as claimed in claim 1, said apparatus comprising : a general input for receiving said input composite video signal; a first and a second filter for supplying said signals L and H respectively, said filters having respective inputs coupled to said general input, and respective outputs; frequency modulating means, including an output filter, said frequency-modulating means having a modulation input coupled to said first filter output, and an output for supplying said signal L' under the form of a wave frequency-modulated by said signal L ; a first adder, having two inputs respectively coupled to said frequency-modulating means output and to said second filter output, and an output; a magnetic writing and reading device having a writing input coupled to the output of said first adder and a reading output; a third and a fourth filter having respective inputs coupled to said reading output, and respective outputs, said third and fourth filters having respectively the same pass-band as said output filter and as said second filter; a frequency demodulator having an input coupled to the output of said third filter and an output; and a second adder having a first and a second input respectively coupled to the outputs of said frequency demodulator and of said fourth filter.

3. , An apparatus as claimed in claim 2, further comprising a crispening circuit inserted between said frequency demodulator output and said first input of said second adder.

4. · An apparatus for carrying out the method as claimed in claim 1 , said apparatus comprising : a general input for receiving said input composite video signal; a first and a second filter for supplying said signals L and H respectively, said filters having respective inputs coupled to said general input, and respective outputs; a first adder having two inputs respectively coupled to the outputs of said first and second filter through a first and a second channel, said first channel having a lower transmission gain than said second channel; a magnetic writing and reading device having a writing input coupled to the output of said first adder and a reading output; a third and a fourth filter having respective inputs coupled to said reading output, and respective outputs, said third and fourth filters having respectively the same pass-band as said first and second filter; and a second adder having a first and a second input respectively coupled to the outputs of said third and fourth filter through a third and fourth channel, the ratio of the transmission gain of said fourth channel to the transmission gain of said third channel being such as to restore the proper ratio between the levels of signals H and L.

5. » An apparatus as clamed in claim 4» further comprising a crispening circuit inserted in said third channel.

6. Ji apparatus as claimed in any one of claims 2 to 5 » wherein a first further filter, bringing back said subcarrier to a constant amplitude,is inserted between said general input and said writing input in the channel transmitting signal H , and wherein a second further filter, having a gain characteristic compensating for the frequency selective action of said first further filter, at least in the frequency band of said subcarrier C , is inserted in the channel, coupled to said reading output, and transmitting signal H .

7. A apparatus for carrying out the method as claimed in olaim 1, said apparatus comprising : a gcnoral input for receiving said input composite video signal; a first filter having an input coupled to said general input and an output, the gain of said filter being substantially uniform in the frequency band of said signal L , and substantially uniform in the frequency band of said signal H , and higher in the band of said signal H then in the band of said signal L ; a magnetic writing and reading device having a writing input coupled to the output of said first filter and a reading output; and a second filter having an input coupled to said reading output, said second filter having a gain characteristic compensating for the frequency selective action of said second filter in said bands of signals L and H ; at least one of said two filters being a band rejection filter in the frequency band separating said bands of said signals L and H i

8. V 8i An apparatus as claimed in claim 7, wherein a first further filter,bririging back said subcarrior to a constant amplitude is coupled in series with said first filter and wherein a second further filter, having a gain characteristic compensating for the frequency selective action of said first further filter, at least in the frequency band of said subcarrier C , is coupled in series with said second filter,

9. As method for recording the luminance and colour information contained in a composite video signal, substantially as described and shown with reference to the appended drawings.

10. A apparatus for recording the luminance and colour information contained in a composite video signal, substantially as described and shown with reference to the appended drawings.