Sun Dec 9 10:13:57 CET 2007
i got very nice reception it looks like.
what about the following algorithm:
* set threshold to an estimate of the noise threshold (say 50)
* wait until something comes in: interpret it as a start bit
* find the max amplitude during the start bit
* start sampling 9 bytes, by waiting for half a symbol length, and
compare to half the dynamic threshold
looking at some sampled data of #x55 + start and stop bits, which is
01010101, with 0 = ON, 1 = OFF, it seems that putting the threshold at
half is not a good idea.. also, the time it takes to reach from going
above noise threshold (50) to the peak of the start bit is exactly the
maybe it should be compared with a lowpass envelope?
tried this, but looks like LPF delay is going to be a
problem. however, it should be possible to keep the same filter, but
perform the comparison with delayed versions?
another possibility is to just save the sample points, and perform the
filtering at a later stage.
or.. it could just be compared to the previous sample point? if lower
it's the reverse? that will probably work just fine: this might give a
problem for stable 0 or 1..
* start sampling s - s_0 after detecting a start bit. s_0 = rise
time to threshold level.
* collect 10 samples.
what i'm doing: s_0 = 0, and watching the output of the sampling with
a #x55 byte. it looks pretty decent. now trying the number station.
* compare with previous (differentiate)
* maybe hysteresis?
differentiate is no good.
i'm probably fighting something else.. maybe the data rate is just too
fast? i had to move from 512 samples to 256.. so looks like something
else is going on..
what about this: change the special purpose lowpass filter so it takes
16 bit coefficients, and then reduce the filter pole a bit.