US2215483A - Carrier wave telephony system - Google Patents

Description

Swt. 24, 1940.

T. S. SKILLMAN CARRIER WAVE TELEPHONY SYSTEM Filed Dec. 17, 1938 3L 6 1101 25 3P5 if:

5 n 10 if L 1! GENERATOR fwbwru? TlSISkz'ZZm/z -Patented Sept. 24, 1940 UNITED STATES oaaamn WAVE mmrnoiw SYSTEM 1 Thomas Samuel -Skillman, Eindhoven, Netherl b, asaignor, by mesne cuts, to Hartford National Bank and Trust Company, Hartford, Conm, as trustee Application December 11, 1938, Serial N... 246,459

, p In Germany December 27, 1937 2 Claims. (o 119-1) My invention relates to a signal-receiver for carrier wave telephony systems.

In carrier-wave telephony systems it is known to transmit signals, such as calling currents or 5 selecting impulses, by transmitting the carrierj wave which is suppressed during transmission of the conversation. These signals are received on the receiving side by a signal receiver which 1' a comprises an oscillatory circuit tuned to the carrier-wave frequency and across which the carrier-wave produces a voltage which, after rectification, controls a relay operating the signalcurrent circuit. 4

To prevent false operation of the signal-current circuit during transmission of the conversation, the oscillatory circuit must have such a high selectivity that only the carrier-wave fre-' quency, but not. the side-band frequencies occurring during transmission of the conversation, sets up a voltage at the oscillatory circuit. As

the lowest audio-frequency to be transmitted is about 200 cycles per sec., the corresponding.

' side-band frequency lies at a distance of only about 200 cycles per see. from the carrier-wave. The higher the carrier-wave frequency, the more diificult his to provide an oscillatory circuit which is capable of distinguishing between two frequencies which differ by only 200 cycles per six The object of the present invention is to overcome the above difliculty, and for this purpose I superpose on the carrier wave emitted during the signal transmission and in the signal-receiver, an auxiliary frequency which differs only slightly from the carrier-wave frequency. The difference in frequency so produced is sieved out by means of a filter or an oscillatory circuit, is rectified, and the rectified current is used to control the relay which controls the signal-current circuit.

As the auxiliary frequency can be made only.

slightly different from. the carrier-wave frequency, the difference-frequency produced is low,

and it is possible without additional means to increase the selectivity of the oscillatory circuit or filter tuned to this difierence frequency to such a degree that the lowest side-band frequency, which differs only by about 200 cycles per sec., does not cause undesirable responses in the signal-current circuit.

The auxiliary frequency is preferably taken from a carrier-wave generatorof another channel, for instance a generator whose frequency differs, for instance by about 8 kilocycles per sec. from the received carrier-wave frequency.

In this case the difference-frequency is also 8 kilocycles per second.

I am aware of the fact that in known signalreceivers including'an oscillatory circuit tuned to the carrier-wave, it has been proposed to avoid 5 undesirable signal responses by making the carrierwave amplitude many times larger than the disturbing side-band frequencies. However, this measure has the drawback that the transmitting, receiving and intermediate amplifiers may 10 be over-controlled, thereby producing non-linear 'cross talk and noises in the other channels of the carrier-wave system. When using the signal I receiver according to the invention this drawback does not occur, because the carrier-wave amplil5 tude is not greater than'the average side-band amplitude need be during the transmission of the conversation.

' In order that the invention may be clearly understood and readily carried into effect, I shall 20 describe the same in more detail with reference to the accompanying drawing, in which the single figure is a schematic diagram of a portion of a carrier-wave telephone system comprising a signal receiver according to the invention. 25

In the system shown in the drawing, in which the signals are transmitted by the emission of a carrier wave which is suppressed during the transmission of conversation, the conversation to be transmitted is received through the lines l 30 and is supplied in the usual manner through a differential transformer 2 with a diagrammatically-indicated balancing network 3, to a modulator 4. The speech currents are modulated by modulator 4 on a carrier-wave of a frequency 35 j, which is produced by a high-frequency generator i.

Modulator 4 is so designed that the carrierwave produced by generator 5 is suppressed, and that there will be only two side-bands available 40 in the output circuit thereof. After passing through band pass- filters 45 and 6 one of these side-bands is suppressed and the other side-band is supplied to a transmission line I. Additional similar transmission channels may of course be 45 connected to line I.

For signaling there is provided a relay 8 which has its coil connected through a single-pole switch 9 to a suitable voltage supply (not shown), and has two single-throw, normally-closed 50' armatures l0 and II adaptedto engage fixed contacts l2 and I3 respectively. The heels of armatures II and II are connected to the terminals of generator 5, and the contacts I: and ii are connected to conductors l4 and I! which inter- 55 connect the band pass filters 45 and 6. Thus, by closing and opening switch 9, call or selective impulses supplied by generator 5 are impressed upon the line I.

The carrier-wave conversation received through line I6 is separated from that of the other channels (not shown) by means of bandpass filters ll and I8 and is demodulated in a demodulator 20. A carrier-wave of frequency f1, produced by a high-frequency generator I9 is added. The demodulator current is amplified by a valve 2| and the amplified current, after being transformed by a transformer 22, is supplied as low-frequency speech current to line Controlled by the anode current of tube 2| is a relay 31 having a single-throw normally-closed armature 38 which controls a signal circuit 39.

The signal impulses having the carrier frequency f1, which come in over line l6, are supplied through conductors 23 and 24 and a. transformer 25 to the grid circuit of a triode 26, which also comprises a condenser 21 shunted by a leakage resistance 28.

Coupled to the anode circuit of tube 26 through a transformer 32 is a mixing circuit comprising a rectifier 29, an auxiliary high-frequency generator 30 of a frequency f2, and the primary winding of a transformer 3|. Connected across the secondary winding of transformer 3| is an oscillatory circuit comprising a condenser 33 and an inductance coil 34, which circuit, in accordance with the invention, is tuned to a frequency is equal to the difference between the frequencies of generators 30 and I9, i. e., f3=f2j1.

The voltage set up across inductance coil 34 is rectified by a rectifier 35 and applied across a resistance 36 in the grid circuit of tube 2|. The voltage drop thereby produced in the grid circuit of tube 2| is in such a direction that the anode current of the tube is greatly suppressed or reduced to zero. As a result, relay 31 is deenergized and a signal impulse is produced in a signal circuit 39.

As referred to above the auxiliary frequency {2 is preferably taken from a carrier generator of another channel for instance from the generator 5 of the sending channel. In this case it is only necessary to remove the generator 30 and to connect the mixing circuit comprising the rectifier 29 to the generator 5.

While I have described my invention with reference to a specific example, I do not wish to be limited thereto but desire the appended claims to be construed as broadly as permissible in view of the prior art.

What I claim is:

1. In a carrier-wave telephony system having a main. channel for transmitting suppressed carrier speech-modulated waves from a transmitter, a receiver connected to said transmitter by a transmission line and comprising a demodulator, a signal circuit, a side channel derived from said main channel at a point before the demodulator and responsive to additional signals transmitted by transmission of the carrier frequency which is suppressed during the transmission of said speech-modulated waves, means for selecting said carrier frequency, means for superposing on said carrier frequency an auxiliary oscillation. of a frequency slightly different from the carrier frequency to produce a difference frequency, means for filtering out the difference frequency, and means to control the signal circuit by said difference frequency.

2. In a carrier-wave telephony system having a main channel for transmitting suppressed carrier speech-modulated waves from a transmitter, a receiver connected to said transmitter by a transmission line and comprising a demodulator, a signal circuit, a side channel derived from said main channel at a point before the demodulator and responsive to additional signals transmitted by transmission of the carrier frequency which is suppressed during the transmission of said speechmodulated waves, means for selecting said carrier frequency, means for superposing on said carrier frequency an auxiliary oscillation of a frequency slightly different from the carrier frequency to produce a difference frequency, said latter means including a carrier-wave generator of another channel of the system, means for filtering out the difference frequency, and means to control the signal circuit by said difference frequency.

THOMAS SAMUEL SKILLMAN.

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