SYSTEM A MODULATOR


Posted on Feb 6, 2014

With the impending close down of the 405-line transmitter network those of us with collections of early TV sets are having to make new arrangements to supply them with suitable signals. Whatever the scheme used, some method of converting baseband audio and video into a form suitable for feeding into the set`s aerial socket is required. The modulat


SYSTEM A MODULATOR
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or described in this article was designed to perform this task with little degradation of the signal. Separate carrier generators and modulators are used for the sound and vision for best possible sound-on-vision and vision-on-sound performance. Crystal control of the oscillators was chosen to ensure stability and reduce alignment problems. The choice of Channel 1 (41. 5MHz sound, 45MHz vision) was made because the vast majority of 405-line TV sets, including all pre-war models, can operate on this channel. Those with one of the few Ch. 4 "Birmingham" models of the late 40s should be able to modify the design for 58. 25MHz sound and 61. 75MHz vision without much difficulty. Fig. 1 shows the vision modulator circuit. The 1V peak-to-peak video input is first amplified by Trl/2 and then fed via the emitter-follower Tr3 to the d. c. restorer Tr4. Tr5 is connected in a Butler oscillator circuit operating at 45MHz. The output from the modulator chip is transformer coupled to the output socket via a lOdB pad and a 6dB combining network. The lOdB pads in this and the sound modulator circuit act as isolators to protect each modulator from the other`s r. f. output. The sound modulator circuit (see Fig. 2) is similar, with IC2 driving the modulator chip in push-pull to reduce distortion to a minimum. The audio sensitivity of 1V r. m. s. = 100 per cent modulation is set by the value of Rl, which can be varied if desired. For example,...




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