US2299390A - Noise suppressor - Google Patents

Description

Oct. 20, 41942. R. s. HOLMES Erm. 2,299,390

NOISE SUPPRESSOR Filed oct; 12, 193s 2 Sheets-Sheet 1 nventors m e v5',

v (ttorneg Mum Oct. 20, 1942. R, s, HoLMEs'l-:TAL 2,299,390

- NOISE sUPPREssoR Filed om. i2, 1939 2 sheets-Sheet 2 e iai/ff cz /PP//va LE VEL l CL PP//VG `Z/LE VEL 1 3u enter Ralph s. Hozne, Circ? C.Mar1fnell Patented Oct. 20, 1942 2,299,390 NOISE SUPPRESSOR Ralph Holmes,

Martinelli, Oaklyn, N.

- Corporation of America,

Ware

Application October 12,

l'5 Claims.

Our invention relates to the suppression of noise in receivers designed for the reception of modulated carrier waves, 'and particularly to-the suppressionof noise in television receivers.

An object of the invention is to provide an improved means for and method of reducing noise in the output of a radio receiver, particularly in either the sound or picture output of a television receiver.

A further object of the invention is to provide an improved noise suppressor circuit in Which the suppressor action is not disturbed by variations in the strength of the incoming signals.

A still further object of the invention is to provide a noise suppressor circuit which removes substantially all the noise signal extending beyond the envelope of a sound modulated carrier wave.

In practicing the invention, a balanced diode circuit is employed which includes a network or filter across which a veloped for opposing current fiow through one of the diodes. This network has a time constant at least fast enough to permit the biasing voltage to vary with variations in the strength of the incoming signals so that the signal to noise ratio will not be less than a predetermined value.

In one of the preferred embodiments of the invention the noise suppressor circuit is applied to the second detector in the sound channel of a television receiver where the pass band of this channel is made much wider than the frequency band of the sound signals. In this application the frequency characteristic of the biasing network preferably is made such that the biasing voltage varies in accordance with the envelope of the modulated carrier wave whereby all noise pulses of greater amplitude than that of the carrier envelope are balanced out. Since these noise pulses are of large amplitude and short duration because of the wide pass band'of the sound channel, the result is that a large partI of the noise signal is removed at'thel second detector and is not passed on to the audio amplifier. 'Ihus at least a one-to-one signal to noise ratio is maintained. l The unusual effectiveness of this embodiment of the invention is based upon the fact that as the frequency pass band of an amplier channel is increased in width, the amplitude of a noise signal or pulse passed thereby is increased while its width or duration is decreased, the energy content of the noise pulse remaining unchanged.

In another embodiment of our invention, the

naaonfiela, and ciro c.

J., assignors to Radio a corporation of Dela- V1939, Serial No. 299,064,

(ci. 17a-5.8)

CII

biasing voltage is denoise suppressor circuit is applied to the picture channel of a television receiver, preferably at the second detector.

The invention will be better understood from the following description taken in connection with the accompanying drawings in which Figure 1 is a block and circuit diagram of a television receiver embodying the invention,

Figure 2 represents a sound signal containing noise pulses as the said signal appears at the second detector in the circuit of Fig. l,

Figure 3 represents the picture and synchronizing signals as they appear at the second detector in the picture channel in circuit of Fig.v 1g and Figure 4 isA a circuit diagram illustrating a modification of the invention. Y

Referring to Fig. 1, there is shown a television receiver ofthe superheterodyne type comprising a first detector and tunable oscillator, represented by the block I, which convert the incoming picture and sound modulated carrier wave to an intermediate frequency picture signal and to an intermediate frequency sound signal. The picture signal is supplied to the picture channel which includes an intermediate frequency amplifier 2, a second detector 3, a video amplier 4 and a cathode ray tube 6. The usual horizontal and vertical deflecting circuits are supplied in the customary Way with horizontal and vertical synchronizing pulses, respectively, through a suitable separating circuit 5 for removing the picture signal and through a suitable filter zontal and vertical synchronizing pulses.

The sound modulated I. F. carrier wave is supphed to the sound channel which includes an I. F. amplifier 1, a second detector Il, an audio amplifier 9 and a loudspeaker II.

From the foregoing it will be seen'that the specific receiver illustrated in Fig. 1 is of the type described in Carlson Reissue April 18, 1938, which is designed to receive picture and sound signals which are transmitted on carrier waves spaced apart by a frequency band of definite width. I

' The second detector 8 is coupled through a turned intermediate frequency transformer I2 to the last I. F. amplier stage of the amplifier 1. The transformer I2 comprises a primary winding I3 and a secondary winding I4 tuned substantially to the mid-band frequency in the usual way by the condensers IS-and I1, respectively. Suflicient damping is provided, as by means'of a resistor I8 connected across the III for separating hori,

20,700, dated rimary winding I3, to make the band-pass gharacteristlc of the tuned transformer, I2 fiat and.v to make the frequency pass band much wider than the frequency band covered bythe v soundsignals..v 'f

'I'he tuned transformers or coup1ing.circuits in thel amplifier stages ofthe ampliiler 1 may be substantially the same as the tuned transformer i2 or they may be any one of several other well known designs which. have a vwide pass bandof the character above indicated.

The second detector 8 comprises a diode 2| and a diode 22, the diode 2| being the detector proper Aand the diode 22 being provided. for the purpose of noise suppression. The cathode ofx one diode is connected to the plate of the Iother diode through a resistor 23 in the specific example shown in 1. Theiunction point of this cathode and the resistor .23 is connected through an output resistor 24 to the electrical mid-point oi' the secondary winding I4, which, in the example shown,is grounded. .The other f electrodes of the diodes 2| and 22 are connected tothe ends of the secondary winding I4.

V'Iheresistor 23 and a condenser 2B connected thereacross are provided for the purpose of applying a variable biasing voltage to the diode .22, this 'bias preferably to vary at substantially the highest sound modulating frequency.. As a rean` amplitude greater than that of the I. F. carrier wave envelope is balanced out whereby the amount of noise energy applied to the audio amplier Qisgreatly reduced r explained with reference to Fig. 2 which shows a sound modulated I. F. carrier wave which has lnoise pulses mixed therewith. Assume, for exl ample, that instead of the RC network 23-26 there is a biasing battery which opposes current flow through the diode 22.` So long as the applied signal has an amplitude below the value v offthis bias voltage the diode 22 is ineffective and the second detector diode 2i functions in the usual way. However, when an incoming signal such as' a noise pulse .exceeds this bias voltage (indicated in Fig. 2 by the' dotted linev32) there is a :How of current through the diodes 2| and 22 which is balanced `out at the outputresistor 24.A 'I'he noise energy represented by this ilow Aof current is dissipated in the circuit 8 and thus does not reach the audio amplifier.

The operation of the circuit with the RC network 23-26 is similar to that .described above except for the important difference that in the preferred embodiment the -frequency characteristic of network 23-26 is made such that the bias producedthereacross will vary with or follow the sound envelope as indicated by the dotted line 33 in Fig. 2. Obviously, such an adjustment of the RC network causes a greater percentage of the noise energy to be dissipated in the circuit 8. By way of example, it may be noted that in a television receiver having a sound channel designed to pass sound having a frequency band lying between approximately 60 cycles and 8000 `cycles and where the sound I. F. channel was designed to pass a frequency band 200,000 cycles wide, the following values were used:

Resistor `23=2o,o0o ohms Condenser `26==1,000 micro-microfarads In this same receiver a .tone control circuit as employed which comprises a resistor 36Y and condenser 31 connected in series and in shunt '(5 suit, that portion of any. noise pulse 'which has 30 The operation of our invention canJ best be35 corresponding to to the RC network 23-2-8. -By decreasing the value of resistor 38 the time constant of the RC biasing circuit is changed whereby the bias mayv be made to,vary slower than the highest modulating frequency. In that case the higher frequency components of the sound signal are not passed on to the audio amplifier. Furthermore,

the amount of noise energy passed on to the audio l amplifier has been still further, reduced. In the example being described, the tone control circuit values were as follows:

' v Resistor 36=100,000 ohms (maximum) Condenser 37 =0;025 microfarad It will be understood that the variable tone control circuit 3G-31 lis not necessary for proper operation of the noise suppressor and. may be omitted. It is described and claimed in the co-f Serial No. 395,346, illed May 27, 1941, in the name of Ralph S. Holmes and assigned to the Radio Corporation of America.

The sound channel may be provided with automatic volume control or AVC in the usual way by utilizing the direct current flow through the detector output resistor 24. The voltage prolduced by this current now is applied through a smoothing filter comprising a resistor. 38 and a condenser 33 to a direct current amplifier 40 which reverses the polarity of the AVC voltage before it is applied to the I. F. ampliiler stages.

In accordance with another embodiment of our invention, an RC biasing network is employed which has a large time constant which may be the same as that employed in AVC circuits whereby the .bias for opposing current ow -in the diode 22 does not follow the sound envelope but, instead, varies only in accordance with the strength of an incoming signal. In this design, the tone control elements 36 and 31 are omitted and, if desired, the iilter elements 38 and 39 may be omitted, the AVC voltage being applied directly from'the junction point of the resistor 23 and the cathode of diode 2| vto the I. F amplifier 1.

Regardless of the-adjustment of the RC biasing network for producing the desired variable bias which opposes current flow through the diode 22 it should be understood that the invention is not so limited.` For example, in` Fig. 4, where parts those in Fig. 1 are indicated by like reference numerals, there is shown a bias producing network which includes an inductance coil 5| in addition to the resistor 23 and the condenser 26.

The use of the inductance coil 5I in the network may be desirable to increase its sharpness of cut-oil at the high frequency and where the network time constant has been made small enough to make the bias follow the sound envelope more or less closely. Unless a tone control adjustment is desired which .will lower the high frequency response, it is preferable that the RC network (23-26-5i, for example) have a 'sharp cut-oil.' Just abo've the highest modulating frecomponent of the suppressor circuit, the

' the RC biasing network quency. For most purposes, the simple RC network such as network 23-26 meets this desirable characteristic closely enough. In certain cases a network consisting of an inductance coil and a condenser only may be desirable, that is, a network as in Fig. 4 with the resistor 23 shorted out.

Referring now to the application of our invention to the picture channel of a television receiver as shown in Fig. 1, the second detector 3 is similar to the sound detector 8 and comprises the detector diode proper indicated at 4| and th'e noise suppressor diode 42. These diodes are connected in series conducting or cathodeto-plate relation and across the tuned band-pass transformer 43. 'Ihe detector output resistor 44 is connected between the electrical mid-point of the secondary of transformer 43 and the junction point of the cathode of one diode and the plate of the other diode.

As in the case of the noise suppressor circuit shown in the sound channel, an RC network comprising a resistor 46 and a condenser 41 is employedv to provide a bias which opposes current flow through the diode 42. work may be located adjacent to the noise suppressor diode Aas previously described or it may be located at another point in the circuit as, for example, between the two halves of the secondary of the transformer 43. The time constant of the biasing network 46-41 is made rather large and such that the bias thereacross will vary at about the same rate as the usual AVC voltage. Forl example, the elementsA 46 and 41 may have th'e values 0.25 microfarad, respectively, in a receiver designed to receive a 441 line interlaced picture with a eld frequency of 60 vertical deflections per second.

one megohm andv put and that it causes the clipping level (or signal level beyond which noise is eliminated) to vary in the desired manner with variations in the strength of the incoming signal. It will also be apparent that we have provided an unusually effective noise suppressor for use in the This RC ne'tsound channel of a television receiver.

Various modiiications of the invention, in addition to those described, may be made without departing from the spirit and scope thereof, and we desire, therefore, that only such limitations shall be imposed thereon as are necessitated by the prior art and are set forth in the appended claims. I

We claim as our invention:

1. In a receiver oi the superheterodyne type for the reception of a carrier wavemodulated by a sound signal, means for converting said modulated carrier wave to 'a modulated intermediate frequency carrierwave, a wide band intermediate frequency amplifier for amplifying said intermediate frequency carrier wave, said ampliiler having a pass band several times the width of the frequency band occupied by said sound signal, noise ,suppressor means for clipping on and dissipating substantially all noise-signal energy which exceeds in amplitude the amplitude v of a biasing or limiting voltage which varies in As illustrated in Fig, 3, the voltage across network 46-41 is equal to the peak value of the synchronizing pulses whereby the clipping level is at this value. Thus all noise energy above this level is dissipated in the detector-suppressor circuit and is not applied to the video amplifier 4. It is assumed that the synchronizing pulses are in the direction of black in the picture, that negative modulation transmission is being employed, and that the direct current picture is being transmitted. peaks of the synchronizing pulses are held at a fixed level, i e., at maximum carrier amplitude, as illustrated. In this embodiment of the invention, as in the embodiment previously described, the I. F. amplifier should be designed with suitable filter time constants in accordance with the teachings of the aboveidentified Martinelli application.

In a television receiver` employing our noise voltage appearing across in the picture channel may be utilized for providing automatic volume control. For example, in the circuit of Fig. 1, the voltage produced across the network 46-41 may be applied through a direct current amplifier 50 to suitable gain control electrodes in the 1 F. amplifier tubes. The amplifier 50 reverses the polarity of the voltage taken off the network 46-41 so that the AVC voltage 'varies with a change in the strength of an incoming signal to decrease the gain of the I. F. ampliiier 1 in response` to an increase in the signal strength,

From the foregoing, it will be apparent that our invention is useful in either a receiver sound channel or picture channel for reducing the In that case, the

amplitude in accordance with the envelope of said modulated intermediate frequency carrier wave, said noise suppressor means comprising a dic-de having a network in series therewith across which said limiting voltage appears, said network comprising a resistor having a capacitor connected thereacross and having a time constant such that said limiting voltage may vary atA approximately the same'rate as the highest frequency component in the said band of frequencies to be pasesd, a reproducing device, and an audio amplier through which the resulting substantially noise-free signal is supplied to said reproducing device.

2. In a receiver of the superheterodyne type for the reception of a carrier wave modulated by a sound signal, means for converting said modulated carrier wave to a modulated intermediate frequency carrier wave, a wide band intermediate frequency amplier foreamplifying said intermediate frequency carrier wave, said amplifler having a, pass band several times the width of the frequency band occupied by said sound signal, a balanced second detector and noisesuppressor circuit coupled to said intermediate frequency amplifier through a coupling network having a winding across which said intermediate frequency signal appears, a pair of diodes connected in series with each other and said winding and in series-conducting relation, an output resistor connected between the electrical midpoint of said winding and the junction point of said diodes, a filter network connected in series with one oi said diodes only for producing a biasing or limiting voltage-opposing current iiow through said one diode, the time constant of said network being such'that said biasing voltage at least approximately follows the envelope of said intermediate frequency signal, and an audio amplifier coupled across 'said output resistor.

3. In a receiver of the superheterodyne type for the reception of a carrier wave modulated by a sound signal, means for converting said modulated carrier wave to a modulated intermediate frequency carrier wave, a wide band intermediate frequency ampliiler for amplifying amount of noise appearing in the .channel out- Q diodes connected in series with each other and said winding and in series-conducting relation.

` an output resistor connected between the electrical midpoint of said winding and the Junction point of said diodes, a network comprising a resistor and a condenser connected in parallel with each otherandin series with one of said diodes only for producing a biasing or limiting voltage-l opposing. current flow through said one diode, the time constant of said network being such y that said biasing voltage at least approximately follows the envelope of said intermediate frequency signal, and an audio amplifier coupled across said output resistor. Y

4. In a television receiver, a picture channel. a balanced circuit in said channel for detection and noise suppression which comprises a pair lof diodes. each having a'plate and a cathode. a

coupling network having which a modulated acrosssaid diodes. said diodes being connected in series with each other and said winding with the plate `oi' oneV diode connected to one end of 'a winding through carrier wave is impressed said winding and withthe cathode of the other diode connected tothe other end of said'windl with veach other andY in ing, an output resistor connected between electrical mid-point of said winding and other cathode and plate of said diodes. and 1 biasing circuit comprisingaresistorandcon denser in'parallelA which is connected in serie with one of'said diodes whereby a bias voltam appears across said resistor which opposes cur rent flow through said one diode. the time constant of said biasing circuit being such that sai: bias voltage varies at substantially the same rati and in accordance with variations in the strengtl: of the incoming signal.

5. In a television receiver for the reception oi picture signals and synchronizing pulses wherein the said'pulses have an amplitude exceeding the amplitude of picture signals of like polarity, a winding across which said signals and pulses appear, a combined detector and noise suppressor circuit which comprises a pair of diodes connected. in series with each other and series-conducting relation, an output resistor lconnected between the electrical midpoint of said' winding and the junction point of said diodes. and a network comprising a resistor and a condenser connected in parallel series with one of said a biasing or limiting current I vthrough said one diodes only 01' Produ voltage opposing RALPH s. Houma. cmo c.

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