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Mon Oct 8 00:26:37 CEST 2007

## frequency decoding

for krikit, the choice to make is to either decode the whole spectrum
(listen to everything at once) or listen only to a single band. this
is a choice that has to be made early on.. some remarks.
* FFT + listening to all bands is probably overkill. it's not so
straightforward to implement, so the benefits should be big. FFT for
point-to-point makes only sense when combatting linear distortion.
* single frequency detection is really straightforward. the core
routine is a MIXER followed by a complex LPF. the output is phase
and amplitude.
* using a sliding window average LPF together with orthogonal
frequencies allows for good channel separation. this works for
steady state only, so some synchronization mechanism is necessary.
* sending out a single message to multiple frequencies: easy to do
with pre-computed tables for 0 and 1. phase randomisation to avoid
peaks is possible here.
* i'm afraid of linear distortion due to room acoustics.. maybe FM/FSK
should be used?
* if non-linear distorition is not a problem, DTMF frequencies are not
necessary.
* using exact arithmetic, it is easy to update/downdate a state vector
for rectangular window LPF. this update can be performed at the
input of the mixer.
* bandwidth limitation for transmission.
http://en.wikipedia.org/wiki/Olivia_MFSK

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