Vorbis - Feb 2002 - Re: An introduction to compressed audio with Vorbis

On <http://cs.leander.isd.tenet.edu/~mitchell/vorbis_intro.html>:

"Each snapshot has a 16-bit number for the "loudness" of the
sound,
meaning that the scale is fairly fine-grained - it ranges from -32,768
(complete silence during that snapshot) to 32,767 (the loudest volume
measurable)."

That's not correct.  A single sample is not meaningful by itself; each
sample merely represents a voltage.  It's the *difference* between
samples that creates vibrations in a speaker, and therefore sound waves.

Pure digital silence is a set of samples whose value is consistently zero
(not -32768).  A "pure" tone is produced by samples whose values
conform
to a sine function -- a "sine wave".  The frequency of the sine wave
(i.e., how quickly the numbers go up and down) determines the frequency
of the sound, which we perceive as pitch.  The amplititude of the sine
wave -- how far the wave deviates from zero -- determines how loud it is.
A sine wave that goes from, say, -1000 to +1000 is much quieter than
one that goes from -5000 to +5000.  A sine wave that goes from -32767
to +32767 is as loud as you can get.

My explanation may leave a bit to be desired; and perhaps this is too
technical/mathematical for your intended audience.  But the sentence I
quoted is simply wrong. :-/


-- 
Greg Wooledge                  |   "Truth belongs to everybody."
greg@wooledge.org              |    - The Red Hot Chili Peppers
http://wooledge.org/~greg/     |


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> "Each snapshot has a 16-bit number for the "loudness" of the
sound,
> meaning that the scale is fairly fine-grained - it ranges from -32,768
> (complete silence during that snapshot) to 32,767 (the loudest volume
> measurable)."
>
> That's not correct.  A single sample is not meaningful by itself; each
> sample merely represents a voltage.  It's the *difference* between
> samples that creates vibrations in a speaker, and therefore sound waves.
        [snip]
> My explanation may leave a bit to be desired; and perhaps this is too
> technical/mathematical for your intended audience.  But the sentence I
> quoted is simply wrong. :-/
Thanks for the correction.  Even as I wrote it, I felt fairly sure that 
-32,768 shouldn't represent silence.

How about: "Each snapshot has a 16-bit number representing the amplitude of
the sound wave being sampled, which is fairly fine-grained: the scale ranges 
from -32,768 to 32,767.  This wide range allows for even subtle volume
differences to be accurately represented."

Still non-technical, but (hopefully) no longer false.


-- 
Graham Mitchell - computer science teacher, Leander High School
"Compassion should never determine our beliefs about sin; it should 
only determine our response to those who struggle with it."
	-- Harry Schaumberg, "False Intimacy"

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