Patent GB-680648-A (GB)
Title: Frequency band reducing system for television transmissions
IPC: H04B1/66
Filed: 1950-01-13
Abstract: 680,648. Pulse code modulation systems. STANDARD TELEPHONES & CABLES, Ltd. (Aigrain, P. R. R.). Jan. 13, 1950, No. 918/50. Class 40 (v). [Also in Group XL (b)] A television video signal is converted into a stepped wave form of approximately corresponding shape, and for each step two pulses are transmitted, one (called an A-pulse) representing the magnitude of the changed amplitude and the other (called a B-pulse) the position along a scanned line at which the change in amplitude occurred. In the transmitter, Fig. 1, the video output from an iconoscope 1 is converted to a stepped wave by apparatus 4 similar to that described in Specification 651,115. At each step a differentiator 5 causes a flip-flop circuit 6 to produce a 1/50 millisecond pulse which opens gates 7, 8. Gate 7 produces the B-pulse by passing a portion of the horizontal scanning sawtooth wave from apparatus 2, the amplitude of the wave at this time representing the position along the scanning line where the step occurs. Gate 8 produces the A-pulse by passing a portion of the stepped wave from apparatus 4. The A-pulse is delayed 1/50 millisecond by network 9 and the two pulses are combined in circuit 10 and "written" or recorded side by side on a storage tube 11. The writing scanning circuit 12, shown in more detail in Fig. 2, comprises a flip-flop circuit 18, also fed with the 1/50 millisecond pulse from circuit 6 and producing in response thereto a 1/25 millisecond pulse which, applied to the control grid of a pentode 20, causes condenser 24 to discharge by an amount sufficient to move the writing beam of tube 11 for the recording of the corresponding pair of A and B pulses. Pairs of pulses are thus recorded side by side in a solid block and at the end of a line scan of iconoscope 1 the writing beam is moved to the middle of the first line of tube 11 under the control of a sawtooth wave of half the line frequency applied over lead 17 to a gate circuit 25 which is opened by a pulse from differentiator 25 during the fly-back period of the horizontal sawtooth wave. The reading scanning circuit 13, Fig. 1, is fed with the horizontal sawtooth wave and produces the half-frequency wave for supply to circuit 12 and the same frequency is supplied to the tube 11 but with 180 degrees phase shift so that the recorded pulses are read off from one half of a line whilst recording takes place on the other half. The read-off pulses are sent out via a low-pass filter 14, and synchronizing pulses of frame frequency are transmitted over circuit 43. At the receiver, Fig. 3, the frame synchronizing pulses are fed to the reproducing tube 32 and to a horizontal sawtooth generator 31 which also feeds tube 32. The apirs of A- and B-pulses are written in blocks as transmitted on a storage tube 29 which has a writing scanning circuit 33 similar to circuit 13, Fig. 1, and a reading scanning circuit 34 similar to circuit 12. The read-off pairs of pulses are passed to a normally-blocked gate 41 and a normally-unblocked gate 40. A B-pulse passing through gate 40 discharges condenser 36 to a potential determined by its amplitude and when the horizontal sawtooth wave reaches the same.potential, a comparison circuit 35 emits a 1/50 millisecond pulse which moves the reading scanning beam through the position of one pair of A and B pulses. The pulse from comparison circuit 35 is also passed with 1/50 millisecond delay to open gate 41 and close gate 40 so that the corresponding A pulse is applied via a storage condenser 39 to the intensity-control electrode of reproducing tube 32.