US2305931A

US2305931A - Noise limiter

Info

Publication number: US2305931A
Application number: US407739A
Authority: US
Inventors: Ciro C Martinelli
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1941-08-21
Filing date: 1941-08-21
Publication date: 1942-12-22
Anticipated expiration: 1959-12-22

Description

Dec. 22, 1942. Q Q MARTlNELL. 2,305,931

NOISE LIMITER Filed Aug. 21, 1941 IQTDET LI.) 2 1 5 E GATE Q g VOLTAGE u VOLTS p/CTQRE VOLTAGE, V v Zhmentor I ,vous Cum C. MARTmELu I BLACK NOISE I WHITE Gttomeg Patented Dec. 22, 1942 NOISE LDIITER Ciro o. Martinelli, Oaklyn, N. 3., assignor to Radio Corporation of America, a corporation of Delaware Application August 21, 1941, Serial No. 407,739

5 Claims.

My invention relates to noise limiters for radio receivers and, particularly, to limiters applied to the picture channel of a television receiver.

In my copending application Serial No. 324,239, filed March 16, 1940, and entitled "Noise limiters, there is described and claimed a preferred noise limiter for use in a picture channel, this limiter having a fixed limiter or gate voltage and being direct current connected in the picture channel. With a limiter circuit of this character, the noise clipping level remains fixed regardless of signal strength with the result that, for the weaker incoming signals, the clipping level is substantially beyond a 100% modulated signal. As a result, a substantial part of the noise is not removed.

An object of the present invention is to provide an improved noise limiter, especially adapted for use in the picture channel of a television receiver.

A further object of the invention is to provide an improved noise limiter for the picture channel of a television receiver, the limiter being supplied with a limiter or gate voltage which varies with variations in the strength of the incoming signal.

A still further object of the invention is to provide an improved noise limiter for a picture signal channel in a television system of the type employing direct current transmission.

In one of the preferred embodiments of the invention *as applied to a television receiver, a series type noise limiter is coupled to the second detector of the picture channel'and supplied with a gate voltage which is obtained from a peak rectifier coupled to the second detector input circuit. The connection is such that the gate voltage depends upon the peak value of the synchronizing pulses and, therefore, changes inresponse to changes in the signal strength such as those caused by fading or by tuning the receiver to a different television transmitter.

Where the limiter is employed in a picture channeL it is preferred that the picture signal be applied to the limiter through a direct current connection in order that the noise may be clipped off at substantially 100% modulation regardless of changes in the picture background.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which Figure l is a circuit diagram showing one embodiment of the invention applied to a television receiver,

Figures 1a and 1b are curves which illustrate the operation of the noise limiter shown in Fig. 1,

Figure 2 is a circuit diagram illustrating one of the Preferred embodiments of the invention as applied to a television receiver,

Figure 2a is a groupof curves illustrating the operation of the noise limiter shown in Fig. 2.

In the several figures, like parts are designated by the same reference characters.

Referring to Figure 1, the invention is shown applied to a television receiver comprising a first detector and tunable oscillator |0, an I.-F. amplifier II and a second detector I2. The detector '|2 may be coupled to the amplifier through a coupling circuit comprising a tunable primary coil I4 connected to a point on a secondary coil I6 through a coupling condenser II.

In series with the detector diode I2, there is a load resistor |8 which is shunted by a condenser 9 for bypassing the I.-F. carrier wave.

The video signal appears across resistor I8 and is supplied to the noise limiter circuit which comprises a diode 2| having an anode 26 connected through a conductor 20 to the plate of the second detector l2 and having a cathode 22 com nected to ground through a signal output resistor 26.

The noise limiter operates on the same principle as that described in my above-identified copending application, namely, alimiter or gate voltage is applied to the diode 2| with the proper polarity to cause a flow of diode current while the signal voltage is applied to the limiter circuit with the opposite polarity. Thus, the flow of diode current ceases ii the amplitude of noise in the signal voltage exceeds the amplitude of the gate voltage and there is no longer a flow of current through the output resistor 26.

In accordance with a preferred embodiment of my invention, the gate voltage is obtained by means of a peak rectifier circuit comprising a diode 2'Ihaving in series therewith an RC circuit consisting of a resistor 28 and a condenser '29, the diode 21 and the RC circuit having I.-F. signal applied thereto from a coil 3| which is close coupled to the secondary coil IS.

The time constant of the RC circuit '28-'29 is such that the voltage produced thereacross is a D. C. voltage which changes only with changes in signal strength. This is the gate voltage which is applied to the limiter diode 2| through a circuit which may be traced-from the cathode end of resistor 28, through a resistor 23 and a conductor 32 to the anode 240i the limiter diode 2|, through the limiter diode and output resistor 25 to ground, and through ground to the other end or resistor 28. The resistor 23 is provided to prevent condenser 29 from bypassing resistor l8.

The second detector output is applied across the series combination of limiter diode 2| and output resistor by means of the conductor 20 and a condenser 33 connected between the lower end of' resistor I8 and ground, the polarity of the applied signal being that the noise pulses (in the direction of black in the picture) oppose current flow in the diode.

The operation of the above-described limiter circuit is illustrated in Figures 1 and If). It is assumed that the incoming signal for the picture channel is a negatively modulated carrier wave and that the modulating signal is the usual mixture of picture signal, including the picture D. C. component, and synchronizing signal such as defined by' the present R. M. A. standards. The latter signal includes horizontal synchronizing pulses occurring at the end of each scanning line and a slotted vertical synchronizing pulse occurring at the end of each vertical deflection. In Figs. 1a and 1b, the horizontal synchronizing pulses are indicated at 36 and 36', the pulses 36 occurring during a dark picture, and the pulses 36' occurring during a light picture.

The curves of Figs. 1a and 1b represent conditions for a strong incoming signal and for a weak incoming signal, respectively. A comparison of the two figures shows that the limiter or gate voltage appearing across resistor 28 (Fig. 1) is greater for the strongsignal (Fig. 1a) than for the weak signal (Fig. lb). Therefore, more noise is clipped off the Weak signal than would be the case if the gate voltage remained at the value required for the stronger signal.

It will be noted that, since the signal is applied to the limiter in Fig. 1 through an A. C. connection (the condenser 33 breaking the D. C. path), the noise is not clipped off as close to the synchronizing pulses for a dark or black picture as fora light or white one. This disadvantage may be avoided by employing the circuit shown in Fig. 2.

Referring to Fig, 2, the gate voltage which appears across resistor 28 and the signal voltage which appears across resistor I8 are applied in series relation to the limiter diode 2! rather than in parallel relation as in Fig. 1. This is done in the example illustrated by connecting the lower end of resistor I8 to the cathode end of resistor 28 through a conductor 4|, instead of to ground through a condenser as in the first circuit. It will be seen that the signal output of the second detector is now applied to the limiter through a D. C. coupling.

The advantage of the circuit of Fig. 2 is illustrated in Fig. 2a. Since the D. C. component of the picture signal is not lost at the-limiter, the synchronizing pulses for pictures having a dark background will remain at the same level as synchronizing pulses for pictures having a light background. Consequently,'thesame percentage of the noise is removed regardless .of the background component of the transmitted picture. It will be understood, also, that the gate voltage varies with changes in signal strength, just as described in connection with Fig. l, to insure removal of most of the noise for weaker signals as well as for strong signals.

I claim as my invention:

1. A television receiver for the reception of a carrier wave negatively modulated by a composite signal consisting of picture signals and synchronizing pulses, said receiver comprising means for demodulating said received signal to produce picture signals and synchronizing pulses, a signal amplitude limiter which is connected to receive signal from said demodulating means and which comprises a diode and an output resistor connected in series, and peak rectifier means for inserting a direct current voltage in series with said series combination of diode and output resistor with the proper polarity to maintain a fiow of current through said diode until the signal to be limited exceeds a predetermined amplitude and with an amplitude that increases in response to an increase in signal strength.

2. A television receiver for the reception of a carrier wave negatively modulated by a composite signal consisting of picture signals and synchronizing pulses and also modulated by a direct current component of the transmitted picture, said receiver comprising means for demodulating said received signal to produce picture signals and synchronizing pulses, a signal ampli tude limiter which is'direc't current connected to said demodulating means and which comprises a diode and an output resist'or connected in se-. ries, and peak rectifier means for inserting-a direct current voltage in series with said series combination of diode and-output resistor with the proper polarity to maintain a flow ofcurrent through said diode until the signal to be limited exceeds a predetermined amplitude and with an amplitude that increases in response to an in crease in signal strength.

3. In a television system of the type in which the direct current component of the picture is transmitted whereby the carrier wave varies in accordance with the changes in background of the transmitted picture and in which the carrier wave is negatively modulated by the picture signal and by synchronizing pulses, a receiver comprising means for demodulating said transmitted picture signals, and a signal amplitude limiter through which the output of said demodulatin means is led to a utilization circuit, said limiter being direct current connected to said demodulating means and comprising a diode and an output resistor connected in series, means for inserting a direct current voltage in series with said series combinationof diode and output :6- sistor with the proper polarity to maintain a flow of current through said diode until the signal to be limited exceeds a predetermined amplitude and with an amplitude which varies in accordance with the peak amplitude of said synchronizing pulses, and means for applying said signal across the series combination of diode, output resistor and D. C. voltage with such polarity as to oppose current flow through said diode.

4. A television receiver for the reception of a carrier wave negatively modulated by a composite signal consisting of picture signals and synchronizing pulses and also modulated by the direct current component of the transmitted picture, said receiver comprising means for demodulating said received signal to produce picture signals and synchronizing pulses, said demodulating means including an output resistor, and an amplitude limiter circuit comprising a diode, an output resistor and peak rectifier means for providing a direct current voltage in series with said diode and said output resistor which increases in amplitude inresponse to an increase in the peak amplitude of said pulses, said series combination of diode, output resistor and direct current means being direct current connected across the output resistor of said demodulating ite signal consisting of picture signals and synchronizing pulses and also modulated by the direct current component of the transmitted picture, said receiver comprising means for demodulating said received signal to produce-picture signals and synchronizing pulses, said demodulating means including an output resistor, and an amplitude limiter circuit comprising a diode,

an output resistor andpeak rectifier. means for providing a direct current voltage in series with said diode and said output resistor which in.- creases in amplitude in response to an increase in the peak amplitude of said pulses, said series combination, of diode, output resistor and peak rectifier being direct current connected in series fed relation to the output resistor of said demodulating means, said diode being connected in such a direction in the circuit that-the synchronizing pulses appearing across the demodulator output resistor oppose current flowthrough the diode, and said direct current voltage having the proper polarity to maintain a flow of current through the diode so long as the signal appearing across the demodulator output resistor having the polarity of the synchronizing pulses does not exceed a predetermined amplitude.

CD30 0. MARTINELH.