Hi,
Thanks for the great explanation Dustin. Is the set of N samples the analog
samples from the soundcard or is "fsk symbol" something else? I see the ratio of
1.8333333*1200 = 2200 but can you explain how you are checking for power at the
1200 Hz frequency? I was also wondering about the effect of doppler shift on the
radio signal. I am putting a radio in a rocket that will be travelling over mach 3
away from the groundstation so will the 1200/2200 fsk frequencies be shifted down
in frequency enought that I should be compensating for this? Also will the UHF
carrier be shifted a significant amount requiring frequency tuning?
Thanks for your time
cheers,
Jamie
http://142.104.54.173
[EMAIL PROTECTED] wrote:
> > I am new to this list and am wondering if anyone here is using the linux
> > soundmodem module with their setup.
> > Is it reliable? Also I was wondering if anyone knows how exactly it works..
> > I tried making a soundcard modem in visualbasic using a fast fourier transform
> > but it only could handle several bits/second.
>
> Before I go into a longish talk about transforms, I's, and Q's, is there
> anyone out there that regularly runs newqpsk? I'm loopback testing the
> modem on vhf fm and I get a 50% error rate.
>
> I've used the afsk1200 baud modem in the linux kernel, I've also written a
> packet modem that works with the 'newqpsk' userspace soundmodem code.
> Both the modulator and the demodulator work on the air, but they're mainly
> a learning tool.
>
> The problem with demoding fsk with dfft's is that your burning cpu cycles
> calculating the power levels for frequencies you don't care about. You
> want to run two I/Q detectors. Take a set S[p] of N samples that contain
> an fsk symbol and generate 4 arrays (assume array indexes start at 0 as
> in C) :
> 1200hz I/Q detector:
> Ia[p] = Sin[(p/N)*2*Pi]
> Qa[p] = Cos[(p/N)*2*Pi + Pi]
>
> 2200hz I/Q detector:
> Ib[p] = Sin[(1.8333333*p/N)*2*Pi]
> Qb[p] = Cos[(1.8333333*p/N)*2*Pi + Pi]
>
> Now to figure out what the power levels are do a convolution
> where a convolution is something like this:
> convolution(X[],Y[]) := X[0]*Y[0]+X[1]*Y[1]+X[2]*Y[2]+...+X[n]*Y[n]
>
> power in 1200 hz = convolution(S[],Ia[])^2+convolution(S[],Qa[])^2
> because sin^2+cos^2=1 or something along that line.
> and
> power in 2200 hz = convolution(S[],Ib[])^2+convolution(S[],Qb[])^2
>
> The dft is the ultimate I/Q detector because it calculates I and Q for
> every possible frequency, but even though there are some fast dfft's it
> doesn't mean your good old 386 will do it at 1200 baud. For reference the
> real part of the dft is the I and the imaginary is Q.
>
> To get the symbol clock do whole convolution thing for every sample. I.E.
> do:
> Go forward one sample;
> make array of last N samples;
> do convolution
> decision = (power in 2200) - (power in 1200)
> if decision != lastloopsdecision then push symbol clock slightly forward
> or backward as necessary to ease the transition into the right place
> if (symbol clock says DING) then take decision and pass it to
> hdlc decoder etc.
> loop
>
> This is essentially what the kernel afsk 1200 code does, minus the dcd
> stuff which I don't think I'll ever get. It's written for speed and not
> legibility. My code low pass filters the decision for pll purposes and it
> does the clocks and detectors in floating point.
>
> To decode 1200b psk drop the 2200 detector and just use the Ia versus Qa.
> Take ArcTan(convolution(S[],Ia[]/convolution(S[],Qa[])) and use that to
> make the decision. The symbol clock is a bit messy, in this case, because
> you need to update the clock on big transitions AND update the
> detector frequency (1200hz) because the carriers probably aren't matched.
> In between transitions the first derivative of ArcTan is how far the
> carriers are mismatched, so that's pretty easy. It the mismatch is more
> than 50 hz or so you may as well for a dfft before you start the
> demodulator to initialize the detector at the right frequency.
>
> -------===Dustin Moore===---------
