GB605109A - Television receiving systems - Google Patents

Abstract

605,109. Television. STANDARD TELEPHONES & CABLES, Ltd. Dec. 14, 1945, No. 33906. Convention date, Aug. 25, 1944. [Class 40 (iii)] [Also in Group XL (c)] A television receiving system comprises means adapted to receive a desired one of a plurality of programmes, each of which is represented by vision signals transmitted by modulation of a corresponding one of a plurality of carrier waves of different frequencies, the necessary synchronising signals, which are common to all vision programmes, being transmitted on a carrier wave of frequency different from that of any of the other carrier waves. The carrier wave with the synchronising signals may also carry modulated pulses representing the sound signals for the several vision programmes. The receiver 11, Fig. 1, includes antennµ 12, 13, directively positioned with respect to the transmitting antenna 5 and adapted to receive respectively the video and multi-channel carriers which after amplification and detection in the tunable amplifiers 14, 15 and detectors 16, 18 have a wave form and pulse train such as a, b, Fig. 2, respectively. The video wave-form comprising vision signals 10 and horizontal and vertical blanking pulses HB, VB together with appropriate synchronising pulses are utilized to reproduce the image in the picture reproducer 17 according to known methods. The synchronising pulses are derived from the pulse train b, Fig. 2, in the pulse width discriminator 19, shown in detail in Fig. 3, in which the pulse train is applied to a limit clipping and inverting valve 20, the output of which shock excites the L.C. circuit 21, which is tuned to a resonant frequency equal to half the period of the pulse V, Fig. 2, so that during .the occurrence of the latter pulses the oscillatory effects produced in circuit 21 by the leading and trailing edges of these pulses add together to produce an oscillation such as 34, 39, (curve c, Fig. 2), whereas, during the occurrence of the narrower pulses H, the additive effect produces an average oscillation such as 34, 39, Fig. 2, and, the still narrower sound pulses 1, 2, 3, oscillations such as 40. Both oscillations are damped out after one complete period due to the action of valve 22, which is shunted across circuit 21 and supplied with a negative blocking pulse during 'the occurrence of the pulses Hand V. The output waveform c, Fig. 2, is clipped at levels 31, 45, Fig. 2, in clippers 30, 32 from which horizontal and vertical synchronising pulses d, e, Fig. 2, are produced in the pulse shapers 42, 46, Fig. 1. In order to select the sound channel corresponding to the desired vision signals the pulse train b, Fig. 2, is applied to the channel selecting mixer 49 which permits the passage of signal pulses only when an additional pulse, such as 52, f, Fig. 2, is simultaneously applied. This pulse is derived from the horizontal pulse shaper 42 and after further shaping in 51 is applied to the channel selecting mixer 49 via a variable delay device 53, which retards the pulses 52 by an amount sufficient to cause them to coincide with the occurrence of the desired sound pulse 1, 2 or 3, Fig. 2, and, consequently to raise their level above the clipping level 54 of the limit amplifier 55. This amplifier is also supplied with pulses d, Fig. 2, from the horizontal pulse shaper 42 which act as reference pulses from which the displacement of the sound pulses may be measured to effect translation of this displacement to amplitude variations in the demodulator 56 for application to the sound reproducer 58 via the filter 59. A suitable circuit for the demodulator is shown in Fig. 4, and comprises a valve 62 in the screengrid circuit of which is a resonant circuit 64 tuned to provide a shock-excited damped wave, the peaks of which pass through the pulse position points of h, Fig. 2, in the absence of modulation. When, due to modulation, the sound pulses are displaced from this latter position, their occurrence coincides with the linear portion of the shock-excited wave, thereby causing the oscillatory swing to vary in amplitude by an amount proportional to their displacement from their zero-modulation position, this position being determined by the reference pulse 43a, Fig. 2, which sets the clipping level of valve 62 at 76, Fig. 2, and produces the pulse train j, the envelope 77 of which defines the envelope of the original modulating signals. Specification 597,461 is referred to.