US2379714A

US2379714A - Audio transmission network

Info

Publication number: US2379714A
Application number: US442894A
Authority: US
Inventors: Hollingsworth R Lee
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1942-05-14
Filing date: 1942-05-14
Publication date: 1945-07-03
Anticipated expiration: 1962-07-03

Description

1 July 3, 1945.

R11. HoLLlNGswoRi-H AUDI() TRANSMISSION NETWORK Filed may 14, 1942 2 sheets-sheet 1 k 1 r Rw 1.Y 1 1 1 mw M 1 1 NN n R m m m kN ...m 1 lo 1mm@ u Q 1 1 11 1 1 n H u 1 1E H u U f1 u.. m 1 1 1 WN U Fl 1 f 1 11 ,1 m. H MEW l M MEW MFV 1 1\ 15H. 1 k ww 1 m. .www1 uf w1 1 1 11\ vhwxmwummw\ 1 MN .Q 1 1 1 1) A V 1 1 1 1 k 1 1 1 2S. YNMNMQNM \N\ M 1 EQYNMQ 1 Q 1 1 Patented July 3, 1945 nsrws;

AUmo TRANSMISSION Ns'rwoax mme noumnwnrth, Riverhead N. Y., mima to Radio Corporation of America, a corporation of Delaware Applicaties May 14, 1942, sei-nu No. 442,894-

t 10 Claims.

v My present invention relates to audio transl .f mission networkarand more particularly to a method of, and means for, resinserting generated harmonics to improve narrow band recep- In the transmission of intelligence -over wires or radio, it would be advantageous oftentimes to reduce the band width of transmission, or of reception, thereby to avoid interference and higher audio frequency noises, as for example, tube hiss.

The band of transmission, or of reception, can

be reduced only to a certain degree without impairing the inteliigibility'l of the modulation signal, should the latter be voice or music.

It may. therefore, be stated that it is one of the main objects of my present invention to provide a method of modulation signal transmission wherein the intelligibility of the modulation signais is reduced to a point where the intelligence sought to be transmitted is impaired; a predetermined higher portion of the transmitted modulation signal being subjected to a distortion acteristlc of my invention are set forth with "process thereby to produce harmonics, and the Vharmonics being utilized in the transmitted modulation signal energy to improve, the aforesaid intelligence. It has been experimentally demonstrated that i deiinite improvement of intelligibility can -be had by receiving a long-distance.'short-wave telephone signal .on a' 4 kilocycle (kc.) band in the intermediate frequency.' (I. F.) amplifier; the higher portion of the received band of audio frequencies being distorted to produce harmonics and the resulting harmonics being re-inserted in the audio frequency'lenergy to give considerably improved intelligence and at the same time avoid strong local interfering signals. Standard broadcast station signals, such as are used in vthe 550-1'100 kc.range, are not full range, and they too can be improved by the present method of harmonic distortion and re-insertion thereby to extend the apparent audio frequency range. The method is, also, applicable to phonograph-reproutilizes an audio. ampliner, a connection being made to the ampliiier input to excite an auxiliary Y amplifier, and the output of the, latter beingl passed through a high-pass `filter to excite a generator of distortion harmonics with voltagepr'oduced by the higher audio frequencies; there being utilized a path between the harmonic generator and the audio transmission' network wherebyV to restore to a desired degree portions of the harmonic content of the original audiofrequency signals lost either in transmission orreception.

The novel features which I believe to be charparticularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with-the drawings, in which I have indicated diagrammatically several circuit Vorganizations whereby my invention may be' carried into eii'ect. i A

In the drawings, Fig. 1 shows an embodiment of the invention; Fig. 2 illustratesamodification; Fig. 3 illustrates various characteristics oi the present system. v

Referring now to the accompanying1 drawings, there is shown in Fig. 1 an audio transmission system which embodies an audio'transformerl a0 whose primary winding may be connected to any stood that the invention may be applied to freduction where the space on the recording disc is limited and does not permit ruil range recording. Similarly.. the method may be applied to sound-on-illm recording, where the speed of the iilm will not permit full range recording, as for is to provide anaudio transmission system which 55 voice or music.

quency .bands lying above the'audio range. Furthermore, the modulation signal source itself may be the detector output resistor ot a radiovbroa'dcast receiver operating in the 550 to 1600 kc. band. 0r, ii' desired, the receiver may be a frequency modulation receiver operating in the assigned 4&-50` megacycleband. Furthermore, the modulation may be thedetector output ora iac-` `simile orv television receiver. v Again, the signal source .can be a,` microphone used in recording.

The source ot modulation la'nai energy can be a microphone used in souncl-on-illmA recording,

where the speed of the iilm will not permit full range rtording. For the purpose of speciilcgillustration, let it be assumed that the signal source provides a band of audiov frequencies, such as The secondary winding of the transformer is connected to the input electrodes of an audio frequency amplifier tube 2. The audio ampliner tube is of conventional construction and connections, as those skilled in'the art will readily recognize. The plate circuit of amplifier tube 2 includes the primary winding of the audio transformer 3. The secondary winding of the transformer 3 is connected to any desired modulation utilization network. The nature of the utilization network will 'depend on the system in whichthe amplifier is used. It may be a reproducer of the loudspeaker type in the case of radio reception; or it may be a recording device; or it can be a television reproducer. The numeral I2 designates the output leads to the utilization network.

It is tobe understood that-the network I-2-3 as shown has the various elements thereof constructed and designed to provide a high fidelity audio transmission line. n formers are chosen so that audio frequencies of th'e entire audio frequency range can be transmitted faithfully, Those skilled in the art are fully acquainted with the manner in which such high fidelity audio transmission is secured. To provide the harmonics to be re-inserted into the transmission line, a potentiometer 4i has its re- I .sister connected across the primary winding of transformer I. The amplifier tube 6 has its input electrodes coupled to th'e secondary winding of an audio transformer 5, and the primary winding of the latter is connected across an adjustable portion of the potentiometer resistor.

In other words, the adjustable slider of the potentiometer d is connected to one end 'of the primary winding of transformer E. The plate circuit of ampliiier 8 includes the primary winding of transformerl. The plate output transformer l is peaked by the retardation coil E whereby the .transformer l can assume the characteristics of a high-pass filter. In other words the coil 8,Y which is a choke coil whose inductance can be varied as indicated by the arrow, functions to adjust. the pass band characteristic of transformer I so that a selected portion of the higher audio frequencies can be transmitted to the input electrodes ofthe tube 9.

Tube. 9 is shown as of the screen grid type. The signal grid bias is so chosen, and the magnitude of the vpositive voltage applied to the screen grid and plate is such, that tube 9 functions to distort the audio wave forms applied thereto. The tube can be operated class B or C, or in any other manner which yields rich harmonics. The output voltage developed by tube 9 is rich in harmonics. Itis not believed necessary to describe the construction of the harmonic generator in further detail. Those skilled in the art are fully acquainted with the manner of constructing a tube circuit so as to distort audio waves supplied theretothereby to provide abundant harmonics in the output circuit.

The harmonic energy is fed through the transformer Ill along with th'e fundamental frequencies which were supplied through transformer 1. The combined harmonic energy and fundamental frequencies are fed into the output leads I2. A phase reversing switch II is provided so that the harmonic energy can be fed into the output circuit in proper phase relation. The reversing switch neednot be used, if so desired. It will be realized th'at the fundamental frequencies developed across the outputof transformer I can be filtered out by using an additional high-pass filter between transformer I0 and switch I I. Such 'Ihat is, the trans-- a filter would permit only the harmonic frequen# cies to be fed to the output connections I2.

In order to prevent the harmonics from becom- Ping too strong during high transmission levels, as

would ordinarily be the case in the latter condition of audio amplitude, there is employed a control circuit whose inputleads I3 are connected to the opposite ends of th'e secondary winding of transf former 3. The audio energy. amplified by tube 2,

There will be developed across resistor I6 a direct;l

currentvoltage whose amplitude is dependent upon the percentage modulation, or input signal energy level. In other words, during high amplitude levels of audio transmission, the voltage developed across resistor I6 `will have a greater magnitude.

It is only necessary to provide the direct current lvoltage connection Il to a point on resistor I6 which becomes increasingly negative with respect to the grounded cathode end of the resistor as the transmission amplitude level increases. The end of lead Il connected to resistor I6 may be made adjustable, and the other end of. the lad Ii is connected to the lower end of the secondarywinding oftransformer l. It will now be seen that the signal grid of tube 3, the harmonic generator, is

variably biased in dependence on the value of the voltage developed across resistor I5. If the transmission level at the input terminals of amplifier 2 reaches, or exceeds, a predetermined am'- plitude level, the output of the harmonic genera' tor Q will be decreased.

In Fig. 2 there is shown a modification of theV invention whereinthe modulation signal input energy is applied to the primary winding of an audio transformer IB. The transformer I9 is provided with a pair of secondary windings I9 and 20.

A tube 23 has its signal grid connected to the high side of winding 2t. The low side of winding 20.

is connected by an adjustable tap to a desired point on the bias potentiometer resistor 25. The potentiometer 25 has one end thereof grounded, while the opposite end may be connected to a direct current sourcewhich will provide the negative bias for the grids of

tubes

22 and 23. In shunt with winding section 20 there is arranged a choke coil ZI whose inductance value may be adjusted, as shown by th'e arrow therethough.

The secondary winding I9 has one end thereof connected to the signal grid of tube 22.\ The opposite end of the winding is connected to` a point on resistor 25 which is at a less negative potential relative to the point on the resistor to which winding 20 is connected. Hence, the grid of tube 23 is given a more negative bias than the signal grid of tube 22. The cathodes of

tubes

22 and 23 ae connectedin common through a resistor 30. The midpoint thereof is connected to ground tov provide additional bias for the tubes, and to provide a commonly used hum balance. rIfhe screen grids and plates of

tubes

22 and 23 are connected to the positive terminal of a common energizing source. The plates of the two tubes are arranged in push-pull relation. The audio transformer 2l couples the output circuit to the push-pull connected plates of the

tubes

22 and 23.

The winding 20 is tuned by the retardation coil 2| so as to function as a high-pass filter. This is similar to the function of transformer I with the Ashunt retardation coil 3. Tube' 22 is quencies over 1000l cycles.

. '2,879,714 biased' normally, and it amplifis the signal energy in a distortionless manner. Thetube 23,

. which is more-negatively biased, functions to characteristics of a high fidelity audio amplifier. i In other words. this is the characteristic of the audio transmission system i 23 in Fig. 1, and i8-'22-24 in Fig. 2. Curve B represents the pass band characteristic of a'good I. F. amplifier with a pass band width-not suiliciently broad to convey 'all of speech intelligence. In other words, letitbe assumed that the `audio networks used in either of Figs. 1 or 2 are employed in a radio receiver of the superheterodyne type, wherein the I. F. amplifiers have pass band widths represented by curve B in Fig. 3. Such an I. F. am-

`piiiler is shown to have a pass band width of 4 kc., since some 2000 cycles is passed on either side of the carrier. Obviously, in such case lthe reproducer would produce slightly unintelligible speech orjmusic.

The curve Crepresents the approximate characteristic of transformer l in Fig. 1 as the result of the action of the retardation coil t. It will be seen from this characteristic that there is What I claim is:

l. A method of transmitting a narrow yband of modulation ysignal l frequencies through a transmission network having a relatively wider frequency pass band which includes, deriving from the narrow band of signal energy. those `frequencies located in the upper portion of the narow band, producing harmonic frequencies from said upper portion frequencies, and combining the harmonic frequencies with the aforesaid narrow band signal energy thereby bto improve the intelligibility ofvsaid narrow band signal energy.

2. In combinationwith a high fidelity audio'I amplier network, a high-pass filter having input terminals connected to the input terminals ofI said amplifier. means for producing harmonic frequencies in the output of said high-pass filter,

and means for injecting said harmonic frepassed to the harmonic generator 9 of Fig. l fre- I The frequencies between 1200 cycles at point a and 2000 cycles at point b represent 800 cycles that is fed into the harmonic generating tube 9 to be later reinserted to improve the intelligence impaired due to the narrow band I. F. amplifier characteristic. represents approximately 800 to 1000 cycles amplified, distorted and theharmonics re-insertedi to extend the apparent band pass to about 5000.

In the broadcast band about 2200 cycles would be passed to the harmonic generator to extend therange from 15,000 to 17,000 cycles.

vFor partially restoring the higher harmonic content of standard broadcast band intelligence,

the highfpass filter feeding the harmonic generator tube 9 would have a characteristic similar to curve D of Fig. 3. same characteristics would be used to build up the higher harmonic content in reproduction that is normally lost in phonograph recording, and in sound-on-iilm recording. demonstrated that the present invention is' oi' particular value in rebroadcasting of .foreign Approximately the,`

The range between curves C and D It has been station transmissions, when reduction of the rev ceived bandwidth is desired to a void interiorence, and when the'transmitted signal does not carry sufficient intelligence. 'I'his is the case when a foreign station being received has a poor remote pick-up due to a poor line.

While I have indicated and described several systems for carryingmy invention into effect, it

@will be apparent toone skilledv in the art that .my invention is by no means limited to the particular organization -shown and described, but that many modifications may be made without departing from the'scope of my invention. as set forth in the appended claims. l

quencies into the output of said amplifier.

3. A method of improving the intelligibility of a narrow band of audio frequencies which-includes deriving from the narrow band energy those frequencies located in .the upper portion of thenarrow band, converting the upper portion frequencies into harmonic frequencies, combining the harmonic frequencies with the original narrow band frequencies, and` controlling the' magnitude of the harmonic freuencies in response to the amplitude level of the narrow :band energy.

4. A method of improving the inteiligibility of a narrow band of audio frequencies which includes deriving from the narrow band energy those frequencies located inthe upper portion of the narrow band, converting the upper portion frequencies into harmonic frequencies, separating the produced harmonic frequencies from the selected fundamental frequencies producing same, combining the harmonic frequencies with the original narrow band of frequencies, and controlling the amplitude of the harmonic frequencies in response to the amplitude level of the narrow band energy. v

5. A method of transmittinga narrow band of audio signal frequencies through a transmission network having a relatively wider audio -frequency pass band which includes-the steps of -deriving from the narrow band of audio signal energy those audio frequencies located in the upper portion of the narrow band, producing harmonic frequencies from said upper portion frequencies, separating `the harmonic frequencies, and combining the harmonic frequencies with the aforesaid narrow band signal energy thereby to improve the intelligibilitylof said narrow band signal energy.

6. In combination with a high ndeluy audm amplier network, a high-pass filter having input terminals connected to the input terminals of said amplifier', a. tube circuit for producing `harmonic frequencies in the output of said high.

pass filter, means for injecting said harmonic frequencies into the output of said amplifier, and additional means responsive to the audio s18- nal level for controllingzthe harmonic amplitude.

7. A modulation signal transmission system comprising a main modulation signal amplifier channel, an harmonic channel having input and output terminals, means for applying to said ,harmonic channel input terminals energy from said main channel, a high-passniter in said harmonic channel to produce energy of frequencies in theupper portion of the mein channel fre' quency band, a distortion generator adapted to be fed with the output of said filter, said genery atorproducing harmonics means for connecting said output terminals of the harmonic channel A amplitude.

8. A method of improving the intelligibility of a narrow band of audio frequencies, which ine. A method or transmitting a. predetermined band of frequencies througha transmission net eludes deriving from the narrow band energy such frequencies as are located in the upper portion of the narrow band, converting the upper portion frequencies into harmonic frequencies, combining the harmonic frequencies with the original narrow band frequencies, and controlling the amplitude of the harmonic frequencies, in response to the amplitude level of the narrow band energy. lby rectifying Said narrow band irequency energy and using the rectified energy to control the harmonic conversion.

work lhaving a relatively wider frequency pall Y band which includes, deriving from the bend those frequencies which are located in` the upper portion thereof, producing harmonic frequencies from said upper portion frequencies, and combining the harmonic frequencies with the total predetermined band.

i0. A method of improving the intelligibility of a band of audio frequencies which includes deriving from the band energy those frequencies located in the upper portion of the narrow band,

converting the upper portion frequencies into harmonie frequencies, separating the produced harmonic frequencies from the selected funda-` mental frequencies producing the same, and combining the harmonic frequencies with tiie original narrow hand of frequencies. v

R. LEE HOHJNGBWORTH.