What 1932 Holds for Radio

Radio Record, Volume V, Issue 43, 6 May 1932, Page 7

What 1932 Holds for Radio

A Review of the features of the modern Set.

we the approach of winter are coming better radio conditions, and it can be expected confidently that this winter will be one of the best yet. It is generally assumed that radip conditions move in cycles, and that we are now almost at the peak.of a cycle. meaning that conditions have been steadily improving over the last few years, and now are almost as good as they will get. Next year it is expected will be even slightly better than this for radio conditions, so we can look forward to two or three years of really good radio, Furthermore the sets are being improved at a marvellous rate. Many new and desirable features are being added and some of the older and less desirable are being done away with. . This, season, howA dat we must look for minor changes in deSign only. Last year saw the introduction of the superheterodyne receiver, and_ this year will probably see.an increase in its popularity. But what is this superheterodyne receiver which has earned such popularity ¢ While it is not desirable in this brief review to go into the technical details of the set, we might say that it is a type of circuitit is not new-whereby a portion of the set acts like a portion of the transmitting station. The waves are collected from the air and slightly strengthened by the first valve in the set. Here they are mixed with other somewhat similar waves, which are generated, not at a transmitting station, but within your own set. When the switch is turned on these valves set up, but they are controlled so that they will not be radiated past your owimpset. In times past this was one of the difficulties of the superheterodyne, and it restricted its use to a loop antenna, from which radiation could not take place. But now that has been overcome and all oscillation, as it is called, is kept within the receiver. This wave is scientifically mixed with the waves coming in from the station, and the two form what is known as a "beat note," or a third wave, different from them both. No matter what the wavelength (or requency) of the broadcasting station may be, it can be tuned in on the superheterodyne and mixed with the wavelength generated in your set, which is likewise altered when your tuning dial is rotated, so that the difference between these two is always the same. It is really a case of simple

arithmetic. Say we decide that the third number is to be 4 and the wave coming in is 9, then we would mix with it a wave of the value of 5. The incoming wave is 15, then the wave generated will be 11, and so is constantly generated a wave of 4. Now the point making the design thus is that any set, or amplifying system, can be made to amplify best on one wavelength (or frequency) only. So, by reducing all incoming signals to the one wavelength, the set can be more scientifically built. When two signals are mixed and the constant product arrived, that product is amplified, or strengthened by what is known

as.the intermediate frequency amplifiers. When they are past this stage, they go through the same treatment as does the incoming wave in the ordinary or, what we call, the t.f. (tuned radio frequency) set. They are detected and then amplified. The superheterodyne has several advantages. It is selective, that is, it can distinguish between two stations operating on very close frequencies. Most of you who have had the older type of set and have lived near the city, know how difficult it is to get past the local station, but with the superheterodyne receiver, it can be tuned out in a very few degrees. The other advantage is its sensitivity, due to the fact that all waves are reduced to the same value at which the receiver is. designed to amplify greatest. And that is why the superheterodyne has become very popular. However, it would not be fair to say that it is the only

type of set which has been popular, or which is likely to be popular during the coming year. ‘The superheterodyne has certain inherent faults, and the ordinary tr.f. set has maintained a certain degree of popularity on account of this. It can be charged against the superheterodyne that it is too sensitive, that it amplifies beyond the noise level. By this we mean that there is a certain degree of amplification beyond which it is useless to go for, when this is exceeded, the noise is so great as to render the broadcast inaudible. Thus, if your receiver is capable of tuning in a 50 watts station in the United States, and at the same time brings in a deafening amount of static, then it is no use being able to bring in that station, and this is the argument that the manufacturers of the t.r.f. set have advanced. Two or three notable manufacturers have not taken up the superheterodyne on this account. They have worked on the selectivity question and, by introducing certain features, generally what is known technically as band-pass tuning, have evolved a very selective set with a degree of sensi- tivity which will satisfy the requirements of most listeners. So far, so good. We have seen the reason for the popularity of the superheterodyne and for the continuance of the t.r.f. set. It is merely a matter for the listeners to judge between the two and see which suits his own needs the. best. ~ ARHE past twelve months i "has seen some notable developments in the valve line. It is an old saying, and not without truth, that a set is no better than its valves; in fact, radio is entirely dependent upon valves. Before Fleming and de Forrest developed their valves, and made possible the amplification of radio signals, radio was at a standstill, Transmitting as we know it to-day was possible only because of the valve, and each development that took place was occasioned by some improvement in this essential piece of apparatus. The introduction of the a.c. set came about only because a valve was found, the filament of which could be operated from the a.c. current. The screengrid valve, with its marvellous sensitivity, has made possible a great advance in the design of radio receivers. Then came the pentode, which did away with audio amplifiers. In fairness to the English and Con- (Continued on page 9%)

The main features of this year’s sets may be summarised as followsI. Superheterodyne circuits, with multi-mu, pentode and sprayshielded valves. . Development in t.r.f. sets of bandpass tuning filters. . Pushpull detectors and pushpull output. Short-wave combinations. Elimination of a large percentage of noise. In the midget sets, fewer valves, employed in a more efficient circuit. 7. Automatic volume control. A nk w NY

Radio in 1932

(Continued from page 7.) tinental manufacturers, it must be placed upon record here that these two, particularly the Continental manufacturers, were using the pentode valves long before they were introduced into the American States. The pentode is a somewhat similar valve to screengrid, but it is designed for the last stage in the set, instead of the first stages. The last stage is called upon to supply power to the speaker, and to do this a valve needs, primarily, to be able to handle without distorting, a large amount of current, both signal and battery. This, the pentode, with its two extra grids, makes possible, but it is marvellously sensitive, more sensitive, in fact, than two ordinary, or as they are called,gtriode valves. Before its development the set employed a detector, a stage following the detector, and then two power Valves (latterly the 245 type of power valve) in pushpull. Then along came the pentode, with sensitivity equalling both the first audio and the power valves. Furthermore, it can handle almost as much volume as the two 245 in pushpull, and give as much power to the speaker. The set then became detector and pentode, doing away, not only with two valves, but also with the large and bulky associated apparatus. With the valves in pushpull it is usual to use transformers, and transformers take space, so, there are now only a few . fisignificant resistances. This has age possible building a set into a very small space, and has made the midget set popular. One of the difficulties of last season’s midgets were that they would not with their one output valve handle the output required. But the pentode can, and so the midget has become as popular as the console. ’ THE Screen-grid valve has been developed still further, and what is known as the multi-mu valve has resulted. The multi-mu valve reduces cross talk-one station breaking in on another, although neither spread themselves over the dial, and are well separated-they make possible more efficient volume control-the local station can now be tuned in without the necessity of having to remove the aerial-they give less distortion owing to the fact that they act as semi-power valves when the grid bias is adjusted correctly, they enable the get to be at a lower noise level-there is not "the amount of parasitic noise in this type of valve ag in the ordinary screen-grid, and decrease of hum. The decrease in hum is a noticeable feature in the new sets, and is due ro more efficient filtering circuits, though the amount of smoothing apparatus has been decreased, due mainly to the use of better valves and the cutting out of the audio frequency stages, which were the source of a great deal of hum and parasitic noises. These are the principal developments _ that have taken place during the last and which will be incorporated in he 1932 radio receivers. Other features incorporated in some sets include .twin detectors and pushpull pentode output. Pushpull circuit has certain advantages. It is capable of handling much greater volume than one of its valves can when used singly. We remarked a moment ago that the single

pentodes had replaced the 245 power valves in pushpull, because they supplied almost the same output, but now two pentodes are being used in pushpull to give even greater output. Of course, when these valves are used in this circuit, it is difficult to build the circuit into a midget form, although such is in many cases being done. The idea of providing the pushpull output is to make a set which can supply adequate output for really loud results, Pushpull Detectors. THE weakest point in any set is the detector, which has been described as the "bottle-neck" of the set. No matter how great the amplification given a signal before it reaches the detector, the output is restricted to the output of the detector valve. Of course, it will immediately be said, "Why not restrict the pre-detector amplification and increase the post-detector amplification," but the point is that the latter -audio amplification as it is called-is always likely to introduce noise, anid pre-detector amplification makes for greater sensitivity and selectivity. Furthermore, it is easier to amplify a weak signal than a strong signal. In order to overcome this bottle neck, a system of pushpull detectors has been evolved, and this is likely to be one of the features of the 1932 radio set. Pushpull detectors and pushpull output valves make possible the delivery of a_ tremendous undistorted output. Another feature is visual tuning, or tuning by a meter or column of coloured liquid. To tuning in a station one must tune either for greater swing of the meter or greater height of the liquid. This is a splendid innovation, because many people have caused their sets to distort through not tuning them accurately. The superheterodyne in particular must be very carefully tuned. When there is a meter to indicate when the set is tuned and when it is not tuned, the human element is greatly decreased, and there is a much better chance of getting good results. An important development is the allwave set. We remarked in the earlier part of this article that the superheterodyne receiver took each wave as it was tuned in, and, through an adjustable oscillator, delivered a constant output. Now the short waves are beyond the reach of the average set. They cannot be tuned in because of the limitations of the apparatus used, but if we could place in front of the set some piece of apparatus which brings them to a value that enables them to be tuned in on the broadcast set, then it would be possible to listen to them, and this piece of apparatus has been developed. It is known as the short-wave converter, and fulfils the requirements exactly. It picks up the short waves through its having speci-ally-designed apparatus and converts them to a signal which can be tuned in by the broadcast receiver. They then go through the same process as does the ordinary signal. Quite simple, isn’t it? A short-wave set cannot, at the present time, be designed to cover the whole short-wave band without altering coils, and this necessitated a season or so ago the pulling out of the coils and their replacement, but changing devices have now been developed so that it is necessary only to adjust a dial on the front of the converter to bring in the various bands. Long and short wave sets are being built into the one cabinet and a com-

pact receiver is the result. Of course, those who. have the type of receiver Without the short-wave conyerter, can always get this piece of apparatus and place it near their set. It will then act as though it were built in. Another device, new this season, is one which was developed in New Zealand, and which will possibly be the means of making radio even more .popular than it is, particularly in areas where interference is bad. The device is fitted between the mains and the radio set, and it is astonishing the amount of noise that it keeps out, The. contention of the designer is that a great deal of the static picked up by the a.c. sets comes in through the power noises, and this has resulted in the well-known: fact that battery sets are quieter than the a.c. ones. Do not think it is a static. eliminator; it is far from that. It is a line noise Suppressor, for it is marvellaus just how much noise is picked up through the mains. Ln

Other features which are being ing corporated into this ‘year’s American sets are panels of airplane fusilage construction, panel key for switching from radio to records, non-glare lials, triplex audio system employing screen grid first audio and pushpull output, biresonator r.f. tuning systems, tri-resona-tor intermediate amplifiers, isolated oscillator valves, image suppressors for discrimination against cross talk, and large baffle area cabinets. To meet situations where the best quality of reception is demanded, the difficulties incident to the use of a single loudspeaker, are being corrected by the employment of two speakers. ‘There are two methods of accomplishing the desired result, one employs a small speaker for the highs, and a large speaker for the lows. Another method is to use two loudspeakers of the same general characteristics of the same size or different. (§