US2515619A - Device for stereophonic transmission of signals by electric means - Google Patents

Description

July 18, 1950 T. J. WEYERS DEVICE FOR STEREOPHONIC TRANSMISSION OF SIGNALS BY ELECTRIC MEANS Filed July 20, 1946 5015000 zaoao 1410a AI/XIZMKY 44mm VIEW/[0H0 50-1600 new "a."

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James A GENII Patented July 18, 1950 DEVICE son STEREO-PHONIC TRANSMIS-' sIoNor SIGNALS BY ELECTRIC MEANS TheodorusJosephus Weyers, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn, as trustee Application July 20, 1946, Serial No. 685,121 In the Netherlands April 21. 1943 Section 1, Public Law 690, August 8, 1946 Patent expires April 21, 1963 This invention relates to a device for stereophonic transmission by electrical means with the use of a common channel for the two stereophonic signals.

It has been proposed in I phonic transmission to transmit the two different sound signals via a common channel and one of the sound signals as the lower sideband of an auxiliary carrier wave. In this case the frequency of the auxiliary carrier wave may be chosen so as to be so high that the lowest frequency of the lower sideband' exceeds the highest frequency of the other sound signal. It has also been proposed to transmit-the two stereophonic signals as the two sidebands of the carrier'wave.

connection with stereo- The great disadvantage of these two methods is that with the transmission of two different sound signals the width of the channel is more than double the bandwidth of the spectrum of one sound signal.

The invention has for'its object to reduce the width of the channel for the transmission of stereo-phonic signals. According to the invention, this object is attained -'by the suppression of the highest and the lowest frequencies of one 1.

signal and by the subsequent transmission of this signal as the lower sideband of an auxiliary carrier wave.

The invention is based on recognition of the fact that for the s-tereophonic transmission of signals it is not essential to transmit stereophonically the highest and the lowest acoustic frequencies.

In order that the invention-may beclearly understood and readily carried into efiect,,it will now be explained more fully .with reference to the accompanying drawing, in which:

Figs. 1a and 1b are graphs illustrative of the theory underlying the invention,

Fig. 2 shows schematically a pref'erredw embodiment of a receiving circuit ,in accordance with the invention, and

Fig. 3 is a block diagram of a transmitting and receiving systemsinziaccordance the invention. w

Fig. 1 makes it clear .that the aforesaid-method permits .of reducing the total width of the -common channel. If use ismade-sof an "auxiliary carrier wave having a frequency :of 35,000 c./s. and if the frequency-spectrum 'of' the twosound signals extends from 50 to 15,000 c./s., the channel shown in Fig 1a is set up. The band (1 represents the frequency-spectrum of one signal, the bands I) and b1 the lower and the upper sidebands of the auxiliary carrier wave modulated 3 Claims. (01. 179-1) 2 by the other signal. If the upper sideband is suppressed, the total width B of the common channel for the stereophonic reproduction is, consequently, about 35,000 c./s. If, however, according to the invention, the highest and lowest frequencies of the signal transmitted as the lower sideband are suppressed, for instance the frequencies above 5000 and below 300 c./s., a width of the common channel of about 25,000 'c./s. will suffice, as is shown in Fig. 11), again under the assumption that the upper sideband is suppressed. The spacing between the bands a and b (viz 5000 c./s.) is chosen to be as large as in the case of Fig. 1a,. An additional advantage of the device according to the invention is further that the selectivity requirements of the filter by means of which the upper sideband is suppressed need not he so severe, the frequency-difference between the highest frequency of the lower sideband and the lowest frequency to be suppressed of the upper sideband having increased materially, viz. flOl'i'l c./s. in the case of Fig. 1a to 600 as. In transmitting, the common channel', which consequently contains the frequencyband: of one signal and the lower sideband of the auxiliary carrier wave modulated by the other signal as well as this auxiliary carrier wave frequency, may be modulated on the carrier wave of a transmitter. This modulation of the carrier wave by the channel may be effected in amplitude equally well as in frequency. In order to secure a greater modulation depth, it may be desirable with amplitude or frequency modulation to reduce the amplitude of the auxiliary carrier wave. This can be secured by means of the same filter as that which. suppresses the upper sideband. The frequency of the auxiliary carrier waveto be reduced in amplitude in the device according to the invention exceeding the highest frequency of the lower sideband by 300 c./s. insteadflof by 50 c./s. the selectivity requirement-of the filter need not be particularly severe. The stereophonic signalstransmitted by means of a' device according to the invention can be reproduced by means of a device as shown in Fig. 2. The latter comprises two channels, one channel I for the reproduction of the signal I) transmitted as the lower sideband of the auxiliary carrier wave (Fig. 1b) and the other channel II for the reproduction of the signal a. The electrical oscillations received are rectified by a detector device I which comprises an oscillatory circuit 2 tuned to the carrier wave of the transmitter and are fed via a potentiometer 3 to the two channels of the receiving device. The channel II is constituted by a low-frequency amplifying device comprising two amplifying tubes 4 and 5. A loudspeaker 6 is provided on the output side. The low-frequency oscillations from 50 to 15,000 c./s. are amplified in this channel. The electrical oscillations having higher frequencies are consequently suppressed in this channel. In the channel I the oscillations are supplied via an amplifying tube 7 to a selective oscillatory circuit 8 tuned to the auxiliary carrier wave. Provision is made for this oscillatory circuit with the object of increasing the amplitude of the auxiliary carrier wave, as is essential for a distortion-free detection of the one-sideband signal. Though now the high frequencies of the lower sideband are more amplified than the lower frequencies, this may be compensated for by a subsequent correcting network 9. The oscillatory circuit 8 is followed by a detector l comprising a potentiometer H, from which the low-frequency signal whose lowest frequencies (below 300 c./s.) and highest frequencies (above 5000 c./s.) fail, can be obtained. In so far as the other signal is not suppressed by the oscillatory circuit (whose highest frequency is lower by 5000 c./s. than the lowest frequency of the lower sideband and differs from the resonance frequency of the oscillatory circuit to such an extent that this other signal is substantially suppressed) provision maybe made for a filter 12 following the amplifying tube 15 to suppress the said oscillations, having now a frequency from 10,000 to 24,950 c./s. Finally, the output amplifying tube 13 is followed by a loudspeaker M which reproduces the signal whose highest and lowest frequencies fail. These frequencies failing in the signal reproduced by the channel I, it is desirable, in order to secure the correct intensity ratio between the low and high frequencies and the medium frequency in the total sound reproduction, to amplify the lowest and the highest frequencies of the other signal supplementarily. This may even occur during the transmission of the two signals, but it is also possible to construct the channel II in such manner that in it the highest and the lowest frequencies are supplementarily amplified.

Apparatus for carrying out the stereophonic transmitting technique set forth in connection with Fig. 1b is illustrated in Fig. 3 wherein blocks and 2| represent signal sources a and 5, respectively, having a like spectrum of 50 to 15,000 cycles. The frequencies of signal 1) below 300 and above 5000 cycles are suppressed by a bandpass filter 22, and the resultant 300-5000 cycle band is combined in a modulator v23 with a 25,000 cycle oscillation from an auxiliary carrier source 24. The upper side band (25,300-30,000 cycles) yielded by modulator 23 is discriminated against by a filter 25 which accommodates only the lower side band (20,000 to 24,700 cycles) and the 25,000 cycle auxiliary carrier. The output of filter 25 and the output of signal source 20 are fed to a modulator 26 where they are imposed on a main carrier from a source 21 and then transmitted by means of a transmitter 28.

What I claim is:

1. Apparatus for transmitting via a common channel first and second stereophonic sound signals having the same frequency spectrum com- 4 prising means for suppressing the upper and lower portions in the frequency spectrum of said first sound signals, an auxiliary carrier wave source, means for modulating said auxiliary carrier wave with the remaining first sound signals to produce output signals having upper and lower sidebands, means for suppressing the upper sideband in said output signals, and means for applying said second sound signals and the remaining output signals as an input to said common channel, the frequency of said auxiliary carrier wave having a value such that said remaining output signals are spaced from said second sound signals in said common channel.

2. Apparatus for transmitting via a common channel first and second stereophonic sound signals having the same frequency spectrum comprising means for suppressing the upper and lower portions in the frequency spectrum of said first sound signals, an auxiliary carrier wave source, means for modulating said auxiliary carrier wave withthe remaining first sound signals to produce output signals having upper and lower sidebands and an auxiliary carrier wave component, means for suppressing said upper sideband and partially suppressing said carrier wave component in said output signals, and means for applying said second sound signals and the remaining output signals as an input to said common channel, the frequency of said auxiliary carrier wave having a value such that said remaining output signals are spaced from said second sound signals in said common channel.

3. A stereophonic system comprising a carrier wave source, apparatus for transmitting via a common channel first and second stereophonic sound signals having the same frequency spectrum including means for suppressing the upper and lower portions in the frequency spectrum of said'first sound signals, an auxiliary carrier wave source, means for modulating said auxiliary carrier wave with the remaining first sound signals to produce output signals having upper and lower sidebands, means for suppressing the upper sideband in said output signals and means for applying said second sound signals and the res maining output signals as an input to said common channel, the frequency of said auxiliary carrier wave having a value such that said remaining output signals-are spaced from said second sound signals in said common channel, and means for modulating said carrier wave with the signals appearing in said common channel.

' THEODORUS JOSEPHUS W'EYERS.

REFERENCES CITED The following references are of record in the file of this patent; 7

UNITED STATES PATENTS.

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