US2129034A - Wireless television receiver for talking television pictures - Google Patents

Description

Sept. 6, 1938. I K. scHLl-:slNGE WIRELESS TELEVISION RECEIVER LFOR TALKING TELEVISION PICTURES 2 Sheets-Sheet l Filed Feb. a; 1935 TLZ Sept. 6, 193% K. scHLl-:slNGER 2,129,034

WIRELESS TELEVISION RECEIVER FOR TALKING TELEVISION PICTURES Filed Feb. 8, 1955 v 2 Sheets-Sheet 2 Plon/R5 7x? 27e/7 70e:

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'Patented Sept. 6., 1938 UNITED STATES WIRELESS TELEVISION RECEIVER FOR TALKING TELEVISION PICTURES Kurt Schlesinger, Berlin, Germany, assignor to Radioaktiengesellschaft D. S. Loewe, Berlin- Steglitz, Germany Application February s, 1935, serial No. 5,575 In Germany February 9, 1934 7 Claims.

The subject matter of the invention relates to a method of using and receiving television images with accompanying sound, which is characterized by the fact that at the transmitter end there are transmitted two adjacent ultra-short carrier waves, which in accordance with the invention possess a defined frequency spacing, whilst at the receiver one local oscillator tuned to a frequency between the frequencies of the two carrier waves and linked up with one, preferably heterodynes the two incoming waves, and the two intermediate frequencies obtained are separated from each other by a filter circuit according to thevinvention and passed to the image and sound ampliers respectively. According to the invention, there may be employed prior to the filter circuit one or more amplifier stages for the simultaneous amplication of both intermediate waves.

The applicant has found, that for the good performance of such a method as described above the special choice of the relations between carrier waves and the modulating frequencies is of very essential importance as interferences between the frequencies have carefully to be avoided.

Fig. l is a diagram of the situation of the frequencies of the transmitted waves and of the oscillator frequency. Figs. 2 and 2a show a sound picture receiving circuit as tested by the applicants with good results. Fig. 3 is a diagram of the frequency spectrum of the different intermediate frequencies resulting by the superheterodyne effect. Fig. 4 shows a circuit of the bandpass filters for the sound and image intermediate frequencies.

By way of explanation reference is made initially to Fig. l, in which there is shown the frequency spectrum of the transmission waves. On the horizontal a represents the carrier wave of the image transmitter, b the carrier wave of the 40 sound transmitter. The frequencies of these two carriers are fs and fb respectively, and fa is greater than fb. The image wave fa is at the same time modulated with synchronizing signals in the form of short and long impulses, whose amplitudes in known manner moderately exceed the peak values of the image potentials, whilst the sound wave fb is modulated only with the sound. The degree of modulation performed at the sound transmitter is preferably made as large or larger than the 50 maximum degree of modulation of the image transmitter so as to reduce the number of amplifying stages required in the sound receiving apparatus. In order to avoid low-frequency distortion at the sound receiving end a rectifier is employed 55` which is characteriized by the fact that the dis- (Cl. Z50-9) tortion is small in relation to the whole output amplitude such as already known per se in the form of the diode or of the twin grid triode detector and of the majority of push-pull detectors.

With a local oscillation frequency fc there are produced two beat frequencies, Viz. f-a) and f(cb). The same local oscillation frequency is fed simultaneously with the two carrier waves a. and b to the one mixing valve and it is desirable to employ the local oscillator as the mixing valve. For transmission of the image the frequency range fac is used, which preferably is at least twice the maximum image current frequency to be transmitted. This in practice, for a 180-line image, preferably amounts to 1.5 106 cycles per second or more. If now the local oscillation frequency fc is in the position C between the two transmitted carrier frequencies fa and fb, and if the spacing fac is as stated, the sound carrier wave b, in accordance with the invention, will require to be so placed that between c' and b a sound frequency range fbc results, which is suciently wide for the transmission of the speech. It is advisable to select this latter frequency range so as not to be excessively wide, and more particularly, in accordance with the invention, to make it approximately one order of magnitude smaller than the image current carrier frequency range. In this way; there is obtained the advantage that owing to the great difference in frequency between the two beat frequencies the separation of the two is considerably simplified and facilitated, and moreover in the case of a long wave represented by the intermediate sound frequency fon-e) possible amplification of this frequency may be effectively performed. In practice operations may be conducted, for example, with fac-:1.5 106 cycles per second fbc=l.5 105 cycles per second i. e., with a relative frequency spacing of the carrier waves amounting to .fsb=1.65 106 cycles per second whereby e. g. the two transmission waves have 45 the frequencies 721:43 106 cycles per second ,fb-41.35 106 cycles per second.

in its length to the duplex transmission. 55

Fig. 2 shows by way of example a sound-picture receiving circuit as tested by the applicant with good results. A half wavelength dipole I of the approximate length of 3.5 metres is coupled so as to be free from radiation with a local oscillator 4 through the medium of an energy line 2 and a transformer 3 in the manner set forth in Fig. 2. The secondary of the transformer 3 is connected to the rotary plate of a differential condenser 5, the two xed plates of which are connected to opposite ends of the oscillatory circuit comprising condenser 6 and coil 'I and the position of the rotary plate is so adjusted that it remains at earth potential as regards the oscillations generated by the local oscillator 4. By utilizing the grid current in the oscillator 4 there is created at a resistance 8 a negative bias, which causes the valve to act as an anode 'bend detector to produce the desired intermediate frequencies f ac and fou-c). These frequencies may be coupled by a resistance 9, and a condenser I to the grid of a common amplifying stage II, The resistance 9 has such a value that it has approximately the same coupling effect for both intermediate frequencies. If its ohmic value is selected to be greater there is obtained a better coupling-effect of the lower intermediate frequency, viz., the sound intermediate frequency fui-c). In cases in which merely small amplifying means are available for the sound reception a great ohmic value of the resistance 9 may be suitable. In the case of approximately equal coupling-effect for both intermediate frequencies the resistance 9 is chosen for example at nearly 4000 ohms, whilst in the case of a greater coupling-effect of the sound intermediate frequency the Value of the resistance 9 amounts to nearly 20,000 ohms. In place of an ohmic resistance there may also be employed the series connection of an inductance I2 with an oscillatory circuit I3. In this case the inductance and oscillatory circuit are tuned to the used intermediate frequencies, and in addition the choke I2 may be damped to the requisite width of band pass by a resistance I4 of, for example, approximately 5000 ohms. It is essential to perform the common amplification of the two frequencies in the valve II on 'linear characteristics. Curvature of the grid current or anode current characteristic might lead to intermodulation between fol-c) and fai-c) andl prevent a subsequent complete separation. If, therefore, at the grid Il a regulation of the amplification is also obtained by a negative bias, it is advisable to employ exponential grids, particularly where large input amplitudes are concerned. In addition, care should also be taken by means ofa protective choke I that traces of the heterodyne oscillations of' 4 do not reach the grid line.

In the anode vcircuit of the valve II, or of a later Valve up to which the two carrier frequencies )Ra-c) and fw-c) should be amplified simultaneously and as equally as possible, it is necessary for the two components of the picture and sound transmission to be separated. 'Ihe arrangement according toFigs. 2 and 2a has proved to be extremely satisfactory. The lter circuit according to the invention comprises the series connection of 1a band passfilter for the long waves fan-c) and an inductance for the wave fol-c), which is ten times shorter. In .this circuit the dimensioning is such that neither of the two frequencies is adversely affected by the operating resistances for the other frequency. The sound-wave band filter is represented, for example, by two circuitsK I6, I1 and I8, I9. The ratio between capacity and self inductance is, in accordance with the invention, very different for these two circuits. In the primary system the capacitative exceeds the inductive admittance very greatly. In practice the condenser I6, for example, is at least 30D-500 cm. while the coil I'I has a correspondingly small number of windings for a resonant frequency of frb-c). On the other hand on the secondary side there is employed for the sake of a good potential transformation a much smaller condenser I8 of approximately 50-100 cm. with a correspondingly larger coil. Whereas both circuits are tuned to the same natural frequency, viz., 1kb-c), their coupling, is so definitely selected, and moreover at the same time the secondary circuit if desired so damped by means of a parallel resistance of the order of 30,000-100,000 ohms, that at the secondary system there is obtained a relatively wide tuning range of several times 10,000 cycles per second. In this way it is accomplished that the unavoidable fluctuations in frequency of the ultra-short wave oscillator 4 which result in variation of the intermediate frequency ft-C) remain without detrimental effect on the reception of the sound. The image intermediate frequency fol-c) proceeds without obstruction via the large condenser I6 as far as the inductance 2|. Owing to the great difference between f(b) and (i1-c) there is produced in parallel with the oscillatory choke 2l merely a negligibly small potential at the sound intermediate frequency fag-c). This residual potential may be reduced to Zero by applying between the band pass filter coils I'I or I9 and the choke 2l a very weak inductive coupling in correct phase opposition to the direct coupling. Normally the two selection systems for image and sound are completely decoupled from each other by the screening means 28 as shown. The inductance 2| (approximately 100 turns, 20 mm. diameter) is approximately tuned to resonance at fra-c), and sufiiciently damped by a parallel resistance 22 of approximately 5000 ohms. The sound intermediate frequency on the o ne hand and the picture intermediate frequency on the other hand may then be tapped separately at the terminals 23 and 24. To both terminals there may be connected additional individual amplifiers for these frequencies, feeding separate rectifiers. The greater the number of stages which are employed for the common amplification of both intermediate frequencies, the smaller will be the total range of amplification necessary. The separation is performed when the continuance of common amplifiers would introduce a cross modulation and. the sound intermediate frequency is then amplified to the same degree as the image intermediate frequency, that is to say-to a television image of brilliant marking there is assigned a full-range and powerful tone. In practice for the sound intermediate frequency it is sufficient to employ, for example, a three-stage valve 25, the first stage of which is connected in the manner indicated as an anode bend amplifier, and the last two stages of which are operating as low frequency amplifiers, whilst for the image intermediate frequency it is necessary to employ approximately one stage more for amplification, i. e., altogether three stages 26 of intermediate frequency amplification and a detector stage 2l. The leads 23 and 24 are required to be screened carefully against each other.

Regulation of the amplification of a receiver of this kind may also be performed in relatively simple fashion andrfor both sound and image intermediate frequency amplifiers simultaneously. The automatically or manually adjusted, variable negative bias for the grids of the image intermediate frequency valves is conducted simultaneously, if desired through the medium of an ohmic potentiometer, to the input grid of the three-stage valve, or generally the amplifier valve for the f(b frequency. In this manner, if no other means of regulation are provided, an equal amplification is ensured for both the picture reception and sound reception. The negative potential itself may conveniently be derived from the amplitudes of the synchronizing signals of the image transmitter in a manner already described previously by theY applicant.

It is equally convenient to derive the negative biassing potential from the carrier frequency of the sound transmitter through the medium of special rectiers in the manner usual in the radio art. For this purpose there may then be employed with advantage the special valves which have been developed and have proved themselves in the radio art, such for example as double diodes. The bias thus obtained is then applied wholly or in part to the image intermediate frequency amplifiers. According to the invention, an additional regulation of the intensity of picture and sound is provided independently of the automatic control, by means of manually controlled potentiometers in the output circuit of each of the two second detectors or in the subsequent low-frequency amplifiers.

As has been found by the applicant the choice of the spacing between the two transmission frequencies and the frequency band necessary for the image is of special importance.

The frequency conditions prevailing in the above described receiving arrangement having a common local oscillator for the sound and picture signals are explained in conjunction with Fig. 3.

In Fig. 3 there is shown a diagram of the frequency spectrum of the different intermediate frequencies resulting by the superheterodyne effect. The line designated @B indicates the situation of the carrier wave for the picture contents frequencies and line in: indicates the situation of the carrier wave for the sound frequencies. By the superheterodyne effect used by the receiving system there result from every value of the oscillator frequency, to which the oscillator circuit is tuned, two beat frequencies, one of which represents the sum of the oscillator frequency value plus the carrier frequency value, the other represents the difference of both values.

This is indicated in the diagram of Fig. 3. In the receiver there is used one oscillator, the frequency of which may be tuned to different frequency-values. If the oscillator frequency I u is changed from high to low frequency-values, i. e, relating to the diagram of Fig. 3 ru is moving from the left to the right through the frequency spectrum there arise on account of the superheterodyne effect in several situations of the oscillator frequency @u designated in Fig. 3 as 29, 3d, 3|, 32, 33 and 34 corresponding beat frequencies to which the two channels for picture and sound are tuned. Above the horizontal line there are marked those values of I u producing effect to the picture-intermediate frequency channel viz., to the picture reproducing device. Below the horizontal line there are marked those values of i u producing effect to the sound intermediate frequency channel viz., to the loudspeaker. The value 29 of the oscillator frequency I u produces in combination with the image carrier wave I B an intermediate frequency, to which the picture-channel is tuned, and therefore the picture-contents frequencies appear on the picture reproducing device. The same effect appears in position 32, in which also the value of I u produces in combination with I B the same intermediate frequency as in position 29. In the positions 30 and 3l of the oscillator frequency @u there arise in combination with im an intermediate frequency, to which the sound channel is tuned, i. e., the picture-contents frequencies are heard in the loudspeaker. Simultaneously in position 3| the oscillator frequency @u produces also in combination Ywith the sound carrier Wave I T an intermediate frequency, to which the picture channel is tuned, i. e., the sound signals are produced on the picture reproducing device. The same effect appears in position 34. In positions 32 and 33 in combination with the sound carrier wave @T there is produced an intermediate frequency to which the sound channel is tuned, i. e., the sound signals are heard in the loudspeaker.

The diagram of Fig. 3 shows that only in the position 32 the picture contents signals are produced on the picture reproducing device and the appertaning sound signals are heard simultaneously in the loudspeaker. In the other positions there are produced either sound signals in the loudspeaker without the appertaining picture signals in the picture reproducing device or the picture appears on the picture reproducing device without the appertaining sound in the loudspeaker, except the position 3l, at which the picture signals are produced in the loudspeaker and the sound signals on the picture reproducing device.

The position 32 of the oscillator frequency im lying between the two carrier waves is therefore the right tuning of the oscillator for picture reception with the appertaining sound.

Now it has been found that under certain circumstances there occur in the case of these arrangements disturbances in the form of mutual interference of the sound and picture frequencies.

In Fig. 3 shaded rectangles are shown at the particular oscillator adjustments indicated below the horizontal line for the sound reproducing device and above the horizontal line for the picture-reproducing device. The Wide rectangles `29', 35), 3l, 32 represent the wide picture-contents-frequency-band and the small rectangles 3l, 32", 33, 34 represent the soundfrequency band. The maximum frequency @Imax occurring in the image amounts to approximately 500 k. c. for ISO-line transmissions. The sound frequency band amounts to several times 10,000 cycles/sec. If now the spacing between the transmission frequencies @B and I T is so small that in the receiving position 32 the frequency of the sound carrier falls within the frequency band of the image receiver in the manner indicated by the broken line 35, an interfering modulation is to be observed in the picture. The modulation corresponds with a frequency of the order of Clar-QT, and occurs in the same fashion as a side band modulating the local oscillator at the point 35.

It is obvious that an interference of this kind must not be allowed to appear in the case of a correctly constructed image receiver with sufiicient width of band.

The interference disappears when the spacing between heterodyne frequency and sound transmission frequency is equal to or greater than tne maximum image frequency ymax in the formula where fr is of course the sound intermediate frequency.

On the other hand, as the applicant has a1- ready set forth on a previous occasion, the difference in frequency between the image carrier and local oscillator, viz., the image intermediate frequency fB, should be equal to or greater than at least twice the image frequency.

By combining the Equations l and 2 there is obtained in accordance with the invention the minimum spacing of the transmission frequencies dependent on the maximum frequency of the contents of the picture:

This means for example in the case of a 180- line transmission a mutual spacing of at least 3X540,000 cycles/sec., i. e., preferably one of 2 megacycles/sec.

It has been found in practice that when selecting a spacing of this, or even a larger width, an interference of the image signals due to the high-frequency beat between im and 'fr no longer takes place, but there occurs in general a new kind of interference, viz., a modulation of the image by intermediate frequency from the low frequencies of the speech modulating the carrier of frequency im The applicant has found the following explanation of this interference.

As the sound carrier im with a given image intermediate frequency fB=( ul is moved away from Du the more the intermediate frequency of the speech signals (fr in Fig. 3) becomes equal to fs.

Itis accordingly more diiiicult to separate the two intermediate frequencies by a filter network. If, for example, the image intermediate frequency is 1000 k. c. the sound intermediate frequency, in accordance with the foregoing should be 500 k. c., so that the frequency ratio amounts only to 1:2. An adequate separation then cannot be performed with a simple separating circuit according to Fig. 2, and it is found in practice that the sound intermediate frequency fr at the choke 2l through the medium of the condenser 2T with the grid of the amplifying tube 26, builds up a sufficient potential to penetrate as far as the second detector for the image intermediate frequency and accordingly appears on the receiver image screen.

In order to avoid this there is selected in accordance with the invention either a correspondingly higher image intermediate frequency JB, e. g., so that the ratio fB:fT is 3, i. e., the carrier frequencies QB, fr being spaced apart to at least the extent of 4X'ymax, or alternately a special circuit according to the invention is employed. Such a circuit is illustrated by way of example in Fig. 4.

In Fig. 4 there is shown a filter circuit for separating the two arising intermediate carrier frequencies consisting of two band-pass filters. The sound intermediate frequency fr is separated by means of an inductively coupled band pass filter 38, 38", 39T, R0 and 41 and for the picture intermediate carrier frequency there is provided a band-pass filter consisting of an inductively coupled pair of coils 36, 36', which in addition are dampedl by resistances 42 and 4'2'.

The ratio C/L of these two filters is selected, in accordance with the invention, to be very large for the sound intermediate frequency band pass filter 40 and 39, and as small as possible for the image intermediate frequency band pass filter 36, 36. For the former there are accordingly employed comparatively large capacity condensers 38, 38 for example of 500 cm. and trimmer lil.

In this manner a high selectivity is obtained, and also on the primary side a very small resistance 'for the sound intermediate frequency fr. The reverse applies to the band pass lter for the image intermediate frequency, as the selectivity is intended to be considerably smaller. The parallel self-capacities 43 and 43', shown in dotted lines are accordingly avoided as much as possible, for example by the use of a single-layer self-supporting cylindrical winding, and the resulting oscillatory circuits are tuned to the frequency ,B with very large L/C by correct selec'- tion of the number of turns of 33, 36. It is then possible, using a fixed coupling obtained by the use of concentric windings and by the use of damping resistances 42, 42 of the order of, say, 10,000 or 50,000 ohms, to transmit properly the necessary frequency band with a step-up transformation l 3 to the grid of the tube 44, without having at the same time any interference from the sound intermediate frequency fr simultaneously traversing the primary coil 36.

The circuit of Fig. 4 allows a faultless separation of the image and sound-signals even with a frequency ratio fBzfT as 2:1. The filter of Fig. 2 calls on the other hand for a considerably greater difference in frequency. The use of the iilter of Fig. 4 according to the invention reveals, therefore, considerable advantages.

I claim:

1. In an arrangement for the reception of a television picture band of a certain image carrier frequency modulated with picture contents frequencies and for the simultaneous reception of an accompanying sound band of a different carrier frequency modulated with sound frequencies, comprising an aerial circuit for simultaneously receiving both said carrier waves, an oscillator tuna-ble to a frequency lying between said two carriers for heterodyning with these carriers thus obtaining two intermediate frequencies modulated with picture contents frequencies and sound frequencies respectively, an amplifier for common amplification of both said intermediate carrier frequencies, and comprising means for separating said two intermediate frequencies, separate rectifying means and separate amplifying means for rectifying said image contents frequency band and said sound frequency band, the spacing between said oscillator frequency and said sound carrier frequency being at least equal to the maximum image frequency occurring in said image contents frequency band and the spacing between said image carrier frequency and said oscillator frequency being at least twice to three times the maximum image contents frequency occurring in said image contents frequency band.

2. An arrangement according to claim 1, in which the means for separating the two intermediate carrier frequencies consist of a series connection of a band-pass filter for the sound intermediate carrier frequency and an inductance for the image intermediate carrier frequency, said band-pass filter having a primary and a secondary circuit, the primary circuit of which possessing a negligibly small apparent resistance for the image intermediate carrier-frequency as compared with the resistance of the inductance.

3. An arrangement according to claim 1, in which the means for separating the two intermediate carrier frequencies consist of a series connection of a band-pass lter for the sound intermediate carrier frequency and an inductance for the image intermediate carrier frequency, said band-pass filter having a primary and a secondary circuit, the primary circuit of which possessing a negligibly small apparent resistance for the image intermediate carrier-frequency as compared with the resistance of the inductance, said inductance and said band-pass lter being arranged to obtain a weakly inductive couplingeffect in suitable phase for completely compensating residual sound intermediate carrier frequency potential.

4. An arrangement according to claim 1, in which the means for separating the two intermediate carrier frequencies consist of two bandpass filters, the primary and secondary circuits of both being coupled purely inductively and the ratio of the capacity C to the inductance L of the band-pass lter for the sound intermediate carrier frequency being large, but the ratio of the capacity C to the inductance L of the band-pass lter for the image intermediate carrier frequency being very small.

5. An arrangement according to claim 1, in which the oscillator and the amplifier for common amplification of both intermediate carrier frequencies are coupled by means of a condenser and a resistance, the ohmic value of said resistance being adjustable in order to have a variable coupling effect for the two intermediate carrier frequencies to obtain if desired a preferred amplication of one of both intermediate carrier frequencies.

6. An arrangement according to claim 1, in which the oscillator and the amplier for common amplification of both intermediate carrier frequencies are coupled by means of a series connection of an inductance with suitable damping in parallel and an oscillatory circuit, said inductance and said oscillatory circuit being tuned each to one of said intermediate carrier frequencies.

7. An arrangement according to claim 1, in which the means for separating the two intermediate carrier frequencies consist of two lter circuits for separating these frequencies, the filter circuit used for transmitting the sound frequency band being tuned not so exactly to the precise frequency band to be transmitted but to a frequency band which is larger than said frequency band to be transmitted, namely to a frequency band of several times 10,000 cycles p. s., so as to allow variations of said frequency band caused by uctuations of the oscillator-frequency.

KURT SCHLESINGER.

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