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The first description of the Marconi-EMI installation. Equipment at Alexandra Palace for television broadcasting is now rapidly nearing completion, and in this article appears the first description of the apparatus installed by the Marconi-EMI Television Co. The system employed is entirely electrical and mechanical methods find no place in the installation.
An artists impression of the aerials.
The time is fast approaching when television transmissions will start as a regular broadcasting service l instead of being merely of an experimental nature. It will be remembered that a full description of the nature of the modulated Waveform to be employed appeared in The Wireless World for October 4th, 1935, but until the present no information about the actual apparatus has been released. Details of the Marconi-EMI equipment installed at the Alexandra Palace have now been given us.
The Marconi-EMI scanning camera.
Four Emitron instantaneous scanning cameras, and two specially arranged with film projectors for use with cinematograph film, are provided. They are fitted with 6.5 in. F/3 lenses, and enable the scenes which it is desired to transmit to be converted into the appropriate electrical impulses without the necessity for using an intermediate film. Their sensitivity is sufficient to permit their being used outdoors under normal daylight conditions.
The scanning camera with a film projector.
Specially designed multi-way cables are used for the connections between the cameras and the control equipment. This last consists of several sections, of which the more important are the voltage supply supply units, the scanning circuits, and the amplifiers. A single unit generates the voltages necessary for focusing the cathode-ray tubes employed in the cameras, The voltages from this unit, however, are fed to the cameras through six sub-units containing the controls necessary for the exact adjustments of the voltages for each tube. A similar arrangement is employed in the case of the scanning circuits. One unit generates the saw-tooth potentials, and six other units distribute them to the cameras.
Each camera is provided with an amplifier for its picture signal, and the outputs are taken to a mixing and fading unit fitted with controls to enable a change to be made from one camera to another, or the outputs of two or more to be superimposed. The signal then passes through further amplifiers and units which enable both picture detail and picture contrast to be controlled. To the output of this equipment a cathode-ray tube is connected so that the picture can be seen for monitoring purposes.
The next step is to inject the synchronising impulses into the signal, and when this has been done further amplification follows. No less than six output channels are provided, and of these four are fitted for connection to monitors and two (one being a spare) for feeding the signal to the modulator of the transmitter.
Before turning to the transmitting equipment proper, the remaining apparatus in this section deserves a brief description. It consists of master oscillators for maintaining the line and picture synchronising impulses at their correct frequencies. Actually, a single mains controlled oscillator is used, and other units divide its frequency by suitable factors for the line and picture pulses so that these always have exactly the same relationship to one another. The outputs of these generators are taken to a number of other units for distribution to the proper points in the system.
Part of the ultra-short wave transmitter which will be used for the transmissions from Alexandra Palace.
In the transmitter itself a master oscillator is used and followed by a frequency-doubler, five stages of amplification at the carrier frequency, and then a single modulator stage. In most stages of the amplifier two valves are employed and connected in a balanced bridge circuit. The later stages are mounted in separate units connected together by means of concentric tube feeders.
The arrangements for cooling the valves are particularly interesting in view of the precautions taken to avoid losses due to the water used for cooling. The water actually circulates through the tubing which forms the tuning inductance and is led out at the electrical centre of the circuit by rubber tubing arranged in spiral form.
Modulation occurs in the last stage, the picture signal being previously amplified by a six-stage amplifier having a flat characteristic over the enormous range of zero to 3 MHz. The modulated high-frequency output is coupled to a concentric tube feeder having an impedance of 75 Ω which carries the currents to the aerial system erected on one of the towers of Alexandra Palace. This aerial system is designed to give maximum radiation in an horizontal plane and consists of a number of aerial units suspended around the periphery of an octagon.
From this brief survey of the apparatus employed for the vision-transmissions, some idea of the complexity of the system can be gauged. There is, of course, a host of essential equipment which cannot be touched upon here - the voltage and current supplies, the control panels, and the studios but which are all very necessary links in the chain. It should be remembered also that this equipment is to be employed for the vision signals only entirely separate transmitting apparatus is needed for the sound accompaniment.
The waveform of the modulation of the signal is shown here. Modulation depths between 30% and 100% convey the picture, while the synchronising signals are transmitted with a lower depth in the intervals of the picture signals.
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