US2448635A

US2448635A - Echo reducing circuit for television receivers

Info

Publication number: US2448635A
Application number: US585688A
Authority: US
Inventors: Smith James Ernest
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1945-03-30
Filing date: 1945-03-30
Publication date: 1948-09-07
Anticipated expiration: 1965-09-07

Description

Sept. 7, 1948. J. E. SMITH 2,448,635

ECHO REDUCING CIRCUIT FOR TELEVISION RECEIVERS Filed March 50, 1945 I l imm-fo, /2 .s/e/m l 015i P I, x I/ mi2 H f== Y Il 13|- INVENTOR. JA MES f. `)3v/H71'.

I I L, l l l I n ATTORNEY Patented Sept. 7, 1948 `,NT-TED STATES PATENT OFFICE ECI-I REDUCING CIRCUIT FOR TELEVISION .RECEIVEBS (c1. 25u-ao) l 1 Claim.

The present'invention relates to television systems, and moreparticularly to means for effectively eliminating in the reproduced image distortion caused by multipath reception.

In certain localities, especially urban areas, a received television image is often distorted by ghost patterns or echoes produced by reflections from buildings or other large objects having dielectric constants which differ radically from that of free space. if the path of the reflected signal is not appreciably greater than that of the direct signal7 the only effect observed will be a slight blurring of the outlines oi the reproduced image. This might be regarded as much the same general appearance in the final image as an absence of the higher signal frequency components. l-Iowever, ii there isian appreciable difference in the lengths of the two paths, separate images often appear on the screen of the reproducing device. Such images are usually termed ghosts YThey be at times either negatives or positives. y

in certain instances, the objectionable eects oi multipath reception may be eliminated bythe use of directional antennas. Such an expedient has proven only partially successful, however, since the reflected signal often arrives at thereceiving antenna from approximately the same clirection as lthe desired signal. Moreover, the orientation of such a directional antenna limits its reception to transmission from a single station. Ii programs are desired from any of severa. stations within the range of the receiver, a compromise adjustment must be found or else a rotatable antenna must be employed.

has been found possible to employ two nondirective antennas to reduce the secondary path signal these two antennas are connected to their respective receivers over a pair of transmission lines the lengths of which are so determined that the received signal arrives at one receiver later than atthe other by an interval equal io the time-delay of the secondary path signal which is to be eliminated. If the time-delay 0i the reflected signal is two microseconds, for example, then one transmission line is selected to be` longer than the other by such an electrical length as to produce two microseconds time-delay. Some ci the disadvantages of such a system are the necessity for using two separate receivers, and the fact that the relative lengths ot the two transmission lines are predetermined for the time-delay oi a particular secondary path signal, or, in other words, for reception of transmissions from a single station.

Anothersystern which has been proposed Yfor l2 reducing the effects` of multipath-receptionmakes use of avportion of the main signal pulse which, after ,being time-delayed, is reintroduced 1 int0 the .receiver circuit `in a manner to balance ,out the secondaryrpathpulse. VThis method includes a time-delay cabletl'ie length oi' which ischosen equal to half ,the delay interval required multiplied by the velocity of electrical Waves lupon the cable. If the cablefhas an electrical'velocity half the velocity of light, `or 150,000,000 meters per second, which is a reasonable value, then `Va' time-delay of -two microseconds requiresia cable meters long. Not-only is a cable -`15'01meters long cumbersome'and difficult toenclose Within a modern receiver, but, in addition, vthe-fixed length of the cable limits its time-delay t-o-a par.- ticularvalue, and does not permit delay `variations `which arerequiredto effect cancellation of reilections selectively received from a plurality oi transmitting vstations and arriving overrpaths of various lengths.

Accordingtoone feature of the present 'invention,`means are provided for eiectivelyeliminating the Veffects of multipath reception, these ineans'lbeing of-relatively small size compared :to the dimensions of thetime-.delay cable .above mentioned, and being-so designed as to forman integral part of the receiver itself. Furthermore, by -makingsuch means easily adjustable by .an observer, distortion resulting from signal reflections vmay effectively becancelled out for each new station received during the time that .the image from that particular station appears-on the screen of the reproducing device.

lOne objectof the present invention, therefore, is to-provide'means for effectively eliminatingfthe effects oi multipath reception in-television-Asystems. i

A further objectofthe-invention isto provide, in a television-system, means whereby the observer of `areproducedimage may eliminate the effects of multipath signal-reception during :observa-nce of the image.

`A still further object of the invention yis to provide a -multisection delay and attenuation networkA associated with the video signal circuit of a ,televisionreceiven such network being-readiiy adjustableby an observer during observance ci the reproduced f image to eiectively include in the network oneor more sections, and thusyproduce a wavegso delayed andfattenuated with-respect to a desired signal waveas to eiectively cancel any reflections which A`may be -includedin the video signal then' being received byreason of multipath reception.

Other objects and advantages will be apparent from the following description of a preferred form of the invention and from the drawings, in which:

Fig. 1 is a circuit diagram of a television receiver incorporating a preferred embodiment of the present invention; v

Fig. 2 is a set of curves illustrating the operation of the circuit of Fig. 1; and

Fig. 3 is an alternative form of the delay and attenuation network of Fig. 1.

Referring first to Fig. 1, there is illustrated a television receiver of the superheterodyne type. The television signal received by antenna lll is fed through an R. F. amplifier l2 to a converter I4, the latter also being supplied with energy from a local oscillator I6.

The video portion of the composite television signal output of converter I4 passes through a video I. F. amplifier I8, a video detector 20, and a video amplifier 22. The output of video amplier 22 is applied to an image-reproducing device, such as a cathode-ray tube 24. The audio portion of the composite television signal output of converter I4 passes through a sound I. F. amplifier 26 and a sound detector and amplifier 28 to a sound-reproducing device, such as a loudspeaker30.

In accordance with the preferred embodiment of the invention shown in Fig. 1, any echo or ghost signal which may be received by antenna I along with the desired video signal is reduced in intensity to a point where it can no longer cause objectionable distortion of the reproduced image. The means for accomplishing this object comprises a delay and attenuation network indicated in the drawing by the reference character 32. This network 32 is connected in parallel with the video signal circuit of the receiver by means of conductors 34 at some suitable point such as the input of the video amplifier 22.

Network 32 embodies the principle of a distortionless transmission line with the far-end impedance mismatched to provide a reflected signal wave having proper amplitude and time-delay characteristics to substantially cancel the received ghost image. To illustrate its operation, let a usual case be assumed wherein the incoming signal contains one pronounced echo of, say, onequarter the intensity of the desired signal. If, at any instant, the desired signal consists of a short pulse, such as might be obtained from the camera tube during the scansion of an isolated dark body, then the incoming signal will appear as in curve (a) of Fig. 2, where 36 is the desired signal of amplitude h and 38 is its image having an amplitude of h/l.

The energy output of video detector 20 is divided between the video amplifier 22 and the delay and attenuation network 32, the latter, as above stated, acting as a distortionless transmission line.` The portion of the energy output of video detector 20 entering the line travels along the line with a velocity determined by the constants thereof. If the far end of the line is short-circuited, this energy wave is reflected back along the line without change of amplitude (assuming the line is lossless). If the timespacing between the desired signal pulse l36 and its image 38 is T seconds (Fig. 2), then the line length is adjusted in a manner hereinafter to be described to give T seconds time delay for the loop transmission of the signal, or, in other words, the time required for the signal pulse to travel down the line plus the time required for the re- 4 flected signal pulse to reach the input terminals of video amplier 22 should be T seconds.

It has been assumed above that the echo pulsehas an intensity equal to one-quarter of the desired signal pulse. If now the amplitude of the reflected Wave is reduced to one-quarter of its original value by means hereinafter to be described, this reflected signal pulse together with its reflected image will appear as shown an 36 and 38 respectively in curve (b) of Fig. 2, the polarity of the reflected wave being reversed with respect to the received signal of curve (a) by the selection of a terminating resistance whichv is lower than the surge impedance of the line..

The actual signal appearing at the input terminals of video amplifier 22 is, therefore, equal to the sum of two components, one being that portion of the divided output of video detector 2l) which is applied directly to the video amplier 22, and which may be represented by curve (a), and the other component being the remaining portion of the divided output of video detector 2U which is returned over conductors 34 after being delayed and attenuated in the manner represented by curve (b) The sum of curves (a) Iand (b) is shown in curve (c). It will be seen that the received ghost image 38 of curve (a) has been com- ,pletely cancelled by the reflected signal pulse 36 0f curve (b), the latter pulse having the same amplitude h/4 as the ghost image 38 and being opposite thereto in phase.

It will als-o be noted that a new ghost image 38' is introduced into the signal input to the video ampliiier 22 as a result of the above mode of operation. However, the amplitude h/16 of this new ghost image 38 is only one-quarter of the amplitude h/4 of the original ghost image 38 or one-sixteenth of the amplitude h of the received signal pulse 36.

Obviously, by employing an additional transmission line of twice the electrical length of that described above and in parallel therewith, the same process may be repeated to cancel the ghost image 38 and introduce a new image having an amplitude equal to h/64. However, such a procedure is unnecessary in most practical cases, since the signal-to-image intensity ratio of the circuit of Fig. 1 is 24 d-b., or considerably better than the signal-to-noise ratio of present day television receivers. In this connection, it has been found that a signal-to-noise ratio of 20 db., is satisfactory.

The delay and attenuation network 32 of Fig. 1 includes a series of low-

pass iilter sections

48, 42, 44 and 46 respectively conected to the contacts of a switch 48 which is so arranged that any desired number of these lter sections may be selectively included in the network 32. The lter circuits shown in Figs. l and 3 are preferred over other arrangements in that they have good linear phase characteristicsthat is, constant time delay over the filter passband. However, any other suitable type of low-pass lter system may if desired be used to simulate the transmission line.

The approximate time delay for each of the

filter sections

40, 42, 44 and 46 is given by the formula Td seconds Where f is the cut-olf frequency of the lter. Thus for the present television signal delity of approximately 4 megacycles, a delay 0f about .08 microsecond is obtainable per section in Ione direction, or a delay of aboutI .16 'microsecond per section for a loop transmission from the out- `put of video detector 20 through the network 32 and back to the input terminals of video amplifier 22. Switch 48 allows variations in delay in steps of approximately .16 microsecond.

Since one scanning line or approximately linear inches of image, on the average, are transmitted in approximately 1/15000 second, it follows that e linch displacement in the reproduced image requires a delay of about 1/1500000 second or approximately .66 microsecond. Since four filter sections each having a delay of about .16 microsecond have a total delay of .64 microsecond, it will be appreciated that four iilter sections can elfectively cancel an echo that is displaced inch in the reproduced image. Similarly, eight l-ter sections can eiectively cancel a ghost displaced 1/5 inch. Inasmuch as the width of a single image element is roughly .008 inch, the -above displacements 4are on the order of several such widths, and their correction by means of the present invention results in a considerable improvement in the detail of the reproduced image.

The above discussion has been concerned only with the time-delay feature of the network 32. This network in addition includes means for adjusting the .amplitude of the reflected signal, such adjustment being .provided by the vari-able terminating resistor 50 connected in series with switch 48. If 'the maximum value of this terminating resistor 50 is chosen equal .to the surge or characteristic impedance of the line, then the reilected wave will always be inverted in phase with respect to the received signal, as shown in Fig. 2. The degree of mismatchthat is, the difference between the line surge impedance and the selected value of terminating resistor 50, determines the amplitude of the reflected wave, this amplitude being chosen to effect cancellation of the received ghost image in the manner above described.

It will be understood that the adjustments of switch 48 and resistor 50 are carried out manually while an image (preferably a test pattern) is viewed on the screen of the reproducing device.

Fig. 3 shows an alternative network 32' corresponding to the network 32 of Fig. 1. The addition of two variable resistors 52 and 54 permits a more precise adjustment of the attenuating characteristics of the network. Furthermore, while only four

lter sections

40, 42, 44, and 46 have been shown in Fig. 1, any desired number of these lter sections may be connected in series as indicated by the broken line 56 in Fig. 3, the number of contacts .on switch 48 being determined accordingly.

From the foregoing description it will .be understood that various omissions, substitutions, and changes in the form and details of the apparatus described may be made by those skilled in the art without departing from the spirit of the invention.

Having now described the invention, I claim:

In a television system in which distortion in the reproduced image results from the multipath reception of the transmitted signal, the combination .of means for lreceiving both the desired signal and the reflected signal, a multi-section delay and attentuation network, manually-adjustable means for applying a portion of the received signal selectively to at least one section of said delay and attenuation network, and

means for .combining the output of said delay and attenuation network with the remaining portion of said received signal, said delay and attenuation network including an adjustable terminating resistor having .a maximum value approximately equal to the surge impedance of the network.

JAMES ERNEST SMITH.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,712,280 Ranger May 7, 1929 2,226,836 Sinninger Dec. 31, 1940 2,227,057 B-lumlein Dec. 31, 1940 2,255,374 Beverage Sept. 9, 1941 2,266,154 Blumlein Dec. 16, 1941 2,271,909 Beverage Feb. 3, 1942 FOREIGN PATENTS Number Country Date 446,663 Great Britain May 4, 1936