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

[Sachi]

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.


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?

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? 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


Thank you very much for your time.

Sachi


                
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