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#18
July 6th 07, 06:13 AM
posted to sci.electronics.basics,rec.radio.shortwave,rec.radio.amateur.antenna,alt.cellular.cingular,alt.internet.wireless
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AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency
"isw" wrote in message ... In article , "Ron Baker, Pluralitas!" wrote: "isw" wrote in message ... In article , "Ron Baker, Pluralitas!" wrote: snip While it might not be obvious, the two cases I described are basically identical. And this situation occurs in real life, i.e. in radio signals, oceanography, and guitar tuning. The beat you hear during guitar tuning is not modulation; there is no non-linear process involved (i.e. no multiplication). Isaac In short, the human auditory system is not linear. It has a finite resolution bandwidth. It can't resolve two tones separted by a few Hertz as two separate tones. (But if they are separted by 100 Hz they can easily be separated without hearing a beat.) Two tones 100 Hz apart may or may not be perceived separately; depends on a lot of other factors. MP3 encoding, for example, depends on the ear's (very predictable) inability to discern tones "nearby" to other, louder ones. I'll remember that the next time I'm tuning an MP3 guitar. The same affect can be seen on a spectrum analyzer. Give it two frequencies separated by 1 Hz. Set the resolution bandwidth to 10 Hz. You'll see the peak rise and fall at 1 Hz. Yup. And the spectrum analyzer is (hopefully) a very linear system, producing no intermodulation of its own. Isaac What does a spectrum analyzer use to arive at amplitude values? An envelope detector? Is that linear? I'm sure there's more than one way to do it, but I feel certain that any Which of them is linear? competently designed unit will not add any signals of its own to what it is being used to analyze. Isaac |
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