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Sat Dec 1 13:43:35 CET 2007

subsampling


A baud rate that sounds like 16th notes would be nice, which is about
8 Hz. A carrier of 600 Hz, this gives a ratio of 75. The sampling rate
needs to be > 16Hz, so let's take the one in the neighbourhood of
32Hz.

Care needs to be taken though when using aliasing: the frequency error
will amplify. Let's see.. using 10 Mhz, the subdivisions become:

2^20 -> 9.5 Hz     baud rate
2^18 -> 38.1 Hz    sampling frequency
2^14 -> 610.4 Hz   carrier frequency
2^12 -> 2.44 kHz   4 x carrier
2^7  -> 78.1 kHz   PWM frequency

the carrier/baud rate here is 2^6 = 64

going from carrier -> sampling frequency is a subdivision of
16. what's the error of the oscillators?

CSTLS10M0G53 has 0.5 % precision. times 16 that becomes 8.0 % which is
quite a lot.. so probably it does need continuous phase compensation??

another reason to not subsample is to get better noise performance and
better frequency rejection due to longer integration time. using 4 x
carrier frequency is still only 2.4 kHz which is at 2^12 subdivision,
or 4k instructions per sample, which is absolutely no problem. this
gives 2^8 samples per symbol.

another thing to think about: synchronization. this can be implemented
using time shifts or phasor rotation. the latter is probably not a
good idea due to problems with filter matching. so actually, the
carrier needs to be significantly oversampled, or at least mixed.. i
think i need to make a new table with variables..



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