US2189916A - Television receiver - Google Patents

Description

' Feb. 13, 1940. M. MESSNER TELEVISIOI! RECEIVER Filed Feb; 29, 1936 fi'ey Patented Feb. 13, 1940 UNITE S'TTES TELEVISION RECEIVER Application February 29, 1936, Serial No. 66,429

y In Germany July 12, 1935 ,2 Claims. (01. 178-75),

The invention relates to circuits for the reception of television and has for its object the avoidance of certain disadvantages of arrangements of this type hitherto developed. These disadvantages and the means by which they are avoided are explained in the following description with reference to the accompanying drawing.

Figs. 1 and 2 are diagrams relating to the method of operation of the hitherto known television receiving circuits. Fig. 3 shows an excruple of the new circuit.

It is well known to r ctify the frequency mixture of the picture signals at suitable points, to amplify them and then convey them to the Braun tube. The amplifier situated between the rectifier and the Braun tube is, in this arrangement, extremely complicated as this amplifier'must be able to amplify the lowest frequencies of about 25 hertz, and the highest frequencies, nowadays 500,000 hertz, equally Well,-a requirement which can only be fulfilled with the employment of expensive apparatus. This method further has the disadvantage that when employing an amplifier (low frequency amplifier) inserted between rectifier and Braun. tube, and which generallycontains coupling capacities, it is impossible to reproduce the mean picture luminosity correctly, since the coupling capacities only transmit alternating potentials for whose mean value the operative point of the Braun tube must be so adjusted that with the picture for dark, that is, 25% of the maximum amplitude, the Braun tube is controlled just to dark. This arises from the method of so-called gap-synchronisation in which the transmitter in the unmodulated condition operates with a carrier value of 25% of the maximum value. The picture signals are transmitted by increasing, and the synchronising sig-' transmitting the synchronising signals the transmitter is preferably controlled to the zero value.

In Fig. 1 the case is illustrated in which the operative point l on the Braun tube is correctly adjusted. The picture value dark modulates the Braun tube to just dark. The characteristic of the Braun tube is designated by K, the value M is the mean value of the alternating potential. The operative point is chosen exactly so that the synchronising signals S are cut oif. If the incoming amplitude is greater, that is, if the transmitted picture is made brighter on the average than in Fig. 1, then this operative point is no longer correct. Parts of the incoming frequency mixture which still contain picture modulations render the control electrodcof the Braun tube more negative than is required for the value dark.

These conditions are shown in Fig. 2. The parts of the picture'indicated by hatchingare not reproduced. The operative point would hav to be displaced from 1 to 2.

In order to avoid this disadvantage it has been proposed to'omit the amplifier which, for example, operates in a frequency range from to 500,000 cycles, and to effect the rectification in the Braun tube itself. One modification of this circuit consists in carrying out'the rectification, not in the Braun tube, but in a rectifier which 'is galvanically coupled with the Braurf string of beads because the half cycles of the intermediate frequency are still visible.

According to the invention an intermediate frequency is chosen which is at least five times the highest picture frequency. In accordance with a further feature of the invention, there is employed as the amplifier, not an amplifier coupled by capacity and resistance, but an amplifier provided with band filters. In the output of the amplifier there is also provided a band filter. The Braun tube is included in the output circuit of this last band filter.

In Fig. 3 all those parts are omitted which serve to supply the feed potentials of the apparatus such as mains apparatus and arrangements for maintaining constant the potentials supplied by the mains apparatus. The tripping apparatus are only shown schematically as they are immaterial for an understanding of the circuit.

The antenna l is in the input circuit of a tube 2, which for example, comprises an octode. ,3 is a generator for supplying the heterodyne frequency. The oscillations generated by 3 are mixed in the tube 2. By the arrangement of the multigrid tube 2 it is ensured that the generator 3 does not radiate into the antenna. The interarrangement is thus very suitable.

stages are coupled to each other by band filters 8 to I I. These filters are similar in construction. In the output of the tube 1 is a band filter l2, which is of the same construction as are the filters 8 to II. Between tubes 5 and l a portion of the amplified intermediate frequency is tapped off and conveyed to control apparatus 13 for effecting synchronization. The tripping apparatus are designated by M and 15. By the tripping apparatus M the line deflection is effected, which takes place electrostatically, and

by the tripping apparatus IS the picture deflection is effected which takes place electromagnetically. The control apparatus l3 contains an arrangement for segregating the synchronizing impulses from the picture signals and also contains inverting tubes in order to obtain a control of the tripping apparatus which is correct in phase.

The filter l2 contains directlyin its output circuit the Braun tube It. .The rectification takes place in the Braun tube itself, that is, by means of the electrode I! which in the case illustrated is a perforated electrode, but a cylindrical electrode can also be employed. A Bra-un tube, as is well known, insofar as its rectifying operation is concerned may be regarded as an ordinary tube. Coupling condensers l8 and I9 are necessary, as in the example shown the cathode of the Braun tube is at a high tension potential, whereas its anode is at earth potential,

as shown in the drawing. The Braun tube must therefore be blocked. in direct potential fashion to the amplifier the cathode of which must be at earth potential and the anode of which must be at high tension potential. Thisis efiected by the condensers l8 and 19, which for example, are of the order of magnitude of 1000 centimeters. The capacity of the Braun tube between cathode and rectifier electrode amounts to about 10 cm. The condensers l8 and I9'must be large with respect to the internal capacity of the Braun tube in order to convey as much as possible of the high frequency potential (intermediate frequency potential) to the Braun tube. In the example shown the potential loss, as will be seen from the capacity values, amounts to 1%. The

The condensers l8 and I9 must be made to stand several thousand volts potential, a requirement which quency iron cores.

can easily be fulfilled. The Braun tube is included in the output circuit of the filter l2 by the fact that the condensers l8 and I!) which are in series with the internal valve capacity are connected in parallel to the condenser C of the filter H.

The condensers C are adjustable in order to enable the output circuits of the band filters to be tuned.

The use of an amplifier, coupled by band filters, for the intermediate frequency has the advantage that a Very high intermediate frequency can be employed without difiiculty. The high intermediate frequency is, as mentioned before, favourable for the picture quality since after the rectification uniform picture lines are formed, that is, not picture lines which are like a string of beads. The construction of the band filter moreover, is simplified by the use of the high intermediate frequency as in television very wide frequency bands have to be transmitted. The higher the intermediatefrequency, the smaller is the relative band width, that is, the more simply can the filters be constructed.

The filters 8 to 12 consist in the well known manner of self inductance coils A with high fre- In addition, however, ohmic resistances are placed in parallel with the condenser C and the coils A in order to obtain sufficient attenuation, that is, a sufficient band width.

What is claimed is:

i. In a television receiver, means for producing an intermediate frequency which is at least five times the highest picture frequency, means for amplifying said intermediate frequency, said amplifying means comprising a multistage amp-lifier provided with band filters for coupling said stages, a Braun tube, and a band filter interconnecting the output circuit of said amplifying means and the Braun tube, this tube being included in the output circuit of said last named band filter.

2. In a television receiver according to claim 1, coils forming part of the band filters and provided with cores of high frequency iron, and at tenuation resistances connected in parallel with these coils.

MAXIMILIAN MESSNER.-

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