US2207775A - Television receiver - Google Patents

Description

A. v. BEDFORD 2, 7,775

' TELEVISION RECEIVER July 16, 1940.

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CIRCUIT 1 B 3noentor HZ/da V Bedford Gttorneg July 16, 1940. A. v. BEDFORD TELEVISION RECEIVER Filed Sept. 30, 1938 2 Sheets-Sheet 2 NN WW M N WW .m N

Snnentor Hlda V: Bedford attorney Patented July 16, 1940 UNITED STATES TELEVISION RECEIVER Aida V. Bedi'ord, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application September 30, 1938, Serial No. 232,490

3 Claims.

My invention relates to television receivers and particularly to the synchronization of the scanning at the receiver with that at the transmitter.

In television systems in which a cathode ray tube is utilized at the receiver for reproduction,

it is the usual practice to synchronize the scanning at the receiver and at the transmitter by transmitting a horizontal synchronizing impulse at the end of each scanning line and a vertical synchronizing or framing impulse at the end of each picture frame. Each framing impulse is of longer duration than a horizontal synchronizing impulse in order to facilitate separation of horizontal synchronizing impulses and framing l5 impulses from each other.

This separation of the two groups of synchronizing impulses preferably is obtained by first removing the picture signals and then employing a differentiating circuit for passing only horizontal synchronizing impulses to the horizontal deflecting circuit and an integrating circuit for passing only the framing impulses to the vertical deflecting circuit. In the past it has been the practice to employ as the integrating circuit a condenser of comparatively large capacity which has the synchronizing signals impressed thereacross through a resistor whereby the framing impulses only appear across the condenser.

However, it has been found desirable to slot the framing impulse as explained in my Patent 2,192,121, issued Feb. 27, 1940, entitled Television systems and the method of operation thereof, and assigned to the Radio Corporation of 35 America. When the framing impulses are slotted, the framing signal appearing across the condenser of the simple integrating circuit previously used will have a negative slope for the duration of a slot whereby unstable synchronization 40 may result as will be more fully explained hereinafter.

In accordance with my invention I integrate the synchronizing signals at least twice instead of only once, and in this way eliminate the 45 above-mentioned negative slope from the framing impulse. As a result, the stability of synchronization of the received picture is improved.

The invention will be better understood from the following description taken in connection with the accompanying drawings in which Figure 1 is a circuit and block diagram of a television receiver embodying my invention;

Figures 2 to 4 are curves which are referred as to in explaining my invention; and

Figure 5 is a circuit diagram illustrating another embodiment of my invention.

Referring to Fig. 1, my invention is shown applied. to a television receiver which comprises a suitable radio receiver l which supplies video signals through video amplifiers 2 and 3 to a cathode ray tube 4.

The cathode ray tube has cathode ray deflecting means such as the horizontal deflecting coils m 6 and the vertical deflecting coils I which are supplied with saw-tooth current from the horizontal deflecting circuit 8 and the vertical deflecting circuit 9, respectively.

In order to hold the deflecting circuits 8 and 9 in synchronism with the scanning at the transmitter, the video signal is taken off a point where the synchronizing impulses are of positive polarity and supplied through a conductor ID to a separating tube H which passes only the synchronizing impulses. The specific separating circuit illustrated for removing picture signals is devalues, respectively, that the tube H is grid-leak biased due to the synchronizing impulses driving the grid of the tube positive periodically, this bias being negative enough to prevent passage of the less positive picture signals.

It will be understood that other picture separating circuits may be utilized in place of the one described above, certain circuits requiring that the synchronizing impulses be negative when applied to the grid of the separating tube.

The horizontal and vertical synchronizing signals are separated from each other by means of frequency discriminating networks, specifically, by means of a differentiating network l6 and an integrating network l1, and then supplied to their respective deflecting circuits. The output of network I! is fed to the vertical deflecting circuit 9 through an amplifier l8 in the specific circuit shown.

It has been mentioned that a receiver embodying my invention such as the receiver shown in Fig. 1, may be employed for the reception of signals transmitted from a transmitting station such as that described in my above-mentioned Patent 2,192,121. It will be assumed, however, in explaining the action of my integrating network I! that signals are being received from a 5 pulses occurring at the line frequency for producing the even" lines are indicated at 23. The

line frequency impulses for producing the "odd lines are indicated at 24, these impulses occurring during the picture frame following the one containing impulses 23 and in such time relation as to fall half way between impulses 23. This produces the so-called odd line interlacing described and claimed in Patent 2,152,234, issued March 28, 1939, in the name of R. C. Ballard, entitled Television systems," and assigned to the Radio Corporation of America.

It may be noted at this point that while a specific form of synchronizing signal is illustrated, my invention is not limited to the reception of such a signal although it is especially useful where the vertical synchronizing impulse is slotted. For example, the double frequency impulses are not essential even though in'terlacing is being employed. Also my invention might well be used in a system not employing interlacing, especially if the framing impulse is slotted for the purposeof keeping the horizontal de-' fleeting circuit from falling out of synchronism during the vertical return line period.

Referring again to Fig. 1, the differentiating circuit it comprises a series condenser 23 of smallcapacity and a shunt resistor 21. The capacity of condenser 26 is so small that the only impulses passed therethrough and appearing across resistor 21 are those produced by the steep fronts of impulses 22, 23, and 24, and by the steep fronts of the impulses making up the slotted impulse 2|. I

The integrating network I! comprises a plurality of integrating sections connected in series,

the first section comprising a series resistor 3| and a shunt condenser 32, and the second section comprising series resistor 33 and shunt condenser 34. Each of the shunt condensers has suiilcient capacity to prevent the horizontal synchronizing impulses from building up a substantial voltage thereacross. In accordance with my invention, at least two integrating stages must be employed. Additional integrating stages may .be employed if desired.

The action of the integrating network I! will be better understood by referring to the curves in Fig. 2. Curves A and B represent the voltage which would appear across the output terminals of the integrating network ll during the even" frames and the odd frames, respectively, if only a single integrating stage were employed. It will be seen that the voltage rises rather sharply upon the occurrence of the framing impulse 25, the curves A and B being nearly identical.

It will be noted that on the steeply rising portion of the curves A and B there are irregularities where the impulse 2! is slotted. Specifically, considering curve B for example, during each slot the curve has a negative slope as at X. It has been found that these irregularities with their negative slopes may cause an irregularity in the synchronization of the television receiver. For example, it the receiver pulls into synchronization near one of the irregularities,

- i. e., near one of the negative slopes, successive frames'may lock in on the positive slope portions, first ahead of and then after the negative slope region. This action will be clear if it is kept in mind that the curves A and B, which differ slightly, are alternate pulses of the same voltage wave.

It might appear that the above-mentioned negative slopes could be eliminated by increasing the capacity of the shunt condenser of the integrating circuit. However, the effect of doing this is shown by the curve C. It will be seen that the negative slopes are still present.

My invention eliminates the undesired negative slopes as shown by the curves D and E. The curve E represents the voltage which appears across the output terminals of my integrating. network ll, 1. e., across condenser 34. This is a smooth curve containing no negative slopes.

Curve D represents the voltage that would appear across the output terminals of the network I! if one of the integrating stages had a condenser of smaller capacity. It will be seen that 'while the slots of the framing impulse introduce a slight irregularity, they do not cause a negative slope.

The reason why a plurality of integrating stages eliminate the undesired negative slope will be explained in connection with Figs. 3 and 4. In Fig. 3 a slotted framing impulse is indicated at 36, its alternating current axis being as indicated. When such an impulse is impressed across an integrating stage, there is a time (during each slot) that the integrating condenser receives no charge and is discharging. The resulting voltage appearing across the integrating condenser is shown in Fig. 4 by the curve 31, its ac axis being as indicated. There is a negative slope interval corresponding to each time the input wave 33 goes below the A. C. axis. It will be seen that the wave 31, which is the input for the second stage, does not fall below the (1-0 axis during the occurrence of a slot" whereby there is always a charging current present. Thus, when this voltage is impressed upon the second integrating stage, the condenser of this second stage does not discharge upon the occurrence of a slot (but continues to charge at a reduced rate) and there is no negative slope in the output voltage curve.

In Fig. 5 there is shown another embodiment of my invention as applied to a television receiver. Referring to this figure, at 38 there is shown an amplifier for amplifying the synchronizing impulses (both horizontal and vertical synchronizing impulses) after the picture signals have been removed, the amplified impulses appearing across a plate resistor 33.

The amplified synchronizing impulses are impressed upon an integrating network 4| compriswas a negligible load on the first stage. This simplified the analysis of performance.)

In the example shown, the framing impulses are impressed upon a blocking oscillator 51 such as described in the Tolson and Duncan Patent 2,101,520. This oscillator is pulled into synchronism by the framing impulses and its output is supplied to a circuit (not shown) for producing saw-tooth waves. The framing impulses are impressed upon the oscillator i, in this particular circuit, across a resistor 52 connected in series with the secondary of the blocking oscillator transformer 53. A suitable by-pass condenser 54' maintains the lower end of resistor 52 at 0-0 ground potential.

On the drawings the values of certain resistors and capacitors have been indicated in ohms, megohms, microfarads and micro-microfarads, by way of example.

I claim as my invention:

1. In a television receiver wherein horizontal synchronizing impulses and framing impulses are to be applied to horizontal and vertical deflecting circuits, respectively, a separating tube for passing said impulses substantially to the exclusion of the received picturesignals, horizontal and vertical deflecting circuits, circuit means between said tube and said horizontal deflecting circuit for supplying said horizontal synchronizing impulses thereto, and a plurality of integrating stages connected in-cascade between said separating tube and said vertical deflecting circuit, each ofsaid stages comprising a series resistor and a shunt condenser.

2. In a television receiver for the reception of a composite signal comprising picture signals,

horizontal synchronizing impulses of comparatively short duration which occur at the scanning line frequency and framing impulses of comparatively great duration which occur at the frame frequency, said receiver comprising separating means for removing at least a portion of the picture signals from the synchronizing and framing impulses, and a framing impulse separating circuit coupled to said separating means for supplying at its output terminals said framing impulses only, said last circuit comprising a plurality of integrating circuits or stages connected in cascade, each of said stages being nonresonant and comprising a series resistor and a shunt condenser.

3. In a television receiver for the reception of a composite signal comprising picture signals, horizontal synchronizing impulses of comparatively short duration which occur at the scanning line frequency and framing impulses of comparatively great duration which occur at the frame frequency, said receiver comprising separating means for removing at least a portion of the picture signals from the synchronizing and framing impulses, horizontal and vertical deflecting circuits, circuit means between said separating means and said horizontal deflecting circuit for supplying said horizontal synchronizing impulses thereto, and means comprising a plurality of integrating: stages eflectively connected in cascade for supplying from said separating means to said vertical deflecting circuit said framing impulses only, each of said stages comprising a series resistive arm and a shunt capacitive arm.

ALDA V. BEDFORD.

Disclaimer 2,207,775. Alda V. Bedford, Gollingswood, N. J. TELEVISION RECEIVER. Patent dated July 16, 1940. Disclaimer filed Sept. 13, 1948, by the assignee, Radio Corporation of America. Hereby disclaims claims 1, 2, and 3 of said patent.

[Ofiicial Gazette October 12, 1948.]

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