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.
Way back in 1979... legal limits were put in place for unintentional radio interference from digital devices (by the FCC).
But many PC manufacturers were not testing for compliance. As the industry grew up, their liabilities became known and were better addressed.
As to Verison vs ATT for Iphone.. they have very different radios. GSM vs CDMA. GSM is more likely to interact with your car radio..(ATT).. but it shouldn't interact much, unless there is something wrong with the phone or radios in question.
GSM will degrade in quality with interference and at the limits of it's range. Basically, it will sound bad before it drops out.
CDMA will hold higher quality ... but it's range (effective range) will be reduced as the cell tower: a) increases in traffic or b) sees an increase in interference. Basically, it wil sound great .. until it drops out entirely.
Often see CDMA radios that work at a given location (fixed) nearly all day but drop out entirely during "rush hour" from a near by freeway (as everyone is using their phones).. .Reason (simplistic version) : the fixed location was near limit of the tower range. The processing gain possible with CDMA is reduced as the channel traffic increases.
Correct, it seems to be the carrier frequency utilized by AT&T, not the specific phone. Some of my co-workers have Sprint iPhone 4S phones which do not generate the sounds, but the one with the AT&T iPhone 4 will generate the sounds. And since we have poor AT&T service in our building, it makes the sounds a lot, since it seems to be constantly searching for better service.
Thanks for the info, Jim. So it's the carrier and not the phone. So it could be that an iPhone with a carrier other than AT&T wouldn't produce the interference. Does that make sense?
Amateur-radio operators routinely find RF-emitting sources that splatter noise across various frequency bands. Often street-lights circuits and malfunctioning power transformers cause the problems. One of the oddest sources: a cordless-phone charger that produced intermittent noise that affected a local repeater in Massachusetts. Members of the local ham club located the source and fixed the problem as described in this newsletter from 2005: http://www.mmra.org/newsltrs/mmnews_200501.lowres.pdf. I belonged to the club at the time.
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.
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.
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.
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.
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.
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.
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