pihpsdr TX audio chain

["Christoph v. Wüllen"]

Dear all,

Recently I had discussions with people from this list about
the TX audio chain (Mic gain, auto-leveller, speech processor
etc) that showed me that this is a field full of myths
and misunderstandings. Therefore, I extended the pihpsdr
manual by a dedicated section on the TX audio chain, since
all the bits are otherwise scattered over the whole
manual.

This "preprint" (5 pages PDF) will soon be part of
the manual on github but is sent separately here
because it can be of interest to those using
host software other than pihpsdr.

P.S.:
Note the change in the semantics of the "TX ALC peak" value,
which now can be both negative and positive (combining
what was previously called "peak" and "gain" into a single
reading).