The Purdue Engineer, Vol. 33, No. 7, p. 157, April 1938
This is Station WBAA by A. G. Hutton, Sc. '40
WBAA was on the air; the time was mid-afternoon; and the main studios were swarming with curious people, wanting to know what made their radios work, how the voices that they heard in the studio went from the studio out into the ether and ultimately into the loudspeakers of the receiving sets in their homes. Yes, there were also questions about all of the dials on the panels of the main transmitter; what was the purpose of each one, why did they work, and what would happen if one were to turn this know or twist that dial.
These people were visiting Purdue as delegates to a convention then in progress on the campus. Many of them had never seen the interior of a radio station, let alone the transmitting equipment that made it possible for them to sit in their comfortable homes and enjoy music, literature, lectures, talks, and entertainment that are projects of the modern radio station. And, as the average normal person is wont to do, this group of visitors was taking advantage of the opportunities that Purdue's radio station afforded. They were utilizing the opportunity to see what a radio station looks like, how it operates, and what makes it operate, by asking questions and listening to the explanations of the guide who was showing them through the local station. Small as WBAA may be, subject to as much criticism as it is; yet it is typical of any large, modern, present-day broadcasting unit; and these guests of the Voice of Purdue were learning something more than was intended for them to learn during their stay on our campus as convention delegates.
Let's follow these guests through the station and see if we can gain an insight into the mysteries of what makes our radio receivers work. As we join the group the guide is just pushing aside the heavy curtains over the large, double glass windows in order that his visitors may watch the program in progress within the studio. It is the Saturday afternoon production of the dramatic show entitled "RADIO STAGE", and the array of sound effects, sound boxes, microphones, and student dramatists immediately catches the interest of the spectators. After watching the program for awhile, one of the men in the group asked the guide about the windows of the studio and why they were of double glass. The guide explained that this was one of the many methods employed to achieve the necessary sound proof effect within the studios. "If the windows were not of double glass, the sound of our voices would penetrate the single pane and would be picked up by the microphone in the studio. There is a space of a couple of inches between the two layers of glass that forms a 'dead air' space, which is one of the chief ways of sound-proofing rooms. It will be noticed that the walls and ceiling are all covered by Celotex to absorb the echoes from the artists' voices in the studios. Some studios are using this Celotex, while others use a composition cork material that is very porous and will absorb the echoes and reverberations within the room.
"But let's go back to the control room to see what makes the sound go from the studios, through the air, and into the loudspeakers in your homes," continued the guide. "For an example of what happens to a speaker's voice, a musical number, or a dramatic show such as you see in the studios now," the guide went on to explain to the visitors gathered around him in the control room, "let's follow the sound of the student dramatic artists who are presenting this show." The guide went on to explain that a radio transmitter consists of two main parts; the R. F. portion, the R. F. standing for radio frequency, and the audio part. Of course these two principal parts of a broadcasting unit are composed of many subordinate and inter-related stages, groups, or phases.
The output of each pre-amplifier is approximately the same level or intensity as the output of a phonograph pick-up. The sound that has thus far come from the artist in the studio, through the mike and into the pre-amp, now goes into what is known as a "mixer". This mixer is a piece of apparatus at which an operator sits at all times while the station is on the air, watching the volume of the voices of the person or persons presenting the program; or, in the case of a musical program, watching the intensity of the selection being presented. The mixer has all of the microphone leads coming into the station, all of the remote control lines, and the turn tables from which phonograph records are played, and the operator is at all times able to adjust the intensity of the signal so as to give the best possible broadcast reception.
Following the program through the mixer, we find that it goes into the speech amplifier. The purpose of this part of the audio system is to build up the signal strength and compensate for the decrease in signal strength occurring in the mixer. It also increases the over-all intensity of the signal sufficiently to drive the modulator tubes. Incidentally, these modulator tubes are of the 849 type and operate strictly in class "A". This type of modulation produces less distortion and makes possible the clearest signal.
The R. F. stage of the modern broadcast transmitter, comparable to ours here at Purdue, consists o a crystal oscillator. There are three stages of isolating amplifiers, which are commonly known to the radio technician as "buffers", the purpose of which is to amplify the crystal stage and isolate it from the rest of the transmitter. The output of the last or final buffer stage goes into the modulated state, consisting of two 204-A type tubes in a push-pull circuit.
The modulator tube line-up consists of an audio output which controls the magnitude of the R. F. energy coming from the two 204-A's. The output of these two tubes, or the modulated stage, goes to the linear amplifier and is reproduced similarly, only in large amplitude. The linear amplifier stage of Purdue's transmitter consists of a Western Electric 228-A tube, which is a 5 kilowatt water-cooled tube. When the output of this 5 kilowatt 228-A is coupled to the antenna, the program that you see in the studio is on its way to your home and to your receiver.
The guests were by this time beginning to look with wonder at the labyrinth of lead cables, dials, instruments, and tubes incorporated in the Voice of Purdue. They stood fascinated by the control mixer, watching the operator intent on his duties of monitoring the program. Suddenly, one of the visitors turned to the guide and asked: "What are the lights that flash on and off as the pitch or the intensity of the speaker's voice is varied?"
"That is a visible indicator of the per cent of modulation," the guide explained. "We have at all times a method of seeing how much the transmitter is off frequency and what the per cent of modulation is. This is made possible through another part of the transmitter that is even more complicated than the R. F. or audio portions. It is called a frequency and modulation monitor."
"You said that you had a method of telling to what extent the station was off the assigned frequency," another of the guests said. "Do you mean that a radio station is permitted to be off its assigned wave length or frequency?"
"Yes," explained the guide, "it is impossible to keep the transmitter exactly on its assigned frequency. Each station is allowed a variation of 50 cycles, plus or minus, of its assigned frequency. Our staff of technicians here at Purdue, however, has the crystal oscillator stage so well and so accurately adjusted that WBAA is never more than three or four cycles off frequency."
Well, how about the 'blanketing' effect of Purdue's signal over that of other stations occupying a frequency near that of WBAA?" inquired another guest. "Some of my friends complain they are unable to get the stations near the same dial setting as Purdue when the Purdue station is broadcasting. On our receiver at home, however, I have never experience that difficulty."
"That is simply because your receiving set in your home is of good construction," explained the staff member. "We have many such similar complaints coming into the offices here; but there is absolutely nothing that our staff can do to remedy that 'blanketing' effect. That is entirely in the hands of the owners of the receivers. If the receiver is of modern construction and is constructed correctly, that inability to tune out WBAA clearly in preference to other stations will immediately be remedied."
"Well, we certainly have learned a lot about broadcasting and the mechanical problems encountered in maintaining and operating a broadcasting transmitter," said one of the guests as they were departing. "Are all of the programs originating from the Voice of Purdue cared for in such a careful and systematic way as this one we have witnessed and followed this afternoon?"
"Yes, they all receive the same care," asserted the guide. "It is our duty, as members of the staff of WBAA, to do our part, no matter how small or how minute, to furnish and insure the listeners with interesting, entertaining, and instructive programs. The mechanical and of this goal is just one part of that job."
Our visitors, with senses of perception dulled by the bombardment of new impressions, filed out of the studio, appreciating if not understanding the complexities of radio transmission. They had seen the obvious parts of program broadcasting,--the performance of the student actors and announcers, the interesting and ofttimes laughable methods of creating sound effects, the role of the control engineers getting the program into the ether. They had observed numerous pieces of complicated and mysterious apparatus; but what they had not seen, the playing of the electrons in the tubes, remains the occult domain of the radio engineer.