Re: [Discuss-gnuradio] Divide frequency by a constant

[CEL]

Ah, I think I see where you're going :)
So, here, we're really talking about digital clock division! That is, a
counter :)

While that'd be totally possible to piece together (counting edges,
then emitting an edge every N input edges), it's not how DSP works: the
things you handle *are* already with respect to a common time source.

That also means that you don't have to worry about phase coherence: If
you start two different oscillators at the same phase, they will stay
coherent, as long as math doesn't fail. 

Because I think this is easier to show with binary signals than with an
oscillation generation, try the following:

1. Add a vector source, output type "byte", values 
[0, 0, 0, 0, 0, 1, 1, 1, 1, 1]
2. Add a vector source, output type "byte", values 
[0, 0, 0, 0, 0, 0 1, 1, 1, 1, 1, 1]
3. use the "And" block from the "boolean operations" category, input
type "byte"; connect the two vector sources to it.
4. add three "UChar to float" blocks. Connect one to the output of the
first signal source, one to the output of the second, and one to the
output of the And block.
5. Add a "Qt GUI time sink", input type float, number of inputs 3. 
6. Connect the outputs of the UChar to float blocks to the inputs of
the Qt GUI time sink.

Let the whole thing run. You'll see how everything stays phase
coherent, the AND of the two clocks just is the let's call it beat
signal of the two clocks and everything is fine.

> Why don't I just switch between 1277 and 1064 signal sources?
> Because 
> the result is not phase-coherent.

Luckily, that's not true :) (I've built something very much like that
as a quick example for some stranger on the internet a couple years
back, see [1]).

This is software! All the signals in GNU Radio are just sequences of
numbers. When you apply an operation to two sequences, the math just
handles the two sequences coherently :) So, all the digital-logic
problems of the real world, namely having things that drift off
arbitrarily, don't apply.

Now, you can imagine that this is not really how you'd build an FSK
modem in GNU Radio.

You can just simply go in,

* "multiply const" and "add const" to your input signal,
  so that it makes sense as baseband frequency values;
  (in your case, I'd go with +- 213/2 Hz as frequencies),
* Repeat (that's a block!) them for as many samples in your samp_rate
  one FSK symbol takes, and then
* use a "Frequency Mod"¹ block straight away.
  (I'd go with sensitivity = 2*3.141 / samp_rate) 

You'd get CPFSK in baseband. Attach a Qt Frequency sink to see! 
If you used an interpolating FIR with a sensible pulse shape as taps
instead of the Repeat block, you'd get CPFSK with a pulse shape.

Best regards, and loads of fun,

Marcus

----------
¹ btw: the "sensitivity" parameter of the Frequency Mod block gives the
factor between input sample amplitude and output "phase advance in
radians per sample"; e.g if your input is 10, 10, 10... , and your
sensitivity is pi/200, then you produce a signal with a phase increase
of 1/40 of a full cycle every sample, i.e. has a period of 40 samples,
which means it has a frequency of f_sample/40.


[1] 
https://stackoverflow.com/questions/36898574/fsk-demodulation-with-gnu-radio/36915550#36915550

On Wed, 2019-06-19 at 12:55 -0400, Barry Duggan wrote:
> Marcus, Chris, and others:
> 
> What I am really trying to do is replicate a time-domain FSK modem I 
> designed in 1972 (using discrete components of course). It used a 
> crystal oscillator at 12,770 hz (square wave) and divided that by 5
> and 
> then ANDed with the input signal. The other path divided by 6 and
> ANDed 
> with the inverted input signal. The two paths were ORed together and 
> divided by 2, giving a fairly good phase-coherent FSK with 1277 /
> 1064 
> frequencies. Low pass filtering followed.
> 
> Why don't I just switch between 1277 and 1064 signal sources?
> Because 
> the result is not phase-coherent.
> 
> My other objective is just to learn what I can do with Gnu radio :)
> 
> Cheers!
> ---
> Barry Duggan

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