The following is taken from a few issues of Television and Shortwave Magazine that was published immediately prior to the start of regular high-definition (>400-line) broadcasts from Alexandra palace in November 1936.
|Alexandra Palace Layout. Link to larger view [239K].|
The London television station stands on a hill in North London 306 feet above sea level. The actual premises are the south-eastern corner of the Alexandra Palace, a well known North London pleasure resort. From the trustees of the Palace the B.B.C. has leased 31,840 sq. ft. of floor space, comprising three large halls floor, and the south-east tower. A further area of 24,525 rooms has also been taken, but up to the present no use has been made of this portion.
The lower floor halls have been converted to the restaurant and kitchen. The rooms on the first floor above have been converted into two large studios with Dressing rooms and make-up rooms for band and artists have been constructed, separated from the studios by a corridor. Adjoining these on the west side is a light well, separating the B.B.C. premises from the rest of the Palace. This has been used to form (a) in the basement, a boiler house for the heating system, which has been installed throughout the premises; (b) a floor at ground level for the preparation of scenery; and (c) another floor at first level for the storage of scenery.
The television mast is erected on top of the south-eastern tower. Its highest point is 300 ft. above the ground, the height of the steelwork above the brick tower being 215 ft. It is tapered for a height of 105 ft. above the tower and is square in section, the sides of the square being 30 ft. at the bottom and 7 ft. at the top of the tapered portion. At this point, to suit the special design of aerials, the section changes from a 7 ft. square to an octagen 7 ft. from face to face, and maintains these dimensions up to the top of the mast. On account of exposure to the force of gales, special means have been adopted to transmit the loads to the brick tower. Four steel lattice girders, 30 ft. long and 7 ft. 6 ins. high, in the form of a square, were placed on top of the existing brick tower. The four legs of the mast were then bolted to the corners of this square, and each corner was then embedded in 17 tons of concrete. In addition to this, at each corner a heavy angle-shaped steel tie bar, 50 ft. long, was carried down inside each corner of the tower, and after being subjected to a tension of 30 tons was firmly connected with the brickwork of the tower with this pull still upon it.
Separate aerial systems are provided—one for vision and one for
sound. Both systems are similar, each consisting of a number of aerial elements
arranged round the mast, those for vision being above and those for sound beneath.
Each aerial consists of eight push-pull end-fed vertical dipoles spaced equi-angularly
round the mast, together with a similar set of dipoles used as reflectors to
avoid induced currents in the mast structure and so increase the radiated field.
The aerials are connected to junction boxes, with which are associated a number of impedance-matching transformers to correct the aerial response. The aerial systems are connected to the transmitters by means of two 5-inch concentric feeders which pass down the mast and along to the transmitting rooms, a change-over switch being provided so that either the Baird of Marconi-E.M.I. vision transmitters can be connected to the vision aerial at will.
The power supply for the whole building is obtained from the mains of the North Metropolitan Electric Power Supply Company at 415 volts 50 cycles 3 phase, and is fed through a main oil-circuit-breaker and distribution switch-gear.
The sound transmitted is capable of operating over a band of frequencies from 35 to 50 mc/s, the working frequency being 41.5 mc/s, and the output power rating 3 Kw, at 90 per cent. peak modulation (Copenhagen rating). Owing to the different method and range of modulation adopted for the vision transmitters, the Copenhagen rating would have no meaning, and these transmitters are rated in terms of the Instantaneous peak power which they will deliver to the aerial at 100 per cent. modulation—about 17 Kw. On this basis, for comparison purposes, the sound transmitter would deliver to the aerial an instantaneous peak power at 100 per cent. modulation of 12 kW.
In accordance with the recommendations of the Television Advisory Committee appointed to consider the development of television in Great Britain, provision has been made for alternate experimental transmission by the systems developed by the Baird Television Company and the Marconi-E.M.I. Television Company, respectively. Each company has provided a complete television system, including" both vision and sound pick-up apparatus and the television transmitter itself. The B.B.C. has been responsible for the sound transmitter and its associated aerial.
The entrance hall is at the base of the tower, and facing it is the main door to the stairway leading to the ground floor corridor which houses the three trans- mitters, projection theatre, restaurant and scenery productions shop. Nearest to the entrance hall is the Marconi-E.M.I. television transmitter which, like its Baird equivalent, operates on a frequency of 45 megacycles per second (wavelength: 6.67 metres). All the apparatus at the station is finished in grey cellulose and chromium.
The sound transmitter hall, which is also on the ground floor, accommodates an ultra-short wave installation of orthodox design for radiating speech and music accompanying the vision signals of both the Baird and Marconi-E.M.I. systems. Its operating frequency is 41.5 megacycles per second (wavelength: 7.23 metres).
The Baird transmitter hall, with its generators and amplification stages, is at the south-west end of the corridor. Beyond this, at the south-west extremity of the B.B.C. section of the Palace, is a large area intended either for scenery construction or for televising such objects as motor cars and animals which cannot be brought into the studio or televised outside.
Between the sound transmitter and the Baird plant is the film projection theatre, or miniature cinema, in which film excerpts can be selected and timed for inclusion in the transmissions.
Another feature is a sloping runway down which the television camera can be moved to a concrete tt apron," of approximately 1,700 sq. ft. area, on the terrace outside, forming a platform for televising open-air performances or special experimental programmes.
The tower staircase leads up to the studio floor, passing en route a first floor on which are the offices of the engineers. Offices on the second or studio floor are occupied by the productions manager, stage managers and secretarial staff.
|Baird studios, showing lighting arrangements.||The Marconi-E.M.I. studios.|
The two main studios, one for use with each of the television systems, are 70 ft. by 30 ft. by 25 ft. high. Acoustically, the studios are rather more "dead" than is general practice for sound broadcasting, since the introduction of scenery necessary for television will, in effect, control the acoustic characteristics.
The walls of the studios are covered entirely with sheets of asbestos compound which has a high degree of sound absorption. As this material has a rather rough surface, it is covered up to about 10 ft. from the floor with a protective fabric which is designed not to affect the sound absorbing properties of the compound. The ceilings of the studios are treated with building board and the floors covered with black linoleum.
Several microphone points are installed in each studio, and they are arranged to allow the use of any type of microphone which may be required. Portable stands of the "lazy-arm" type are also provided.
Each studio is fitted with two stages equipped with curtains, the detailed arrangements of the stages and curtains being different in the two studios on account of the different requirements of the two systems. A number of overhead battens, each of which carries several lighting circuits, has been provided in each studio. There is also a large number of wall sockets for portable lighting.
In each studio a large lighting- switchboard has been installed, with provision for the separate control— dimming, etc.—of every circuit. In addition, there are arrangements for pre-selective switching and bankdimming of any number of circuits, and the whole equipment has been designed to give the maximum possible flexibility. A lighting bridge has been erected across the Marconi—E.M.I, studio to give further lighting facilities.
All the lighting in both studios is at present of the incandescent lamp type, using spot and flood lighting, but modifcations are contemplated with developments in television technique.
Ventilation has been provided in the studios by means of extract fans, the intake for fresh air being provided by openings in the upper part of the windows fitted with filters that clean the air and deaden extraneous noise; the lower parts of the windows are covered by sound-proof shutters during performances. Sound deadening ducts are connected with the outlets. The ventilation is sufficient to keep the studios at a moderate temperature when full lighting, reaching a maximum of approximately 50 kW, is used, and to allow the temperature to be adjusted within normal limits.
The Baird Company considered it advisable to install three different types of scanner, namely:—
The number of lines used to form the image for the Baird system is 240; this was chosen by the Baird engineers after experiments and public demonstrations up to 700 lines in the picture.
The spotlight scanner is employed for televising subjects in the studio, either as close-ups or semi-extended views. This equipment may be divided roughly into two main sections :—
|(a)||The projection room contain- ing the light source, scanning unit, line synchronising im- pulse generator and its asso- ciated amplifier equipment, and the "B" console amplifier unit.|
|(b)||The studio, containing four multiplier photo-electric cells with associated amplifiers and monitor rack, and the "A" console amplifier unit.|
A beam of light from an automatic high-intensity arc lamp is focused through a small water-cooled rectangular shaped window situated at the top of the scanning unit. This unit has two discs running in vacuum, each disc being driven by a separate synchronous motor also run in vacuum. The scanning disc driven by a water-cooled motor revolves at 6,000 r.p.m., and has 240 minute apertures arranged in four spiral traces, of sixty holes in each trace, near the outer rim.
The second disc has a slit arranged in a spiral trace near the outer edge, and acts as a rotating shutter so that only one scanning disc hole is exposed to the light beam from the arc lamp at any single instant.
Associated with the scanning unit is a line synchronising impulse generator. This consists of a light source, optical system and photo-electric cell and 240 synchronising slits arranged in a circular trace on the scanning disc itself. This, in conjunction with a special amplifier system, produces square topped synchronising impulses at the end of every scanning line.
The spotlight beam from the scanning unit is focused through the window of the projection room into the studio, being reflected from the subject being televised on to four 5-stage photo-electric multiplier cells mounted on stands. The output from each multiplier cell is fed to the "A" amplifier console unit housed in the studio. From here the signal passes to the " B" amplifier console unit located in the projection room. From the "B" amplifier the signal is fed to an output control amplifier, from which the signal together with the line frequency synchronising signal, passes to the control room.
A full description of the Baird intermediate film scanner was published in the September issue of this journal and it will suffice therefore to give a brief outline of the more salient features of this system. The intermediate film equipment is used for televising scenes in the large Baird studio. It may be divided roughly into two main sections as under:
|(a)||Film processing unit comprising recording cameras, sound head, processing tanks, arc lamp, scanning unit and associated equipment.|
|(b)||Amplifier desk console unit, containing the "A" and "B" amplifiers, control amplifier, complete with all power supplies, decoupling units, and monitor rack.|
The film processing section consists of a tank divided into six compartments, each compartment being used for one stage in processing the film as follows: (1) developing, (2) washing, (3) fixing, (4) washing, (5) scanning, (6) outer jacket containing warm water to maintain the developer and fixing compartments at the correct temperature.
The subject to be televised is photographed on 17.5 m.m. film (half standard 35 m.m. film), with a motion picture camera of the intermittent type, mounted directly above the developing compartment. The film passes through the camera at a rate of 47 ft. per minute, the whole unit being driven by a synchronous motor running at 1,500 r.p.m.
The film, coated with a rapid and sensitive emulsion, after passing through the picture camera, is fed to a sound recording camera situated immediately below, where the sound track is recorded between the perforations and the edge of the film. After leaving the recording camera the film passes into the developer, is then washed, after which it is fixed. It is then finally washed and passes into the water-filled scanning compartment, where it runs over a guide, the complete operation taking thirty seconds.
A beam of light from an automatic arc lamp is focused through the window in the scanning compartment on to the slit in the guide. The image of the moving film passing over the guide is projected by the lamp on to the scanning unit through a combination of lenses.
The scanning unit consists of an encased scanning disc having a circular trace of sixty minute apertures near the outer rim. This disc revolves at 6,000 r.p.m., i.e., four times every picture frame, so as to provide a 240-line picture dissection.
The disc is driven by a water-cooled synchronous motor, both the motor and the disc being run in vacuum. The light variations passing through the apertures of the scanning disc are focused by a lens on to a ten-stage multiplier photo-electric cell contained in a head amplifier, housed on top of the scanning unit.
Associated with the scanning unit is a line synchronising impulse generator, somewhat similar to that used in the spotlight scanner, which produces square-topped synchronising impulses at the end of every scanning line.
The film, having been scanned, passes to a sound head which is mounted directly above the scanning compartment. There, the film runs over a guide located in a small container supplied with a constant flow of water from the water main. A beam of light is concentrated on the sound track of the film as it passes over the guide, the variations of light being focused on to a photo-electric cell.
The output from the head amplifier is fed to the "A" amplifier, which together with the "B" amplifier and control amplifier, is housed in the amplifier desk console unit. Each stage of amplification is provided with a separate H.T. unit and decoupling unit to prevent instability due to back coupling.
The signal from the "A" amplifier is fed to the"B" amplifier, and thence to the output amplifier, control again being provided between these amplifiers. The output from the control amplifier, together with the output from the high-frequency synchronising amplifier, is then fed to the control room. A standard monitor rack is included in the equipment to permit the outgoing picture to be viewed.
The Telecine scanner is capable of providing tele- vision pictures from any standard 35 m.m. sound film. It consists of two main units:
|(a)||The projector unit consisting of light source, film drive mechanism and associated optical system, line synchronising impulse generator and its associated amplifier and auxiliary equipment.|
|(b)||The amplifier unit consisting of the "A" section and "B" section amplifiers complete with all necessary power supplies, decoupling units, together with control and monitor racks.|
The film is fed through a projector, the drive being provided by a synchronous motor running at 1,500 r.p.m., the projector having been modified so that the film runs at a steady uninterrupted rate of 25 frames per second. The film passes through the picture gate, then into the sound head located immediately below the projector, being reloaded finally in the bottom spool box. The shutter on the machine is dispensed with, and the picture gate is water-cooled, a small pump providing a continuous flow for this purpose. A beam of light from an automatic arc lamp is focused on to the gate. The image of the moving film is projected by the lamp on to the scanning unit through a combination of lenses, the actual scanned images being exactly the same size as the original frame. The scanning unit is almost identical in all respects with the intermediate film machine already described.
One stage of the amplifier is arranged to have unity gain, and may be switched in or out in order that a negative picture may be scanned should this be desired.
The vision signals, line and frame synchronising impulses, and the sound signals from each scanner are fed to the Control Room where any one signal source modulates the two ultra-short wave radio transmitters. Provision is made to handle five programme sources.
The control room equipment consists of the following units:—
Main vision control desk; main sound control desk; vision monitor rack; vision radio receiver rack; check loudspeaker; vision signal termination amplifier rack; line synchronising impulse termination amplifier rack; frame synchronising impulse termination amplifier rack; sound distribution rack; sound rack containing the B.1. and B.2. amplifiers; and two H.T. power unit racks.
The vision signals, line and frame synchronising impulses from the various programme sources are fed to three termination racks with five termination amplifiers in each rack. Each amplifier is provided with a gain control enabling any adjustment in level to be made when required.
For the operation of the Marconi-E.M.I. system at the Alexandra Palace six Emitron television cameras and six complete Emitron supply and amplification units are provided, arranged to feed two alternative channels to the vision transmitter.
The signals from the Emitrons are amplified two million times, that is from two thousandths of a volt to 2,000 volts before being supplied to the radio transmitter. A special unit supplies all the necessary pulses for synchronisation. The signals from the Emitron are first amplified in a unit built in the camera itself and the amplified signals then pass via a special cable to the amplifiers in the control room. The arrangement of the equipment allows a selection from any two of the six Emitrons to be made at the same time so that the producer can use one for transmission and then fade over to the other.
The special mosiac plate of the Emitron camera receives the image via a lens and creates small potential differences between the mosaic particles on the front of a mica plate and a metal plate on the back. These signals are of the order of 2 millivolts. The signals are produced in sequence by the scanning action of a cathode-ray beam and are fed to the input of the first amplifier valve via an electrical contact to the back metal plate.
The camera is sufficiently sensitive to enable it to be used under conditions of normal daylight or studio lighting.
In order to obtain sufficient detail the focus of the cathode-ray
beam has been reduced to a spot size of less than one millimeter in diameter.
This fine focus is necessary to produce the necessary delicacy of detail. The
order of mosaic element size to spot size is such that the camera is capable
of greater detail than the 405-line definition of the Marconi-E.M.I. transmission
The picture signals from the camera plates are fed straight into the head amplifier housed in the camera itself. This amplifier amplifies the minute signals from the photo-sensitive plate of the camera sufficiently for them to pass down as much as 1,000 ft. of cable until they reach the main valve amplifying equipment.
The head amplifier has four stages comprising an input valve, two resistance capacity coupled amplifier valves, and a pentode output valve. The oulyut stage is designed to match up with the characteristic impedance of the cable, which connects the camera to the later stages of amplification.
The movement of the scanning ray is controlled electro-magnetically both for line and frame scanning frequencies, the main cable entering the bottom of the head and carries within it 18 conductors. This cable not only carries to the main equipment the picture signals from the camera, but supplies to the camera the filament, high-tension and scanning pulses, which are generated for it on the main equipment.
After passing through the multi-core connecting cable from the head amplifier in the camera, the picture signals enter the picture illumination corrector unit, one of which is provided for each of the six cameras. Emitron picture signal trains constituting for example, one line scan, tend to fall in mean level with regard to true black. In order to correct this condition which produces a light or shadow effect over part of the picture, compensating impulses are added to the picture signals in this unit in order to maintain a true "black level" throughout the scan. The picture signals are still A.C. on leaving this unit.
After correction for evenness of illumination the signals enter the phase reverse unit, the purpose of which is for compensating for the use of negative or positive film. This unit consists of a single valve, the output from which can either be taken from the cathode or anode circuit, thus providing alternative phase reversal. There is one phase reversing valve for each of the six camera channels. No magnification is employed in this unit.
The equipment so far described is common to all the cameras, but at this point the signals from any of the six cameras can be caused to pass into the mixer unit. This unit sorts out the picture signals from the six cameras by means of an electrical remote control situated at the programme producer's control desk. Electrically, the mixer unit consists of two banks of valves, each of which have a common anode circuit. The anode output of each bank of 6 valves is passed to a separate B amplifier. Control of picture signals from individual cameras is by variation of the grid bias on the grids of the individual camera valves. By connecting together the anodes of each bank, it is possible to select and pass on signals from one, or if necessary, a combination of cameras in any desired sequence.
From the mixer unit there are two outputs to two separate channels, each being identical. These two channels are interchangeable.
The provision of two complete channels make it possible not only to watch the channel actually in use for transmission at the moment, but also to observe the picture on the camera which is to come into use next.
The signals selected by the mixer unit under remote control of the producer now pass to the duplicate B amplifiers. This unit consists of three resistance-capacity amplification stages and one output stage. A potentiometer is provided on each B amplifier controlling the variable Mu first valve in order that the changing from one channel to another does not alter modulation conditions.
The partially cleansed and amplified picture signals are fed into the duplicate C amplifiers which comprise 3 stages of R.C. amplification. Indivdual valves are so arranged that each exercises a limiting effect to ensure that there is no possibility of the next valve running into grid current.
Final removal of any interference from the picture signals is effected in the suppression mixer units (a five-stage amplifier). Signals leaving the suppression mixer unit are freed from any spurious interference which may have succeeded in getting past the cleaning up processes in the "B" and "C" amplifiers. The output of the unit is D.C.
The vision radio transmitters at the London tele- vision station were provided and installed by the two companies responsible for the systems used, namely, Baird Television, Ltd., and The Marconi-E.M.I. Television Co., Ltd. The B.B.C. provided the sound transmitter and aerials for use with either television system. The three transmitters are housed in separate halls on the ground floor and all the apparatus is uniformly finished in grey cellulose and chromium plating.
In the Baird transmitter hall are situated the final stages of the modulation system used by the Baird Co. and the transmitter and motor generators necessary. The motor generators supply high tension up to 10 kV and some of the lower voltages used for radio lighting equipment and subsidiary supplies. The machines are of a special design to ensure silence in running with a maximum speed of 750 r.p.m., and are laid upon"floating" beds.
The modulation panels are fed from the C panel installed in the control room, separate amplifiers being provided for vision and synchronising, the output in each case being 600 watts.
The transmitter comprises three main panels together with an associated cubicle which holds the smoothing circuits. These panels are :—
|(1)||Constant-frequency drive equipment having crystal oscillator in a suitably controlled double oven. The frequency at which the crystal oscillates is 1.406 megacycles and it is followed by the necessary frequency doublers and amplifiers which give an output of 100 watts at 45 megacycles. This output is fed through a concentric feeder to the grid circuit of the drive stage.|
|(2)||Drive Stage. This stage comprises one demountable water-cooled tetrode having an input of approximately 8 to 10 kW, which is supplied from a 7 kV D.C. generator, the screen being fed from a separate 2 kV generator on the same shaft. On this stage is injected from the appropriate modulation amplifier, the line and frame synchronising impulses which are then amplified and passed to the output stage.|
|(3)||Output Power Stage. This stage comprises one demountable water-cooled tetrode having a maximum output of 40/50 kW supplied from a 10 kV D.C. generator and as in the previous stage, the screen is fed from a separate channel coupled to the anode supply machine.|
The high-frequency circuits associated with the anode and grids of the last two stages are of special design and where necessary embody novel features in water cooling.
The output of the transmitter is coupled to the B.B.C.'s concentric feeder, through an intermediate circuit which is also water cooled.
In addition to the machines, panels, etc., a control desk is situated in the transmitter hall from which the running of the transmitter, and an observation of the outgoing picture by means of a check oscillograph and Televisor receiving set can be made.
A special feature of the transmitting system is the use of water-cooled demountable tetrode valves which, due to their extreme stability under the exacting conditions called for ensure reliability. In addition, the valves have replaceable electrodes, and although continuously evacuated it is only necessary to use the evacuating pumps for a maximum of 15 minutes per day. All water-cooling, H.T. and L.T. filament sup-plies, are covered by interlocking and visual device indications.
A sound chain is included to work in conjunction with each type of scanner. In the Telecine scanner a standard sound head is mounted immediately below the scanning gate, and the signal from the photo-electric cell is taken to an amplifier rack, a separate amplifier being employed for each machine. The output from each of the amplifiers is taken to a potentiometer enabling a smooth fade-over to be made from one machine to the other. From this change-over control the output signal passes to the break-jack in the control room jackfield, and from thence to a channel fade potentiometer mounted in the control desk. The fade unit is used either alone or in conjunction with other programme fade units for the insertion of effects, etc.
All incoming lines from programme sources are fed through the jackfield, a device similar to a small telephone exchange, permitting easy coupling and manipulation of the various units.
The output from the fade control unit is fed to a B amplifier and from thence via the jackfield to the transmitter, programme checks and modulation meter amplifier.
In the large studio, three microphones are provided together with their own single-stage amplifiers, and the output of each of these amplifiers is fed to a floor control desk. This has three fade units for microphone mixing together with the main recording control for the intermediate-film scanner.
The output from the floor control desk is fed to an amplifier rack and from thence to the control room, and finally, to the intermediate-film recording amplifier rack. The output from this recording amplifier produces a variable density track on the film via the recording lamp.
After passing through the intermediate-film processing plant, the film sound track is fed into the reproducing sound head, and from thence the signal is passed to the control room B amplifier and transmitter via the jackfield as described in the Telecine chain.
For the spotlight studio one microphone of a similar type is employed. For the purpose of checking the input to the transmitter, and for rehearsal purposes, four loudspeaker checks are provided.
The Marconi-E.M.I. transmitter consists essentially of a master oscillator, frequency doubler, five stages of carrier frequency amplification and a single-stage modulated amplifier, with the addition of the necessary rectifiers for the main high tension and grid negative supplies having a linear band width of zero cycles (D.C.) to two million per second.
The most important unit of the Marconi-E.M.I. vision transmitter is the master oscillator unit which generates at low power a wavelength which corresponds to double that on which the pictures are finally transmitted, representing vibrations of 22½ millions per second.
A modern version of a Franklin temperature compensated coil is employed in the master oscillator, which consists of a single valve (MPT4) circuit of special design, the frequency of which is maintained at an accuracy of the order of one in 20,000, when the anode supply or filament voltage does not vary more than plus or minus 5 per cent. The output frequency of this unit is 22½ mc. Expansion or contraction of the inductances mechanically varies tuning condensers which are designed to keep the carrier frequency constant.
Drive Amplifier Unit
The signals from the master oscillator are fed into the drive amplifier unit. The output from the master oscillator on 22½ mc. is frequency doubled by an MPT42 valve following which are 2 MPT42 amplifiers in parallel, followed by one ACT9 stage, the output of this unit being two ACT9 valves in push-pull. The output volttage sweep of this unit is of the order of 1,200 volts.
The magnified H.F. is now passed to the intermediate amplifier which contains two CAT15 valves in push-pull. The output of this unit is fed to the final high-frequency magnifier stage which feeds the aerial. This is modulated by the camera-plus-synchronising signals from the camera channel on approximately 3 kilowatts.
The small but accurate pulses of the master oscillator, having been magnified as stated above to a power of approximately 3 kilowatts, are fed into the final power amplifier unit, which consists of two water-cooled valves having about 6,000 volts high tension supply, and which are arranged to give a linear power output to a specially tuned aerial feeding system.
The power amplifier unit consists of two CAT9 valves in push-pull, inductively coupled to the aerial feeder and modulated by the final modulator stage of the camera channel. At "peak white" the power delivered to the aerial is of the order of 17 kW.
The B.B.C. sound transmitter is built in four separate units, each unit being housed in a metal cubicle. The master oscillator (ensuring a stability of plus or minus one part in 100,000) operates at half the carrier wave frequency, and is followed by one frequency-doubling stage and five high-frequency amplifying stages. Modulation is effected at the anodes of the final high-frequency amplifier by choke control; modulator, sub-modulator and sub-sub-modulator stages of the conventional type being employed.
In the final high-frequency stage, two CAT9 water-cooled valves in push-pull are used, and in the main modulator stage three CAM3 valves in parallel. The transmitter is designed to give high-quality sound reproduction and enable full advantage to be taken of the wide frequency band which is available at this low wavelength. The frequency response, of the transmitter is substantially flat between 30 and 10,000 c/'s., the maximum departure being less than 2 db. over this range, while the low-frequency harmonic content introduced by the transmitting apparatus is very low.
The low-frequency input stage (the sub-sub-modulator) is designed to operate from a signal which has an amplitude equivalent to that employed as a standard at all B.B.C. transmitters.
All the valve filaments are heated by direct current from a motor-generator set, having an output of 300 amperes at 20 volts, the filaments of the early stages being fed through voltage-dropping resistances.
The main H.T. supply at 6,000 volts D.C. for the penultimate high-frequency amplifying stage, the power-output stage and the modulators is obtained by means of a hot-cathode mercury-vapour type rectifier fed by a step-up transformer and provided with adequate smoothing circuits. Control of the high-tension voltage is carried out by means of a remotely controlled induction-regulator.
All auxiliary H.T. and grid-bias supplies are obtained from metal rectifiers, fed from transformers and provided with suitable smoothing circuits.
The main controls are grouped on a control table so that one operator is able to manipulate all the power supplies to the transmitter. All switching operations are effected by remote control, and the switch-gear is fully interlocked to prevent damage to the transmitter by the application of power-supplies in the wrong sequence. In addition, there is a sequence-starting switch which ensures that sufficient time elapses between the application of each succeeding voltage, so as to allow valves and other apparatus to become properly warmed up before the mains H.T. power is applied.
All electrical apparatus is fully protected by means of over- and under-voltage relays and water-flow monitoring devices so that, in the event of the failure of any supply, the whole apparatus is automatically shut down and cannot be restarted until the deficiency is remedied. Additional interlocks ensure that the whole operation of "running-up" is restarted at the beginning, in the event of such a failure.
|The Baird Vision Transmitter||The Marconi-E.M.I. Control Room|
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