US2007985A - Radio transmission and receiving system - Google Patents

Description

July 16 1935.

5. W. SEELEY RADIO TRANSMISSION AND RECEIVING SYSTEM Filed Jan. 19, 1931 :5 Sheets-Sheet 1 TRANS.

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RADIO TRANSMISSION AND RECEIVING SYSTEM s. w. SEELEY Filed Jan. 19, 1951 Wifnsss 3 Sheets-Sheet 2 U a 4b JD 100 1 1,000 10,000

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12 Claims.

This invention relates to improvements in radio transmission and receiving systems, and particularly to such improvements that eliminate interfering disturbances as static, tube noises, etc.

the major portion of all interfering noises encountered in the reception of broadcast signals consists of frequencies in the upper end of the audio frequency spectrum. An analysis from the probability standpoint of interfering impulses, will throw some light on the reasons for this condition. I

It is commonly known that the tonal scale of the ear is a logarithmic function of the frequency and, thereforaif the said spectrum consists of frequencies of from 50 cycles to 5000 cycles, 500 cycles will be the central point or half-way mark of such a tonal scalef Thus we see that the lower end consists of frequencies varying from 50 to 500 cycles or a difference of 450 cycles, while the upper half consists of frequencies from 500 to 5000 cycles or a difference of 4500 cycles.

In solving the problem of disturbance elimination, we must consider the probable frequencies of random impulses which might heterodyne the carrier frequency of an incoming broadcast sta tion and thus produce interference in the audio output system. It is obvious that such random impulses will be fairly evenly divided, if distributed according to frequency from 5 kilocycles below the carrierfrequency to 5 kilocycles above the carrier frequency, and since only such impulses as fall in the region of 500 cycles plus or minus the carrier frequency, will produce interference in the lower half of the audio band, it is obvious that only 1/l0th or 10% of such interference from this cause 'would fall in the lower half of the audio band.

Consideration of audio frequency impulses caused byheterodyning of two radio frequency impulses other than the carrier frequency of a transmitting station, shows the same condition to be true. Further consideration of the physiological characteristics of the ear, tend to make more objectionable interference in the upper half of the audio spectrum. This is because of the well-known fact that the threshold value of audibility of the human ear is far higher at the lower frequencies than atthe frequencies ranging from 1000 to 5000 cycles. It is, therefore, desirable to eliminate more particularly the interference present in the upper half of the audio spectrum.

It is well-known by all versed in the art that present day radio receivers designed to amplify signals of as low as 2 to 10 micro-volts per meter 1931, Serial No. 509,693 (01, 250-6) field strength, are never free from the influences of vacuum tube and tube circuit noises. At present, these form quite a definite limit to the sensitivity which can be incorporated in a re- I have found by empirical investigation that .ceiver. I, therefore, propose to alleviatethis difficulty so that the sensitivity of a receiver may be increased approximately 100 times beyond thi limit.

Investigation of the modulation of the present day broadcast transmitter, when broadcasting 10 various types of programs, disclosesthe fact that this modulation consisted chiefly of frequencies 'in the lower half of the audio band and that while modulations of from to by single frequencies in the lower audio scale are numerous, 15' no instance was found of modulation greater than about 15% by a frequency higher than 500 cycles nor more than 1% above 2000 cycles. This is due to three causes. First, lower notes of'necessity' carry the higher air pressures because they origi- -20 nate with the larger instruments; second the threshold value of audibility of the ear is such that much lower impulses on the higher frequencies produce equal sensation with the much larger low frequency impulses; third, frequencies 25 above 2000 cycles .are seldom, if ever, funda mentals and exist simply as harmonics of the lower frequencies.

It is, therefore, an object of my invention-to provide a radio sending and receiving system 30 that reduces outside interference in the receiver as well as reduces the interferingnoises originating in the circuits and tubes of the receiving system.

It is a further object of my invention to pro- 35 vide a sending and receiving system that will. facilitate a so-called flat frequency response. In the present day art of permanent recording of music, particularly on disc records, it is wellknown. that the cutting mechanism for such 0 records must of necessity be so governed that the lateral velocity of the cutting tool is not absolutely proportional to the applied voltage at all frequencies. This is because of the fact'that the lower frequencies would tend to overlap from one 45 groove to the other and thus cause the reproducing needle to jump from one groove to the next. An examination of records designed to give constant ouput at various frequencies in the audio 50 hand, discloses the fact that the amplitude of the wave form of the track is approximately inversely proportional to the frequency and thus the lowest notes to be reproduced must be attenuated before entering the cutting tool in order that the 55 amplitude of the wave form not be too great.

It is also well known in the art that the reproduction of phonograph music is invariably accomplished by a certain amount of surface noise or what is commonly known as record scratch. If this is allowed to remain, in the reproduced music, it is exceedingly objectionable, and if it is removed by means of electrical or mechanical filters, certain desirable portions of the recorded frequencies are lost.

It is a further purpose of this invention to overcome these difliculties in the art of phonograph recording and to facilitate fiat frequency response in the phonograph reproducing system.

Other advantages and objects of the invention relate to the particular circuit arrangement of of the path would my apparatus and will be apparent from the fol lowing description, taken in connection with the accompanying drawings in which:

Figure l is a schematic view of my sending apparatus.

Figure 2 is a schematic view of my receiving system.

Figure 3 shows the gain curve of my audio amplifier in the sending system.

Figure 4 shows the gain curve of my audio amplifier in the receiving system. I

Figure 5 shows theresultant output characteristic of the receiver.

Figure 6 is a fragmentary schematic view of my recording system.

Figure 7 is a fragmentary schematic view of my reproducing system.

Figure 8 is a schematic view of a modified receiving system.

Figure 9 is an attenuation curve, characteristic of the receiver circuit shown in Figure 8.

I have found that the undesirable disturbances which normally predominate in the upper half of the audioscale previously mentioned, can be appreciably decreased by raising the energy level of the higher frequencies to a point well above the energy level of the accompanying noises or disturbances in this region. This is accomplished by means of a special amplifier in the sending apparatus that has a rising gain characteristic which is directly proportional to the frequency. Modulation of the carrier wave will not be uniform with frequency as in the present day systems, but rather increases as the frequency rises and such increase will be approximately proportional to the frequency from 50 to 5000 cycles. Then, in order to facilitate a fiat response in the reproducing apparatus of the receiver, I- propose to utilize an audio amplifier in which the gain is approximately inversely proportional to the frequency from 50 to 5000 cycles.

A frequency characteristic of the sending system as above described, and shown in Figure 3,

can be obtained in a number of different ways.

I propose to use the circuit shown in Figure l. The desired sound to be transmitted, is picked up by microphone l. microphone current and feeds the, same into a special amplifier comprising-vacuum tube 3 and the necessary coupling units to produce a rising gain characteristic. Tube 3 may be of the well known screen grid type having an amplification factor of approximately 400 and a'mutual conductance of about 1000.

The output circuit of tube 3 includes resistance 0, inductance 5. and a blocking condenser 6 which is used to prevent the passage of direct current through inductance 5 from the 250 volt source An amplifier 2 amplifies the used to energize the plate of tube 3. Inductance 5 is of such a value that the total output impedance of tube 3 does not vary over 2% from 50 to 5000 cycles. Connections 1 and 8 are taken across inductance 5 and lead to theinput of a succeeding tube included in amplifier l2. The input circuit to amplifier [2 comprises a grid condenser 9, a grid resistor I0 and, the conventional grid bias battery I I.

The output of amplifier I2 is connected to the radio transmitter l3 which may include a modulator, an oscillator and power amplifier in any conventional arrangement. Antenna is shown as a means for radiating the modulated carrier frequency.

In practice, I have used a resistance of 600,000 ohms for resistance 4 and a coil 5 having 1.59 henries inductance. Such an inductance has approximately 50,000 ohms impedance at 5000 cycles. This special stage shows a gain of about 20 units at 5000 cycles and 0.2 units at 50 cycles, or a variation of 40 decibels. The gain of tube 3in connection with the above-described circuit will be Mu w L p+ which indicates a rising gain characterstic where mu is equal to the voltage amplification of the tube, w is equal to 21l' times the frequency being amplified. L is the inductance of coil 5; Rp is the internal plate impedance of the tube and r is the external resistance. 4.

It will be noted that the input voltage to amplifier I2 is taken across inductance 5 in the output circuit of tube '3. Thus the input to amplifier 12 will be proportional to the voltage drop across this coil. This voltage will vary approximately in accordance with the above formula expressing the gain of' the circuit.

In the formula, mu, L, Rp and r remain sub- 'stantially constant over the frequency range from 50 to 5000 cycles. The term w, which is 2 pij will vary directly with the frequency f being The gain will be directly proportional to the frequency from 50 to 5000 cycles by using the constants above-mentioned.

The receiving system that I propose to use is schematically illustrated in Figure 2. The system comprises an antenna l5 for receiving the radio waves. These waves are selected and detected by the combined amplifier and detector unit Hi. The audio amplifier of my system comprises, in addition to amplifier 25, a special stage including a screen grid tube Il having an output resistance l8 and condenser l9. This special stage is adapted to have an output, the gain of which is inversely proportional to the frequency over a frequency range from 50 to 5000 cycles. The particular gain curve is shown in Figure 4.

Tube I1 is of the screen grid type and, in this instance, may have similar characteristics as tube 3 of the sending apparatus. The value of resistance l8 that I have chosen may be 600,000 ohrs. The capacity of condenser I9 is such that the output circuit of tube l1 maintains practically constant impedance throughout the audio band. This capacity has been found in practice to be equal to .064 mfds. This capacity has an impendance of approximately 50,000 ohms at 50 cycles.

The input to the succeeding amplifier tube of amplifier 25, is taken by means of leads 20 and 2| across condenser is. In addition, this output circuit also includes a conventional grid coupling condenser 22, a e'rid'resistor 23 and a. grid bais battery 24. The output of amplifier 25 may be connected in the usual manner to a reproducing device or loud speaker 26..

From the previous circuit diagram, it is apparent that the gain of the special amplifier stage in the receiver, is represented by the formula where .mu is equal to the voltage amplification'of tube l'l, R1) is the internal plate impedance of the tube; r is the external resistance l 8; w is equal to 21r times the frequency being amplified and c is the value of capacity 19.

It will be noted that the input voltage to amplifier 25 is taken across condenser IS in the output circuit of tube II. The voltage drop across condenser IQ is proportional to the output of tube I! as given in the approximate formula above. In the formula, mu, Rp, r and C remain approximately constant throughout the frequency range for 50 to 500 cycles. The term w which is 2 ply will vary directly with the frequency -f being amplified. Now, since the gain of tube l1 varies inversely as the frequency, then as the frequency increases, the voltage drop across condenser l9 will decrease.

Using the constants given in the previous discussion and calculating the gain by means of the formula, we find'that the gain varies from 20 unitsat 50 cycles to 0.2 units at 5000 cycles, or a loss of decibels at 5000 cycles over the 50 cycle level. It will be noted that this is just the reverse of the gain of the transmitting amplifier. It is apparent that the resultant response at the loud speaker will, therefore, be of the form shown by the curve in Figure 5. This is what is commonly wave and any marked attenuation of the socalled side bands eliminates certain portions of these modulating signals.

With my system, I have been able to eliminate the necessity for band passing since a decreased response with frequency is the desired operation of my receiver. Withproper design and sufficient selectivity, it is possible to produce a, receiver which needs no further attenuation or disproportional amplification in the audio frequency end. to. produce the desired response curve, as shown in Figure 5.-

I Such a receiver is schematically illustrated in Figure 8. The circuit comprises a receiving antenna 40, a radio frequency amplifier 4| and a selective circuit 42 which may be designed to attenuate the received modulated carrier wave as shown in the attenuation curve of Figure 9. The receiver also includes the usual detector 43, audio frequency amplifier 44 and loud speaker 45. From the curve of Figure 9, it will be noted that the side bands of the received wave are attenuated in proportion-with the frequency of the.

modulating, signal wave. Although the selective circuit 42 is here shown asa separate circuit for aoomsc pended claims.

3 illustrative purposes, it isto be understood that the selective characteristics may be obtained by proper radio frequency amplifier design and that the provision of any radio frequency amplifier which gives the desired attenuation is clearly within my invention.

It will be apparent by those skilled in the art that means may be employed other than the use of special gain amplifiers for changing the characteristic of the transmitter and receiver. For instance, the 'same results may be accomplished by properly designing .the microphone of the .transmitting apparatus and in the receiver by employing a properly designed loud speaker.

The method in which I apply my invention to the art of sound recording on permanent disc records, is illustrated in Figures 6 and 7. In this instance, only a fragmentary view is shown as the apparatus for recording may be identical to that shown in Figure 1 up to and including amplifier 12. The amplified special output from amplifier i2 is fed to a record-cutting tool 21 which records the sound waves on a' wax record 28 which is rotated by turntable 29 in any conventional manner.

An increasing gain characteristic with frequency of the special amplifier, as previously mentioned, will eliminate the tendency to overcut the record for thelower frequencies which will allow a closer grooving of the record than was heretofore practical. In order to. facilitate a fiat frequency response in the reproducing system, I propose to utilize the special amplifier as shown in Figure 7.

The circuit in this instance, will be similar to that-shown in Figure 2, with the exception that the antenna and receiver amplifier detector unit It is replaced with a disc record 30 and a pickup'device 32 which may be' conveniently connected to the amplifier tube'll by means of a transformer 33. A turntable 3i of any conven tional design may be utilized to rotate the record. The gain characteristic of the special amplifier in the reproducing system, as previously described, varies inversely with the frequency and, therefore, reproduces the recorded sounds with a fiat frequency response as illustrated by the curve in Figure 5. This gives a record of constant amplitude of track wave rather than constant voltage of pick-up as is used at present. Furthermore, record surface noise and needle scratch are reduced without impairing the quality of the recorded sound.

' It isto be understoodwhat the apparatus herein disclosed is clearly illustrative of my invention and constitutes no limitation thereof, as many changes and modifications can be made withoutdeparting from the spirit and scope of the apriations than would be possible with proportionately amplified sound waves, receiving said highly modulated wave and de-modulating the same,. ahdlamplifying said demodulated wave in a an inverse proportion with the frequency for producing a substantially uniform response over the sound frequency range and for substantially eliminating local and static disturbances.

2. A system for the transmission of intelligence comprising means for producing a high frequency carrier wave, an audio frequency amplifier having a rising gain characteristic with frequency for amplifying electrical sound waves to be transmitted, and means for modulating said carrier wave with said electrical sound waves, said amplified electrical sound waves producing a. per cent modulation by the higher frequency portions of said waves higher than is possible to produce with a proportionately amplified electrical sound wave.

3. A system for the transmission and reception of intelligence comprising means for producing a high frequency carrier wave, an audio frequency amplifier having a rising gain characteristic with frequency for amplifying electrical sound waves to be transmitted, means for modulating said carrier wave with said amplified electrical sound waves, said amplified electrical sound waves producing a per cent modulation higher than is possible to produce with a proportionately amplified electrical sound wave, means for receiving and demodulating said waves, means for amplifying the modulating component of said wave in an inverse proportion with the frequency for producing a substantially uniform response over the sound frequency range and for substantially eliminating local and static disturbances.

4. A system for the transmission and reception of intelligence comprising means for producing a high frequency carrier'wave, an audio frequency amplifier having arising gain characteristic with frequency for amplifying electrical sound waves to be transmitted, means for modulating said carrier wave with said amplified electrical sound waves, said amplified electrical sound waves producing a percent modulation by the higher frequency portions of said waves higher than is possible to produce with a proportionately amplified electrical sound wave, means for receiving said wave, means for amplifying the modulating component of said wave in an inverse proportion with the frequency for producing a substantially uniform response over the sound frequency range and for substantiaily eliminating local and static disturbances.

5. In the transmisison of intelligence, the meth od which consists in amplifying electrical variations corresponding to sound'wave frequency variations in direct proportion with the frequency and modulating a high frequency carrier wave by and in accordance with said amplified electrical variations for utilizing said carrier wave to its maximum.

6. In the transmission of intelligence, the method which consists in amplifying electrical variations corresponding to sound wave frequency variations in direct proportion with the frequency and modulating a high frequency carrier wave by and in accordance with said amplified electrical variations for enabling a higher percent modulation by the higher frequency portions of cal variations for enabling a higher percent modulation by the higher frequency portions of said electrical variations than would be possible with proportionately amplified electrical waves of sound frequency, and for eliminating disturbances in the higher frequency range of the sound frequency band.

8. In the transmission of intelligence, the methodwhich consistsin amplifying electrical variations corresponding to sound wave frequency variations in direct proportion with the frequency and modulating ahigh frequency carrier wave by and in accordance with said amplified electrical variations for enabling a higher precent modulation by the higher frequency portions of said electrical variations than would be possible with proportionately amplified electrical waves of sound frequency, receiving said highly modulated wave and de-modulating the same, and amplifying the modulating component of said carrier wave in an inverse proportion with the frequency for producing a substantially uniform response over the sound frequency range and for substantially eliminating local and static disturbances.

'9. In the transmission of intelligence, the method which consists in amplifying electrical variations corresponding to sound wave frequency variations in direct proportion with the frequency and modulating a high frequency carrier wave by and inaccordance with said amplified electrical variations for enabling a higher percent modulation by the higher frequency portions of said electrical variations than would be possible .with proportionately amplified electrical waves of sound frequency, receiving said highly modulated wave and attenuating the modulating side bands of said wave in proportion to the difierence in frequency with the carrier frequency and the side band frequency. 1

10. In the transmission and reception of intelligence, the method which consists in amplifying electrical variations corresponding to sound wave frequency variations in direct proportion with the frequency and modulating a high frequency carrier wave by and in accordancewith said amplified electrical variations for enabling a higher percent modulation by the higher frequency portions of said electrical variations than would be possible with proportionately amplified electrical waves of sound frequency, receiving said highly modulated wave and attenuating the signal portion thereof in proportion to the modulating frequency to produce signal waves of substantially fiat frequency response.

11. In a system for transmitting intelligence, the combination with means for producing a high frequency carrier wave, of an amplifier for amplifying electrical waves corresponding to sound waves to be transmitted, said amplifier having at least one audion tube, said tube having an output circuit, said output circuit having a resistance, a condenser and an inductance in series, and means for modulating said high frequency carrier wave by and in accordance with the voltage variations across said inductance, the constants of said output circuit beingsuch as to produce across said inductance, a rising gain in direct proportion with the frequency for producing a higher percent modulation of said carrier wave by the higher frequency portions of said voltage variations than would be possible with proportionately amplified electrical variations of audio frequency.

' 12. In a system for receiving intelligence,

means for receiving a high frequency carrier wave modulated by and in accordance with waves of audio frequency amplified in direct proportion with frequency, means for de-modulating said carrier wave, an amplifier for amplifying the modulating component of said carrier wave, said amplifier having at least one audion tube, said tube having an output circuit, said output circuit having a resistance and a condenser in series,

and a translating device actuated by voltage variations across said condenser, the constants of said output circuit being such as to produce across said condenser, voltage variations varying in magnitude inversely with the frequency to produce a substantially flat frequency response at said translating device.

STUART W. SEELEY.

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