Patent GB-1393674-A (GB)
Title: Telephone image transmission system
IPC: H04N7/12
Filed: 1971-06-01
Abstract: 1393674 Television RCA CORPORATION 31 May 1972 [1 June 1971] 18337/71 Heading H4F To enable one frame of the video signal from a TV camera 10, Fig. 1, viewing a moving object, to be transmitted over a narrow bandwidth link, the television camera 10 continuously feeds the video signal to an information storage unit 12, comprising a silicon storage tube, at standard line and frame scan rates. A stored single frame is analysed in terms of columns and each column sampled at a reduced rate and fed to a transmitter TV converter 14 for conversion into an audio frequency signal capable of being transmitted over the narrow bandwidth line, e.g. telephone line. At the receiving location the receiver TV converter 20 processes the audio frequency signal to produce a sample train signal which is then stored in information storage unit 22, also utilizing a silicon storage tube. Upon completion of the transmission, the single video frame can be viewed with a TV monitor 24. Upon pressing the start button at the transmitter TV converter 14, the stored video frame is divided into 134,400 elements, Fig. 3, which are transmitted sequentially column by column in the order shown at the rate of 2,100 elements per second, i.e. a column of 525 samples is transmitted from 8 successive TV frames. The converter generates an audio signal having a frequency determined by the brightness of the element. This signal is transmitted to the receiver TV converter 20 where it is changed into a pulse with an amplitude proportional to the element brightness and with a width the same as the original element. This pulse turns on the beam of the storage tube in the information storage unit 22 and thereby deposits a charge on the target which is proportional to the brightness level of the original element, and is located on the target at the same geographic location as the original element. Thus, after 65<SP>5</SP>/ 8  second transmission time, the storage tube will have an image corresponding to that at the transmitter. In the transmitter TV converter, Fig. 2, a 4 MHz crystal clock 30 determines all timing. Its output is divided by 256 to produce horizontal drive which in turn is divided by 262-1/2 to produce vertical drive. These horizontal and vertical pulses are used to operate the TV camera 10. When the transmission of the single frame begins, the vertical drive frequency to the storage system is reduced by a factor of 16 to bring about the 2100 element/sec. sampling rate. The horizontal driving rate is not changed. Since prolonged reading of a stored image results in gradually reduced contrast, the blanking signal to the storage system is also changed during transmission to blank the electron beam during the lines which are not sampled. The modulating signal is developed from the stored image video by a sample-and-hold circuit. The sample-and-hold output remains at the gray level of the first element of the column (Fig. 3), for ¢ ms. at which time it assumes the gray level of the second element of the column &c. To reduce transmission time, the elements corresponding to the original TV horizontal blanking (i.e. 15% of the picture), are not transmitted. A sampling pulse that automatically steps from column to column every “ second, beginning at the left of a video frame, is derived by a counter 48 whose inputs are the master clock and a counter clocking every 8 TV frames. During the first 8 TV frames the sample pulse occurs during the first master clock cycle along the TV lines. During the second group of 8 TV frames the sample pulse occurs during the second master clock cycle along the TV lines, &c. The sample-and-hold output signal is filtered with an active 1 kHz low-pass filter 44 and applied to a frequency modulator 46. The FM signal has a frequency of 2À2 kHz for a black video element, 1À2 kHz for a white video element, and varies linearly between these extremes for gray video elements. Any element can be precisely located by specifying the horizontal line containing the element and the clock cycle along that line. It is necessary for the receiver to know the rate at which successive elements are transmitted and the order in which they are transmitted. If this is specified as a standard, then only the starting time need be transmitted for each individual picture to provide synchronization. Starting time information is furnished by transmitting (adder 50) a known tone burst at the beginning of each transmission. The received FM signal is limited at 60, Fig. 4, and demodulated (62) to reproduce the original modulating waveform. This waveform is sampled (65) with a sample pulse generated in the same way as at the transmitter. The second sampler 65 output is a train of amplitude modulated pulses spaced “ ms. apart. Every eight TV frames a column is completed and the position of these pulses steps “ ms. to the right across the picture. These pulses are used to modulate the beam current in the storage tube and turn on the beam only when a pulse is present. With the normal horizontal and 1/16 normal vertical deflection frequencies, the pulses reconstruct the original matrix charge pattern on the target. When the transmission is completed, normal reading of the stored image reproduces the original TV video signal. Master timing is produced with a 4 MHz crystal clock identical to the transmitter clock. The input signal is analysed with band-pass circuitry which detects the presence of the starting tone burst. Upon recognition of the tone, the sampling and storing of video pulses begins. If the object viewed by the camera 10 is fixed, the first storage tube can be dispensed with and the 1/16 reduced vertical scan/sample rate can be directly used in the television camera tube.