In article .com,
Radium wrote:

On Sep 1, 1:12 am, (Paul Schlyter) wrote:

In article .com,

Radium wrote:
Sorry, I meant to ask whether 3,438 GHz is the highest radio frequency
used to receive audio signals from outer space. I should have made my
question more specific. Radio-astronomers study sounds from the sun as
well as visual data.

Radio astronomers study EM radiation, not "sounds", from the Sun.
Since there's a vacuum between the Sun and us, no sound waves would be
able to propagate from the Sun to us.

The radio-frequency EM radiation emitted from the sun does translate
to sound when it is picked up by a radio receiver of the same carrier
frequency.

Here you make the silent assumption that the electric signal from the
radio receiver is fed to a loudspekarer. But that's just *one*
possible way of converting the EM radiation. You could use other ways
too. For instance displaying it on some video screen - those who do
so could claim that "The radio-frequency EM radiation emitted from the
sun does translate to light when it is picked up by a radio receiver
of the same carrier frequency" (with the silent assupmtion that the
output from the receiver is displayed on a video screen). It's the
translator who decides what the EM radiation translates to....

Btw did you ever try to *listen* to a TV transmission? I mean, to feed
the *video* signal (not the audio signal) to a loudspeaker instead
of a video screen? Yep, the sound changes with the contents of the
picture - but of course one hears only the lowermost part of the 5 MHz
of bandwidth a normal video signal has.

Another interesting experience is to feed a digital signal directly to
a loudspeaker instead of decoding and converting it to an analog
signal first. That of course requires that the digital signal is
within the audible range of frequencies -- the signal from a
traditional telephone modem would be quite suitable here. The old 300
bps modems produced a signal with a quite clear structure (the signal
jumped between two frequencies 300 times per second), but the more
modern telephone modems which can handle bit rates up to 57600 bps,
they sound pretty much like white noise to the human ear.

Otoh careeful studies of
Doppler shifts have enabled solar astronomers to study sound waves
*within* the Sun. But these sound waves never reach us - we can only
study them indirectly because they move matter near the solar surface.
And their frequencies are usually well below what the human ear can
hear, i.e. it's infrasound.

That's why audio software is often used to speed up the infrasound
until it is at least 20 Hz so that humans can hear it.

:-) ....there's no need to speed it up just to convert the frequency
into the audible range.... the frequency can be bumped up even if
the original speed is maintained.

I wonder if a space station with a 3,438 GHz AM receiver could pick up
any extremely-distant audio signals between 20 to 20,000 Hz [from
magnetars, gamma-ray-bursts, supernovae and other high-energy but
cosmic objects] after demodulating the 3,438 GHz AM carrier wave.

They could certainly try .... but if they did, and succeeded, it would
sound just like noise. This radiation does not originate as audio
signals, and they're certainly not put on an AM modulated carrier.

Well, most natural sources of EMI and RFI are amplitude-modulated.

They're probably frequency modulated and phase modulated as well,
since their contents are pretty random. I strongly doubt they consist
of one single frequency whose amplitude varies while its frequency and
phase remains unchanged (that's the way a properly modulated AM signal
would be). In particular it won't have symmetrical sidebands with the
same content, the way a real AM signal should have.

The audio signals are not put on the carrier wave, however if the
variations in the peak-to-peak amplitude of the 3,438 GHz
electromagnetic waves correspond to frequencies between 20 and 20,000
Hz [and the peak-to-peak variations are sufficient in power], then the
signal can be picked up of 3,438 GHz receiver and demodulated. The
result would be audio signals.

Trivially true -- but these audio signals would be created by us
humans. They're not inherent in the original signal.

Therefore it's hardly useful to try to demodulate these waves as if
they were AM modulated signals - there's e.g. no AM carrier (i.e. one
single frequency which is stronger than all the others within the
frequency band).

Also, any audio (= pressure waves within a gas) which are formed
outside the Earth is certainly *not* limited to the 20 to 20,000
Hz frequency range..... that frequency range is merely the limits
of what the human ear can hear.

Audio waves from 20 to 20,000 Hz can be derived from demodulating
radio waves.

You can create audio waves also below 20 Hz and above 20,000 Hz as
well. Humans won't hear them, true, but dogs and bats might enjoy them... :-)

Since most natural radio disruptions are amplitude-
modulated it would be easier to listen to cosmic sounds

These sounds aren't "cosmic" - they're created here on Earth by us humans.

using an AM receiver as opposed to an FM receiver. FM is immune to the
disruptions that normally affect AM.

Did you ever try to tune an FM receiver between radio stations on the
FM band? Also turn off any "muting" or "squelch" the receiver may have.
What do you hear? Silence? Or perhaps noise?

You say "FM is immune to the disruptions that normally affect AM". If
this is to work, you must have an FM carrier which is strong enough
for the receivers amplitude limitation circuits to work well. Cosmic
radio noise is far too weak for that.


description of AM and definition of frequency snipped
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