GMSK channel filter

Hello,

I have a bit of a confusion concerning the decimating Channel filter in
the
stock receive_path.py script that I’m using with the GMSK rx and tx
python
script:

The filter coefficients for the chan_filt filter are determined by the
gr.firdes.low_pass() function, which takes, among other arguments, the
midpoint and the width of the transition band. In the gr_firdes.h file
it
is specified that the units for these arguments is Hz, but in the code
it
says that the midpoint of the trans. band is “1.0”. The script
correctly
receives data on a much larger band than 1 Hz, so what am I missing? Is
the
value somewhere being changed to designate a different value at the
filter’s
level?

Thanks,
Ben

Ben Olsen wrote:

the code it says that the midpoint of the trans. band is “1.0”. The
script correctly receives data on a much larger band than 1 Hz, so
what am I missing? Is the value somewhere being changed to designate
a different value at the filter’s level?
In many instances we use a normalized frequency, since these are sampled
systems. A normalized frequency of 1.0 is equal to the sampling rate.
You can think of the units as cycles per sample. The first nyquist zone
would be from 0 to 0.5 if these are real samples, or -0.5 to 0.5 if
these are complex samples. You

In some other instances we may use radians per sample, which would be
2PI times the normalized frequency.

Matt

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