ON ALL WAVELENGTHS

Evening Star, Issue 23325, 22 July 1939, Page 4

ON ALL WAVELENGTHS

AUTOMATIC VOLUME CONTROL Cause of "Rushing Noises" Change From Sensitivity Control “ Something terrible appears to be wrong with my set. When I tune off a station, it makes the most awful rushing sound you ever heard. How can I stop it? ” This question is still asked by nontechnical readers. In the early days—say four years ago—it was a regular thing to find such a letter in the mail.

There was the case of one woman who had juSt purchased a new and powerful set; She turned it on one morning, when it happened not to be tuned to any station. She opened up the volume control while her daughter was operating the vacuum cleaner in the next, room. . . As ■ might be imagined, the set set up such a frightful dm that the woman ran from the room, scared to deatji, and she wouldn’t come in again until someone had turned off the sett “ Yes, yes,” you reply, ” but that’s all very well. It can’t be right for a set to rpake such a row. What is the cause of it ? ” THIS " A.V.C." If I wanted to silence you with science, I would just say “ Blame the A.V.C.’*, and leave it at that. Being kind-hearted, however, I will endeavour to explain what is meant by. that brief but quite correct statement. A.V.C. is short for automatic Volume control. ' You are all familiar with the volume control on your sets—how it makes .the signals loud of soft, according to its Setting. This control is literally a vol-ume-control,. but it doesn’t control the ■ehsitivity of the set. The sensitivity of » set may be defined as its ability to pick Up weak signals. Now the older types of sets con* trolled volume by controlling sensitivity. If you wanted to tone \down your local station,’ found went the knob of the sensitivity . control, and down went th* ability of the set to pick, up weak stations. If you left the control knob in that position and rotated the tuning dial, only stations as strong as, or stronger than, the one you were listening to, would be heard at good volume. In other words, there was none of that rushing noise between stations which is characteristic of the modern set. NEW CIRCUIT This is, of course, a disadvantage in many ways. If you set the sensitivity control so that a fairly weak local was heard at nice Volume, you would just about be blown out of house ahd homO if you tuned over a very much stronger one without turning down the control. And while hunting for other stations, you had to be working the sensitivity control all the time. So some clever engineer devised a hew circuit by which the sensitivity of the set was controlled by the strength of the stations themselves, Difficult? Not at all.

NEWS and COMMENTS \

by “HENRY” \ '

Without going too far into technicalities, we now had sets witli autdmatio sensitivity control, or wrongly but popularly known at automatic volume control. With such seta, when a strong local was tuned in the sensitivity was automatically and almost instantly reduced to bring the signal down to a reasonable level. If you were tuned to a weak station, the fact that the station's signal was weak meant that it could not cut the sensitivity back so far, and thus the set was able to tune it in. This _ automatic change in sensitivity according to the strength of the station being received meant that in general all signals tended to level themselves out to the same strength. If they were strong they were cut down to norma! and if they were weak they were automatically brought Up. BUT THE NOISE ? A little thought, and now you will begin to understand why the noise is apparent between stations with Such sets. Imagine tuning to a station which was so weak as to bo practically inaudible. The automatic control would allow the set to fun “ flat out ” in an endeavour to bring in the signal, In other words, the maximum sensitivity of the set would bo used. Now, a radio set in such a condition is very, very sensitive. Not being human, it can’t tell the difference between that much-desired signal and the electrical interference from a transformer down the street—from the neighbour’s’electric egg-boater, radiation from power lines, and so on. All these infinitesimal sources of interference*-coupled with noises generated In the valves themselves • when operating this way, combine to produce that background of “ rush ” which you notice whenever there is no strong Signal to automatically cut down the gain* The volume control oh the set. you will see now, is there simply to determine how loud you want all signals to become. If you want them full bore, and there doesn’t happen to be a signal there at all, you’ll get instead of signal all. the noises the set can rake in at a volume something like hn express engine blowing off steam 1 The bigger tho set the bigger the noise. So nest time you get stuck between stations and your receiver produces such sounds don’t run for a service man convinced that the set lias blown up. Just turn tile tuning dial round a little further, and as soon as you Strike a station of any strength tho rushing noise will disappear like magic.— ‘ Wireless Weekly,’