Re: 16-Bit Stereo PCM Standard?

On Mon, Apr 5, 2010 at 12:02 PM, Patrick M. Rutkowski
<[email protected]> wrote:

> 3) How does sound get captured from a microphone? Do you get 16-bit
>  PCM input as well? But if you do get something like 16-bit PCM data
>  from a microphone then there _must_ be some mapping going on from
>  "decibels -> 16-bit PCM", since the microphone really can be
>  guaranteed to capture exact decibel levels (even if the speakers
>  aren't guaranteed to output exact decibel levels). Additionally,
>  is there some maximum decibel level that a mic can usually capture?
>  Might that get mapped to something like the 32767 PCM value?
>
> Apologies in advance if any of these questions seem strange, my head
> is still spinning :-)
>
> -Patrick

Sound waves push on an element in the microphone.  The microphone is
an analog device hooked up to an electrical circuit.  The element in
the microphone that gets pushed on modulates the electrical current so
that the voltage on the circuit is proportional to the pressure level
on the microphone.  That circuit is hooked into a Digital / Analog
Converter.  This DAC on your sound card switches the raw voltage on
the circuit into a digital number at any given instant.  (It can be
with 8 bits of precision, 16, 24, etc.  That doesn't change it
conceptually.  Just think of it as percentages.  It goes from the
minimum voltage that the DAC expects to the maximum using whatever
number of bits.)  This DAC is sampled many times every second so that
you get a series of numbers which correspond roughly to what the
pressure level on the microphone was at that instant.  That sequence
of numbers is raw audio in memory.

When you want to get noise out of the computer instead of into it,
this all happens in reverse.  You get no guarantees about dB levels at
any point.  It's basically all relative.  You can use an imaginary
super-sensitive microphone to catch the sound of an ant walking along,
or a very bad microphone that picks up a Jet engine with the same
loudness when you play back the two sounds on the same speakers.

The goal at every step is usually that everything should have a linear
relationship.  In practice, nothing is ever perfectly linear.
Perceived loudness is very non-linear, but that's a separate issue
from sound pressure levels and line voltages.
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