Noise suppression circuit

July 31, 1934.

F. B. LLEWELLYN 1,968,460

NOISE SUPPRESSION CIRCUIT

Filed Dec. 29, 1932

1500 TO 2000

1000 TO 1500

500 TO 1000 too TO 500

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3/ OUTPUT FIG. 8 INVENTOR

F.B.LLEWELLYN

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ATTORNEY

Patented July 31, 1934

1,968,46®

UNITED STATES PATENT OFFICE

1,968,460

NOISE SUPPRESSION CIRCUIT

Frederick B. Llewellyn, Montclair, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York

Application December 29,1932, Serial No. 649,316

Claims. (Cl. 179—78)

This invention relates to the transmission of waves for signaling or similar purposes and relates more especially to the reduction of interference or noise effects in wave transmission sys5 terns.

A general object of the invention is to secure an overall discrimination against the interference energy and in favor of the signal.

In the case of speech and some other forms 10 of signal waves, such as music, which are ordinarily spoken of as comprising a band of frequencies, it is an observed fact that the signal does not at all times contain components of all frequencies within the. band. At some instants 16 the signal may have principal components at only a few frequencies with perhaps less important frequencies also scattered in the band. At another instant the signal may comprise other frequency components but components which 20 likewise do not fill up the whole band. Interference or noise, on the other hand, is often either of a random character as regards frequency distribution or, as in the case of resistance noise, may be substantially uniformly distributed over 25 the frequency band.

A receiver connected to respond to the energy coming in over the entire frequency band will in such a case often receive a sum total of noise energy greater than the signal energy whereas 30 the actual signal components at the instant of time under consideration may have energies greater than that of noise of the same frequencies.

In accordance with the present invention, the 35 receiver does not have impressed upon it at all times the energy coming in over the entire signal band, but is controlled by the signal that is being received at any instant so that the signal and only something less than the total noise energy 40 distributed over the entire band are impressed on the receiver. In particular, selective circuits are provided to prevent noise from being impressed on the receiver in portions of the frequency band not at the moment occupied by important signal 45 components. In this way the signal to noise ratio is increased.

A more complete description of the invention and its various features and objects will be given in connection with the accompanying drawing in 56 which:

Fig. 1 is a schematic circuit diagram of a general manner of carrying out the invention;

Figs. 2 and 3 show graphs representing the principle of operation of the invention; and

Figs. 4 to S, LLJusive, show modified types of relay circuits that may be used in place of those shown in Fig. 1.

As stated above, observation shows that in ordinary speech only a few frequencies are actually present at any one time. For instance, a 60 vowel sound may contain frequencies of 300, 900 and 1200 cycles. In order properly to reproduce this vowel sound it would be necessary to transmit only those three frequencies. On the other hand the random noise energy present in the 65 receiving system is proportional to the total frequency band which is passed by the system. Consequently, an improvement in the signal-to-noise ratio may be obtained if the output of the receiving system is arranged to pass only those ire- 70 quencies which are actually employed in transmitting speech components at the time in which they occur.

A circuit operating in this manner is shown in Fig. 1. An input source of waves is indicated 75 at 10 in the form of a microphone on which speech or music waves may be impressed. Any other suitable source of input waves of a band of frequencies may be used in place of the microphone 10. This microphone or other source is 80 connected to any suitable transmission network 11 which may contain, for example, amplification as indicated. A signaling channel such as a telephone line or any other suitable connection extends from the transmitting network 11 to the 85 network 12 which may be at a distance from the transmitter 11 and is representative of any suitable type of receiving system terminating in a receiving instrument such as the headset 13.

It is assumed for convenience that the waves 90 to be transmitted are comprised in the frequency band extending from 100 to 2000 cycles. The output waves from the transmitting network 11 are divided by means of four band filters 14 to 17 into subbands each about 500 cycles wide, the 95 first band extending from 100 to 500 cycles, the second from 500 to 1000 cycles,’ etc. For simplicity in showing, only four filters are represented although it will become evident as the description proceeds that the more filters which are 100 used and the narrower the band of each filter the greater will be the improvement in the signalto-noise ratio. On the output of each filter is a relay 18, 19, 20 or 21, respectively, which in the absence of signal frequency in the band of the 105 respective filter effectively cuts off the output network 12 from the corresponding filter. In Fig. 1 these relays are indicated as vacuum tubes which are biased slightly below the cut-off point, but these are intended to represent any suitable

1,968,460 type of relay and in later figures various other types of relays suitable for use at this point in the system will be disclosed.

The bias of the relays 18 to 21 is adjusted to 5 such value that residual noise impressed on the filters 14 to 17 is insufficient to overcome the bias and is therefore hot transmitted through any of the relay circuits. The essential speech components passing through any given filter, how10 ever, have sufficient amplitude to overcome the bias on the respective relay circuits so that they are transmitted through the relays and into the output network 12. The final energy in the output network contains only the output of those 15 filters which carry sufficient signal energy to operate corresponding relays. The noise output is only that in the wave bands whose energies actually operate the corresponding relays, whereas the signal is the same as that in the input.

The operation of the system may be made more evident by reference to Figs. 2 and 3. In these figures nine filters are indicated by showing in broken lines their transmission frequency bands. The noise from the input is proportional to the 25 areas of the transmission bands of these filters, that is, to A+B+C+D+E+F+G+H+I. No attempt is made in this diagram to show the actual level of the noise present in the system. It is assumed that the noise is of random character 30 as regards both frequency distribution and energy content. The speech is assumed for purposes of illustration to consist at any one instant of three frequency components, one in each of the transmission bands B, E and H. It will be ₃₅ understood that each of the nine filters A to I, inclusive, has associated with its output a suitable relay, as has been indicated in Fig. 1. The signal component in the instant under consideration operates only the three relays attached to 4Q the filters B, E and H so that the output noise energy is proportional only to the areas B, E and H instead of the areas A to I, inclusive, as would be the case in the absence of the filters and relays. The signal energy, however, is the 45 same in the two cases except, of course, for unavoidable transmission losses. For this example the improvement in the signal-to-noise ratio would be 4.77 decibels.

If thirty filters instead of nine had been used ₅₀ to cover the same overall transmission band, the gain would be 10 decibels for the three-frequency signals assumed. If the signal consisted of only a single frequency the improvement in this latter case would be 14.77 decibels. In general, the 55 gain in decibles is equal to ₁₀ j ___________number of filters___________ number of speech frequencies present.

It will be obvious from the foregoing descrip₆₀ tion that the relays to be suitable for use in the system should preferably be capable of operating very quickly and susceptible of adjustment to a desired bias or operating margin in order to discriminate sharply between noise and sig65 nals. Suitable relay circuits alternative to those shown in Fig. 1 are illustrated in the figures now to be described. In Fig. 4 each of the subdividing filters, such as filter 14 in Fig. 1, has its output connected to the grid circuit of the gas-filled 70 tube 25 the grid of which is normally biased so far negative as by battery 26 that the tube transmits no current when only noise from filter 14 is impressed on its input. The bias is so adjusted that a signal component transmitted through ₇₅ filter 14 drives the grid sufficiently far positive to cause tube 25 to break down and transmit energy into the output circuit 31. The tube 25 is of the type in which the space current is caused principally by gas ionization within the tube, the ' grid serving to initiate the discharge, but there- 80 after having no further control on the discharge current so long as the externally applied plate voltage remains unchanged. In order to enable the grid to have effective control over the space current at all times an ultra-audio frequency 85 generator 27 is coupled to the plate circuit for periodically making the plate positive and negative. When the source 27 drives the plate negative, the discharge through the tube is interrupted and will not resume unless the grid is still 90 being influenced in a positive direction by the signal component in the filter 14.

Fig. 5 is similar to Fig. 4 but employs an amplifier tube 30 in addition to a gas-filled tube circuit 25. When tube 25 is broken down, as 95 by a signal transmitted through coupling 37, space current flows through resistance 28 and a drop of potential is developed across resistance 28 sufficient to overcome the normal negative bias on tube 30 from battery 32 so as to change 100 tube 30 from its normally blocked to its transmitting condition. The speech component present in filter 14 is then allowed to pass through tube 30 to the output 31, this component being impressed on the input of tube 30 through in- 105 put transformer 33. When the speech impulse in filter 14 ceases, tube 25 is restored to its non-transmitting condition so that no potential now appears across resistance 28 and negative bias battery 32 is enabled to block the tube 30. jjg

Fig. 6 is somewhat similar to Fig. 5 but employs a solid type rectifier 36 for controlling the unblocking of amplifier tube 30 instead of using the gas-filled tube for this purpose as in Fig. 5. The signal component from filter 14 is im- 445 pressed through both couplings 33 and 37 on the rectifier circuit 36 and on the grid of tube 30 simultaneously. The resulting rectified current from 36 develops the potential difference across resistance 39 which overcomes the nor- ₁₂₀mal bias due to the battery 32 and changes tube 30 from its normally blocked condition to its transmitting condition so that the signal component is enabled to pass through into the output 31. A smoothing condenser 38 is shunted ₁₂iacross resistance 39.

In Fig. 7 a polarized relay 40 is controlled by the current from rectifier 36 to change amplifier tube 30 from its normally blocked to its transmitting condition. In the normal condi- ,οθ tion the armature of relay 40 maintains a shunt across the grid circuit of tube 30 making that tube inoperative. When a speech component is transmitted through filter 14 it is impressed through both couplings 33 and 37 as in the case ,05 of the previous figures. The portion of the wave impressed through coupling 33 is rectified at 36 and causes relay 40 to shift its armature opening the shunt across the amplifier input so that the impulse transmitted through coupling 37 is allowed to pass through the tube 30 into the output. Relay 40 is shown with an adjustable biasing circuit for facilitating adjustment of the circuit to the desired marginal characteristics. -.Fig. 8 differs from Fig. 7 only in having the ¹armature of relay 40 included in a series circuit with the grid of tube 30 rather than in a shunt across the grid circuit. A peak impulse coming in through filter 14 as in the case of Fig. 7 459

1.868.460 3 causes relay 40 to shift its armature to close the lower contacts thus establishing the grid circuit of tube 30 which is normally open-circuited. The impulse impressed on the grid cir5 cuit through coupling 37 is thus allowed to be transmitted through tube 30 to the output 31.

In each of the relay circuits shown in Figs. 1 and 4 to 8, inclusive, the marginal effect can be readily adjusted by controlling the grid bias 10 or other constants of the circuit including in the case of Figs. 7 and 8 a bias of the relay 40 or other of its constants.

It will be understood that the invention is not dependent upon the use of any particular 15 type of relay circuit, but that any suitable type may be used, several different types having been shown by way of illustration in the various figures. The invention is not to be construed as limited to the specific form shown, nor by 20 the frequencies or other quantities referred to, but only by the scope of the claims.