US2179414A - Contrast amplifier - Google Patents

Description

Nov. '7, 1939. P. 1.. KONKLE 2,179,414

CONTRA'ST AMPLIFIER Nov. 7, 1939. P J. KONKLE 2,179,414

CONTRAST AMPLIFIER Filed May 25, 1937 2 Sheets-Sheet 2 Patented Nov. 7, 1939 UNITED STATES PATENT OFFICE CONTRAST AMPLIFIER Application May 25, 1937, Serial No. 144,733

8 Claims.

This invention relates to a novel amplifier for compensating for the non-linear characteristic of certain electrical signal devices or circuits. The principal object of the invention is to provide a novel amplifier having a non-linear characteristic which may be made to compensate for the non-linear characteristic of a device or circuit with which it is employed. The invention is useful, for example, in television systems and the like for the purpose of improving the contrast of the picture and to overcome certain inherent objectionable characteristics of the apparatus used to translate light images into electric signals and vice versa. In a conventional television system, the video signal is formed in a camera tube which produces an electrical signal that is proportional to the intensity of light at a particular point in the image and at a certain time. As the image is scanned, the output of the camera tube varies in accordance with the light intensity. It is desirable that the output signal of the camera tube be proportional to the actual light intensity but, unfortunately, in the camera tubes which are now available, the amplitude of the output signal tends to fall off during scanning of the high light .portions of the image. lin other words, while the camera tube has a sub.- stantially linear characteristic over a portion of its operating range, its characteristic tends to become non-linear over a portion of its range corresponding to the white portions of the image and, as a result, the amplitude of its output signal tends to fall off in that region. While the camera tube is the principal onender in this respect, the picture tube which translates the video signal into the desired picture has a similar tendency but to a lesser degree than the camera tube.

The present invention provides a novel wide band amplifier whose gain may be caused to v increase over a particular portion of its operating range to impart to the amplier a non-linear characteristic which is complementary to the non-linear characteristic of devices such as those above mentioned, and which may be employed to compensate for the non-linearity of such devices to obtain substantially linear over-all response therefrom. Thus, the invention may be employed to correct the non-linearity of a camera tube, in which case the gain of the amplifier provided by the invention is caused to increase in the region where the output of the camera tube tends to fall off. Another object of the invention is to provide such a non-linear amplifier which has a at frequency response and which is ca-Il pable of transmitting the wide range of frequencies obtained in the video signal of a high definition television system.

Still another object of the invention is to provide such a non-linear system which has a substantially instantaneous response, i. e., one 5 in whichthe gain of the amplifier at any instant is determined by the signal level at that instant and thus one in which the gain may be changed at a rate comparable with the highest frequency required to be transmitted. lil

The invention comprises essentially two units, viz. a variable-gain amplifier and a gaincontrol amplifier. The variable-gain amplifier is adapted to be included in the signal channel of the device whose characteristic is to be cor- M rected, while the gain control amplifier is connected in a manner to derive a control signal from the signal channel and to supply such signal to the variable-gain amplier so as to vary the gain thereof. The gain control amplifier may be connected so as to derive the control signal from a portion of the signal channel subsequent to the variable-gain amplifier, in which case the control signal is fed back to the variable-gain amplifier, or the gain-control amplifier may be connected so as to derive the control signal from a portion of the signal channel ahead of the variable gain ampliner. By these different modes of connection, different operating characteristics may be obtained, as described hereinafter. 30

In the case where the backward or feed-back connection is employed, regeneration is avoided by virtue of the fact that the variable-gain amplifier is so constructed that the control signal applied thereto cannot appear in the output cir- 35 cuit of the amplifier. lin other words, the control signal cannot traverse the loop which is provided by the particular connections employed. The control signal serves only to control the gain of the variable-gain amplifier. This is a very important feature of the invention and it is, therefore, a further object of the invention to provide a novel amplifier which is not subject to instability due to regeneration.

Since the invention is particularly useful as a 45 means to overcome the above-mentioned objectionable characteristics of a television system, it will be described herein with particular reference to such use. it will be understood, however, that the invention is not thus limited but is capable of use in any instance where it may be desireci to compensate for non-linearity of a particular device or circuit.

In the accompanying drawings:

Fig. 1 is a block diagram illustrating the backward connection of the two units as above mentioned: f

Fig. 2 is a similar diagram illustrating the forward connection of the two units; Fig. 3 is a circuit diagram illustrating in detail a certain form which the invention may take; Fig. 4 illustrates certain characteristics of the amplifier provided by the invention; and Fig. 5 is a block diagram illustrating the application of the invention to a television system.

Referring to Fig. 1, the line l represents a signal channel through which the signal is passing in the direction of the arrowheads. A variablegain amplier 2 is included in the signal channel as illustrated, while the gain-control amplier 3 is connected to the signal channel and to the variable-gam amplifier, as illustrated, so that a control signal is derived from a portion of the signal channel subsequent to the variable-gain amplifier and passes through the feed-back circuit including' the gain-control amplifier, as indicated by the arrowheads.

In Fig; 2, the gain-control amplifier is connected so as to derive the control signal from a portion of the signal channel ahead of the variable-gain amplier, so that the control signal traverses the control circuit including the gainoontrol amplifier as indicated by the arrowheads. In Fig. 1, the control signal is rst amplied by the variable-gain amplifier and is then fed through the gain control amplifier and applied to the variable-gain amplifier. This serves to impart an exponential characteristic to the device. In other words, the amplification of the signal traversing the variable-gain amplifier is approximately proportional to the amplitude of the amplied signal itself. On the other hand, in Fig. 2, the control signal is not amplified prior to itspassage through the gain-control amplier and in this case, the gain of the variable-gain amplifier is proportional to the amplitude of the input signal and, consequently, the device has a square law characteristic. The device of Fig. 1 which has an exponential characteristic is prei'erred for the reasons set forth hereinafter.

The variable-gain amplifier may be of the type disclosed in my prior Patent No. 2,081,127, dated May 18, 1937. In Fig. 3, there is illustrated a 'specific form of a variable-'gain amplifier of this type which may be employed. 'Ihere is also illustrated a specic form of gain-control amplifier which is connected according to the method of Fig. 1. Referring to Fig. 3, the signal source E is representative of any means from which the input signal of the device may be derived. The signal is supplied to the control grids of tubes V1 and V2 in equal and opposite phase relation by means of the phase-inverter tube Va. Such phase-inversion circuits are well known in the art and need not be described further. The variable-gain amplifier comprises essentially the four tubes V1, Vs, V4 and V5. The cathodes of the tube V1, V2 and V4 are commonly connected through a resistor R to ground. Thus, the resistor R is connected in the input and output circuits of the tubes V1, V2 and V4. It will be seen that any voltage appearing across the resistor R will be applied to the control grids of tubes V1 and V2. Any signal developed across the resistor R is, therefore, supplied in the same phase to both of the tubes V1 and V1.

The output signal from tube V1 has its phase inverted by the tube V5 which may be coupled to tube V1 by resistance-capacitance coupling as shown. The output of tube Vs is combined with the output signal from tube V1 and the combined output signal is taken from across the common resistor R1. It will be seen then that the tubes V1, Vn and V5 are included in the signal channel and the opposite phase signals sup- 5 plied to tubes V1 and V1 from tube V3 are added together by virtue of the phase inversion of one of them and the combining of the signals as described. It will be apparent, however, that any signal appearing across resistor R, since it is ap- 10 plied to tubes V1 and V1 in the same phase, will not appear in the output circuit of the variablelgain amplifier 2. In other words, signals of like phase applied to tubes V1 and V1 are caused to oppose and neutralize each other in the variable- 15 gain amplifier so that they do not appear in the output circuit of the amplifier. This is a very important feature of the device as discussed further hereinafter.

The gain-control signal is derived from the sig- 20 nal channel and supplied by the gain-control amplifier 3 to the input circuit of tube V4, whose anode ls connected directly to the anode supply source. Hence the voltage developed across resistor R will be essentially the same as the signal 25 supplied to the grid of tube V4. The tubes V1 and V2 preferably have identical characteristics and each Anf these tubes has the' property that its mutual conductance is proportional to the voltage between its grid and cathode. The input 30 signals supplied by tube Vs are preferably small as compared with the total operating range of the tubes V1 and Va, and hence the gain of the variable-gain amplifier is determined and is substantially proportional to the voltage across re- 35 sistor R which, of course, is determined by the voltage in the grid'circuit of tube V4.

The output signal of the variable-gain amplifier is the same as its input signal amplied, however, by an amount determined by the sig- 40 nal supplied to the grid of tube V4. A variablegain amplifier of this type has the important characteristic above mentioned, viz., that the control signal supplied to it produces no corresponding component in the output circuit but 45 serves solely to determine the gain of the ampliiler. In the absence of an input signal, no output signal is produced by the amplifier regardless of any control signal supplied to tube V4. When an input signal is present, the only effect of the control signal is to determine the amount by which the input signal is amplified. This characteristic of the variable-gain amplifier permits the use of the feed-back connection shown 1n Figs. l and 3 without incurring instability. Furthermore, the input circuit for the input signal 55 4and the input circuit for the control signal are effectively completely isolated one from another so that there is insufcient coupling between them to cause instability in a device connected as shown in Fig. 2.

The gain control amplifier 3 may comprise tubes Vs and V1 as shown in Fig. 3. Where the device is used for transmitting signals covering a wide frequency range and including high frequency components, such as obtained in a television video signal, it will be found that potentiometer type gain controls are not satisfactory. A suitable gain control circuit may be obtained, however, by varying the bias on an amplifier tube of the remote cut-off type. the tube V1 may be of the remote cut-oil type and may have the characteristic that its mutual conductance is variable and determined by its grid bias. The grid bias may bevdetermined by a gain control 75 potentiometer G shunted by a bias battery. The movable arm of the potentiometer may be connected to the grid leak and preferably Vshould also be grounded through a blocking condenser. Varying the bias will vary the gain of amplier stage including the tube V1.

It will be lnoted that resistance-capacity coupling is employed throughout both amplifiers in order to obtain the desired uniform frequency response range. Both amplifiers should be capable of transmitting the highest useful frequency component in the video signal and hence there will be no appreciable time delay between the signal in the transfer circuit of the variable-gain amplifier and the signal in its gain control circuit. This instantaneous response characteristic,

which is a desirable feature of the invention,y

would not obtain if filter circuits, inherent or otherwise, were employed to restrict the frequency range of the signal supplied to the gain control circuit.

It will be understood that the two units may take forms other than those specifically disclosed.

The variable-gain amplifier shown in Fig. 3 is representative of any amplifier having the characteristics above described, that is a unit for amplifying one signal by an amount determined by a second signal and in which means are provided for preventing the transfer of the second signal tothe output circuit. The gain control ampliiier illustrated is representative of any suitable gain-control device which may be employed for the purpose of the invention.

In Fig. 4, there are illustrated characteristic curves illustrating the relation between the input and output voltages which are obtainable by employing an amplifier of this type. Curve A represents the desired linear relation, while curve B illustrates the normal characteristic of a camera tube of a television system. llt will be seen that in the region near the origin D, the output voltage is approximately proportional to the input which, in the case of the camera tube, would be the illumination of the image being scanned. llt will be seen from curve B, however, that as the amount of illumination increases, the output voltage does not increase proportionately but falls off somewhat. 'lihis may be overcome by providing a nonlinear amplier in accordance with the present invention in which the output voltage increases more rapidly than the input voltage for the region away from the origin O, as illustrated by curve C. Where the characteristic C is required,

as in the case of a camera tube, the polarity of the control signal supplied to the gain-control am plier should be such that, as the signal traversing the amplier changes in response to an increase in light intensity, the gain of the amplifier likewise increases. The amount by which the curve C departs from the linear characteristic .a may be determined by the gain of the amplifier. In the device of Fig. 3, the gain of the amplifier may be controlled by theu gain-control device Gr.

As described above, a device of the type shown in Fig. 1 has an exponential characteristic, while a device of the type shown in Fig. 2 has a square law characteristic. The diffrence Ibetween these characteristics resides largely in the fact that the exponential characteristic corresponds more closely to a linear characteristic near the origin of the curves and diverges more rapidly from the linear characteristic away from the origin than does a square law characteristic. While the difference between these two characteristics is not very marked for the operative region in which the device is used, nevertheless it has been found that the exponential characteristic is preferable and should preferably be employed. For this reason, a device of the type shown in Fig. 1 is preferable to that shown in Fig. 2.

In Fig. 5, there is shown a block diagram embodying the essential elements of a television re-- ceiver which employs the device of the present invention. While the device of the invention may be used at either the transmitter or the receiver, it is preferably employed at the receiver in order that it may serve the additional useful purpose of minimizing the range of modulation required to transmit effectively a given number of shades in an image being televised.

As shown in Fig. 5, a television receiver ern-4 ploying the device of the invention may comprise a unit l which, when supplied with television signals, produces a video signal and also the proper defiecting signals for the picture tube ti. The video signal may be supplied to the variablegain amplifier t and the output of this amplifier may be supplied to a second video amplier t and thence to the picture tube. .a control signal may be derived from the output of the amplifier t and fed baci: through the gain-control amplifier d to the variable-gain amplifier, as above described. d television receiver embodying the device of the invention is characterized by improved clarity of the received image and by the fact that the image contains a larger number of light gradations or shades than it has been possible to obtain in prior devices. Generally speaking, it has been found that by employing the device of the present invention, the number of distinguishablelight gradations or shades may be increased by approximately to 25%. It will 'be apparent, therefore, that a marked improvement in the performance of apparatus, such as television apparatus, which has heretofore had the objectionable characteristics above mentioned may be obtained by employing the present invention.

As indicated above, the invention contemplates broadly the provision of a variable-gain amplifier, the gain of which may be varied by means of a control signal without causing any component of the control signal to appear in the output circuit of the amplifier, and the control of the gain of the said amplifier, utilizing a control signal derived from the signal channel, preferably from a portion of the signal channel subsequent to the said amplifier. Further, the invention contemplates the utilization of such a device to compensate for non-linearity of apparatus with which the device is employed. lit will be understood, therefore, that the present disclosure is merely exemplary and is not intended to limit the scope of the invention.

I claim:

l. In an electrical signaling system, an input circuit, an output circuit, signal amplifying means connected between said input circuit and said output circuit constructed and arranged to transfer signals over a wide frequency range, means for deriving a control signal from one of said circuits, signal amplifying means for said control signal constructed and arranged to operate over said frequency range, means for applying the amplified control signal to said first amplifying means to control the gain thereof, whereby the gain of said first signal amplifying means varies instantaneously with the amplitude of the signal in said circuit at the point where the control signal is derived, and means for preventing said control lib lli

dll

signal or any component thereof from appearing in said output circuit.

2. In an electrical signaling system, an input circuit, an output circuit, signal amplifying means connected between said input circuit and said output circuit constructed and arranged to transfer signals over a wide frequency range, means for deriving a control signal from said output circuit, signal amplifying means for said control signal constructed and arranged to operate over said frequency range, means for applying the amplified control signal to said ilrst amplifying means to control the gain thereof, whereby the gain of said rst signal amplifying means varies instantaneously with the signal in said output circuit, and means for preventing said control signal or any component thereof from appearing in said output circuit.

3. In an electrical signaling system, a signal channel including an ampliiler having an input circuit, an output circuit, a gain control circuit, and means for preventing a gain control signal from appearing in said output circuit, an amplier for supplying a control signal to said gain control circuit, both of said ampliers being constructed and arranged to transfer signals over a Wide frequency range, and means for deriving a control signal from the signal channel and for applying said signal to said last-named amplifier, whereby the gain of said iirst amplifier varies instantaneously with the amplitude of the signal in said channel at the'point Where the control signal is derived.

4. In an electrical signaling system, a signal channel including an amplier having an input circuit, an output circuit, a gain control circuit and means for preventing a gain control signal from appearing in said output circuit, an amplifier for supplying a control signal to said gain control circuit, both of said amplifiers being constructed and arranged to transfer signals over a wide frequency range, and means for deriving a control signal from the signal channel at a point subsequent to said first-named amplifier, and for applying said signal to said last-named amplifier, whereby the gain of said first amplifier Varies instantaneously with the amplitude of the signal in said channel at the point Where the control signal is derived.

5. In an electrical signaling system., a signal channel including an amplifier having an input circuit, an output circuit, a gain control circuit, and a pair of signal transfer paths adapted to balance out a control signal passing therethrough, an ampliiier for supplying a control signal to said gain control circuit, both of said ampliiers being constructed and arranged to transfer signals over a wide frequency range, and means for deriving a control signal from the signal channel and for applying said signal to said last-named amplifier,

whereby the gain of said rst amplier varies instantaneously with the amplitude of the signal in said channel at the point where the control signal is derived.

' 6. In an electrical signal system, a signal transfer channel including a signal amplifier, said ampliiier including a plurality of space discharge devices each having an input circuit, an output circuit, and an eifective mutual conductance, the mutual conductance of each of said space discharge devices being dependent upon the voltage in the input circuit of the respective discharge device, means for applying an input signal to said signal amplier, means for deriving an output signalfrom said amplifier, another space discharge device having an input circuit and an output circuit, an impedance having a voltage thereacrcss in said input circuits and said output circuits, for establishing said voltage in each of said input circuits and each of said output circuits, means for deriving a control signal from said channel, and means for applying said control signal to the input circuit of said other space discharge device.

7. VIn an electrical signal system, a signal transfer channel including a signal amplifier, said amplliier including a plurality of space discharge devices each having an input circuit, an output circuit, and an eiective mutual conductance, the mutual conductance of each of said space discharge devices being dependent upon the voltage in the input circuit of the respective discharge device, means for applying an input signal to said signal amplifier, means for deriving an output signal from said amplifier, another space discharge device having an input circuit and an output circuit, an impedance having a voltage thereacross in said input circuits and said output circuits, for establishing said voltage in each of said input circuits and each of said output circuits, means for deriving a control signal from said channel, means for amplifying said control signal, and means for applying the amplified control signal to the input circuit of said other space discharge device.

8. In an electrical signal transfer system comprising apparatus having an inherent non-linear output-input response characteristic, the method of compensating for the non-linearity of said apparatus which consists in passing the signal through a signal channel including an amplier, deriving a control signal from the signal channel having the same frequency characteristics the signal in the channel, and utilizing said control signal to vary the instantaneous gain of said amplier in complemental relation to the nonlinear response -of said apparatus, to thereby obtain over-all linear response of said system.

PHILIP J. KONKLE.

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