Re: [Discuss-gnuradio] FIR filter in fsk_tx.py

[Sachi]

Hi, Matt, 

Thanks a lot, now I am able to understand the FSK code
completely.  Now the only remaining problem is the
program doesn't run as it should.  Quite often, the
receiver can detect something while actually nothing
is there, as a result, it receives some messy data
into the file.  For example, if I only run the
receiving program fsk_rx.py, without running the
fsk_tx.py, the receiver can still get some data and
write it into the file.

The ideal situation appearing in my mind is we send a
text file, and we should be able to recover it
correctly at the receiver and read it.  Have you ever
really achieved this?  

Do you have any suggestion that we can improve the
realibility of the FSK transmisstion?  such as
increase the length of the synchronization bits? or
decrease the hamming distance?

Thanks
Sachi



--- Matt Ettus <address@hidden> wrote:

> Sachi wrote:
> > Hi, Matt
> > 
> > Thanks so much, your reply is so helpful.  Now I
> think
> > I can understand the FSK code well.  However, I
> still
> > have  some confusions. Hope you or Eric could help
> me
> > solve them.
> > 
> > 1. In gr_simple_correlator.cc, you calculate the
> > hamming distance, using
> > hamming_dist = gr_count_bits32 (d_shift_reg[d_osi]
> ^
> > GRSF_SYNC);
> > Why you choose the threshold as 3? Do you choose
> it
> > from your experiments?  If I understand correctly,
> > there should be several "osi" when the hamming
> > distance is less than 3 and we pick up the center
> one.
> 
> 
> I'm not sure what "osi" is.  The threshold is a
> compromise between false
> positives (i.e. we correlate when nothing is
> actually there), and false
> negatives (i.e. we miss real packets).  3 was chosen
> somewhat arbitrarily.
> 
> > 2. In fsk_rx.py, you use:
> >         u.set_rx_freq (0, -options.cordic_freq)
> > Why a negative IF frequency is used here? What
> happens
> > if I use the positive one?
> 
> Negative means translate the frequencies lower.  So
> a signal centered at
> cordic_freq and then translated by -cordic_freq ends
> up at DC.  If you
> had used +cordic_freq, you would have translated
> whatever is at
> -cordic_freq to DC.  If you are using a dboard with
> real sampling, then
> this would be a frequency-inverted version of your
> signal, and ones
> would be swapped for zeros and vice versa.  If you
> were using a dboard
> with IQ sampling, then your image would be there. 
> You would see a much
> weaker spectrum reversed version of your own signal,
> plus any
> interferers at your image frequency.
> 
> > 3. In fsk_tx.py, the +1, -1 sequence is
> interpolated
> > by 8 and then processed using a lowpass filter.
> Why do
> > we have to do this in software? Can we directly
> send
> > the +1 -1 sequence to the FM modulator? What's the
> > difference?
> 
> The lowpass filter gives a cleaner signal, which
> otherwise would extend
> very wide.
> 
> > Does it have anything to do with the
> > receiving side?  If so, why we have to interpolate
> it
> > using FIR lowpass filter? Can we duplicate each +1
> or
> > -1 8 times? i.e.
> > 1 -1 1 1 -1 becomes
> >
>
11111111-1-1-1-1-1-1-1-1111111111111111-1-1-1-1-1-1-1-1
> 
> You could do that.  That would be the equivalent of
> filtering with a
> boxcar or sinc filter.  It would give relatively
> poor filtering of out
> of band interference (about -18dB), but better than
> nothing.
> 
> Matt
> 


                
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