Signal translating and correcting system

June 11, 1935.

H. G. TASKER

2,004,253

SIGNAL TRANSLATING AND CORRECTING SYSTEM

Filed April 20, 1932

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RESPONSE IN DECI-BELS RESPONSE IN DECI-BELS

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FREQUENCY

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a CHARACTERISTICS DUE TO SHUNTING ώ EFFECT OF RESONANT CIRCUIT

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FIG.4

FREQUENCY

FIG.6

COMBINED CHARACTERISTICS OF CELL AND TRANSFORMER

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COMBINED CHARACTERISTICS OF CELL TRANSFORMER AND RESONANT CIRCUIT

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FREQUENCY

FREQUENCY

INVENTOR

Hornet' G. Tasker

Patented June 11, 1935

2,004,253

UNITED STATES PATENT OFFICE

2,004,253

SIGNAL TRANSLATING AND CORRECTING SYSTEM

Homer G. Tasker, Douglaston, Long Island, N. Y., assignor to United Research Corporation, Long Island City, N. Y., a corporation of Delaware

Application April 20, 1932, Serial No_; 606,337

Claims. (Cl. 250—41.5)

This invention relates to signal translating systems, and, particularly, to means for amplifying and equalizing the response of a signal translating system.

⁵ The reproduction of optical sound records is conveniently done by means of a light sensitive cell·, which may desirably be a selenium cell, suchas is described in Patent No_; 1,552,926 to Russell Hart, issued Sept. 8th 1925, because of the sim¹⁰ plicity and low cost thereof. Difficulty is, however, experienced with the selenium cell, as well as other types of cells in that the response of the cell is not proportional to, but varies with, the frequency of the incident light pulsations from the IB optical record, in the selenium cell, the response being much greater at low frequencies than at high.

An object of this invention is to equalize, and compensate for, the deviations from proportion20’ ality of response of a light sensitive cell to changes in frequency of modulated light of constant amplitude.

Another object is to modify the characteristic response-curve of a selenium cell reproducing sys25 tern to obtain substantially linear response over a wide range of frequencies.

It has been found possible to compensate, by means of a transformer having a low inductance, for the preponderant response of a selenium cell ³⁰ to low frequencies of modulated light from an optical record, but the resulting response curve is not strictly linear, but shows a maximum at a point near the middle of the useful frequency range; that is, between 500 and 2,000 vibrations 35 per second.

This invention provides a simple auxiliary circuit, which flattens the maximum of the characteristic curve and produces substantially a linear proportional reproduction over a widerange of fre40 quencies;

The invention; therefore, provides for correcting the frequency response of a selenium cell by attenuating the low frequency response thereof to a value appropriate to the higher frequency re⁴⁸ sponse thereof, auxiliary means being provided for attenuating, the middle frequency response to correspond to the high and low frequency response.

_r Other objects of this invention will be appar⁵⁰ ent from the following description, when read in connection with the accompanying drawing, . wherein:'

Fig. 1 is a diagrammatic representation of circults and apparatus of this invention, and ⁵⁸ Figs. 2 to 6, inclusive, are characteristic curves, indicating the characteristic quality of operation of various members of the system;

Referring to the figures, an exciter light I is provided, cooperating, through suitable devices to illuminate an optical sound record 2; which record 5 serves to modulate light from the exciter light I, which is thrown upon a light sensitive cell 3, which may desirably be a selenium cell. The cell 3 is energized by a source of current; such as a battery I, and is connected to the primary coil 10 of a transformer 5.

The secondary coil of the transformer 5 is connected to an amplifier tube 6, as shown, which, in turn, is connected to a transformer 7, as indicated. The transformer 7 desirably supplies a 15 second vacuum tube 8, which is coupled to a third vacuum tube 1I, which is adapted to supply a load circuit such as a loud speaker, or other translating device 12.

It is, of course, obvious that the number of vac- 20 uum tubes and transformers may be varied according to the needs of the load circuit, and that more than one may be of low inductance as before described.

The transformer 7 is desirably one of low im- 25 pedance, to attenuate the pulsating currents, corresponding to the low vibration frequencies in the record 2 more than it attenuates the higher frequencies from the record 2, as disclosed in the co-pending application of John Nielsen, Serial 30 Number 606,394, filed April 20,1932. >

Connected in shunt to the primary coil of the transformer 7 is a series resonant circuit, consisting of an inductance 14, a condenser 15 and a resistance 16. 35

The selenium cell has a frequency-responsecharacteristic of the type shown in Fig. 2. It has been found that the response is sufficiently great at vibration frequencies as high as 6,000 cycles to be usable, and increases to a number 40 of times that value at low frequencies, such as 100 vibrations per second. The transformer 7 is made with low inductances as disclosed in the above mentioned Nielsen application, and has a frequency transfer characteristic as indicated in 45 Fig. 3. The resulting response characteristic of the combination of cell; transformer and amplifier is shown in Fig. 4.

The series resonant circuit is designed by adjustment of the inductance and capacity to have 50 a natural resonant response point, coinciding with the maximum indicated in Fig. 4, as shown for the resonant circuit in Fig. 5. The resistance is adjusted to broaden the resonant peak by an appropriate amount. The effect of the series 55

2,004,283 resonant circuit is then combined with the effect of the transformer and produces an influence upon the characteristic curve as shown in Fig. 6.

It will be noted that over a range of frequencies from below 100 vibrations per second to a frequency above 5,000 vibrations per second, the characteristic curve is substantially, linear. and horizontal, indicating that this system will reproduce at constant volume level a constant amplitude optical record over this range of fre-. quencies.

In the operation of the device and method of this invention, the exciter light provides a steady beam of light, which is modulated by the optical record 2, converted by the cell 3 into electrical vibrations, which are corrected by the transformer 7 to equalize the frequency response at the high and low ends of the useful frequency spectrum; which pulsations are then further corrected by the series resonant circuit to equalize the middle frequencies to those at the top and bottom of the frequency range.

In the embodiment shown in Fig. 1 the desired correction of the curve at the mid point is obtained by the use of a series resonant circuit connected in shunt to the coupling transformer, and is the preferred form, since it introduces a minimum of losses in the circuit. Similar results can, however, be obtained by the use of a parallel resonant circuit, consisting of an inductance connected between the plate of the tube 6 and the primary of the transformer 7, the inductance being shunted by a condenser to form a parallel resonant circuit with a resistance in series either with the inductance or the condenser to broaden the resonant peak. This con35 struction imposes a greater impedance to the transfer of pulsations from the tube 6 at points near its natural resonant frequency, which is de40 sirably adjusted to match the maximum of the characteristic curve of Fig. 4. This system, however, is less desirable because of the greater losses encountered therein.

The preferred embodiment of the invention 45 utilizes but a single auxiliary resonant circuit, since in most cases but a single maximum is encountered in the characteristic curve. If, however, equalization of the high and low frequencies is obtained in two steps as described in the 50 above mentioned Nielsen application, the equalization of the middle frequency may also conveniently be performed in two steps, thereby permitting the system to compensate for two maxima if such should occur.

By the device of this invention there is thus obtained a more uniform response in electrical vibrations from an optical sound record by a simple, convenient and inexpensive light cell and _ri amplifier system.

⁶⁰ While there is herein disclosed but a single embodiment of the device of this invention, it is possible to produce still other embodiments thereof without departure from the inventive θ- concept herein disclosed, and it is, therefore, desired that only such limits shall be imposed upon the appended claims as are stated therein or required by the prior art.