US 2304713 A
Description (OCR text may contain errors)
Dec. 8, 1942. V2,304,713
NOISE REDUCTION IN ELEoTIc TRANSMISSION SISTEM v D. B. ,SMITH y Filed Aug; 19, 1939 H @HP Ulti @ PATENT NHSE RWUCTION IN'EHECTRIQ TRNSMSSEON SYSTEMS David Smith, Philadelphia, Pa., assigner Philco Corporation Application August 19, 1939, Serial No. 291,09? En Great Britain AWC 23 1938 laima This invention relates to noise reduction in electric transmission systems and has for its general object to provide improved means for limiting the amplitude, steepness of wave front, and energy content of interfering noise signals and/or pulses in communication or control systems and the like.
The invention is particularly adapted for use in the synchronizing signal circuits of television and facsimile receivers. The noise limiting means disclosed herein are of particular importance when the desired synchronizing signals are relatively weak and where they are accompanied by erratically spaced short, sharp, noise pulses of an amplitude comparable to or greater than the amplitude of the synchronizing signals.
As is well known, in modern electronic television systems, means are normally provided at the receiver for generating the required horizontal and vertical scanning waves. These waves are generally of a sawtooth shape. Synchronism between the scanning operation at the transmitter and at the receiver is usually maintained by means of suitably tiined and spaced synchronizing signals which are transmitted withthe video signal, as is well known in the art. It has been found, however, that operation of the scanning wave generators may be initiated upon the arrival of ordinary noise or static pulses, thereby destroying the desired synchronism between the scanning operations at the receiver and transmitter. Under these conditions it has been necessary, in the past, to employ relatively insensitive circuits and to operate the receiver within a restricted radius from the transmitter where the desired signals are capable of overriding the undesired noise signals. By means of the present invention, however, it is possible to secure satisfactory television reception even in areas where the noise signal amplitude is considerably greater than the amplitude of the desired signals.
1t has been found that a majority of the noise signals commonly encountered inthe reception of carrier wave signals are ni short duration as compared to the shorter periods of the modulation envelope, but they are of considerable amplitude as compared to the maximum amplitude o that envelope. Thus noise pulses produced by atmospherics, motor car ignition systems, the interruption of electrical circuits generally, and the like, may often have an effective duration of considerably less than a microsecond; yet because of their high amplitude suchshort pulses still possess a high energy content and when impressed on signalling circuits may give rise to transients of considerable magnitude.
According to the present invention there is provided in a television receiver or the like, a source of signals, wide-band signal transfer means, signal detector means, amplitude responsive means for deriving a synchronizing signal from the detected signal, a video signal channel, a synchronizing signal channel,amplitude limiting means disposed in said last-named channel for rendering said channel unresponsive to voltages exceeding a predetermined level, narrowband signal transfer means, and a synchronizing signal utilization means.
Accordingly it is an important object of this invention to provide means for preventingl the appearance oi undesired noise or static pulses in the various synchronizing circuits and deilecting wave generators employed in television receiving systems and the like.
Another object of this invention is to provide an amplitude limiting means preceded by a wideband signal transfer means and followed by a narrow-band signal transfer means for limiting the amplitude of undesired noise pulses to a value substantially less than that of the desired signals, and to provide thereafter an amplitude responsive device for completely removing said `undesired pulses from said desired signals.
'I'he invention may best be understood by reference to the accompanying drawing, the single ligure of which is representative of the invention as employed in a conventional superheterodyne type television receiver.
In the drawing, the rectangle l represents the carrier frequency portion of a superheterodyne television receiver. In accordance with the inventionf the radio frequency and intermediate frequency amplifier circuits contained therein should be capable of passing a Wide-band of frequencies, not less than two and preferably three or more megacycles in width. The provision of such a wide-band system permits the passage of short, sharp, high amplitude noise pulses, with ical expression. Accordingly the invention is not to be limited to the specific disclosure, but only by the scope of the kappended claims.
1. In a noise limiting circuit, a modulated carrier wave source, said carrier Wave having modulated vthereon at least one desired signal, band-pass iilter means connected to said source for transferring said Waves, said filter being capable of passing a band of frequencies much wider than is required to pass said desired signals, signal detector means for demodulating said carrier wave, amplitude limiting means arranged to receive the demodulated signal from said detector and adapted to limit the amplitude of noise voltages accompanying the demodulated signal to substantially the amplitude oi the desired portion o! said signal, low-pass lter means following said amplitude limiting means and having a cut-off frequency just high enough to pass the most important frequency components of said desired signal, whereby the noise voltages are greatly attenuated, and an amplitude responsive transfer means adjusted to transfer only a predetermined and selected portion of the signal passing through said low-pass filter. thereby substantially to eliminate the attenuated noise voltages.
2. A noise limiting circuit in accordance with claim 1, characterized in that said amplitude responsive transfer means is adjusted to transfer only those portions or the desired signal which extend above the average noise level.
3. In a television receiver employing wideband signal transfer means, a signal detector, a video signal channel and a picture reconstituting device coupled to the output of said detector,
the combination of selector means for deriving synchronizing signals from the .output of said detector, amplitude limiting means coupled to said selector means for reducing the amplitude of disturbance signals to a predetermined amplitude level corresponding substantially to the maximum amplitude of the synchronizing signals, low-pass iilter means coupled to said amplitude limiting means for greatly attenuating the disturbance signals without detrimentally aiecting the synchronizing signals, whereby the` disturbance signals by virtue of their greater high frequency content are greatly reduced in amplitude relative to the synchronizing signals, means coupled to said lter means for generating deflecting Voltage waves in synchronism with the synchronizing signals, and means for applying said waves to said picture reconstituting device.
4. In a television receiver employing wideband signal transfer means, a signal detector, a video signal channel and a picture reconstituting device coupled to the output of said detector, the combination of selector means for deriving synchronizing signals from the output of said detector, amplitude limiting means coupled to said selector means for reducingv the amplitude of disturbance signals to a predetermined amplitude level corresponding substantially to the maximum amplitude of the synchronizing signals, low-pass lter means coupled to said amplitude limiting means for greatly attenuating the disturbance signals without detriment-.ally affecting the synchronizing signals, whereby the disturbance signals by virtue of their greater high irequency content are greatly reduced in amplitude relative to the synchronizing signals, amplituderesponsive mea'ns coupled to said lter means for selecting the synchronizing signals substantially to the exclusion of the disturbance signals, means coupled to said last-named means ifor generating deflecting voltage waves in synchronism with the synchronizing signals, and means for applying said waves to said picture reconstitutingv device.
5. A television receiver in accordance with claim 4, characterized in that said selector means and said amplitude-responsive means eacl.l includes a vacuum tube biased below plate current cut-oil, whereby only voltages exceeding a predetermined amplitude'level are transferred by the tube, Vand said amplitude limiting means comprises a vacuum tube diode in shunt with the synchronizing signal channel and biased so as to be conductive only to signal voltages exceed- DAVID B. SMITH.