"MODULATION"

Evening Post, Volume CVII, Issue 16, 19 January 1924, Page 22

"MODULATION"

The reference in last week's notes to the quality of the modulation of Farmers' Ltds. new broadcasting station in Sydney has prompted ."Musicus" to write asking what the term means. He says he understands it in the musical sense, but he assumes, (and rightly) that the wireless expert has a different use for the word. In the musical sense 'the word usually means to change the key;! but "vary" is perhaps the nearest synonym for the word as used in connection with wireless telephony. The whole operation of wireless "telephony re- ! quires .first of all a continuous emission of electric waves from the transmitting station. These are of themselves quite inaudible, and they play no direct part in the production of the sounds heard by the receiving operator. Tliis stream of oscillations is called the "carrier wave." It is produced by a valve or valves which sets the transmitting circuit into oscillation with such vigour that a large amount of energy is discharged at each oscillation, the oscillations following one another at the rate of a million per second or' thereabouts, this rate being fixed, of course, by the wave-length at. which the transmitter is operated. When ,the station is "on the air," as tho American phrase has it, but no sound is being transmitted, these waves or oscillations are all of precisely the same magnitude, resembling in that respect the ripples that run across a pond under the influence of a light and steady breeze. A receiving set, properly adjusted for the reception of telephony, produces no sound as the result of picking up such a continuous wave. In that case, sound will be produced in the receiver (a) if the carrier wave .is. rapidly interrupted. This produces clicks, and the method is often used for Morse telegraphy; or (b),. if the carrier wave is varied in strength, or "modulated," at such a frequency as , produces an audible note. i Modulation is therefore the process of altering the strength, but not the fre- j quency, of the carrier wave. It can be ] done in various ways, but the most im- j portant is to use a duplicate valve in . which the current is controlled by a' microphone, and a combination of cir- i cuits such, that the current from thi3; valve is combined with the carrier wave. ■! The effect is alternately to increase and I diminish the strength of the carrier j wave; and,the quality of the modula- , tion is judged by the extent of this modification. Obviously the decrease cannot exceed the full 'strength of the . carrier, or there would be less than j nothing left; and the increase being equal to the decrease, the maximum, strength value of the oscillations (technically called the amplitude) cannot be! more than double that of the carrier oscillations. The frequency of the variations is governed by the note of the i sound being transmitted; the magnitude | Iby the 'loudness of the sound. Actually the modulation does not reach the zero-to-double magnitude, -even on the loudest'notes. Good modulation means the fullest use of variation within the practical limits. Over-modulation is nob common, for the excellent reason that it produces intolerable faults of reproduction. Under modulation,' resulting in signal strength- below what the station ia capable' of, is not and its presence is indicated if the music or speech is very weak compared with the sound made by tho carrier wave in ieNceiver which is oscillating. An exact estimate of the degree of modulation can only be made by instrumental measure- • n ente. An average of about 60 per, cent, appears to be best. This enaMrs .pud sounds to be reached without distortion.