Re: /p/ versus devoiced b?

I'm quite late to this topic, but what the heck...

I don't know if by "devoiced b" you are referring to the practice of
writing an IPA [b] with a <devoiced> mark below it (the little circle
thing), i.e [b^0] in SAMPA. I think so. This is a tradition in some
countries, certainly Iceland (my home country), and I gather China too.

The IPA chart has pairs of symbols for each point of articulation
(bilabial, alveolar, etc) and each mode (nasal, plosive, etc), where one
represents the voiced and the other one, next to it, the unvoiced. Thus you
have [b] beside [p], and their relationship is only a matter of +/- voice.

The chart arbitrarily supplies two symbols for some types of sounds, like
[p] and [b], but not for others, like [n] (no unique -voice symbol for the
same type of sound). A practical matter, having to do with the frequency of
unvoiced versions of the individual sounds, I suppose. Since unvoiced [n]
still occurs (though rarely), IPA supplies a small modification mark, an
empty circle, which can be attached to any symbol like [n], which doesn't
have a -voice counterpart, to indicate a state of -voice. Thus you get
[n^0].

This being how I understand the IPA system, adding the -voice mark to a
symbol that *does* have an unvoiced counterpart, such as [b], makes no
sense. How can there be a difference between [b^0] and [p]? If anyone can
tell me, I'd be most interested :)

The reason that I would see, is that phoneticians, being human, may have a
kind of bias for their native orthography's rendering of fortis vs lenis.
Icelandic has two sets of plosives, +/- aspirate, both -voice; they should
be phoneticized [p] and [p^h]. But our orthography writes <b> for the -
aspirate, and <p> for the +aspirate (which is afterall practical, and
etymological too). In the process of transcribing a word with orthographic
<b>, with a [p], it is rendered irrecognizable (<bagga> becomes [pAkkA],
which is confusable with <pakka> [p^hAhkA]). Therefore Icelandic
phoneticians would be prone to "cheating", using the IPA [b] symbol, but
attaching a -voice marker to it (since they'd otherwise be in fault), to
make the word more recognizable.

But this is just my theory; if I am missing out information here, I'd be
delighted to be proven wrong :)

Óskar