this version is more tested. i'll merge _decode back
once it's better tested. it contains a lot of invasive
changes, whereas recv is much closer to the original GNU
code that i inherited, that i also know works quite well.

Signed-off-by: Leah Rowe <leah@libreboot.org>

Signed-off-by: Leah Rowe <leah@libreboot.org>

Signed-off-by: Leah Rowe <leah@libreboot.org>

for pulses, we currently use amplitude detection.

edge detection is better, because weak / low gain
signals will be more reliable. if audio is coming
in on/from a system that does automatic gain
adjustment, this once again is more robust too.

microphones and speakers (which people often use
with spkmodem if nothing else available) often
clamp amplitude, to an extent that this software
may not detect those pulses reliably that way.

so we detect slope edges instead. this causes
very little performance penalty (use of abs(),
that's about it)

however, edge detection is inherently vulnerable
to noise, so we will also detect amplitude. this
acts as an effective noise filter, while still
improving pulse detection.

Signed-off-by: Leah Rowe <leah@libreboot.org>

in select_separator_tone, i never reset these
after computing their average.

Signed-off-by: Leah Rowe <leah@libreboot.org>

this check no longer applies (never triggers)

is_signal_valid already guarantees that the separator
tone is valid.

Signed-off-by: Leah Rowe <leah@libreboot.org>

Signed-off-by: Leah Rowe <leah@libreboot.org>

otherwise, calibration could collect garbage data.
this improves noise mitigation.

Signed-off-by: Leah Rowe <leah@libreboot.org>

bits are currently assembled even on invalid frames. this
patch fixes that - the bug is also in the GNU version.

this reduces the chance of noise/calibration from creating
corrupt character output during operation.

Signed-off-by: Leah Rowe <leah@libreboot.org>

improves reliability in the case when audio cuts out,
mic glitches, laptop audio power saving, etc.

Signed-off-by: Leah Rowe <leah@libreboot.org>