[RFI] RFI on 1.8 mhz

[Jim Brown]

On 1/22/2013 9:14 AM, David Robbins wrote:
> what would distinguish the sound of that sweep from that from something else sweeping across a different frequency range?

What we "hear" on a recording and "see" in a spectrum display is the 
MODULATION produced by the noise source. I don't have the experience 
chasing noise sources that many of the RFI professionals on this list 
have, so I yield to those who do in noting the possible value of 
recordings in identifying sources.

In addition to the examples cited (street lights, treadmills), I can 
add, from my own experience, the sound of a cell phone transmitting back 
to the tower and detected in a susceptible audio product.  The data is a 
series of short duty cycle pulses with a repetition rate on the order of 
220 Hz, which puts its detected spectra in the middle of audio, and the 
"chatter" sound is quite distinctive. Until the Pin One Problems in 
commonly used mics were fixed, it was quite common to hear this sound on 
the air  If you haven't heard this sound on news and interview programs, 
you haven't been listening. :)

With the wide range of noise sources, everything from switching power 
supplies to control circuitry for motors in various products, to 
lighting of all sorts, I don't find it at all hard to believe that audio 
recordings and spectrum recordings ARE potentially useful in giving 
clues as to the source.  Indeed, one of my colleagues at Motorola went 
so far as to generate a series of "noise signatures" of cell phone 
products for use by the pro audio community.

73, Jim Brown K9YC
Chair, Technical Committee on EMC, Audio Engineering Society