US2805282A - Automatic printing telegraph receiving system - Google Patents

Description

United States Patent AUTOMATIC PRINTING TELEGRAPH RECEIVING SYSTEM Joseph A. Krcek, Arlington, Va., assignor to the United States of America as represented by the Secretary of the Navy Application August 2, 1954, Serial No. 447 ,448

9 Claims. (Cl. 178-69) (Granted under Title 35, U. S. Code (1952), sec. 266} The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates primarily to telegraph receiving systems of the automatic printing type. More particularly, the invention concerns a novel filter selector means which is adapted to be used in such telegraph receiving systems for improving the operation thereof.

In telegraph systems of the automatic printing type, intelligence is usually transmitted between the various units through the medium of marks and spaces while the receiving system is controlled by start-stop or other transmitted synchronization signals which may also be in the nature of marks and spaces. In a telegraph system employing a start-stop system, a complete transmitted signal for one character or symbol will in a predetermined period of time comprise a start or synchronizing signal, a plurality of code or intelligence signals, and a stop signal. When a signal of this type is received by a telegraph receiver, the leading edge of the start signal, which is generally transmitted as a space, is recognized by the receiver whereby a portion of the receiver system is initiated to process the signal in a manner to determine the mark and space conditions thereof and hence, to interpret the intelligence of the signal. This processing, which is referred to as signal sampling, involves the taking of a narrow portion of each code or intelligence signal and determining its potential level to provide the necessary information. Since the initiation of this sampling process by the receiver is governed by the time at which the leading edge of the start signal is recognized, it is highly important that the leading edge of the start signal be recognized by the receiver at a reasonably precise time relative to the received intelligence signals. Otherwise, should the edge be recognized at variable times, sampling will occur at variable times relative to the several intelli ence signals, whereby the receiver may sample and interpret the sloping edge of a mark signal as a space and produce an error in the printed output.

Generally, telegraph systems which employ wire transmission circuits do not encounter such severe difficulties since the signals are usually free of atmospheric noise. However, magnetic induction and line impedances may produce somewhat similar results. In wireless telegraph systems, particularly when atmospheric or noise conditions are unfavorable, noise and other undesired spurious signals are superimposed on the telegraph signals. If the telegraph signals are permitted to pass through the receiver with such spurious signals present, the leading edge of the start signal and the sampled position of the intelligence signals may be falsely interpreted by the receiving system with a resulting inaccurate and unreliable receiver printer operation.

To overcome this difiiculty, a restrictive low pass filter is usually incorporated in the receiver circuit for eliminating much of the undesired spurious signals from the tele- Patented Sept. 3, 1957 graph signals. Since only the center porton of the intelligence signals need be properly reproduced for accurate operation, it is permissible to provide severe or highly restrictive filtering of these signals. However, severe filtering of the start signal will cause the leading and trailing edges of the start signal wave-form to have long rise times or sloping edges. Such sloping leading edges makes it difiicult to obtain precise recognition of the start or" the start signal relative to time. As a practical measure, the filter in a receiver system is generally compromised such that a filter is used which has medium filtering characteristics. Because of this compromised filter, some noise and spurious signals remain on the intelligence signals after filtering and the leading edge of the startsignals still slopes somewhat so that the recognition of the leading edge by the receiver and subsequent initiation of the sampling process varies in time relative to the incoming signal; all of which causes relatively unreliable operation of the telegraph system.

Under the provisions of the present invention, a novel automatic filter selector system is provided for a telegraph receiving system which permits relatively broad filtering of the start signals, and highly restrictive filtering of the intelligence signals. Because of such optimium filtering of the signal elements, the sampling or interpreting operation by the receiver is initiated at a precise time relative to each incoming telegraph signal and further, undesirable spurious signals as well as noise are substantially eliminated from its intelligence signals.

Accordingly, it is one object of the present invention to provide a filter selector system for automatic printing, telegraph receiving systems which increases the reliability and accuracy of such systems.

Another object of the invention is the provision of a filter selector system for processing the various elements of an incoming signal in the most advantageous manner.

A further object of the invention is the provision of a filter switching system which operates automatically to separately filter different portions of an incoming signal.

Still another object of the present invention is to provide a simple and reliable filter selector system which can be readily incorporated in existing telegraph equipment.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Figure l is a diagrammatic view of a typical automatic printing, tele raph receiving system having a preferred embodiment of the instant invention incorporated therein; and

Figure 2 is a waveform diagram of an ideal start-stop telegraph signal composed of mark and space elements.

Referring now to the drawing, there is shown in Figure 1 a block diagram of a telegraph receiving system 19 which functions to receive transmitted telegraph signals from a remotely positioned telegraph unit (not shown), interpret the intelligence of such signals, and provide an indication of such intelligence, usually in the form of printed characters or symbols on a sheet of paper. Conventionally, telegraph receiving system 10 includes a signal receiving apparatus which in the case of a radio-propagated signal comprises an antenna 12 and a suitable receiver 14, generally of the frequency shift keying or FSK type. It will be apparent that the signal may be communicated to the receiving system by other means as, for instance, communication lines or the like. As such, the particular type of receiving apparatus employed in conjunction with the invention is basically immaterial; the only requirement of the apparatus is that the transmitted received.

The received signal, which in a start-stop telegraph system may comprise a frequency modulated wave having two predetermined frequencies representing the marks and spaces of the telegraph signal, is amplified and heterodyned in receiver 14 to produce a beat signal having lower frequency components than the input signal. Usually, an intermediate frequency stage is also provided in receiver 14 for providing a limited filtering action on the. signal.

Operatively connected to receiver 1 -3 and forming an integral portion of telegraph receiving system in is a receiver converter 16 comprising an amplifier and limiter circuit 18, discriminator detector circuit Zil, low pass filter circuit 22, and a telegraph line circuit 24. The amplifier and limiter circuit 18 serves to amplify the telegraph signal and to clip or remove either or both extremities of the signal waveform for eliminating some of the noise and undesirable spurious signals carried upon the extremities of the telegraph signal. The function of discriminator detector 29 is to receive the signal output from circuit 18 and to develop in its output, a potential signal having an amplitude that varies in accordance with the frequency deviations of the applied input signal.

Inasmuch as the signal of the mark-space telegraph system comprises only two predetermined frequencies, one of which represents a mark and the other a space, and each of these is transmitted for a set time period of, for instance, 22 milliseconds, the output from discriminator circuit 20 will comprise a D. C. voltage having two relative amplitudes. These amplitudes, which have a time duration equal to said set time period, serve to define the individual elements of the signal. The waveform of such a signal is shown in Figure 2. This figure illustrates, in full lines, a complete telegraph signal having an ideal waveform and, as shown, includes a start signal in the form of a space, a plurality of code or intelligence elements 1 to 5 in the form of variously arranged marks and spaces, and a stop signal in the form of a mark. it will be noted that in the illustrated ideal signal waveform, the signal is completely free of noise or other spurious signals and that the leading and trailing edges of each signal element are sharply defined by a vertical edge. In practice however, some noise and other undesirable signals pass through the preceding stages and are superimposed upon the signal. Also, due to circuit constants, the leading and trailing edges of the elements are necessarily slopedto a small degree.

The discrimator detector output, as will hereinafter more fully be described, is passed through a low pass filter circuit 22 to eliminate noise and undesired signals therefrom, and thus, to reduce the influence of such signals on subsequent circuits. Connected to filter circuit 22 is a line circuit 24. This latter circuit, which preferably comprises a conventional D. C. amplifier, takes the signals from filter circuit 22 and amplifies the same. The signals developed across the output of circuit 24 are then fed through line 28 to telegraph printer 26.

With the exception of a slight modification, as will hereinafter more fully be described, telegraph printer 26 is conventional. As such, it is not believed necessary that a detailed description of the printer be provided for a complete understanding of the invention. However, it will be noted that upon receipt of the start signal from line circuit 24, the printer mechanism recognizes the start of the start signal and releases a clutch-constrained main shaft (not shown) for operation. Upon release, this shaft, which is driven by a suitable drive motor, begins rotating and makes one complete revolution in a time period equal to the time period taken by a complete telegraph signal. Conventionally, after making one revolution, the shaft stops.

During rotation of the main shaft and after a precise time period which is determined by the time that the start signal be properly of the start signal is recognized, the telegraph printer initiates the signal sampling process. This process inolves the taking of a selected short portion of each code element, which desirably is the center portion, and determining the potential or relative amplitude of the selected portions. Generally, this sampling process is a mechanical procedure utilizing a printer selector mechanism having electrical switches which are controlled, as to their open or closed conditions, by the main shaft and cornp ses a series of spaced, sampling time periods, each of which involves a total time period of a few milliseconds, such as for example, 4 milliseconds. By sampling the potential of these selected portions, the telegraph printer selector mechanism is capable of differentiating between z the mark and space elements of the entire telegraph signal and indicates or prints the interpreted intelligence.

l ixediy mounted on the main shaft of telegraph printer 2% is the usual selector cam sleeve 32. This sleeve functions, upon rotation of the main shaft, to conventionally control certain operations of the telegraph printer. The cam sleeve of the printer is modified in the instant invention by incorporating a filter selection cam 34 thereon. Because of the length of the cam sleeve, in the usual printer, this may be accomplished with little ditficulty. Cam 3% is fixedly mounted on cam sleeve 32 for rotation therewith for reasons which will hereinafter appear.

From the above, it will be apparent that the leading edge of the start signal is important in initiating the mechanical sequence of the printer selector mechanism at a reasonably precise time relative to the incoming telegraph signal so that the sampling time period will occur during the selected, narrow center portion of the code signals. Moreover, it will be apparent that any noise or undesirable signals appearing on an incoming signal may prematurely initiate the sampling process or may be present on the code signals and hence, upon sampling thereof, cause an error indication. This, as indicated in the foregoing, has been provisionally provided for by filter circuit 22.

Under the teachings of the instant invention, filter circuit 22 is formed with a plurality of individual filter units, each of which is of such size as to provide an optimum filtering action for a particular portion of the telegraph signal. These filter circuits include a relatively broad, low pass filter circuit 36 and a restrictive, low pass filter circuit 38. Low pass filter circuit 36, which may be of any suitable and conventional construction, is designed to provide optimum filtering characteristics for the start signal. This filtering action, which is relatively broad filtering for the transmission rate of the telegraph system, permits the passing of the start signal with a minimum of waveform distortion such that the slope of its leading edge remains relatively steep in nature. Preferably, low pass filter circuit 36 is designed to permit all frequencies up to cycles per second to readily pass through but to prevent the passing of any frequencies above this value. While some lower frequency noise and undesired spurious signals will pass through filter 36, all undesired signals carried on the start signal which have a frequency above 80 cycles per second will be eliminated therefrom.

On the other hand, filter 38, which is also of any suitable and conventional construction, is designed to provide a highly restrictive filtering action for the code or intelligence signal elements. While such restrictive filtering tends to introduce large sloping edges on the waveform of the code elements, the selected center portions of the code elements retain their proper shape and are substantially free of all noise and spurious signals. The frequency range of restrictive filter 38 will depend on the transmission rate and other circuit features of the receiver although an optimum low pass filter circuit for a telegraph system having five code elements is one which only permits frequencies up to approximately 40 cycles per second to pass through.

It will be apparent therefore that since the steep leading edge on the start pulse is unaffected by filter 36, accurate and positive recognition of the start pulse, relative to time, is assured in the printer mechanism. Moreover, because of such accurate recognition and the precise relation of the signal sampling time to the instant of recog nition, the sampling time process will occur only during the selected center portion of the code elements. Therefore, the large sloping edges of the code element waveform will have no effect on the receiver operation.

Filters 36 and 38 are accurately related to the telegraph receiver system and to an incoming telegraph signal by a switching unit 40 which is electrically connected to filter circuits 36 and 38 and mechanically operated by telegraph printer unit 26. Switching unit 40, preferably comprises a pile-up type spring-leaf switch having a center resilient switch arm 42 and a pair of relatively flexible contact arms 44 and 46; one of which is arranged on opposite sides of switch arm 42. Arms 42, 44 and 46 are spaced and electrically insulated from each other by a series of stacked, fiat, insulating members 48 interposed between one end of arms 44 and 46 and arm 42. The elements are secured together by a suitable fastening arrangement having a component such as a bolt 50 which passes through members 48 and arms 42, 44 and 46. If desired, an insulating bushing of suitable design may be inserted between bolt 50 and the switch arms for preventing the fastener from electrically bridging the arms.

A pair of electrical contacts 51 and 52 are secured on opposite sides of center arm 42, inwardly from its outer end. These contacts are adapted to engage contacts 53 and 54, secured on the outer ends of arms 44 and 46, respectively. The contact arrangement of the switching unit is such that when arm 42 is in its unstressed normal position, contact 51 engages contact 53 to form a normally closed switch and contacts 52 and 54 are separated from each other to form a normally open switch. The extreme outer end of arm 42, which eXtends outwardly beyond arms 44 and 46, is adapted to be flexed by cam 34 for closing contacts 52. For this purpose, cam 34 is formed with a narrow notch 56 in its periphery for receiving the extended portion of arm 42 whenever cam 34 is in its central or stop position. Electrically, switching unit 40 is connected in receiver converter circuit 16 by a line 69 extending between discriminator detector circuit 20 and center arm 42 and by lines 62 and 64 which extend between arms 44 and 46 and filters 36 and 38, respectively.

Operation Assuming that the telegraph receiving system is energized, the system is in operative condition, and a telegraph signal of thestart-stop type is transmitted from a remotely positioned telegraph unit to the receiving system. This signal is received by antenna 12, heterodyned, and then partially limited in the intermediate frequency stages of receiver 14. The output from receiver 14 is amplified and limited in circuit 18, and detetected in circuit such that the output from detector 20 comprises a D. C. potential wave having two potential levels which define the individual components of the signal. It will be noted that at the instant the signal is received, the mechanical drive of the telegraph printer unit and the other detection components of the receiving system are in standby condition and conact 51 is in closed engagement with contact 53.

As the telegraph signal is developed across discriminator circuit 20, it is fed through line 60, switch arm 42, normally closed contacts 51 and 53, line 62, to band pass filter 36. The start signal, which is the first portion of the telegraph signal in time, passes through filter 36. While passing through, a filtering action occurs such that some of the noise and other spurious signals are removed therefrom, but, as indicated in the foregoing, the leading edge of the start signal waveform is substantially unaffected. After the start signal passes through filter 36, it enters the telegraph printer selector mechanism through line 28 where the leading edge of the start signal is accurately recognized relative to time. Upon recognition, the mech anism acts to permit rotation of the main shaft and sleeve 32 in telegraph printer unit 26. This rotational movement, in turn, acts to drive cam 34 mounted on selector shaft sleeve 32.

When cam 34 begins its movement, the rim of notch 56 engages center arm 42 of switching unit 40 and moves the arm in a direction so as to break electrical contact between contacts 51 and 53 and to make contact between contacts 52 and 54. This action serves to disconnect discriminator detector circuit 20 from filter 36 and to connect the output of circuit 20 to restrictive low pass filter 38 for channeling the remaining code elements of the telegraph signal through filter 38. It will be noted that the initiation .of the mechanical components of the telegraph printer and the completion of the switching operation is relatively instantaneous compared to the relative length in time of the start signal. Because of this, and due to the length in time of the start signal which is in the order of 22 milliseconds in a 5 code element telegraph signal, the code elements do not reach filter 36 until after the switching operation has been completed. Upon engagement of contacts 52 and 54, the code elements pass through filter 38 where substantially all noise and undesirable spurious signals are eliminated therefrom, and through line circuit 24. At the instant this occurs, which time is accurately related to the recognition of the leading edge of the start pulse, the printer mechanism initiates the signal sampling process to determine the mark or space conditions of the code elements. According to the presence of a mark or space, the signals are acted upon by the telegraph printer for indication and/or printing of the transmitted intelligence.

After all code elements have been processed, the stop signal appears and acts to provide a period of inactivity for the receiving system in preparation for activation upon receipt of a subsequent start signal. Meanwhile, cam 34 has completed one complete revolution and has returned to its initial position with the end of center arm 42 resting in notch 56.

It will be apparent from the foregoing that an automatic filter selector system is provided for introducing relatively broad filtering for a first portion of a telegraph signal and restrictive filtering for the remaining portions of the telegraph signal. As such, each of the signal portions, which inherently have different waveform requirements, are capable of being filtered in an optimum manner for providing optimum functioning of the equipment. Consequently, a higher order of reliability and accuracy by the system is obtained.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What I claim is:

1. A telegraph receiving system having, in combination, a telegraph signal receiving means, signal actuated means, a signal converter means connected between said receiving means and actuating means, and means in said converter means for selectively filtering the different portions of an incoming telegraph signal in accordance with their particular waveform requirements comprising, a first and a second filter circuit, each of said filter circuits having different filtering characteristics, and means controlled by said actuated means for selectively channeling the different portions of an incoming tele raph signal to said filter circuits.

2. In a telegraph receiving system having a telegraph signal receiving means, signal actuated means, and a signal converter means connected between said receiving and actuated means, means in said converter means for selectively filtering the different portions of an incoming telegraph signal in accordance with their particular wave form requirements, said last named means including a first relatively broad low pass filter circuit and a second narrow low pass filter circuit, and a selective means electrically connected to each of said filter circuits and to said converter means, said selective means being mechanically controlled by the actuated means for selectively connecting each of said filter circuits to said converter means in accordance with the particular portion of the signal being received at that instant.

3. In an automatic printing, telegraph receiving system of the type employing a start-stop signal system and having a signal receiving means, a signal actuated means, and a signal converter means connected between said receiving and actuated means, a plurality of filter circuits having their outputs connected in the circuit of said converter means, each of said filter circuits having difierent filtering characteristics with the filtering characteristic of each filter circuit being such as to provide optimum filtering for a particular portion of a start-stop signal, and control means controlled by said actuated means connected between the input circuit of said filter circuits and said converter means for selectively placing said filter circuits in the circuit of the converter means upon the presence of a particular portion of an incoming start-stop signal.

4. In an apparatus as defined in claim 3 but further characterized by said control means comprising a switching unit mechanically connected to said actuated means for operation upon receipt of the first portion of an incoming telegraph signal.

5. In an apparatus as defined in claim 4 but further characterized by said switching unit comprising a normally open and a normally closed switch, said normally open switch being electrically connected between the input of one of said filter circuits and the signal converter means, and said normally closed switch being electrically connected between a second filter circuit and the signal converter means.

6. In an automatic printing, telegraph receiving system of the type employing a start-stop signal system, a signal converter means including a signal detector means, a filter means and a line circuit all of which are serially connectable together, said filter means including a first relatively broad low pass filter circuit and a second narrow low pass filter circuit, each of said filter circuits having an input and an output with their output being connected to said line circuit, a switching means,

said switching means including a pair of normallyclosed contacts connected between the input of said first filter circuit and said detector means and a pair of normally open contacts connected between said second filter circuit and said. detector means, means electrically connected to said line circuit, said means including a rotatable shaft means and a cam carried thereby with said cam being in operative engagement with said switching means, whereby said switching means is operative to selectively transfer an incoming telegraph signal after detection in said detector means to said filter circuits.

7. In an apparatus as defined in claim 6 but further characterized by said switching means comprising a pileup spring leaf switch having a first resilient center arm in engagement with said cam and a pair of switch arms, one of which is disposed on each side of said center arm, said first arm being electrically connected to said detector means and each of said second arms being electrically connected to one of said filter circuits.

8. In a system utilizing mark space signal groups to convey intelligence, said groups comprising a synchronizing signal and a plurality of intelligence signals spaced in a predetermined time sequence, means for filtering said signal groups comprising a first filter for passing said synchronizing signal, means actuated by said passed synchronizing signal, selective means, and a second filter, said selective means operable by said actuated means to pass the intelligence signals through said second filter a predetermined time after passage of said synchronizing pulse through said first filter.

9. In a radiotelegraph system utilizing mark space signal groups to convey intelligence, said groups comprising a synchronizing signal and time space intelligence signals, means for receiving said signal groups, means for selectively filtering the synchronizing and the intelligence signals of a group comprising a first broad band low pass filter, a second narrow band low pass filter, first means actuated by said synchronizing pulse in said group after filtering in said first filter, and second means responsive to said first means for switching the intelligence pulses of said group to said second filter.

References Cited in the file of this patent UNITED STATES PATENTS

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