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Visitors to the recent Olympia Radio Show saw demonstrations of television reception from the Alexandra Palace, here we describe the two competing systems.
Now that the transmitting apparatus is installed in the Alexandra Palace, and experimental transmissions have commenced, technical details of the apparatus employed are available. It will be remembered that the Baird and Marconi-EMI companies have each installed separate equipment, which is to be used alternately for transmissions by the different methods developed by these firms. In each case the gear can be divided into sections performing similar functions, but the actual methods employed are very different.
The Baird Spotlight projector.
In the Baird system three different types of scanning apparatus are provided. For studio work 'Spotlight' scanning is used; a beam of light is focused through a small water-cooled rectangular window situated at the top of the scanning unit, in which there are two discs running in vacuo and driven by separate synchronous motors also running in vacuo. The scanning disc revolves at 6,000 rpm and has 240 apertures arranged in four spiral traces. The second disc has a spiral slit and acts as a shutter, so that only one of the 240 holes is exposed to the light at any instant.
The line-synchronising impulse generator is associated with this apparatus and consists of a light source, optical system, photocell, and the necessary amplifiers. There are 240 holes arranged in a circular trace around the scanning disc, and the passage of light through these causes the photo-cell to give a pulse of current for every hole.
The Baird control desk for vision and sound.
The spotlight beam from the scanning unit is focused through the window of the projection room into the studio. It is the light reflected from the subject being televised which actuates the photo-cells, and there are actually four of these of the multiplier type. The output from each is amplified in the studio, and again in the projection room, where it is mixed with the synchronising signal and passed to the control room.
The Baird Intermediate Film Scanner.
Intermediate film scanning employed when large scenes have to be dealt with. The subject is photographed on 17.5 mm. film with a motion-picture camera of the intermittent type, the sound also being recorded on the film. The film then passes into a developing chamber beneath the camera, and, after being developed, is washed, fixed, and washed again, the whole process taking only 30 seconds. The film then passes through a scanning compartment which is not unlike the one already described, although there are naturally many minor points of difference. A similar scanning system is also provided for dealing with standard 35 mm, films, although here the film runs continuously at the rate of 25 frames per second. After amplification the combined picture and synchronising signals are fed to the control room, in which the outputs from the various scanners can be selected and faded. The output from the fader is taken to a low-gain amplifier, which feeds the sideband-corrector amplifier to which the line and frame synchronising pulses are also fed.
The Baird control desk and part of the transmitter.
In the transmitter itself crystal control is used, the crystal oscillating at 1.406 MHz, and its output being passed through amplifiers and frequency-doublers. The output from this stage is about 100 Watts at 45 MHz, and is taken through a concentric feeder to the drive stage, in which a water-cooled tetrode is employed and in which modulation is effected. The modulated signal is then passed to the output stage, in which there is also a single water-cooled tetrode. The circuits associated with this stage are also water-cooled, the cooling system being also employed to provide the necessary damping.
At various stages through the chain monitory devices are employed, and in certain instances these take the form of cathode-ray tubes. A considerable amount of additional apparatus is employed for providing the operating potentials for this gear.
A view of the Marconi-EMI Studio showing the Emitron camera in use.
The Marconi-EMI system is entirely electronic, and the Emitron camera is employed as the link between the visible and the electrical. The basic unit, however, is the pulse generator, for this provides the necessary pulses for operating the camera and also for the synchronising signals. It consists essentially of an oscillator system providing output signals in the form of black bars for testing, line and frame synchronisation pulses, interference suppression pulses, black-out pulses for camera-gun flyback, camera-gun line and frame scanning pulses, and picture shape-correction, pulses.
The Marconi-EMI Emitron Instantaneous Television Camera unit and film projector.
The Emitron camera operates on the cathode-ray principle and contains a mosaic plate of photoelectric material upon which the image is focused. This is scanned by the electronic beam, and an output of about 2 mV is obtained; the sensitivity being adequate to allow the camera being used under conditions of normal daylight or studio lighting. The output of the camera is fed straight into the head amplifier, in which two resistance capacity coupled stages are used in addition to the input and pentode output valves. The output is designed to match the impedance of the 18-way cable connecting the camera to the later amplifiers.
Two leads, which are screened and of low capacity, carry the picture signal from the camera to the main amplifiers, while four other leads supply the high- and low-frequency scanning impulses , for the electro-magnetic deflection of the electron beam of the camera. The remaining leads carry the necessary supply voltages for the camera and head amplifier.
A portion of the Marconi-EMI control room.
Six Emitron cameras are employed, and their outputs taken to the mixer unit, containing two banks of valves with commoned anodes. In this unit the output, of any camera, can be selected, or the outputs of two or more mixed and passed on to the B amplifier, which has three RC coupled valves in addition to the output valve. One stage is employed for interference reduction; this is done by feeding the signal with any interference on to a portion of the valve curve which tends to rectify the interference peaks, leaving the picture signal un-rectified. The output of the B amplifier is fed into the C amplifier, which contains three further RC stages.
The Marconi-EMI transmitter control desk.
Following this is the Suppression Mixer. The first valve is a diode which rectifies the AC input and corrects it into a varying DC signal, and there are then two stages of amplification. This unit is followed by the Synch Mixer Unit, in which the vision signal is combined with the synchronising pulses, and this in turn is followed by the Distribution Amplifier, the output stage of which feeds the transmitter.
The Transmitter Circuits
Included in the transmitter are the modulators, which are fed from the Distribution Amplifier, and modulate the carrier of the transmitter. The final modulation stage employs a CAT6 valve with 5,000 Volts HT Supply.
The transmitter proper starts with a master oscillator at 22.5 MHz. A temperature compensated coil is used, and the frequency is maintained with an accuracy of one in 20,000. A frequency-doubler then follows and is in turn followed by stages of amplification employing valves of increasingly large power-handling capacity until the final power amplifier is reached, in which two CAT9 valves are used in push-pull. Modulation is effected in this stage, and the output is coupled to the aerial feeder.
The sound equipment provided includes five Marconi-EMI moving-coil and three ribbon microphones, in addition to gramophone and-sound-on-film reproducing equipment. The distribution system is unusual in that no valves are employed for isolating the various circuits, although they are naturally used in the amplifiers.
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