US2423263A - Signal peak limiter - Google Patents

Description

Patented July 1, 1947 2,423,263 SIGNAL PEAK LIMITER Robert M. Sprague, Hicksville, N. Y., assignor to Press Wireless, Inc., Chicago, 111., a corporation of Delaware Original application December 30, 1941, Serial No. 424,946. Divided and this application March 26, 1943, Serial No. 480,618

This invention relates to electric signalling systems and more especially to control arrangements for use in telegraph systems, telefacsimile systems and the like.

Facsimile transmission can be roughly divided into two classes; one where the subject matter comprises a gradation of shades or half tones between black and white; the other where the subject matter consists only of two shades, e. g., black and white. Ordinary printed or written matter, line drawings and the like, would fall within the second class. In one known system of transmitting half tones by telefacsimile, the various shade values are converted into a frequency-modulated carrier which is required to handle a range of frequencies, for example be tween 1800 C. P. S. representing black shades and 3000 C. P. S. representing white shades, the intervening shades being represented by intervening portions of the 1800-3000 cycle spectrum. Such frequency converters can be used to transmit either class of subject matter. However, the electrical elements of the converter introduce the equivalent of a time lag, for example in scanning sudden changes from black to white and vice versa.

A principal object of this invention is to provide an improved signal peak-limiting arrangement.

A feature of the invention relates to a keying a control arrangement for transmitting marking and spacing signals, wherein one signal is raised to a very much higher level than the other and 4 Claims. (01. 178-44) Fig. 1 is a schematic wiring diagram of a facof the invention.

both are passed through a special limiting device so that in the output only one signal is present even though both signals are present at the input.

A further feature relates to a keying arrangement employing two sources of oscillations, one representing marking condition and the other representing spacing condition, or vice versa and with the relative levels of the two signals greatly different; in conjunction with a power limiting arrangement whereby switching from spacing to marking and vice versa is controlled by the swamping action of one signal on the other in passing through the limiter.

A further feature relates to a novel form of threshold and power-limiting arrangement.

A still further feature relates to the novel organization, arrangement and relative interconnection of parts whereby a simple and efficient transmission of black and white facsimile subjects can be effected.

Other features and advantages not specifically enumerated will be apparent after a consideration of the following detailed descriptions and the appended claims.

In the drawing which shows one preferred form,

Figs. 2, 3 and 4 represent modifications of a portion of the system of Fig. 1.

Referring to the drawing, numeral l represents any well-known form of signal generator such for example as a facsimile transmitting machine of known type having means to scan the successive elemental areas of the subject matter to be transmitted whereby the shade values of such areas are translated into a corresponding electrical signal, e. g., an audio frequency carrier signal. For a detailed description of such a device, referen'ce may be had to Patent No. 2,209,719. The generator I is connected through an adjustable resistancefpad 2 to a coupling transformer 3. In the case of black and white subject matter, devices I and 2 are adjusted so that the facsimile signal is of some predetermined fixed maximum level when a black area is being scanned, and thislevel drops substantially near to zero when a white areais being scanned. In other words, for black areas there appears at the primary winding of transformer 3 an audio frequency signal, e. g., 1800 C. R8. of high level, e. g, 30 decibels, whereas for white areas there is substantially no signalapplied to transformer 2, or if there is, itis of the lowest possible amplitude. f

' In theevehtthesignal from device I corresponding to white areas is not at the desired minimum amplitude, for example because of leakage conditions in the facsimile equipment, the signalsjare passed through a threshold limiter. This limiter comprises a pair of diodes 4, 5, which may be in separate tubes or they may be mounted within the same tube but electrically isolated from each other. The signal from transformer 2 is applied to cathode 4a directly, and to anode Eb through condenser G'and resistor 1. For negative half cycles the space between cathode 4a and anode 4b is conductive thus applying the signal to grid 13a of the triode I3. On the positive half cycles, diode 5 becomes conductive between cathode 5a and anode 5b applying the signal through the condenserfl and resistor 9. Resistors II! and ll are merely grid return, resistors for a tube is. The anode I31) is supplied with steady positive potential from a suitable power supply, the positive terminal of which is indicated in the drawing. A potentiometer l2 and voltage divider resistor I4 are connected across the power supply source, and another set of voltage divider resisters l5, it, are connected across the power supply for purposes to be described.

When a positive bias is applied from potentiometer l2 through the secondary of transformer 3 to'cathode 4a and through resistor 9 to cathode 5a, each diode is non-conductive until 3 the applied signal from transformer 3 is greater than this bias. This condition obtains during the negative half cycles on diode 4 and for the positive half cycles on diode 5. Thus the diodes act as threshold limiters for both the negative and positive half cycles of the signal waves. Instead of providing the cathodes 4a and 5a with a positive bias and tying the respective anodes to ground, the cathodes can be tied to ground and a negative bias applied to anodes 4b, 51); or in the alternative the cathode of one diode may be positively biassed while simultaneously a negative bias is applied to the anode of the other diode. These two alternative arrangements are schematically illustrated in Figs. 2 and 3.

The tube I3 is coupled through a transformer H to a pair of balanced diodes l8a, I8b, for fullwave rectification, the cathodes l9 and 20 of the two diodes being connected in balanced relation across the secondary of transformer whose electrical midpoint is grounded.

Since maximum output is desirable from tube l3, transformer should be a step-up transformer and tube l3 should be biassed by grid bias resistor 33 sothat tube l3 operates as a class A amplifier or as a class AB amplifier. Tubes H311 and |8b feed a tube 2| which is of the double-triode type, the triodes being arranged to cut off plate current to their respective anodes for relatively low grid-bias voltages on their respective grids 2|a, 2|b. The plates of tube 2| are connected in balanced relation to the primary of transformer 22 and the plate voltage is applied through voltage divider l5, l6, so that tubes 2| operate at a much reduced plate voltage. The common bias resistor 23 is adjusted so as to provide a bias for grids 2 la, 2|b, approximately half way between zero bias and plate-current cutoff bias. Resistor 24 is preferably of the same value as resistor 23 and plates 25 and 26 of the diodes are connected in balanced relation through resisters 21 and 28. The positive bias on the anodes 25 and 26 of the two diodes will equal the negative bias on the grids 2 la and 2|b thus providing a balanced and symmetrical arrangement. The tubes 18a, I81) and 2| comprise the peak limiter. When the grids of tube 2| swing negatively beyond plate current cut-off, complete and effective limiting results. However, if the grids swing positively into the grid current region, only partial limiting results and high distortion occurs. Tube 2| is biassed by resistor 23 half way between zero grid bias anda negative grid bias sufficient to cut off the plate current of tube 2|, then tube 2| will be operating in a substantially linear portion of its characteristic. On signal swingsv which would tend to drive the grids into the conducting region, limiting is produced by the diodes |8a and Hit). Since the anodes of diodes |8a and |8b are biassed positively by resistor 2.4. which is equal in value to resistor 23, the anode to cathode voltage of the diodes is equal to the cathode to grid voltage of each portion of tube 2|. Thus when the signal appears across the secondary of transformer assume that the upper half swings ne atively and the lowerhalf positively. Diode |8a will conduct current on the furthest excursion of its negative swing and this'negative voltage is applied to grid -2|-a. When this voltage is suflicient to cut off the upper half of tube 2|, peak limiting occurs. At the same time the cathode of diode |8b was swinging: positive. This voltage will appear on the anode of diode |8b only up to the point wherethe cathode voltage is brought. up equal to the fixed bias on the anode 26, due to resistor 24. As the positive voltage on cathode 20 becomes still further positive, diode |8b will cease to conduct and peak limiting on the positive swing occurs. Since the voltage across resistor 23 and resistor 24 are equal, peak limiting on the negative half cycle on the upper half of the cir- Therefore, on positive half cycles the diodes lfia.

IBb, will be respectively conductive up to the point of grid current flow in tube 2! thus limiting the signal waves in the positive direction. On negative half cycles the diodes Illa, 581), are conductive, but the grids of tube 2| are driven beyond plate current cutoff, thus limiting the signal waves in the negative direction. Consequently, the peak limiter is symmetrical to both halves of the signal wave cycles, as well as being balanced because of the push-pull arrangement. This symmetry has the advantage that only odd harmonics are generated and filter 29 may be made to attenuate only the third and higher harmonics so that even the poorest filter will have little efiect on the fundamental frequencies. The signals from the filter 29 are therefore substantially sinusoidal and can be transmitted over any channel such as a radio channel to the receiving apparatus 30. If the apparatus 30 is a facsimile receiving machine, it will be operated in synchronism with the machine l as is well understood in the art and will be provided with a recording mechanism which is selectively responsive to the marking and spacing signals so as to reproduce the Whites and blacks of the original subject matter in the known manner.

Connected through a resistance 3| to a suitable point between resistors I0 and I is a source 32 of oscillations of substantially uniform amplitude but of a different frequency from the black facsimil signal frequency. For example, source 32 may produce a 3000 C. P. S. signal for purposes to be described. Merely for explanatory purposes the 1800 C. P, S. signal from device will be called the marking signal, while the 3000 C. P. S. signal from source 32 will be called the spacing signal. The level of source 32 is adjusted to the point of limiting by the peak limiter, but not beyond. I have found that for complete squelching or blanking of the signal from source 32, the level of the signal from machine I must be about 20 decibels greater than the level of the signal from source 32 or in a voltage ratio of 10 to 1.

Since the local oscillator 32 is permanently connected in circuit, when the machine is in operation, two signals are applied to the peak limiter, namely an 1800 C. P. S. marking signal from machine I representing black elemental areas of the subject matter being scanned, and a 3000 C. P. S. spacing signal from oscillator 32. I have found that when the power level of the marking signal is many times higher than the power level of the spacing signal, the presence of the marking signal completely masks the spacing signal in the output of the limiter. For the best results, the marking signal should be about 20 decibels or more higher than the spacing signal. Thus the marking signal may be of the order of 30 decibels while the spacing signal may be decibels or less.

The operation of the system is as follows. The spacing signal from source 32 is adjusted to such a level that it is just barely limited on its peaks by the limiter. In other words, it just reaches the saturation point of the limiter so that any drop in the spacing signal will give a drop in the output from tube 2! but any increase of the spacing signal will ive no further increase in the said output. Consequently, when no marking signal is being received from the device I, a slightly flattened signal wave appears across transformer 22, at the spacing frequency but of a fixed maximum amplitude. When the device I is to be set in operation, it is adjusted so that its power level at the input to the limiter is many times the level of the spacing signal, preferably of the order of 20 decibels higher. When a marking signal is applied to tube I3 from the facsimile device I, it is preferably at least 10 times the voltage level of the signal applied at the grid of tube I3 from source 32. Under these circumstances, at the output of the peak limiter, namely, at transformer 22, there will appear substantially only the marking signal and it will be of the same amplitude as the spacing signal would be in the absence of the marking signal. With a level ratio of 10 times between the marking and spacing signals, there remains substantially no vestige of the spacing signal in the output of the limiter,

Because of the characteristics of the particular limiter described, there will be substantially no time delay or transients in the keying or switching between the marking and spacing signals, nor will there be any chance of interaction between the two signals since one is always many times the other. The spacing and marking signals are transmited over the channel L to a suitable facsimile receiver 30 which may be of the type described in Patent No. 2,299,937, the electrical portions of which may be as described in said patent and the mechanical portion of which may be as described in Patent No. 2,209,719. As a result, the 1800 C. P. S. signal is translated into a black area on the receiving paper or film while the 3000 C. P. S. signal is translated into a White area on this paper or film, it being understood that the facsimile transmitting machine and the facsimile reproducing machine are operated in synchronism as well-known in the art.

While in the foregoing, the system has been described as employing a threshold limiter between the facsimile machine I and the peak limiter I8aI8b-2I, in certain instances, the threshold limiter may not be necessary. Thus, regardless of the actual signal level corresponding to white and black as delivered by the machine I, if the difference between the two signals is relatively great, e. g., 40 decibels or better, the level of the signals from the facsimile machine with respect to the level of the signals from source 32 can be adjusted so that a marking signal from machine I will squelch or blank the signal from source 32 while on a spacing signal from machine I the signal from source 32 takes control. For example, if the marking signal from the facsimile machine is adjusted to 40 decibels greater than that required to reach saturation in the peak limiter, and if the source 32 is adjusted to 20 decibels greater than the said saturation level, then the marking signal from the facsimile machine being 20 decibels greater than the signals from source 32, the latter would be blanked.

However, on a space signal from the facsimile machine, the source 32 would be 20 decibels greater and would then squelch the space signal from the facsimile machine. This modification however is not as desirable as that illustrated in Fig. 1 since it requires a peak limiter of much greater range'and the adjustments for the relative blanking conditions are more critical.

Various changes and modifications may be made in the disclosed embodiment without departing from the spirit and scope of the invention. While in the foregoing the tubes I3 and 2| are shown as of the triode type, it will be understood that multigrid tubes may be employed. Furthermore, while tube 2| is shown as a single tube containing two sets of triodes, it will be understood that two separate triodes can be employed. Consequently, in the claims the expression grid-controlled tubes includes arrangements where the sets of electrodes, e. g., triodes, are in a single envelope or in separate envelopes. Fig. 4 shows a modification of Figs. 2 and 3 wherein the cathode 4a of tube 4 and the anode 5b of tube 5 are connected directly back-to-back, with the cathode 5a positively biassed with respect to its anode 5b, and with the anode 4b negatively biassed with respect to its cathode 4a.

This application is a division of application Serial No. 424,946, filed December 30, 1941, now issued as Patent No. 2,356,361, August 22, 1944.

What I claim is:

1. A signal peak limiter comprising a pair of diodes, a pair of grid-controlled amplifiers, a signal input circuit, means connecting said diodes to said input circuit for full wave rectification, a pair of load resistors connected in series across the plates of said diodes, a diode biasing resistor connected from the junction point of said load resistors to the midpoint of said input circuit, another bias resistor for said amplifiers connected between said junction point and the cathodes of said amplifiers, said bias resistors being proportioned with respect to each other so as to bias the anodes of the diodes and the grids of the amplifiers substantially equally but opposite in sign so that peak limitation will occur on substantially the same voltage for positive half waves applied to one diode as for negative half waves ap lied to the other diode.

2. A signal peak limiter according to claim 1 in which the control grids of said amplifiers are normally biased between zero bias and approximately one half plate current cutoif bias.

3. A signal peak limiter according to claim 1 in which both of said bias resistors are approximately equal.

4. A signal peak limiter according to claim 1 in which all of said resistors are proportioned so that the diodes become conductive of positive input signal cycles only up to the point where the amplifiers begin to draw grid current.

ROBERT M. SPRAGUE.

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

UNITED STATES PATENTS Number Name Date 2,277,000 Bingley Mar. 17, 1942 2,171,671 Percival Sept. 5, 1939 2,298,657 Smith et a1 Oct. 13, 1942

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