US2300501A - Television system - Google Patents

Description

Nov. 3, 1942. e. L. GRUND'MANN TELEVISION SYSTEM Filed Feb. 23, 1940 2 Sheets-Sheet l FREQUENCY I i I 72512-4126 I III? 2 .o 1 Is 6 WW wo o wo a mma az CHEF/ER.

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00 0 00 mwwwwsaazz 3nventor Gustave L, Grundmann -2 1942. G. L. GRU 'NDMANN TELEVISION SYSTEM Filed Feb. 25, 1940 2 Sheets-Sheet 2 m .0 xi \HQQwSQmQR kNEEB Nkini kNEkRQ 8.8a?

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TELEVISION SYSTEM Gustave L. Grundmann, Westmont, N. 1., casino:-

to Radio Corporation of of Delaware America, a corporation Application February 23, 1940, Serial No. 320,235

3 Claims. (cans-1.3)

My invention relates to television systems and particularly to systems of the type in which one side band and only a portion of the other side band are utilized in transmitting picture intelligence. t

It is an object of my invention to provide a television system in whicha picture of the maximum detail and of the most pleasing appearance is reproduced when the system is operating over a television channel lying between fixed frequency limits.

Another object of my invention isto provide an improved television receiver for reproducing a picture of the best detail and appearance in a system of the above-mentioned type.

As shown in a paper by Poch and Epstein, entitled Partial suppression of one side band in television reception, RCA Review, vol. 1, p. 19, 1937, there are certain advantages in a system of the above-mentioned type. It is also shown in this paper that it is advantageous to locate the carrier frequency at fifty percent response on one side of the overall frequency reponse curve for the television system.

I have found that, with a certain frequency spectrum allotted to a television channel, it is important that the above-mentioned frequency response curve have the proper slope on the side where the carrier frequency falls. If this slope is too gradual, the picture will not contain as much detail as it should. If the slope is too steep, there will be undesirable transients which may put a following black edge and a. leading white edge on a gray area of the reproduced picture, for example.

The invention will be better understood from the following description taken in connection with the accompanying drawings,'in which Y Figure 1 is a group of curves which are referred to in explaining the invention, Figure 1a is a diagram indicating the character of the transmitted television and accompanying sound signal,

Figures 2, 3 and 4 are frequency response curvesof televsion receivers as designed according to my invention, the abscissa being marked in megacycles, K

Figure 5 (solid line) shows the square wave re-- a response in this way.

Figures 7 to 11 are curves showingthe frequency response characteristics at diflerent points in the receiver of Fig. 6.

Referring to Fig. 1, there is shown an idealized frequency response curve ill for a television system. It will be understood that the selectivity as shown by this curve may be due in part to the transmitter characteristic and in part to the receiver characteristic. In the specific example being described, it is 'due solely to-the receiver characteristic, the transmitter radiating a signal of the character indicated in Fig. 1a having.

It is assumed that the allocated televisionchannel lies between the frequencies f1 and f2, as indicated in Fig. 1. It is' apparent that the maximum high frequency response is J: minus the carrier frequency, that is, fzfc4 in the case of the curve Ill.

If the slope of the curve In is made more gradual as represented by the dotted line Illa, this high frequency response is reduced to fz-fea. If the slope is made steeper, as indicated by the dotted line lllb, the high frequency response obviously is increased to ffl-fcS. However, as previously stated, it has been found that there is a limit to increasing the high frequency In Figs. 2, 3 and 4, there are shown frequency response curves for three television receivers embodying my invention, the curve of Fig. 2 being for a wide band receiver (high fidelity) and the curves of Figs. 3 and 4 being for comparatively It will be understood tem including transmitter and receiver. Circuit details for these three; receivers will not be described since they may be substantially the same sponse of an amplifier having too large a tran-' sient response, while the dotted line portion shows how this response is reduced when the abovementioned. slope of the frequency response chartelevision receiver embodying my invention, and

as those for a fourth receiver which embodies my invention and which is described hereinafter in connection with Figs. 6 and 7.

The square wave response curves in'Fig. 5

wave and a transient trailing the square wave as shown by the solid line curve at Al' and A2, respectively. In an actual picture the transient AI will usually appear to produce a following black edge providing the picture is not already black at this point, while the transient A2 will appear to produce a leading white edge on the picture. It will be understood that these black and white edges in anactual picture appear where there are abrupt changes in the light intensity.

With the characteristic shown in Figs. 2, 3 and 4, however, the square wave response is as represented by the dotted line curve of Fig. 5. It will be seen that the transients AI and A2 have been substantially eliminated. Any further 'decrease in the steepness of the carrier side of the frequency response characteristic will cause a comparatively small improvement in the transient condition while, at the same time, it will cause a decrease in the high frequency response.

A comparison of the curves in Figs. 2, 3 and 4 will show that, in Fig. 2,the picture carrier side of-the curve (the double side band region) occupies substantially two and one-half megacycles of the frequency spectrum, while, in Figs.'3 and 4, this side of the curve occupies substantially one and one-half megacycles. It will be noted that, in these two and one-half megacycles and one and one-half megacycle regions, there is both-an upper side band and a lower side band. Beyond these regions, there is the upper side band only.

In terms of the ratio of lower side band to upper side band, the ratio in the case of Fig. 2 is 1 to 4 or 0.25, in the case of Fig. 3 it is 0.75

to 1.88 or 0.4, and in the case of Fig. 4 it is 0.75

to 2.5 or 0.3.

filed February 28, 1939, and assigned to the Radio Corporation of America, in which'there is described and claimed a compensated wide band amplifier. In order to simplify the drawings in this patent application, certain of the amplifier stages have been represented by blocks.

Referring to Fig. 6, the receiver is of the superheterodyne type comprising a first detector stage and a tunable oscillator represented by 'a block III, a plurality of intermediate frequency amplifier stages II, I2, I3 and I4, and a second detector I6.

The coupling network of the first detector and the coupling network of the I. F.. stage II are alike except for a difference in adjus ent, each stage including a rejector circuit for rejecting the sound signal which accompanies the picture signal. Referring to the stage I I, which is shown in detail, it comprises an amplifier tube I7 and a coupling network consisting of a tunable primary coil I8, a tunable secondary coil I9, and a coupling coil 2I the primary and secondary circuits being tuned by the output capacity of the tube I7 and by input capacity of the tube 22, respectively. The coupling coil 2I, which iscommon to the two tuned circuits, provides the proper amount of coupling to provide the desired band- It can be shown that, in a system where the carrier wave is located 50 percent down, the ratio of lower side band to upper side band preferably should not be less than about 0.2 or greater than about 0.35, this being on the assumption that the phase is a linear function of frequency over the pass band and also on the assumption that the amplitude characteristic of the partially suppressed side band spectrum varies linearly from 0 to 100 percent. In practice, the phase is not exactly a linear function of frequency and it has been foimd that in some receivers the lower to upper side band ratio should be greater than ing the selectivity characteristics shown in Figs.

2 to 4 passes a lower side band at least 0.8 megacycle in width. It has been found from tests that in the case of the wide band receiver having the characteristic shown in Fig. 2 where the upper side band is 4 megacycles, wide the lower side band should be at least 0.8 megacycle, thus giving a side band ratio of 0.2, while a lower side band width of l megacycle is preferred.

In Fig, 6, there is shown one particular television receiver designed in accordance with my invention which has the overall frequency responsecharacteristic shown. in Fig. 7. This receiver is designed the same as the one described in my copending application Serial No. 258,990,

' the above-mentioned limit of 0.35, the receiver pass characteristic.

This coupling network and the preceding coupling network are designed in accordance with the teachings of my copending application Serial No. 138,684, filed April 24, 1937, and assigned to the Radio Corporation of America, in which said network is claimed. As explained in this dopending application, the network includes a rejector circuit of the bridge-T type.

The rejector circuit for the I. F. stage II is shown located in the primary circuit and it consists of a coil 23 in series with the primary and secondary coils, the coil 23 being shunted by a coil 24 and a condenser 26 in series whereby the rejector circuit may be tuned to parallel resonance at the frequency to be rejected. As taught in my above-mentioned copending application, coupling between the coils I8 and 23 is provided, the coupling being such as to provide a negative mutual inductance whereby a resistor 21 maybe connected from the junction point of coils I8 and 23 through a blocking condenser 28 to ground to form a rejector circuit of the bridge-T type. By giving the resistor 21 the proper value of resistance, the effect of the resistance necessarily present in the parallel resonant circuits 23, 24 and 26 may be balanced out to provide infinite rejection at the unwanted frequency. is supplied to the amplifier tube I! through a suitable filter resistor 3i and through the resistor 21 and primary coil I8. The amplifier tube 22 is providedwith the usual grid leak resistor 32 while the plate voltage of the tube I7 is kept off its control grid by means-of a blocking condenser 33.

As pointed out in my copending application,

the coupling network-of the amplifier stage II is so designed that it has a peaked response close to one end of the desired 1. F. amplifier pass- The succeeding I. F. amplifier stage I2 comprises the amplifier tube 22 and a coupling network for feeding into an amplifier tube 34. This Plate voltage coupling network includes no rejection circuit and comprises a tuned primary circuit having therein a primarycoil 36 and a tuned secondary circuit are associated in the usual manner with the filter resistors. The usual blocking .condenser 4| is provided for keeping plate voltage off the grid of the amplifier tube 34.

Preferably, a certain amount of damping is provided in the form of a resistor 42 connected across the coupling and primary'coils. The coupling network is so designed that the gain over the greater part of the pass range is held at a comparatively high level, with the result that the gain falls off rather rapidly at the low frequency end. As explained in my copending application,

. this lack of gain at the low frequency end is compensated for in the preceding stages by utilizing the rejector circuits accentuating the gain at the low frequency end of the pass range.

The succeeding I. F. amplifier stage I3 is the same as the preceding stage and, therefore, is not shown in detail. The last I. F. amplifier stage I4 is of the same general character, but it is preferred that it be given the specific form described in the Grundmann and Allen Patent No. 2,157,170, issued May 9, 1939, and assigned to the Radio Corporation of America.

The last amplifier stage l4 feeds into the second detector indicated at .IB, which supplies the video signal to suitable video amplifiers not shown. There also may be obtained from the second detector I8 8. direct current which is a measure of the average carrier of the incoming signal and this may be applied through a suitable direct current amplifier indicated at 43 to provide automatic volume control. In the example illustrated, the AVG voltage supplied from the amplifier 48 is impressed upon the control grid of the first I. F. amplifier I! through the secondary circuit of the coupling. network included in block l and, therefore, not illustrated specifically, and also upon the control grid of the amplifier tube 34 of the stage l3.

By referring to Figs. 7 to 11, it will be seen that, by causing a coupling network, such as the network of the I. F. stage II, which includes a reiector circuit, to resonate in the proper manner, the band width of the amplifier is increased without sacrificing amplifier gain. Fig. '7 shows the overall selectivity curve for the I. F. amplifier, this including the detector stage i0. This is the selectivity curve of the amplifier from the grid of the first detector tube in the block i 0 to the plate or plates of the second detector in the block I8 I omitted.

Fig. 9 shows the overall frequency response for the I. F. amplifier stages I2, I3v and I4. It will be seen that the gain between 8.5 and 10 megacycles is below the desired response. It will also be noted that the picture carrier frequency of 13 megaoycles falls at a point on the curve where the response is close to100%, actually about 90%.

Fig. 10 shows a frequency response for the I. F. amplifier stages l3 and I4 in cascade. while Fig. 11 shows a frequency response for the amplifier stage i4 alone. It will be noted that in both Fig. 10 and Fig. 11 the picture frequency carrier falls at substantially the same point on the curve as in Fig. 9, namely, at about 90% full response.

fall on the overall selectivity curve of Fig. 7 at 50% full response. This is indicated in Fig. 7 by the legend and dotted line 44. A comparison of the curves in Figs. 7 to 11 will show that the side of the selectivity curve on which the picture carrier frequency falls has been given the desired shape and steepness by a proper adjustment of the two amplifier stages which include rejector circuits. Specifically, it will be noted that the picture carrier frequency falls at the same point on the curves of Figs. 9, 10 and 11, that is, at about 90% response; that it falls on the curve in Fig. 8 at about full response; and that it falls on the curve in Fig. 7 at the desired point of 50% response. Thus the shaping of the picture carrier frequency side of the selectivity curve obviously is accomplished primarily in the I. F. stage I I and in the preceding stage.

It will be understood that the receiver of Fig. 6 may be adjusted to have the overall selectivity shown in Fig. 2, Fig. 3, or Fig. 4 if desired, ratherv than the characteristic shown in Fi '7.

Comparing the lower to upper side band ratio of the curve in Fig. '7 with that of the curves in Figs. 2 to 4. it will be seen that this ratio in Fig. 7 is 1.25 to 4.5 or .3.

I claim as my invention: 1. In a television picture receiver having a predetermined pass-band of frequencies and a response characteristic which slopes in the regions of the upper and lower cut-off frequencies of the pass band and which is substantially uniform between the upper and lower cut-ofl frequencies. the method of obtaining maximum picture detail with minimum transient response which in-' cludes the steps of receiving television signals comprising a carrier wave and upper and lower side bands of picture signal frequencies, adjusting the receiver to locate the carrier wave on one of the slopes of the characteristic to give substantially 50% of the response given through the uniform region of response and regulating the slope of the characteristic on which the carrier wave is located so that the ratio of frequencies included between zero per cent and fifty per cent of the uniform response to the frequencies lying 2. The method claimed in claim 1 wherein the 0 ratio lies between 0.25 and 0.35.

3. The method claimed in claim 1 wherein the ratio is substantially 0.3.

GUSTAVE L. GRUNDMANN.

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