Spiderant wrote:



Hello Nick,

I know this is a very basic question (and I will be hitting the library
today to see if I can improve my basic knowledge), but could you tell me
exactly what or how much information is going from a microphone to a
recording device? I always just assumed that, at any given point in time,
there nothing more than a polarity difference between two wires. If
possible, could you tell me what is happening at each given point?

Much appreciated,

Roland Goetz.



Hi,

I think Jim can do this much better, but the little I know. To actually
state how much informatoin is being sent, you need to know a couple of
things, the range of frequences being transmitted, and the signal to
noise ratio, given that you can actually calculate the amount of
information, Lookup Shannon in the text books. However, i don't you are
using the word information is such a formal sense.

The simple and quick answer, is yes, at a particular point in time there
is only a single voltage being produced by the source, but thats just
one part of the story, at a point in time just before that, the voltage
was at a different level, and at a point in the future it will be at yet
another voltage. So you could regard the signal as a sequence of
instantanious voltage levels, and the information is encoded in this
ever changing level.

To try and put it into context with your original question, consider two
instruments, lets use a pair of flutes, as they can produce nice pure
tones. If one flute is playing a A above middle C (just gessuing, don't
know the actual range of a flute), thats a 440hz sine wave (for the sake
of argument), that means the wave goes up, and then down and back again,
440 time sin s second. So if that was recorded and played back, the
speaker cone would folow the sine wave and move in and out 440 times a
second. Now if we play that recording, and look at in on a scope, we see
a continuious sine wave on the screen, thats showing the signal in the
time domain, it displays how the voltage changes with respect to time.

If we feed the same signal intp a spectrum analyser, we see a very
different display, we see a single line, at the frequency of 440hz, this
is showing the recording only contains a single frequency, that of
440hz, thats showing the signal in the frequency domain, how the signal
is conposed of frequences of sine waves.

Now lets take a second flute, and this time, play a A one octive above
the other, this is a note that has a frequency of 880hz (each musical
octave is a doubling in frequency). Now if we record both flutes playing
their notes together, and play tyhem back, the speaker, does not have to
move in and out 440 times a seconds and 880 times a second at the same
time, it moves to follow the signal thats the combined two frequencys.
(this would be so much simpler to show in person, with a copy of cool
edit). Then if we play this through a scope, the display shows a trace
thats the combination of the two frequencies, imagine a sime wave, that
while it wobbles up and down, is also wobbiling at twice the speed. And
if we feed the same signal into the spectrum analyser, now we see two
lines, one at 440 and one at 880, showing the signal is a combination of
the two seperate frequencies.

Not sure if any of the above makes sense of helps, but there you go.

In a way (and not to offend, we all had to learn this once), some of
this stuff, is so basic, that its hard to explain, you are just used to
it, and take it as read. Mind you its always good to go back to basics,
you often end up with a better understanding of something you thought
you fully understood allready :-)

--
Nick