Land-Lines for Broadcasting in Germany 

10 February 2023 tbs.pm/76588

 

Cover of World-Radio

From World-Radio for 9 March 1934

This article describes the land-lines used for broadcasting purposes in Germany and deals with the measuring and supervising technique employed

 

IN radio-broadcasting the electrical oscillations produced by the sounds which are received by the microphone are usually conveyed over wires to the transmitter. Running commentary broadcasts sometimes offer an exception, as in this case small short-wave transmitters are occasionally used for bridging a short distance. The transmission lines are not necessarily short connexions merely running from the studio to the transmitter in the same building or in the vicinity, but they have often to bridge very great distances, which may exceed the normal service range of a large transmitter. It is now a common practice to relay programmes between a group of broadcasting stations or to broadcast the same programme from all the radio stations in the same country; hence many hundreds, if not some thousands, of miles have to be bridged, while even greater distances are involved in international broadcasts. These international programmes invariably have a very momentous aspect, and so it is imperative that the transmission should be reliable and immune from disturbance; this can only be achieved by way of cable lines.

As overhead lines are very much exposed to the hazard of interference, underground long-distance cables are exclusively used as being the most reliable channel of communication. The German Administration utilises for the transmission of programmes principally the central quads — i.e., four cores, situated in the middle of the cable, which are mutually insulated from each other and screened by a separate lead sheathing from the other cores. These four conductors, originally intended for measuring purposes and special services, are not subjected to any disturbing influences; in particular, the cross-talk from the other cable cores is extremely low.

In the communication art, a transmission of frequencies up to about 2,400 cycles per second is sufficient to give good reception, but in wireless frequencies up to about 6,000 cycles per second are required for good speech and music transmissions. This requirement is fulfilled, inasmuch as these cable cores have a cut-off frequency of about 9,500 c/s, new cables even possessing a cut-off frequency of roughly 11,500 c/s. For undistorted transmission, it is important that the line attenuation be the same at all frequencies, and that with very long transmission lines the various velocities of propagation are equalised. The cable lines are improved in sections by amplifiers known as repeaters. The various attenuation values are simultaneously equalised in the repeaters; that means the lines are corrected. The effect of this correction is that a practically undistorted frequency range of from 50 to 6,400 c/s can be transmitted with the older types of cables and of from 30 to 8,000 c/s with modern types.

General Arrangement of Circuits

From the microphone in the studio, a short connexion first of all runs to the microphone amplifier whence a local line leads to the first repeater station (junction station); then follow the actual long-distance cable lines and finally the local line, which connects the last repeater station to the transmitter. The first repeater station contains a main amplifier with several supplementary amplifiers which permit the currents arriving from the microphone amplifier to be despatched simultaneously to several transmission lines. The long-distance line passes through intermediate stations; of these stations those which do not undertake the distribution of the programme to be broadcast only possess main amplifiers, whereas those stations from which other transmission lines radiate are also equipped with auxiliary amplifiers. From a station which feeds a transmitter the local line passes via the transmission terminal amplifier to the transmitter.

It is important in respect of layout and replacement of parts that all main amplifiers for the transmission system are of the same kind; in Germany, therefore, they are so devised as to correct a long-distance cable section of 72.5 km. long (average length). Should a repeater section not attain this normal length, a long-distance cable extension line (an artificial connecting member which is so arranged that it has all the electrical characteristics of the missing piece of cable) is inserted in order to supplement the cable in respect of its electrical values as though it were of normal length. If the length of a repeater section exceeds the normal, the normal attenuation is again established by means of the surplus amplification available in the repeater. For local lines (cable) the German Postal Authorities have standardised the normal length at 10 kilometres and use local extension lines which can be altered in levels of 2 kilometres as supplementary members.

The necessary efficiency of transmission is safeguarded by various kinds of auxiliary equipment. At the transmitting and receiving ends, anti-distortion devices again suppress the distortion of the local line. When distributing over several lines the auxiliary amplifiers sene the purpose of producing the requisite electrical input for the corresponding line. Auxiliary equalising devices and temperature-attenuation compensators for the transmission lines are also connected in circuit to ensure the most exact correction of the transmitting lines between the individual repeater stations as well as between these and the junction stations.

In a standard repeater section 72.5 km. long, an additional attenuation distortion of roughly 0.1 neper occurs with a frequency range of about 50 to 6,000 cycles per second at a temperature fluctuation of 10° C. Accordingly, apart from the net attenuation between summer and winter, it is possible for a line of, say, 10 repeater sections (725 kilometres) in length, to experience an alteration in attenuation of more than 2 nepers in the range from the lowest to the highest transmitting frequency. Since much greater transmission distances are already in use, it is essential to incorporate equalising arrangements. These are temperature attenuation compensators in the form of variable artificial lines. System correction of the lines is undertaken at +20° C. The additional attenuation distortion occurring at +10° C. and at 0° C. is then obviated in service by a corresponding adjustment of the temperature-attenuation compensators.

Measurement and Supervision

The extensive system such as that outlined has always to operate reliably. It is therefore continually supervised and often checked by measurements. The measuring and supervisory apparatus necessary to this end form inseparable components of the entire plant. Of prime importance is the measurement of the effective attenuation and of the transmission level (measure for the magnitude of voltage), the normal value of which is 0.775 volts (=zero level). The simplest measuring method is one whereby a large number of frequencies, extending over the entire range of transmission, is agreed upon between the participating terminal stations, measurements being then effected in succession at these frequencies. This method, however, entails much loss of time, particularly when it extends over the wide transmission range (frequency range) of such transmission lines. Endeavours were therefore made, especially for long-distance lines, to supplement the earlier apparatus in such a manner that the transmission of measuring frequencies and their measurement takes place automatically. This aim has been realised by the development of automatic measuring and supervisory apparatus.

Frequencies of from 30 to 10,000 cycles per second are automatically transmitted and received in a continuous sequence with modern measuring apparatus. An automatic apparatus of this kind was demonstrated by Siemens and Halske for the first time in 1931 on the occasion of the convention of the C.C.I. (Comité Consultatif International des Communications Téléphoniques a grande Distance) at Prague. In the full meeting of the C.C.I. which followed, the technical pre-suppositions necessary for such measurements were framed, so that these measurements could be carried out uniformly, and, for long-distance transmissions, conjointly, in the following manner.

If a long-distance cable network has been interconnected for broadcasting, the level or attenuation measurement is effected prior to transmitting. At the outgoing point is a source of audio-frequency oscillations (beat oscillator with additional power amplifier), the transmitting frequency of which is altered steadily from 0 to 10,000 c/s by means of a rotary condenser driven by clockwork. The actual measuring apparatus, a transmission level measuring set with a recording instrument, is connected to the other end of the transmission line. Here the magnitudes of the incoming voltages are recorded on a chart on which the frequency is marked on the abscissa and the voltage (in nepers) on the ordinate. The action of the transmitting and receiving equipments is automatically made to coincide so accurately that the frequency read off the chart at any instant always coincides with the transmission frequency.

Prior to the actual measurement of the transmission level, the indicating instrument is adjusted with the aid of a calibrating equipment to the level ∞ and to the voltage level zero (=0.775 volts) at a constant frequency of 800 c/s, so that the attenuation values are accurately indicated.

A tall cabinet with switches and dials

Fig. 1 – Transmission-level recorder rack

The level recorders (Fig. 1) are installed at the most important points of the transmission plant — e.g., in the studio — at the distribution points of the cable lines (junction stations) and at the transmitters. In the case of the transmitting stations the transmitted measuring frequencies can either be tapped off direct at the end of the long-distance line for the level recorder or following their radiation by the transmitter, so that the frequency characteristic of the latter can then either be included in the measurement or determined separately. In the latter case the modulated high-frequency carrier of the transmitter has to be rectified in order to obtain the audio-frequency oscillations again.

With a view to simple and easy operation and attendance, all accessories for level measurement and the apparatus at the transmitting and receiving ends are either united in stationary racks or in portable cases (Fig. 2) in such a manner that they can be conveniently transported and quickly assembled for measurements. The latter type is especially important for laboratory and testing purposes, as attenuation measurements can be undertaken on all network configurations—in so far as they are four pole — with the level-recording equipment. The recording of an entire measuring curve, including calibration, only takes four minutes.

Apart from the level recorder, yet another supervising apparatus, the equipment for measuring the non-linear distortion factor, performs signal service at the transmitter. When transmitting sound frequencies over lines, harmonics, which were not originally present, are produced owing to the inherent characteristics of the system, especially the amplifiers. The nonlinear distortion factor is the ratio of the effective value of all these harmonics to the fundamental oscillation. It must not exceed certain values otherwise it will have a disturbing effect. The device for measuring the non-linear distortion factor enables those factors in the range of 0.1 to 100 per cent, at frequencies of from 50 to 5,000 c/s to be determined. This measuring equipment also is combined with the necessary auxiliary apparatus, such as an auxiliary condenser, rectifying voltmeter, and a current filter, on a measuring rack.

Besides the above measuring equipment, which is intended for testing or measuring the transmitting plant prior to a broadcast, there is also equipment for supervision during broadcasting. The difference in the volume of sound between music and speech broadcasts is very great, so that extreme care has to be taken that the amplifiers are not overloaded by an excessive voltage amplitude, but also that the softer passages are not drowned by the line noises which are always present to some extent. It is also desirable to be able to measure these interference noises of the lines immediately. An efficient monitoring equipment, consisting of amplifier and loudspeaker, is equally necessary for supervisory purposes. With this equipment all faults occurring during transmission—such as interference noises, crosstalk on the lines, faulty connexions, unauthorised interference, etc.—can be recognised at once, and immediate counter measures can be taken. Measuring and supervisory equipment of this kind is installed at the most important points of the line (junction stations and transmitter), and incorporated in a supervising rack.

 

A squat machine with buttons and dials

Fig. 2 – Receiving portion of the portable transmission-level recorder

 

Programme Monitoring

In order to prevent over-modulation and insufficient modulation the voltage amplitude of the programme travelling over the line during transmission is continuously supervised by a maximum and minimum voltmeter. Prior to a transmission it is usual to predetermine which relation is to be maintained between the maximum and minimum voltages. Certain regulators are adjusted accordingly, and the supervising official has then only to see that the pointer on the minimum voltmeter does not return to a red section, and that the pointer of the maximum meter does not deflect too often into a section also marked in red. In principle, these measuring instruments are impulse meters — i.e., valve voltmeters which also correctly indicate a voltage impulse of but short duration by means of sensitive moving-coil instruments. These indicated values can also be recorded on a chart with the aid of an ink recorder.

For service supervision at the transmitter the same supervising apparatus is supplemented by a measuring rectifier and a modulation meter. The measuring rectifier can also be used to check the transmitter. The modulation meter corresponds in principle to the maximum impulse meter, but it employs an auxiliary valve which can be calibrated for constant amplification so that this low level input to the terminal amplifier can be measured also. The measuring range of the modulation meter is adjusted by means of a potentiometer in such a manner that the beginning of the red section on the scale indicates the voltage at the output of the terminal amplifier when the transmitter is modulated to a maximum. During transmission, therefore, the pointer must not, or very rarely, enter the red section. Service interference of a mechanical kind, as, for instance, that due to the burning out of a valve filament, is indicated by acoustic or optical signals.

The measuring and supervising equipment practically forms one unit with the transmitter terminal amplifier at this point. All measuring apparatus can be connected at will to the incoming or outgoing side of the transmitter or to the end of the line. A second set of the indicating instruments can be accommodated on the switchdesk of the transmitter.

 

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