Jens David wrote:
> Look at it like this:
> You theoretically could sample 44.1KSymbols/sec, but that would require
> the software demodulator to tell the soundcard ADC _when_ to sample the
> signal. However, usually the conversion process is controlled by an
> on-board
> crystal oscillator.
Yes, I figured this out today. :)
> Thus you need to do at least 2 times physical oversampling,
> interpolating
> the signal using a fractionally spaced FIR filter to an oversampling
> ratio
> of at least 16 to guarantee a sufficiently small clock recovery PLL
> phase
> shift resolution. We wanna sample the eye at the position where it is
> maximally opened, right?
Yes. I understand.
> Note that for QAM modulation you need I/Q modulation and linear TX and
> RX
> signal paths (due to non-constant envelope). This is non-trivial to
> implement,
You can do the I/Q modulation in entirely in software. This shouldn't be
too hard.
As for the linear signal paths.. I'm assuming you mean linear power
responce in frequenceny space. This is corrected using a FIR filter.
Modems do this.
I dont know how you would generate the EQ on a non-point to point link
though. :(
Even if we could only use QAM over point-to-point links, I think that
the prospect of 150Kbit/s (22050 symbols/s, 4I+3Q) over a point-to-point
radio link using nothing other then a commonly available Celeron 300a
equipt system
with soundcard is quite attractive.
>
> As I said some weeks ago, we *really* ought to design a new mac protocol
> for this kind of stuff. We could use syncronization preambles to help
> clock recovery a bit and integrate FEC negotiation. That way, we could
> support
> those more-complicated modulation schemes on half duplex (user access)
> channels.
It certantly would be intresting.
