Think back to the 1980s when there was no Internet and the most advanced microprocessor for personal computers was the Intel 8088, which required a minimum of four clock cycles per instruction and used a 4MHz clock. Computers were slow!
The Massachusetts Institute of Technology got a contract to study the feasibility of dissemination of data using FM radio broadcast stations. It leased some of the spectrum used by the FM station owned by the Emerson Radio School in Boston. The data rate was 1,200 baud, and was encoded onto a 92kHz carrier by shifting the frequency a few kilohertz up and down to send a mark and space. The 92kHz carrier was mixed with the monaural audio, the stereo subcarrier, and the 67kHz subcarrier radio reading for the blind. The composite signal modulated the FM transmitter. The power dedicated to the transmission of the data was in the order of 25W or less.
MIT contracted my company to design and build the radio modems needed to conduct the tests. MIT students volunteered to use their computers to receive data, which consisted of national news. Since my office was about 30 miles from Boston, I could receive the signal, but it was not very strong. Since I already had a high-quality crystal controlled FM receiver, it was only necessary to design the 92kHz FSK demodulator board. The design was simple, but highly effective, complete with an RS-232 output.
I thoroughly tested the production prototype with my own equipment and then observed the signal from Boston. Both tests were excellent, and I was ready to send the prototype to MIT for final approval. It was only a few days before that I received a call from the MIT professor who said the modem put out only garbage! It was embarrassing. I checked a receiver in my office, and the data was clean.
A few days later, I visited the computer lab at MIT. There were computers everywhere. They were even experimenting with robotics. I asked to go to the men’s room, and it was full of computers. Finally, the professor showed me the receiver that I had sent and the display on the oscilloscope. Yes, it was, in fact, garbage. I asked him if he was getting a strong signal. He pointed out the window to the tower on the other side of the Charles River. I explained that the power near the transmitter was not always as great as 10 or 20 miles away because of the gain of the antenna, but we both knew that was not a good explanation.
As I tried to solve the puzzle, I gazed around the room and realized that there were more than a dozen computers, all powered up and doing something or other. I asked the professor if he could turn off the computers. One by one, he turned them off. Soon, on the oscilloscope, a waveform started to appear. It got cleaner and cleaner until it was perfect.
Around 1980, computers were not regulated by the FCC, and no effort was made by the manufacturers to control the slew rate on the data lines, even those leaving the computer. The noise spectrum from some computers extended into the hundreds of MHz.
Shortly after my meeting, I got a confirmation to construct the balance of the order, and MIT successfully conducted its experiment.
This entry was submitted by Frank Karkota and edited by Rob Spiegel.
Frank Karkota worked with power transmitters in the range of less than 1MHz to 5GHz. He designed and built equipment for radio stations and eventually started a company that made commercial and consumer receivers that covered 500kHz to almost 1GHz.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
Nice look back at life prior to the onslaught of computers and devices. Going into that MIT lab and seeing the spread of computers must have been eye-opening then. Today, you'd see a comparable set up in an office and even in some homes. Also pretty eye-opening that the FCC wasn't governing the computer spectrum back in those days. Shows you that Bill Gates' vision (and others) of a computer on every desk was still pretty much considered a pipe dream.
Frank, that was an iteresting experience. Today we have a similar issue with cell phones. I am not talking about the regulatory situation, but about interference with audio equipment. My son first told me about it in regard to a situation in middle school. Students were not to have cell phones in school. If they had them, they were to be turned off. Often, though, they were not. In classes with computers the teachers noticed a buzzing sound coming from the computer speakers. They soon figured out that it was the cell phones, and it allowed them to "catch" those students who had their phones with them. Now this happens only with the at&t GSM phones. I had a Verizon phone which used a different frequency and transmission type (CDMA). I did not notice interference with audio equipment with those phones.
I have the same problem in my car. When my ATT phone talks to the tower to change towers, it transmits and meses with my radio. I have to have it far away from the receiver to avoid the irritating buzzing. But I don't blame the cell phone. After all, who expects to place a transmitter so close to a receiver without interference? That's why we squelch the radio during transmit, normally, in ham radios. Plus, we aren't always duplex, but that is another story.
Back in the mid '60's I remember hearing Christmas carols "played" by an IBM 1620 computer via an AM radio. Someone said a program caused certain sequences of clock signals that radiated the "music" the radio picked up.
Tonight I found this: "From Bill Principe, 16 Sep 2005:
I saw your 1401 and 1620 pages on the Internet. As an undergrad at Berkeley in the 60s, I had part-time jobs working on both machines. I'd like to share a 1620 anecdote.
There used to be a program for the 1620 that worked line this. You put an AM radio on the CPU console, and tuned it for the loudest noise. (They generated a lot of random RF noise that could play havoc on nearby electronic equipment.) Then you fed a deck of cards with the program. The radio would play "Stars and Stripes Forever" and the line printer would play the drum rolls. I'd like to see a Pentium IV laptop do that!"
Here's the source: http://www.columbia.edu/cu/computinghistory/1620.html, so it seems as though programmers pretty well mastered creative uses of computer generated RFI some time ago.
Back in the early days of computing we did some pretty crazy stuff to make music. Most small computers had cassette ports for saving programs and that was hacked into for music. There was another one for uploading code into a Commodore floppy's RAM so that the stepper mode could sing, but my favorite was using a dot matrix printer.
No knowledge is wasted, I still use the singing motor and sometimes solenoids in appliances as a beeper.
Yes, audio interference from AT&T GSM phones is very common if the phones are close to a speaker. My wife's old RAZR flip phone does it ,as does my co-worker's iPhone 4. As the other poster mentioned, it seems to happen when the phone is searching for towers. My wife's phone will also generate the noise just before her phone rings. Quite annoying.
Since I switched to a Sprint phone, I haven't heard the sound once, so it seems to have something to do with the frequency that the AT&T GSM phones operate on.
The GSM poll rate is 277 Hz, right in the audio spectrum. I live in a low-signal area, so my GSM cellphone is always transmittting near maximum power. It's tone can be heard on the laptop speakers and on my wireline phone if I don't place it carefully, and I am sure I could hear it on AM radio if I tried.
The previous few generations of ATM (cash) machines generated a fanfare at the conclusion of a transaction by cycling the receipt printer printhead back and forth at the appropriate rates and distances.
Years ago, my wife would call out to me that the TV was messing up. Living in a rural community, but during the height of CB popularity, we assumed it was a neighbor using their CB radio, possibly with an (illegal) power amplifier. This went on for weeks and we coined the phrase DCB for "Damn CBers". One night it hit me though - the only time she complained was when I turned on my new Osborne computer. Apparently there were enough harmonics coming from the blazing fast 4 mHz Z80 processor chip to interfere with the television rooms away. That solved the mystery of who the DCB was, it was me. In those days, I only occasionaly used the computer unlike today when it is hardly ever turned off and since I was constantly fiddling with something in my basement workshop, it took some time to make that connection.
From Dell / Intel® New Paradigms in Design Work Scott Hamilton, vertical market strategist for Dell Precision workstations, 5/2/2013 3
Early in my career, I worked as a draftsman and remember the days of drawing on vellum with numbered pencils and Mylar with plastic lead. This was a fun experience in the sense that I ...
I've been using workstations for more than 10 years and love finding ways to get more performance from my system. With demanding professional applications that require more power each ...
A lasting memory from my first job as an engineer in an auto assembly plant is standing on hard concrete at six in the morning, vending-machine coffee clutched in hand, listening to ...
A quick look into the merger of two powerhouse 3D printing OEMs and the new leader in rapid prototyping solutions, Stratasys. The industrial revolution is now led by 3D printing and engineers are given the opportunity to fully maximize their design capabilities, reduce their time-to-market and functionally test prototypes cheaper, faster and easier. Bruce Bradshaw, Director of Marketing in North America, will explore the large product offering and variety of materials that will help CAD designers articulate their product design with actual, physical prototypes. This broadcast will dive deep into technical information including application specific stories from real world customers and their experiences with 3D printing. 3D Printing is
To save this item to your list of favorite Design News content so you can find it later in your Profile page, click the "Save It" button next to the item.
If you found this interesting or useful, please use the links to the services below to share it with other readers. You will need a free account with each service to share an item via that service.
Tweet This
4 
