Possible Causes of Fading and Distortion

Radio Record, Volume I, Issue 32, 24 February 1928, Page 12

Possible Causes of Fading and Distortion

The Part of the Monitor in Governing Modulation

By

M.[?].R

E.

HE question of fading and distortion of broadcast signals has long been, and is still, a vexed one in New Zealand, as well as ’ in other parts of the world, Due to the fact that there ts only the one company in New Zealand, and they are looked upon as a monopoly, they have come in for severe criticism. -Putting asidc the question of whether the power, wavelengths, and location of the stations are strictly technically right (and such a question bristles with technicalities, both radio and commercial, and is one which the saying concerning the fools aud the angels is applicable), the fact remains that in certan districts, where good, reliable signals from the nearest.station might have been reasonably expected by listeners, there have been disappointing results. The transmitting stations have come in for the blame, and in most cases they have been as blameless as a lighthouse giving its usual output, hut blinded by fog or clouds from giving its light to passing shipping. The nature of the fog of clouds which blind a radio transmitting station will be briefly outlined, but before dealing with this question the possibilities of the transmitting apparatus or personnel being blameworthy will be discussed. THE MONITOR'S DUTIES. AS is well known, there is in the studio of the broadcasting station a sound pick-up device called a microphone, which converts air waves into electrical impulses. The air waves, of course, represent the sounds of music or speech created by the artist or speaker. The electrical impulses are taken into a control room, in which is located electrical apparatus for the purpose of checking the purity and strength of signals, and the duty of the operator or attendant there (who is usually caHed the "monitor’"’) is to maintain the purity of signal delivered from the microphone, and check its issue from the transmitter. He has in his charge amplification apparatus and a volume or "monitoring"? control whereby. the output from the studio and the input to the actual transmitting set (which "may be a mile or more away and con-

nected by wites) may be varied at will. Nov , the monitor can vary the power of the transmitter from zero to maximum by a simple rotation of the volume control in the monitoring room and by a turn of a switch he can listen to the purity and volume of signals anywhere up to the time they leave the monitoring room as well as after they leave the transmitter as ether waves because a standard radio receiver is installed in the monitoring roon..

It is a fact which is very seldom appreciated that the power of a broadcast station is not the power of the carrier wave, but is the depth of amount of niodulation applied to, the carrier of a transmitter of a certain capacity. Tor instance a transmitter rated at 100 kilowatt emits a carrier wave of a certain amplitude or strength, and this carrier wave is capable of travelling large distances. If it were possible in practice to modulate this carrier wave 100 per cetit., then the whole energy of that carrier wave would be devoted to carrying the broadcast signals to maximum distance. Actually in practice a depth of modulation of 70 to 80 per cent. can be obtained, and this means at once that the actual telephonic power available on the carrier wave is that due to an expenditure of 70 to 80 kilowatt instead of the 100 k.w. used in setting up the carrier. If the telephonic input to the transmitter is cut down to such an extent that the depth of modulation is 1 per cent. only, then the equivalent signa] strength in a receiver tuned to the 100 k.w. carrier is only the equivalent of a 1 kay. station fully | modulated. °

THE question might be asked whether the 100 k.w,. station’s carrier would travel further than that of the 1 k.w., and that the 1 per cent. modulation of the former would deliver a greater signal strength than that of the 100 per cent, modulation of the 1 k.w. carrier with its limited range. Actually ‘in practice other factors would have to be considered, and the results would be problematical, but theoretically the. expenditure of telephonic power is the) ‘same, and the results would be approximately tle same so far as signal

strength in the receiving aerial is concerned, SUBSIDIARY CONTROLS. qt is seen therefore that the power output of the tramsouutter is in the hands ot the monitor because the carrier wave is mandible in the receiver (providing the recetyer 1s correctly adjusted), and merely provides the connecting link between transmitter and receiver just as though a wire were joined between the two as in the case ot ordinary telephony. Whatever the monitor allows to be put into the carrier becomes available in the receivers tuned to that carrier, The monitor is consequently a very responsible unk in a broadcast system,

ie will be seen at once that any lack of attention on the part or the monitor. to his job may result in fading or distortion. If the input to the transmitter is cut down during an item an apparent ‘‘fade" at relatively distant poimts wil be noticed. If m changing from studio to relay, or vice versa certain necessary readjustments are not made to compensate for different strengtlis of pick-ups, then fading or blasting will result so far as observations at distant points are concerned, Sometimes there is a sub-monitor engaged in the case of an outside relay, and the operator in charge of the transmitter has the ability in an emergency of regulating the mput to his transinitter. In a properly regulated broadcast system neither the sub-moni-tor nor the transmitting operator should tamper with volume controls, and this duty shouid be. strictly confined to the regular imonitor while programmes are being transmitted.

THE OBJECTIVES AIMED AT. BVIOUSLY the ideal the monitor is aiming at is to put out the maxinium amount of power compatible with purity. The check on signal purity is made by observing the radio signals returning from the transmitter, and the monitoring adjustments are always made using the radio signals as a guide. If distortion is noticeable, a turn of a switch enables the monitor to check the

input to the transmitter. If the input to transmitter is right and the output distorted, while the strength of input is normal, then it is the duty of the moui‘tor to call the transmitting operator’s attention to the fact. ‘The operator should have noticed the trouble as soon as it oceurred in any case, as he is in a position to listen to the input and output of his transmitter. Wit modern apparatus very little trouble shuuld be occasioned by distortion actually in the electrical portion of the transmitting system, Particular and constant attention has to be paid to the studio, where the studio attendants have to arrange the relative positions of artists and microphone. Naturally the momitor can.give material assistance in increasing the flexibility of the system by compensating for errors on the part of the studio by an increase or decrease of amplification. The monitor attendant has to be on the job the whole time, and has to be experienced to enable a professional finish to be imparted to the material | broadcast.

OTHER FACTORS AT WORK. @ ‘THE foreguing has given some idea of possibilities of distortion and fading likely to be traceable to the actual transmitter, ‘Io the casual reader it may be likely ta appear as though the possibilities are so great that the percentage of ‘‘fades’"? observed are very largely the result of slips on the patt of the monitor, while the number of times the signals become mushy are merely due to the fact that the transmitter attendant is sucking his thumb instead of attending to his job. In reality the blame definitely traceable to variations from the trans--mitter is an extremely small proportion, the majority being due to atmospheric conditions, which will form the subject matter of this column next week. Of course, the broadcasting company are alive to the necessity of minimising unnatural fades, and a constant check is kept on the station output. It is the practice in many modern stations to use mechanical or electrical instruments to check the station output, and to keep photographic records of modulation percentages taken

periodically. At the same time, visual idicatious of average modulation percentages may be constantly available in the transmitting room, monitoring room, and also in the studio managet’s" coutrol room. ALL PARTS LINKED OP. NATURALLY every section of a broadcast system is in quick telephonic communication with every other section, and in the case of the studios, control room, and monitoring room, not only is there quick conununication, but by means of large plate-glass windows it is possible to see everything that is going on. Even artists awaiting their turn to perform can see what is going on in the particular studio in opera-. tion and by means of a loudspeaker also hear the performance.

ALL that organisation and electrical ~ and mechanical devices can do 1s done in a modern broadcast station to cut out human error and the minimising of the latter is naturally dependent on quality of personel. So far as New Zealand is concerned, there is a much greater necessity for care than in other countties, where more stations are available per head of population or per square mile to be covered. Owing to absence of population, and large areas to be covered, the very most has to be made of the power available from the four stations, and if listeners close to the stations notice a certain amount of furriness of signals, due to slight overloading of the transmitter, in order to make the most of the power available, and get programmes into distant parts of the coutttry, they should sympathise with the ambitions of the monitor in this direction. Nevertheless, it is naturally most desirable to limit ‘the modulation of the station to such safe limits as to ensure faithful reproduction over a_ limited area, rather than take the ede off good music, and make it available over larger areas. In next week’s issue, therefore, at attempt will be made to explain in as popular a manner as possible the cause of fading and distortion, as well as the reason that signals transmitted on the standard broadcast wavelength carry to greater distances in the night than dur ing the day.