DE2447150A1 - DEVICE FOR TRANSMISSION OF TELEVISION IMAGES - Google Patents

Description

7732-74 / Kö / S

FS-SN 403,085

Convention Date:

October 3, 1973

RCA Corporation, Hew York , HY, V.St.A.

Device for the transmission of television images

The invention relates to a device for the transmission of television pictures, in which a television frame to be transmitted is converted into corresponding frequency-modulated audio frequency signals and clock pulses are generated, by means of which the audio frequency signals are converted back into a video signal for the reproduction of the television frame.

In U.S. Patent Application Serial Ho. 257 412 (filed May 26, 1972) a device is described with which Feraseh still images of three-dimensional objects via message channels such as telephone trunk lines with voice quality and without equalization can be transferred. With the help of a television camera, a video signal is continuously fed to a storage tube, in which any frame of the video information can be "frozen". The stored frame, i.e. the image to be transmitted is then converted into an audio-frequent Signal (audio frequency signal) converted over the telephone network is transmitted to a remote receiving station, where the transmitted audio frequency information in a second Storage tube is saved. After the transfer is complete the audio information stored on the receiver is converted into a Video signal for viewing on a monitor or control

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receiver reconverted * The transmitted signal is essentially frequency-modulated by adding its instantaneous frequency to the brightness level of the stored picture element being transmitted is directly proportional.

As described in the aforementioned US patent application, can the video frame to be transmitted is viewed as being made up of a matrix of horizontal rows and vertical columns will. This matrix of information elements (each of which has a gray value that corresponds to the video brightness) is transmitted in columns and in two rows until the gray value of all elements has been sent «Before the exact transmission this stored video image according to a touch and hold method in which the transmitted output signal has its gray value for one element of the matrix until it assumes the gray value of the next element to be transmitted is open clearly a precise keying is necessary in order to select the individual elements according to their column and row position in the Capture and discharge matrix.

At the receiving station, the recovered audio signal is converted into a pulse signal ₉ in which the pulse amplitude is proportional to the brightness of the individual picture elements and the pulse width is essentially the same as that of the original video element. With the pulses of this pulse signal, the scanning beam of the second storage tube is switched on and controlled in such a way that charge elements are generated on the screen or the storage plate of the tube which are in each case proportional to the brightness level of the corresponding original video elements and are located in the same geographical or local locations like the original elements concerned. A correspondingly precise keying is therefore also required on the receiver side in order to ensure that the image contained in the storage tube of the receiver, which is intended for later viewing as a single video image on a television monitor device, corresponds to the image on the transmitter side.

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This precise keying during image reproduction is then all the more important if for the storage for the subsequent reproduction of the frequency-modulated video information representing Signals (EM signals) audio tape recorders (tape recorders) can be used. The transmitted image information or, at the receiver end, the recovered image information can be recorded with such devices. In this way one can set up a usable registry system.

However, problems can arise when using audio tape recorders to record and play back television images appear. Because of mechanical or structural conditions, there may be slight deviations in the Tape speeds on the magnetic heads result. For example the tape slippage during recording may differ slightly from that during playback; or the resistance of the The roles of the device can vary over time; or the tape itself can stretch with changes in temperature. aside from that There can also be fluctuations in the mains voltage and / or the network; frequency change the belt speed. Such changes are usually more noticeable if and when playback occurs many hours after recording Recordings made with one device can be played back with another device. By such Fluctuations or deviations can cause geometric distortions in the displayed image, both in the form of local shifts in the image information as well as in the form of a distortion or distortion of the entire image.

One way to overcome these difficulties is to use tape decks with a servo mechanism that controls the tape speed so that it remains substantially constant. However, such devices are quite expensive and complex. On the other hand, for devices that cost only about US $ 100 or less, the band ¹ 'speeds considerably, namely by 1 to 2 $>

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vary. Experimental investigations have shown that when image transmission is carried out with devices from the US patent application serial no. 257 412, the mean tape speed errors normally need to be less than 1/100 $> in order for an acceptable image reproduction to be achieved.

The invention is based on the object of creating a device which meets this requirement.

A device of the type mentioned is fiction. according to characterized by a first arrangement with a frequency-adjustable oscillator for generating the clock-wise Tactile pulses; by a second arrangement for generating a reference signal of predetermined frequency, the one substantially has a fixed relationship to the audio frequency signals, such that undesired frequency fluctuations of the audio frequency signals become noticeable as corresponding frequency fluctuations in the reference signal? through an audio tape recorder for storage the reference signal and the frequency modulated signals; by a third arrangement for recovering the frequency modulated Signals during playback of the tape recorder and for providing the audio frequency signals for conversion into the video signal; by a fourth arrangement for recovering the reference signal in playback of the tape recorder; and by a fifth arrangement, which feeds the recovered reference signal to the first arrangement, such that the frequency of the oscillator at the Generation of clock pulse pulses depending on deviations the target signal frequency is adjusted between recording and playback on the Bend recorder.

In the device according to the invention is therefore to achieve a correct or faithful image reproduction influences the electronic scanning process during image re-education, instead of the tape speed of the recorder being regulated mechanically. For this purpose, the Tape recording of the frequency-modulated signal together with

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this recorded a control or pilot tone of essentially fixed frequency. The pilot tone is then used when playing detected and used to control the frequency and / or phase of a control oscillator that controls the timing or timing for the signal key when switching on the beam of the storage tube worried. Changes or fluctuations in the belt speed Speed, regardless of what it is caused, is expressed in corresponding fluctuations in the frequency and / or phase of the recovered Pilot tones so that tape speed changes the frequency and / or phase of the frequency-modulated signal components affect the video information, which is in a fixed relationship to it. The regained pilot tone can then used to adjust the timing or clock of the system to reflect the changes that these signal components experienced as a result of deviations in tape speed between recording and playback, compensated will.

The invention is described below with reference to the drawing in individually explained. Show it:

Figures 1A and 1B are simplified block circuit diagrams of parts a telephone picture transmission system employing the invention j

Figure 2 shows a block diagram of the processing stages on the transmitter side the device according to the aforementioned US patent application Serial Ho. 257 412 in inventive training} and

FIG. 3 shows a block diagram of the processing stages at the receiver end the establishment of the aforementioned US patent application in training according to the invention.

In FIG. 1A, the video information is included frequency-modulated audio signal that is transmitted to terminal 10 (either the remote end of the telephone line on the receiver side or the transmitter-side input end of this line) is pending, fed to a notch filter 12, the image frequency components in the A pilot tone neighborhood of essentially tighter

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Frequency that is fed to a second terminal 11 is suppressed. In a practical, proven embodiment of the invention, this notch filter was dimensioned so that it frequencies suppressed in the vicinity of 8 kHz (the pilot tone frequency), the pilot tone then being algebraic in an adder 13 is added to the FM output signal of the notch filter 12. The resulting sum signal is fed to an audio tape recorder (tape recorder) 14, where it is stored in a single Channel or a single track on the tape.

When playing, the recorded signal becomes parallel two filters 22, 23 supplied (Figure 1B). The first filter - a pilot tone notch filter - separates the 8 kHz pilot tone and its neighboring frequencies from the Yideo part of the input signal and couples this signal part to an image processing and display arrangement 24 for image reproduction. The second filter 23 - a pilot tone pull-out filter - also separates the recorded one Signal into its two components, but in this case the pilot tone is frequency and phase synchronized Control oscillator 25 is fed, which in turn controls the sequence of operations in the image processing and Display arrangement 24 controls. The stage 25 contains a frequency and phase detector 26 and a voltage controlled one Oscillator 27, the frequency and phase detector 26 control signals for setting the frequency and phase of the Oscillator 27 generated in a known manner. For this purpose, the control oscillator 25 can, for example, be a stable voltage-controlled one 8 MHz oscillator included, which is suitable for frequency and phase comparison with the perceived pilot tone Way is counted down. The 8 kHz pilot tone added during recording can be adjusted by counting down from a highly stable, quartz-controlled 8 MHz clock generator can be obtained in a suitable manner.

As you can see, the frequency and phase of the control oscillator 25 follows the center frequency and phase of the extracting

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ten pilot tones. The image processing and display arrangement 24 thus adapts to the average belt speed in its mode of operation on, whereby deviations in the tape speed between recording and playback are compensated. As will be seen later, when a single image is in The form of a stored video signal over the telephone line for the purpose of "real time" playback without saving Tape recording is transmitted to the control oscillator 25 at a frequency controlled by a highly stable 8 MHz crystal and is only variable by about 13 parts per million. Proper synchronization of the operation of the image processing and The display arrangement in the receiver arises essentially when an 8 MHz crystal of the same type is used in the transmitter will control the required image sampling and discharge of the IM audio signal components onto the telephone line.

As in the aforementioned US patent application Serial No. 257, the transmitter part of the system includes a television camera, a storage tube coupled to its output and a converter for converting the information taken from the storage tube Video signal to audio frequency signal for input to a telephone line. The receiver part contains a second Converter for converting the audio signal received from the output of the telephone line into a pulse modulated signal, a second storage tube for recording the pulse modulations corresponding to the transmitted image and a television monitor device to play back the received stored image. Figures 2 and 3 show block diagrams of the converter in the transmitter and of the converter in the receiver part.

In FIG. 2, the entire time control is provided by an 8 MHz clock generator 50 with a quartz oscillator, the oscillations of which are divided by 512 in a synchronization generator 52, so that horizontal control signals are generated at the output 54 will. The frequency-reduced clock oscillations are further divided by 262 1/2, so that at the output 56 vertical control signals be generated. With the two

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generated horizontal and vertical control signals the television camera is controlled in generating the video information from which the frame selected for transmission is entered into the first storage tube.

The stored video frame is in 525 horizontal lines and 512 vertical columns can be dismantled. This matrix of 268,800 information elements (each with a specific Gray value) can be transferred column by column, starting with the upper left corner of the matrix, whereby for the transfer a time of 1/8 of a second is required for each column of 525 lines, i.e. a transmission frequency of 3933 elements per second for transmission over a telephone line with a bandwidth on the order of 2 kHz and one almost twice as high video sampling frequency. The modulated signal is derived from the stored image video content after Touch and hold method developed in which the output signal retains the gray value of the first element of the column for 1/4 millisecond, then the gray value of the second element of the Column assumes, maintains this gray value for the next 1/4 millisecond, and so on. If you use a Matrix of 525 horizontal lines and 256 vertical columns, so you can use a 4 MHz clock generator, with the matrix then 134 400 information elements and the transmission of a column of 525 lines 1/4 second with a modem with IkHz Bandwidth used.

In Figure 2, this sample and hold output generated by a pulse generator 58, which is connected to the output of the clock 50 and in turn a first Sensing unit 60 controls so that the gray value of the video element fed to its input 62 from the transmitter storage tube for the interval of 1/4 millisecond is recorded. This key and hold output signal is then filtered through an active 2 kHz low-pass filter 64 is fed to a frequency modulator. This frequency modulator 66 can be set up in this way be that it delivers an output signal that is suitable for a video

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element of minimum brightness (i.e. black) has a frequency of 2.4 kHz and for a video element of maximum brightness (i.e. white) has a frequency of 1.4 kHz and its frequency is for gray signal components according to the brightness of the individual elements linearly between these cutoff frequencies changes.

The stored single video image is divided into the matrix of 268 800 elements by identifying the 512 columns in each of the 525 horizontal lines. Since the horizontal sync signal is derived by dividing the 8 MHz clock signal by 512, each contains Horizontal line 512 clock periods by means of which each of the 512 columns within the individual line is easily identified will be. Any element in the array can then be precisely localized by both the horizontal line to which the element in question belongs and the corresponding clock period is specified in the relevant line. To change the gradation from column to column every 1/8 Microsecond, starting on the left side of a video frame, to generate, the horizontal frequencies and the Vertical frequencies pulses fed to a counter circuit 68, the output variable of which corresponds to that of the clock generator 50 in the pulse generator 58 is compared to generate the key signal for the key and hold circuit 60.

With this arrangement, the system according to the aforementioned US patent application Ser.No. 257 412 with an F _r FREQUENCY of 1 element per 8 TV line intervals (horizontal intervals), so that successively transmitted sample values in the imager of the receiver can be reproduced spatially adjacent. Synchronized information about the starting time is conveyed in that a known audio frequency burst (audio frequency oscillation pulse) is transmitted at the beginning of each image transmission, in that an output signal from the counter circuit 68 is fed to an adder 70 together with the FM signal generated by the frequency modulator 66.

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In addition, the adder 70 is fed the output signal of a + 8- "circuit 72 which, upon receipt of the horizontal control signal the receiver-side key pulse is synchronized in such a way that that it occurs at the right time with respect to the transmitter-side key pulse. The output of adder 70 is via the output terminal 74 in the telephone line for the Transfer entered ^ being for the purpose of transferring acoustic and induction input / output devices as well Standard telephone company data couplers can be used.

As far as previously described ₉ , the transmitter-side converter according to FIG. 2 is essentially identical to that described in the US patent application Ser.No. 257 412 is identical to the arrangement described, the only difference being the use of a clock generator with 8 MHz instead of 4 MHz, and in particular in the generation of a matrix with 512 instead of 256 columns, which is achieved as a result. According to the invention, however, a second adder 76 is also provided _9, the sum signal to be fed into the telephone line via the output terminal 74 at a first input and a pilot tone with a frequency of essentially 8 kHz from a 42-circuit connected downstream from the synchronization generator 52 at a second input 78 receives. In a practically tested embodiment with an 8 MHz quartz oscillator as the clock generator 50, the actual counting in the synchronization generator 52 results in a pilot tone for the adder 76 at a frequency of approximately 7.867 kHz. Since the quartz-controlled clock generator 50 controls both the generation of the pilot tone and the keying of the video signal, the output signal of the additional adder 76 contains these two components in an essentially fixed frequency relationship. The given frequency spacing between the pilot tone of essentially 8 kHz and the FM signal of 1.4 to 2.4 kHz ensures a sufficient separation between these two signals for perception and detection.

The output signal of the additional adder 76 is used

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as a microphone input for a tape recorder which, according to the invention, is an inexpensive cassette recorder with a price of approximately Can be $ 20 to $ 100. With such an arrangement the video signals to be transmitted over the telephone line can also be recorded on tape in the transmitter. Together the pilot tone is recorded with the signals, which, as explained below with reference to FIG. 3, during playback is used to reproduce the image with the medium tape speed instead of some crystal oscillator synchronize. The tape recording can later be played back with a playback device at the sending station or by post, Messenger or the like to be sent to other remote recipients for reproduction there.

However, the tape speed at the recording heads during the recording of the pilot tone and the video signal from the tape speed to the Playback heads, be it the same or a different device, deviates during the playback of the recorded information. Such deviations in the belt speed can result from differences in the mains voltage or mains frequency between the time of recording and the time of playback and / or due to mechanical irregularities in the equipment used, e.g. in the belt stretch, the roller pressure force, etc.

The transducer at the receiving station is shown in Figure 3 where the EM signal received from the telephone line is fed to a limiter 80 and then to a demodulator 82 for recovery of the original modulation signal. The output signal of the demodulator 82 passes through a second low-pass filter 84 for the purpose of keying to a second keying circuit 86, which is controlled in largely the same way as the first keying circuit in the transmitter. An 8 MHz clock generator 88 supplies an output signal to a pulse generator 90, which output signal is compared with a pulse signal from a counter 92 which is derived from applied horizontal and vertical control signals which are generated by a synchronizing

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Signal generator 94 after corresponding frequency division of the supplied 8 MHz clock signal can be generated. A tone detector 96 connected to the output of limiter 80 controls the Operating time for the generation of the key pulses for the key circuit 86.

The output signal of this second key circuit 86 exists from a sequence of amplitude-modulated pulses with a mutual spacing of 1/4 millisecond, corresponding to the Audio signal transmission frequency of a telephone line with a bandwidth of 2 kHz. Every 8 television frames will be a column of video elements completed and the position or Position of the generated impulses advanced by 1/8 microsecond to the right across the image «. Each generated impulse therefore represents represents the output video size of one element of the original 268,800 element matrix, where the amplitude of each Pulse is proportional to the gray value of the corresponding matrix element. With these output pulses at output 98 is then the beam current of a storage tube is modulated by, for example the beam is only switched on when impulses are present. Under the action of the horizontal and Vertical deflection frequencies, the original matrix charge pattern on the storage disk is then reconstructed by these pulses, so that when the transmission is complete, normal reading of the stored image retains the original television video signal information can be reproduced. As with the arrangement according to FIG. 2, by changing the clock frequency of the clock 88 in 4 MHs get a matrix with 256 columns, in which case the output of the key circuit 86 consists of amplitude modulated pulses with a mutual spacing of 1/2 millisecond.

According to the invention, the arrangement according to FIG. 3 also contains a single pole changeover switch 100, an FM = and pilot tone separator 101 and a tape speed detector 102. When the switch 100, the contact "B" is switched to the contact "A". is, the FM audio signal from the TeQephonleitung is den

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Converter stages for processing and reproducing the video signals in the manner described above. If, on the other hand, the Contact "B" of switch 100 switched to contact "C" is, the recorded derived in the separation stage 101 comes FM signal from tape recorder to limiter 80 for further processing. The FM and pilot tone separator 101 corresponds a combination of the pilot tone pull-out filter 23 and the pilot tone blocking filter 22 in FIG. 2 and splits the output signal of the tape recorder into its two separate FM and pilot tone components. The tape speed detector 102 is also used only when the image recorded on the tape is played.

The 8 MHz clock generator 88 contains one in the present case Crystal oscillator, which is used when receiving telephone signals, and a frequency-controlled oscillator, which is used when playing the recorded signals can be tuned by changing a DC voltage on a capacitance diode. That The signal from the tape recorder is first band-pass filtered in the separator 101 at a frequency of 7 »867 kHz to avoid the pilot tone pull out. This pilot tone is then used in the belt speed detector amplified and frequency divided by 32 to provide the one input variable for a digital 3? frequency / phase detector to win. The 8 MHz output frequency of the frequency controlled The oscillator in the clock generator 88 is also divided in the belt speed detector 102 to the same nominal frequency as the pilot tone and then divided again to be the second To serve input variable for the digital frequency / phase detector. This detector is specially designed so that it causes the 8 MHz control oscillator to synchronize with the pilot tone frequency. As the tape speed increases, then exceeds the {! Frequency of the frequency-divided pilot tone the frequency del frequency-divided clock signal, so that the duty cycle (Pulse duration / pulse period) of the square-wave output oscillation in the sense of an increase in the value of the direct current component, is changed. When the tape speed decreases 'so that the frequency of the frequency-divided clock-'

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signal exceeds the frequency of the frequency-divided pilot tone, the direct current component is the square wave with proportional duty cycle lower. This direct current component is filtered out through a filter and then the capacitance Zitätsdiode in the clock 88 fed to the frequency-controlled 8 MHz oscillator so that it is on the The frequency of the pilot tone locks in or is dragged along by the pilot tone frequency. The time constant of the filter can be approximately 1 second to make the 8 MHz oscillator insensitive to band tremors or flutter during the phase comparisons at a divided frequency, e.g. 250 Hz, instead of the higher pilot tone frequency to the higher frequency Average out tremors or flutter. In this way, the scanning operations of the key circuit to the middle Belt speed is synchronized instead of the current belt speed. Any deviations in the belt speed between recording the FM audio signals and playing back the information recorded on the tape compensated so that the otherwise resulting spatial image distortions are minimized.

The inventive principle of timing the receiver-side image processing stages of the image transmission system as a function of the average running speed of the tape on which the corresponding information is recorded, instead of as a function of the vibrations of a stable oscillator, can also be implemented by means other than those described above as the preferred embodiment . The invention enables the use of a tape recorder with a tape speed varying by 1 or 2 fo in a picture display system which requires an accuracy of 1/100 # for perfect picture display.

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Claims (5)

Claims T) M / device for the transmission of television images, at which a television frame to be transmitted is converted into corresponding frequency-modulated audio frequency signals and Clock pulse pulses are generated, by means of which the audio frequency signals in a video signal for the reproduction of the television frame are converted back, characterized by a first arrangement (27) with a frequency adjustable Oscillator for generating the clock pulse pulses j by a second arrangement (50) for generating a reference signal of a predetermined frequency, which is a substantially borrowed fixed relationship to the audio frequency signals, such that undesired frequency fluctuations of the audio frequency signals become noticeable as corresponding frequency fluctuations in the reference signal5 by means of an audio tape recorder (14) for storing the reference signal and the frequency modulated signals; by a third arrangement (22) for retrieval of the frequency-modulated signals when playing the tape recorder and for providing the audio frequency signals for the conversion into the video signal; by a fourth arrangement (23) for recovering the reference signal when playing the tape recorder; and by a fifth arrangement (26) which feeds the retrieved reference signal to the first arrangement such that the Frequency of the oscillator when generating the clock pulse pulses as a function of deviations in the reference signal frequency is adjusted between recording and playback on the tape recorder. 2. Device according to claim 1, characterized in that g e Kennzeich.net that the first arrangement (27) and the second arrangement (50) each have a quartz-controlled oscillator and that the fifth arrangement (26) comprises a detector circuit to compare the frequency and phase of the oscillations of the first arrangement with the frequency and phase of the Contains the reference signal after playing from the tape recorder (Η). 509815/0990 3. Device according to claim 2, characterized in that g e that the frequency ™ and phase comparison takes place with a follow-up frequency that is lower than the repetition rate of the retrieved reference signal of the predetermined frequency. 4. Device according to one of claims 1, 2 or 3, characterized in that the third arrangement (22) contains a filter which passes frequencies in the vicinity of the frequency-modulated audio frequency signals and blocks frequencies in the vicinity of the reference signal; and that the fourth arrangement (23) includes a filter which Frequencies in the vicinity of the reference signal passes and frequencies in the vicinity of the frequency-modulated Blocks audio frequency signals. 5. Device according to claim 4> characterized that the first arrangement (27) contains a voltage-controlled oscillator and that the fifth Arrangement (26) voltage signals corresponding to the frequency and phase deviations between the oscillations of the first arrangement and supplies the reference signal for supply to the voltage controlled oscillator such that the oscillation frequency of the voltage-controlled oscillator is adjusted in the sense of a correction of the deviations. 509815/0990