US2443434A - Automatic signal bias control means and apparatus - Google Patents

Automatic signal bias control means and apparatus Download PDF

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Publication number
US2443434A
US2443434A US536871A US53687144A US2443434A US 2443434 A US2443434 A US 2443434A US 536871 A US536871 A US 536871A US 53687144 A US53687144 A US 53687144A US 2443434 A US2443434 A US 2443434A
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United States
Prior art keywords
frequency
carrier
signals
signal
voltage
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US536871A
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English (en)
Inventor
Robert M Sprague
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PRESS WIRELESS Inc
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PRESS WIRELESS Inc
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Priority to US536871A priority Critical patent/US2443434A/en
Priority to FR917421D priority patent/FR917421A/fr
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Publication of US2443434A publication Critical patent/US2443434A/en
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/10—Frequency-modulated carrier systems, i.e. using frequency-shift keying
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/00095—Systems or arrangements for the transmission of the picture signal
    • H04N1/00103—Systems or arrangements for the transmission of the picture signal specially adapted for radio transmission, e.g. via satellites

Definitions

  • the signals being transmitted are the usual telegraph signals of the mark and space kind.
  • filtering action however, very frequently destroys or suppresses the harmonic content of the original square-topped signal waves and leaves only the fundamental thereof. therefore, in such systems, to follow the filter with some kind of a peak limiter or the like, whereby the smoothly rounded fundamental frequency Waves can be reconstituted into equivalent squaretopped waves.
  • Fig. 1A shows the original wave shape of telefacsimile or telegraph signals which, for purposes of explanation, are assumed to represent a series of dots in the original subject matter. After these signals are passed through a lowpass filter at the receiver, they assume the shape shown in Fig. 1B. The signals of Fig. 1B are then applied in the usual way to a peak limiter or the like whereby they are reconstituted into square-topped waves as shown in Fig. 10. If in the meanwhile, the transmitter frequency has drifted, or if the frequency of the local beat oscillator in the receiving radio set has drifted, the base line B of Fig. 13, will no longer be symmetrical with respect to the positive and. negative excursions but will have a non-symmetrical appearance as represented for example in Fig. 1D. I have found that by employing a special form of capacity and rectifier network ahead of the limiter, the undesirable effects of frequency drift are automatically compensated for.
  • FIG. 2 there is shown in schematic form a telefacsimile system employing the invention, but with well-known parts of the system omitted for the sake of clarity.
  • the picture or other subject matter I', to be transmitted, is wrapped around the usual scanning drum 2, which is rotated so that the picture is completely scanned in successive elemental areas in the wellknown manner by means of an elemental light spot from source 3.
  • the reflected light excites the photocell 4, whose output is amplified in any suitable amplifying system 5.
  • the amplified signals are then applied through a suitable switch 6 to a radio transmitter preferably of the carriershift type comprising a carrier source 1 arranged to generate a normal carrier frequency which will be referred to as the base frequency, and a frequency shifting arrangement 8, whereby the picture signals cause the frequency of the carrier to be shifted between predetermined limits.
  • a radio transmitter preferably of the carriershift type comprising a carrier source 1 arranged to generate a normal carrier frequency which will be referred to as the base frequency, and a frequency shifting arrangement 8, whereby the picture signals cause the frequency of the carrier to be shifted between predetermined limits.
  • the output from device 8 consists at any given instant of a substantially single frequency carrier which however can be varied in frequency.
  • the carrier may have a frequency of 9999.6 kc.
  • the carrier may have a frequency of 10000.4 kc.
  • the shifted frequency carrier is applied to any well-known type of radio receiver, preferably of the beat frequency type, so as to produce beats representing the frequency excursions of the transmitted carrier.
  • the beat oscillator ID at the receiver is designed to operate at a frequency which when beat against the signal from oscillator I will produce signal frequencies to operate the succeeding equipment.
  • the beats from receiver 9 which for example can be 2000 c. p. s., and 3000 c. p. s., are passed though a suitable input filter and limiter and then through a discriminator ll whereby they are converted into corresponding pulsating D. C. signals. These signals are then applied to a lowpass filter I2 which suppresses the A. C. components and produces at its output variable amplitude D. C. signals corresponding to the original signals from cell 4.
  • a discriminator and filter arrangement that is suitable, reference may be had to U. S. Patent
  • the signals applied to the discriminator l I are substantially square-topped waves (Fig.
  • Fig. 1A rich in harmonics, but at the output of filter l2 they are substantially sinusoidal (Fig. 1B).
  • the waves are applied to a peak limiting arrangement I3 or similar device which is so adjusted that for a small voltage above the base line B (Fig. 1B), the limiter is keyed on, and at a small voltage below the line B, it is keyed off; thus producing the square wave of Fig. 1C.
  • This square wave can then be used to control the recording element or printing hammer of any well-known form of telegraph reproducer or facsimile reproducer.
  • the half waves of the reconstituted keying signal (Fig. 1C) are symmetrical, both as to amplitude and time duration.
  • the limiter I3 will no longer operate symmetrically in the plus or minus directions, but rather will produce a wave form such as shown in Fig. 1D.
  • the negative bias D will be much greater than the positive bias D. This will result in improper operation of the recording element or keyerof thedevice l4.
  • the :re'sistor :I-B represents the iload fresistoriin the output of the filter I2, the voltage across *which :isalso changing between :the :two transmitted values :corresponding to'th'e'signals fromcell *4.
  • condenser "I '1 is charged through rectifier Hi :to the peak voltage-of the signal. The time constant.
  • the limiter'lfi can then beadjusted "softhat this zero or quiescent condition holds the local circuit of the telegraph keyer 26 or recording element of the facsimile reproducer M in -e'ithermark" or space position 'asde- *sired. "The mean or average voltage 'across resistorTfi will not disturb this balancdbecause no direct current is transmitted'through condensers .l"! 'or2l. Therefore, it does "not matter whether the voltage across resistor B is varying about-zero or'abouta plus or negative'value. 'Only changes involtagesimpressed across resistor [6 determine the extremes of the'voltages at point 23. Thus,
  • the transmitter or.receiver can'drift any amount
  • the transrmitted carrier isstatisticallyshifted betweenits "extreme frequencies.
  • some means must be provided to shift the carrier between its extreme frequency excursions a multiplicit of times during the transmission. This can be effected by providing contrasting Eblackf, :and'afiwhite?strips li-fieon-t el-drum, thus a ,ibla-ckfz and.
  • iwhiteu strip be vanterposed :between-ithe; spaced edges at the 221 101311116 ll whichJiswrappediaroundztheidrum.
  • said network whereby the potential of said point changes only when 'thesignal 'vdltages impressed .on said network are'yaryingrandindependently of frequency drift in said "local oscillator, said network including a .pair .of rectifiers .having a .commonreturn .and each connected 'throug'h respective condensers to the said receiving means, a, pair of rectifier load impedahces "having a common junction connected to-said common lreturn and an outputimpedancebridged across both said rectifier fload 'impeda'nces, said --.p.ointTbeing flocated,at 'the electrical m'idpointloi said. output, impedance.
  • a signalling system means to scan a subject matter having black and White areas to produce symmetrical and substantially squaretopped signal waves, a carrier generator, means to shift the frequency of said generator symmetrically around a mean frequency and between predetermined limits corresponding to said black and white areas, means toreceive and detect said signals and including 'a frequency discriminator and filter for the" detected signals, a frequency-drift compensatingv network having at least one input point and one outpoint point, said network including a pair of rectifiers'having a common return and connected back-to-back with respect to said common return, means connecting said rectifiers to the receiving means so that only the pulsating components of the detected signals are applied to said rectifiers'said 130 oppositely poled with respect to said common output being symmetrically connected to the combined output of saidrectifiers sothatsaid output point is maintained in a'quiescent potential condition even though there exist undesired carrier frequency drifts in the system.
  • electro-opticalmeans to scan a picture or the like having a range of tone values between black and whitel .toipro quiz electric picture signals means to shiftthe frequency of a carrier between predetermined limits corresponding to the extreme ranges of said tone values, means independent of the'picture to subject the carrier to alternate frequency excursions between said predetermined 'limits, means to receive and detect the signals from said carrier, a picture shade reproducer, and-means between therreceiverand reproducer for reproducing a range of signal voltages corresponding to said picture signals'with the mean valueof the reproduced signals corresponding withthezmean value of the original signals and independently of any accidental drift in the mean frequency of said carrier, said last-mentioned means including a pair of rectifiers having a common returnand each connected through respective condensers to said receiver, a load impedancefor eachrectifier, said load impedances having a common junction connected to said common return, and another impedance bridged across both said-load impedance
  • a system according to' claim 6 in which the means for subjecting the carrier to alternate frequency excursions independently of the picture comprises longitudinal black and white strips which are scanned in cyclical recurrent intervals between the ends of each scanning line of the picture.
  • carrier frequency drift compensating means comprising a pair of rectifiers connected in parallel with an impedance, a common ground return connection between the junction of the said rectifiers and the electrical midpoint of said impedance, said rectifiers being oppositely poledlwith respect to said common return, a pair of condensers connected in series across said rectifieraa signal input impedance connected between the common junction of said condensers and said common return, wherebysignal voltagesare impressed in parallel on said condensers, a load impedance connected across said first impedance, a signal responder, and means connecting one terminal of said responder to the electrical midpoint of said load impedance and the other terminal of said responder to said commonv ground return.

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  • Engineering & Computer Science (AREA)
  • Signal Processing (AREA)
  • Physics & Mathematics (AREA)
  • Astronomy & Astrophysics (AREA)
  • General Physics & Mathematics (AREA)
  • Multimedia (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Digital Transmission Methods That Use Modulated Carrier Waves (AREA)
US536871A 1944-05-23 1944-05-23 Automatic signal bias control means and apparatus Expired - Lifetime US2443434A (en)

Priority Applications (2)

Application Number Priority Date Filing Date Title
US536871A US2443434A (en) 1944-05-23 1944-05-23 Automatic signal bias control means and apparatus
FR917421D FR917421A (fr) 1944-05-23 1945-11-14 Système automatique de contrôle des signaux

Applications Claiming Priority (1)

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US536871A US2443434A (en) 1944-05-23 1944-05-23 Automatic signal bias control means and apparatus

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US2443434A true US2443434A (en) 1948-06-15

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FR (1) FR917421A (fr)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2999925A (en) * 1959-01-28 1961-09-12 Page Comm Engineers Inc Variable decision threshold computer
US3619493A (en) * 1968-12-20 1971-11-09 Graphic Sciences Inc Automatic gain control for graphic data transmission system

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2005111A (en) * 1931-04-09 1935-06-18 Rca Corp Amplifier
US2233165A (en) * 1938-06-30 1941-02-25 Gen Electric Automatic frequency control
US2292100A (en) * 1940-08-30 1942-08-04 Rca Corp Square wave generator
US2299945A (en) * 1940-11-27 1942-10-27 Rca Corp Direct current reinserting circuit
US2366363A (en) * 1943-03-06 1945-01-02 Times Telephoto Equipment Inc Frequency modulation system
US2411853A (en) * 1941-07-24 1946-12-03 Rca Corp Photo radio signaling by frequency modulation

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2005111A (en) * 1931-04-09 1935-06-18 Rca Corp Amplifier
US2233165A (en) * 1938-06-30 1941-02-25 Gen Electric Automatic frequency control
US2292100A (en) * 1940-08-30 1942-08-04 Rca Corp Square wave generator
US2299945A (en) * 1940-11-27 1942-10-27 Rca Corp Direct current reinserting circuit
US2411853A (en) * 1941-07-24 1946-12-03 Rca Corp Photo radio signaling by frequency modulation
US2366363A (en) * 1943-03-06 1945-01-02 Times Telephoto Equipment Inc Frequency modulation system

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2999925A (en) * 1959-01-28 1961-09-12 Page Comm Engineers Inc Variable decision threshold computer
US3619493A (en) * 1968-12-20 1971-11-09 Graphic Sciences Inc Automatic gain control for graphic data transmission system

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Publication number Publication date
FR917421A (fr) 1947-01-07

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