US2298190A - Suppression of cross talk - Google Patents

Description

,Oct 6, 1942.- B. G. BJoRNsoN AEmu.V 2,298,190

SUPPRESS ION vOF' CROSS -TALK N 2 N e Q a I a v7 :s A:

/NVENroRs-GBJORNSON ATTORNEY Lf VQIISQI /6/ PRIVACY I 45 RADIo 47 /5 Rec I I /8/ I AD PRIVACY I L2 I T L GAIN DECREASCR /6 CHANNEL Na/ L U3 0 Y E/C. 3 L/ 'l /V' 53 ADT* CHANNEL No,/

- l I 55 I 60 BPF A I 1L RADIO 50 TRANS (TWIN CHANELId" 5/ I /2 59 U56 BPI' I I 57 H I CHANNEL No.2 1 H Anp f Y CHANNEL N0./ 2 I 60 Essl T //v Oct. 6, 1942. B G' BJORNSQN. ETAL Y 2,298,190

SUPPRESSION OF CROSS-TALK y Filed May s, 1941 2 sheets-sheet 2 GAIN GAIN Y K DEcREAseR INcREAsgR A A F/G. 2

, VVYY CHANNEL .2

Y B CJo/PNSON A TTORNEY Patented Oct. 6, 1942 "UNITED STATES 'ATE N T O F F I CE SUPPRESSION OF CROSS TALK Application May 3, 1941, Serial No. 391,660

V6 Claims.

The present invention relates to cross-talk reduction between telephone circuits, particularly between the two channels of a twin-channel or twin-single side-band radio system.

In this type of system, of itself known in the art, one telephone .message is sent as the upper side-band and another telephone message is sent as the lower side-band of the same radio frequency wave. Such .a Idouble side-band wave is usually built up by a series of modulation steps involving, in certain stages at least, use 4in common by both messages of modulating and power amplifying apparatus which is liable to give rise to intermodulation products appearing as crosstalk from one channel into the other. n

During normal use of such a system there will be times when speech is being received by both channels sin'iultaneously and other times when speech is being received by one channel while the opposite receiving channel is in condition to receive but is momentarily idle. A working assumption is made that when both channels #are receiving speech, the received speech effectively masks the cross-talk so that no cross-talk reducing means need be used under s-uch condition.

It is a general object of the invention to reduce cross-talk into an idle channel which is in receiving condition.

In the detailed disclosures of this specification several ways of realizing this object in practice will be illustrated and described. This and the various subsidiary objects and features of the invention will be made more apparent in such detailed disclosures and will be particularly pointed out and distinguished in the claims.

Since both terminals of a radio telephone system of the type to which the invention is particularly, though not exclusively, adapted will comprise a radio transmitter and a radio receiver and may in practice b'e Vduplicates of each other, it is deemed sufficient in the drawings to 'show only one terminal in each illustrative embodiment, it being understood that the opposite terminal may be the counterpart of the one shown.

In each of the embodiments for reducing crcssv talk to be disclosed herein, it will be observed that when one channel lis idle andthe other is carrying speech, the idle channel has its loss increased to suppress the cross-talk currents therein. When speech is applied to such idle channel the increase in loss is removed.

In the drawings:

Fig. l is a simplified schematic sketch of the essential lcir-cuit features employed at one twoway terminal of a twin-channel radio telephone if system in accordance with one form of the invention;

Fig. 2 is a similar circuit diagram showing a modification applied to the receiving channels. In this ligure the transmitting channels are not illustrated;

Fig. 3 is a simpliiied schematic circuit diagram of one manner of applying cross-'talk suppression to a transmitter, according to the invention; and

Fig. 4 shows amodied form of one `portion of the-'circuit of Fig. 3.

In each of the figures of the drawings the showing of the relays and control circuits has been simplified. In practice various means known and commonly employed in the art would be used to facilitate operation, provide the necessary timing and hang-over characteristics `and otherwise meet the technical requirements of the particular service conditions.

Fig. 1 shows land lines L1 and L2 connected to transmit out over respective channels III and II leading to twin-channel radio transmitter I2 and to .receive incoming radio communications from twinchannel radio receiver I3 over respective channels III and I5. The transmitting and receiving channel branches are associated with the given line by means of the customary hybrid coil I-I and balancing articial line N. The radio transmitter and radio receiver may be of suitable known construction such as is indicated in Taylor and Wright Patent 2,179,106, November 7, 1939. Each transmitting channel III, II and each receiving channel III, I5 is shown as including a privacy circuit. This may be a frequency inver-ter or a split band privacy or other suitable type such as are known in the art, and the arrangement may follow the more detailed disclosure .of the Taylor and Wright patent.

Each channel branch includes adjacent the land line an amplifier I6, I'I, I8 or I9 (vcgad) whose gain is automatically adjusted to constant output volume by known means such as disclosed in Hogg-Doba Patent 1,853,974, April 12, 1932, or in application of H. L. Barney, Serial No. 370,600, filed December 18, 1940. These are useful where lines L1 and L2 lead to a toll switching point where they may be connected to lines of varying length or where they may carry conversations of weak talkers at some times andstrong talkers at other times.

Receiving channel I4 includes a noise reducer 20 having a control circuit 2|, and, similarly, receiving channel I5 includes a noise reducer 22 with control circuit 23. Each of these may suitably be of the type shown in N. C. Norman Patent 2,063,334, December 8, 1936, especially Figs. 3 and '1. They are used here as in the patent to improve the signal noise ratio by variably amplifying incoming waves in such a way as to depress in the energy scale weak oscillations relative to strong. This function is performed by means of the control circuit which develops from the incoming oscillations a rectified control voltage for determining the gain of a variable gain amplier in the box 20 or 22 such as to provide a large loss to weak oscillations and no loss or even a gain for stronger incoming waves. In the present disclosure the noise reducers areused to suppress cross-talk, as will presently be described.

If speech is incoming on channel I from radio receiver I3 it is likely to be heard as cross-talk on line L1 provided that line is neither transmitting nor receiving speech over its receiving y channel I4. The coupling producing the crosstalk may be anywhere in the system ahead of the noise reducer, but, as noted above, is most likely to be in the distant transmitting station. If apparatus is arranged under these conditions, however, to introduce a high loss into channel I4 at this point, the cross-talk will be effectively suppressed. The manner in which this result is accomplished may be seen from following through a sequence of operations now to be described.

The incoming speech on channel I5 from radio receiver I3 after passing the privacy circuit goes through noise reducer 22, amplifier I8 and into line L2. Relay 24 is in receiving or normal condition with its back contacts closed. Receiving AD (ampliiier-detector) 25 is energized by the received speech and operates relays 26, 21 and 28. Relay 26 disables transmitting AD-29. Relay disables the circuit branch leading from AD-Bll of channel I4 to relay 3| preventing operation of the latter. Relay 28 conditions noise reducer 20 for operation by substituting for normal bias source 32 the large negative battery 33 in series with the bias control resistor 49 of the control circuit 2| (corresponding to resistor 49 of the Norman patent) The effect of relay 28 in shifting its armature is to change circuit 20 from a zero loss circuit to a ZO-decibel loss circuit for weak oscillations of cross-talk level.

If under these conditions, speech from the distant talker with Whom the subscriber on line L1 is communicating should come in via radio receiver I3 and channel I4, some of this speech will pass into the control circuit 2I which will then adjust the circuit 20 to low loss permitting the speech to pass through to line L1. Speech is now being received in both channels I4 and I5 and it is assumed that the speech in each channel is strong enough to mask the cross-talk. When the received speech in channel I4 operates AD-30 disabler relay 34 operates to restore the noise reducer 20 to its normal fixed bias condition by releasing relay 28. If desired, control circuit 2I may be left to function during speech reception, relays 21 and 34 being optional.

Condenser 35 is for the purpose of allowing the loss to be introduced and removed gradually in noise reducer 20 and may be proportioned to provide any desired time constant.

The system is entirely symmetrical so that noise reducer 22 serves in the same way to suppress cross-talk in line L2 when channel I4 is receiving and'channel I5 is in receiving condition but idle.

When a receiving vogad (voice operated gain adjusting device) is used in a multichannel system, cross-talk may cause false operation of the gain increaser circuit of such vogad. To prevent this, a circuit as shown in Fig. 2 may be used in accordance with one feature of this invention. In this gure only the receiving channels I4 and I5 are represented, the corresponding transmitting channels being omitted for simplicity. Also the various switching relays for disabling or enabling respective sides of the circuits have been omitted. Otherwise the system may be considered the same as in Fig. 1 but omitting the noise reducers and inserting the cross-controls now to be described.

The vario-amplifiers or vogads I6 and I8 have been shown more fully in Fig. 2 by indicating in connection with each a gain increaser and a gain decreaser, which, of course, are assumed present in Fig. 1 also. Ahead of gain increaser 40 of vogad I6 is a resistance pad 4I normally shortcircuited by relay 42 in its unenergized condition. Similarly associated with vogad I8 are gain increaser 43, pad 44 and relay 45.

It is a property of the vogad that it does not raise its gain until the incoming waves exceed a certain minimum amplitude, so that it will not readjust its gain in response to noise of lower volume level than speech. The sensitivity is set for a given noise condition so that the proper marginal operation is obtained. The sensitivity can be reduced by inserting loss in the gain increaser input and the sensitivity is increased by removing this loss.

Referring to Fig. 2, if speech is being received in channel I5 from radio terminal I3, cross-talk may be produced in channel I4 under the conditions outlined in the description of Fig. 1. Such cross-talk would have a tendency to cause adjustment of the gain of the vario-amplifier IB, which would mean an increase in its gain since such cross-talk is weaker than normal speech and the vogad normally adjusts its gain to a given level of output corresponding to normal or acceptable receiving level for normal speech. Such tendency toward false operation of the vogad is reduced, however, by operation of AD-41 and relay 42 which inserts pad 4I in the gain increaser circuit of the vogad I6, thereby decreasing its sensitivity and preventing a change of its gain in response to the cross-talk.

Since the circuits are symmetrically arranged, an entirely similar action takes place to prevent false operation of the gain increaser circuit of vogad I8 by cross-talk from channel I4 when channel I5 is idle, by operation of AD-46 and relay 45 to insert pad 44 in the gain increaser circuit by vogad I8.

If desired, disablers (not shown) similar to relays 21 and 34 of Fig. 1 may be added to this circuit each adapted to be energized in parallel with a respective relay 42 or 45 to disable the energizing circuit of the opposite relay 45 or 42, respectively.

Several advantages (as discussed below) can be secured where it is found feasible to suppress the cross-talk .at the transmitter, such as in fairly low frequency systems that permit of separation of the transmitted side-bands by filtering, between the output of the final or power amplifier and the antenna. Systems for cross-talk suppression at the transmitter are illustrated in Figs. 3 and 4.

In Fig. 3 only the transmitting channels I0 and Il are shown, since the receiving channels are not directly involved and may be supplied from the earlier figures. It is assumed that the wavelengths are such as to permit of separating channel No. 1 on the output side of radio transmitter l2 from channel No. 2 by the band-pass filters 53 and 5| inserted between the output side of the transmitter and the radiating antenna 52. Speech in either channel interrupts the other (in the absence of any speech input to the latter) by disabling the corresponding filter. Assuming that the cross-talk products occur in the modulating or amplifying circuits of the transmitter such cross-talk products from an active channel into an idle channel are prevented from being radiated or are at least reduced to harmless level.

For instance if speech is present in the input of channel No, l but no speech is being applied to the input of channel No. 2, A13-53 operates relays 54, 55 and 55, the latter interrupting the circuit through filter I of channel No. 2, so that cross-talk from channel No. 1 is not transmitted. If speech comes on to the input of channel No. 2, AD-El operates relays 58 and 59, the latter restoring the circuit through filter 5l. Relays 54 and 53 are part of the regular vodas equipment. The Xs in the output leads from ADs 53 and 5l indicate disabling points operated from the receiving channels (not shown).

When speech is present in channel No. 2 but channel No. 1 is idle, relay El) is operated from CII What is claimed is:

l. In a 'twin-channel radio telephone system in which respective side-bands of the radiated wave carry different speech messages and in which cross -talk from an active channel to an idle channel occurs, means operated under control of speech-representing waves in an active channel for introducing loss into the opposite channel when idle to'reduce the magnitude of the cross-talk currents therein, and means operated in waves impressed on the latter channel for removing said loss.

AD-5'l (along with relays 58 and 59) disabling channel No. 1 at the input to filter 50.

In Fig. 4 the elements shown may be substituted for those to the right of broken line Y--Y in Fig. 3. Elements 5G and 5l are band suppression lters or they may be tuned circuits, two-terminal or four-terminal, such as ordinary anti-resonant or rejector `circuits to give a requisite degree of suppression of frequencies most effective in producing cross-talk. To secure beneficial results a high degree of suppression may not be necessary in some cases, but a 10-decibel discrimination may vbe very helpful. simple character can be used.

Relays and B are operated in the same manner as in Fig. 3 and when either relay is operated it interrupts a normal short circuit around the -corresponding band suppression circuit thereby in effect inserting the suppression circuit in the corresponding channel.

A suppression of the cross-talk at the transmitting end would have many advantages, the following being the most important, (1) the operation of the cross-talk suppressor would be more positive at the transmitting end than at the receiving end, where the signal amplitudes are influenced by fading, etc.; (2) the ratio between cross-talk amplitude and signal amplitude for a given signal intensity would be approximately constant at the transmitting end, whereas at the receiving end fading introduces variability; (3) cross-talk suppression at the transmitting end is not subject to trouble from locked-out speech; (4) in one operation all the receiving devices, vodas, receiving vogads, etc., would be protected from being unduly influenced by crosstalk; and (5) eificient cross-talk suppression, where possible at the transmitting end, might allow more distortion in the ampliiers.

The invention is not to be construed as limited to the particular circuit arrangements that have been disclosed as illustrative examples, but the scope is dened in the claims, which follow.

Thus, circuits of rather f 2. A system according to claim l in which said channels are receiving channels, said first means comprising a variable loss circuit normally having low loss to received Waves and a conditioning circuit operative to increase the loss under control of received speech in the opposite channel, and said second means comprising a control circuit for decreasing the loss in response to received'waves only in excess of cross-talk level.

3. A system according to claim 1 in which said channels are transmitting channels, said first means comprising frequency selective loss devices each adapted to introduce loss in the frequency band of one respective channel but not the other and a circuit controlled from an active channel for effectively inserting a said device into the opposite channel when idle, and said second means comprising switching means controlled by speech-representing energy in the input of the latter channel for disabling said circuit.

4. in a multichannel telephone system cornprising physically adjacent channels adapted for transmitting different speech messages in the same direction, a variable loss device in each channel, such device having a control circuit for automatically adjusting the loss of the device under control of waves in the respective channel and dependent upon the magnitude of such waves, and means controlled by speech waves in4 either channel for decreasing the sensitivity of the control circuit of the loss device of the opposite channel.

5. In a telephone system comprising physically adjacent channels adapted to transmit separate speech messages in the same direction and to cross-talk into each other, a vario-gain amplifier in each channel having a control circuit normally set to zero loss in the absence of speech in the opposite channel, said control circuit operating to insert gain in the device in response to input waves of greater than cross-talk magnitude, and

means controlled from the opposite channel for inserting loss-in the device in the absence of input. waves of greater than cross-talk magnitude. 6. In a twin-channel, single side-band radio telephone system employing a noise reducer in the receiving circuit of each channel at a terminal of the system, means for reducing the effects of interchannel cross-talk comprising a separate detector controlled by the input to the receiving noise reducer of the respective channel at said terminal, responsive to incoming telephone signals to condition the receiving noise reducer in the other channel for operation.

BJORN G. BJORNSON. HAROLD J. FISHER.

response to speech-representing'

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