US2280740A

US2280740A - Sound recording apparatus

Info

Publication number: US2280740A
Application number: US326836A
Authority: US
Inventors: Belar Herbert
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1940-03-30
Filing date: 1940-03-30
Publication date: 1942-04-21
Anticipated expiration: 1959-04-21

Description

April 21, 1942. E-L' BELAR SOUND RECORDING APPARATUS Filed March 30, 1940 2 Sheets-Shet l Bnventor He rbe rt B e la attorn eg April 21, 1942. H, BELAR 2,280,740

SOUND RECORDING APPARATUS Filec} March so, 1940 2 Sheet sSheet 2 P we I Suventor Hei'be rt Be Z ar ttomeg Patented Apr. 21, 1 942 sown RECORDING APPARATUS Herbert Belar, Philadelphia, Y Pa., assignor to Radio Corporation of America, a corporation of Delaware Application Marcli30, 1940, Serial No. 326,836

7 Claims.

This invention relates to sound recording apparatus, and-more particularly to a ground noise reduction amplifier for use in such recording apparatus and the like.

It is known that thetransparent unmodulated areas on sound film tend to produce noise during the reproduction of sound due to extraneous materials which may adhere to the surface of the clear portion of the film and due to developed silver grains in the emulsion. The reduction of such noise, by rendering the unmodulated portion of the background or. positive film opaque, is'

described and claimed, for example, in McDowell Reissue Patent No. 21,389. The said patent discloses the use of a rectifying amplifier into which a portion of the audio frequency current is fed. This amplifier, commonly referred toss a ground noise reduction amplifier, rectifies and filters the that the background in the positive will be 4 printed opaque.

There have heretofore been difficulties in the construction ofsuch amplifiers due to the fact that the filtering circuit which is capable of 5 filtering out the low audio frequencies necessarily produces a correspoiiding time delay. This time delay causes, of course, a corresponding delay in the response of the shutter in following the-envelope of the sound waves and causes the l clipping of the peaks when there.is a sudden increase in the amplitude of the sound.

With early equipment, the frequency range was i not very great and this clipping did not become serious. However, improvements in the sound recording and film moving equipment have permitted the frequency range of recording to be greatly extended so that the recording of frequencies as low as 50 or even 30 cycles has become customary. The recording of such low frequencies has necessitated the improvement of the filtering in the ground noise reduction amplifier,

while, at the same time,-the improved quality of 4 the sound reproduction has caused the audiences to become more critical of any defects in the.

ground noise reduction. Improving the speed of operation of the ground noise reduction apparatus requires the use of less filtering, which is completely inconsistent with the requirement for better filtering due tothe increased frequency range in such a circuit as that disclosed by Mo- Dowell, for example. p

The apparatus of the present application accomplishes either a speeding up of the operation of the ground noise reduction'apparatus, or an improvement of the filtering produced, or both. This result is accomplished by passing the audio frequency input current to the ground noise reduction amplifier through a series of frequency doublers consisting of full wave rectifiers and transformers so that the lowest frequency to be filtered is several times the lowest frequency which is recorded, thereby permitting use of a smaller filter with correspondingly less time delay to secure the same degree of filtering.

One object 'of the invention is to provide an improved ground noise reduction amplifier.

Another objectof the invention is to provide an apparatus which will derive a direct current corresponding to the envelope of an audio frequency current with maximum filtering and at minimum time delay. 7

Another object of the invention is to provide an amplifier including one or more frequency doublers for increasing the minimum frequency fed to a filter circuit, thereby improving the efficacy of the filter circuit.

Another object of the invention is to provide the sound recording apparatuswhich will produce a record having better ground noise reduction and higher fidelity of recording than has heretofore been produced.

Other and incidental objects of the invention will be apparent to those skilled in the art from a reading of the following specification and an inspection of the accompanying drawings, in which Figure 1 is a schematic diagram of one form of the invention in which the direct current out put increases on an increase in the audio or A. C.

Figure 2 is a schematic diagram of a second 1 III, the secondary of whichis provided with a potentiometer 20 or other equivalent device for controlling the input to the first amplifier tube ll. Grid bias .ior the first amplifier tube may be provided by the usual cathode resistor 12 bypassed by the usual cathode condenser 3. The output from the tube H is fed through the transformer I 4 to the rectifiers i5 and 16, which constitute a full wave rectifier and may be in a common envelope, as, for example, in the type of tube known as the 61-16. The output from the rectifiers l5 and I6 is fed to the primary of the transformer l? which, like the transformer I4, has a push-pull secondary. It will be apparent that each half wave passing through the transformer It will produce a separate impulse in the same direction through the primary of the transformer ll, thereby inducing in the secondary of the transformer i'l alternating currents of twice the frequency of those in the trans former l4. Theoutput from the transformer ill is fed to a second full wave rectifier I8 and I9 which again may be a type 6H6 tube; It will be apparent that each half impulse from the transformer l1 produces a separate impulse through the rectifiers in the same direction, thereby again doubling the frequency. The output from this second pair of rectifiers is-fed into the filter circuit including the capacitors Z6, 22 and 23', and the resistorsifl, 25 and 28. It will be apparent that if a frequency of, say, 50 cycles is applied to the transformer ill, the input to the filtercircuit due to the two doublings will be a pulsating direct current with a pulsation frequency of 200 cycles, which can be smoothed out to its average value by a filter circuit having a much faster time constant than would be the case for the original 50 cycles, if rectified.

In the form of the invention shown in Fig. 1,.

the grid lead of the amplifier 21 is connected to the cathodes of the rectifiers l8 and i9, and therefore the output of the rectifiers will cause ent recording it is often desired to record a fre-- quency range from 30 to 10,000 cyclesand the the range of output frequencies would become the grid to become more positive, thereby increasing the plate current on an increase of A.-C. input. 'An appropriate meter 28. may be inserted in theplate. circuit of the tube 2'1 to indicate the D.-C. output and thereby indicatethe operation of the ground noise reduction shutter which later may be constructed, for example, in accordance with Baker Patent No. 2,102,776.

In the form of the invention shown inF-ig. 2, the cathodes of the rectifiers I8 and iii are connected to the cathode of the D.C. amplifier tube 35 through a potentiometer 34, while the center tap of the transformer I lie connected through the filter circuit to the grid of the tube 35. This filter circuit is quite similar to that shown in Fig. 1 consisting of the capacitors 30 and 3] and the

resistors

32, 33 and. Inthis circuit, due to the reversal of connections, an increase in audio are necessary to secure the required speed of operation by decreasing the time constant of the filter circuit, v

In the form of the invention shown in Fig.3,

a multiplicity of frequency-doubling stages are used in -thiscase five, which gives a correspondingly'high speed of response. However, the use of such a number of doubling stages by themselves is impractical particularly where a wide frequency range is to be handled. For example, in pres- 960 to 320,000 cycles, It will be apparent that the higher frequencies would not be transmitted by the audio frequency transformers used and a separate circuit is therefore used to transmit the higher frequencies.

In the operation of this circuit, the audio fre quency input passes through the transformer ID, the potentiometer 20, the amplifier tube II and the transformer M, as in the other forms of the invention. Instead of the single interstage transformer ll of Figs. 1 and 2, there are provided, however, four such transformers, Ha, H2), H0 and lid. In the present case, the full wave rectifier tubes are indicated in the manner con ventional for the type 6H6 tubes at 3B, 31, 38, 39 and 40 and it will be apparent that the output of the tube 40 will-havebeen doubled in frequency a corresponding number of times, as pointed out above.

A portion of the output from the transformer I0 is also applied to the amplifier tube 53 through the high pass filter including resistors 50' and 5i and the capacitor 52. This high, pass filter is designed according to the characteristics of the circuit from the transformer H to the tube 40 so that the frequencies of the input to the tube 53 will correspond to those frequencies which have been attenuated in or rejected by the doubier circuits. The output of the tube 53 passes through thetransformer 56 to the rectifier 5,1 which, in this case, is shown used as a halfwave rectifier so as to avoid any frequency doudoubling. The output from the rectifier tube 40 is passed through the filter circuit including the elements pointed out above and also the capacitor 5! and the resistor 24 in addition to the resistors 59 and B0 and the capacitor 5!. Appropriate grid bias may be applied to the tube 21 through the potentiometer 53cwhich is connected to one side of the plate supply and the resistor 62 which is connected to the other side of the plate supply. It will be apparent that in thisform of the invention all of the lower frequencies will be doubled five times leaving the lowest frequency in the output of the tube 40 something of the order of 900 to 1000 cycles with the other frequencies increasing correspondingly. The frequencies selectedby the filter circuit in the upper portion of this circuit and the degreeof amplification secured through the amplifier 53 are so chosen that they exactly offset the high frequency losses through the doubler circuits and the output from the tube 21 accordingly corresponds acv curately with the envelope of the audio impulses introduced through the transformer Hi. This output further is'fiitered to a very high degree but without any appreciable time lagas' the filter I circuit in the input to the-tube 2'! can be made 1 frequencies applied thereto.

very small, having no frequencies applied to it less than about 900 cycles.

It will be apparent that the modification of the apparatus as shown in Fig. 3 can be extended to any desired degree. For example, the first doubling amplifier may have five doubling stages, as illustrated in Fig. 3, and be fed only with the lower group of frequencies. A seconddoubling amplifier might be fed through a band-pass filter transmitting a group of frequencies slightly higher than those fed through the first doubler and the \second doubler having a smaller number of doubling stages, as for example, three. Still a third doubling'amplifier may be provided having, for example, only a single doubling stage and likewise fed through a band-pass filter transmitting a .higher group of frequencies and the final rectifying amplifier of the group would be small to that shown in the upper portion of Fig. 3 but fed with only the highest audio frequencies which require no doubling.

All the outputs of the rectifying amplifiers in .such an arrangement would be combined to provide the desired envelope current preferably being combined through a single filter circuit as shown in Fig. 3although separate filter circuits could be provided if desired and the outputs thereafter combined in the D.-C. amplifier. The number of doubling amplifiers to be used, the

number of doubling stages in each doubling amplifier and the provision of a rectifying amplifier without a doubler must, of course, be chosen with proper regard to the circumstances under which the circuit is to operate. It will be apparent, for

example, that an apparatus using a large-num-- and at least one frequency doubler preceding said filter, said filter circuit having such characteristics that it will not properly filter A.-C. input frequencies of the order of 50 cycles but will adequately filter the doubled frequencies applied thereto. 4

2. An amplifier of the type described having an A.-C. input and a D.-C. output corresponding to the envelope of the A.C. input, said amplifier including a filter circuit and at least one full wave rectifier serving as a frequency doubler preceding said filter circuit, said filter circuit having such characteristics that it will not properly filter A.-C. input frequencies of the order of 30 cycles but will adequately filter the doubled 3. An amplifier of the type described having an A.-C. input and a D.-C. output corresponding to the envelope of the A.-C. input, said amplifier including a filter circuit and a plurality of full wave rectifiers serving as frequency doublers preceding said" filter circuit, said filter circuit having such characteristics that it will not properly filter A.-C. input frequencies of the order of 30 cycles but will adequately filter the doubled frequencies applied thereto.

4. An amplifier of the type described having an A.-'C. input and a D.-C. output corresponding to the envelope of the A.-C. input, said amplifier including a first amplifying stage adapted to amplify the A.-C. input, a plurality of full wave rectifier frequency doubler stages, a filter circuit, and a D.-C. amplifier stage actuated from the output of said filter circuit, said filter circuit having such characteristics that it will not properly filter A.-C. input frequencies of the order of 30 cycles but will adequately filter the doubled frequencies applied thereto.

5. Apparatus of the class described including an amplifier having an A.-C. input and a D.-C. output corresponding to the envelope of the A.-C. input, said amplifier including at least one frequency doubler, rectifying means and output filter means, said apparatus including a second amplifier connected to the same A.-C. input, having a D.-C. output corresponding to the envelope of the A.-C. input and including rectifying means, the output of said second amplifier being connected to the same output filter means as the first amplifier. l

6. Apparatus of the class described including an amplifier having an A.-C. input and a D.-C. output corresponding to the envelope of the A.-C. input, said amplifier including at least one frequency doubler, rectifying means and output including at least one amplifying stage, a filter,

filter means, said apparatus including a second amplifier connected to the same A.-C. input, having a D.-C. output corresponding to the envelope of the A.-C. input and including rectifying means, the output of said second amplifier being connected to the same output filter means as the first amplifier, and high-pass filter means connected between the said A.-C. input and said second amplifier.

to the envelope of the A.-C. input and includin rectifying means and a less number of doublers than the first amplifier, the output of said additional amplifier being connected to the same output filter means asthe first amplifier.

HERBERT BELAR.