Hi,
Just a small message to notify that I have today updated the gmsk modem
on github:
https://github.com/on1arf/gmsk/
The main change is simular to the last change and both are the result of
a small side-project: to help port the WSPR software to the raspberry pie.
The last change deals with adding a version of the gmsk modem using
floating-point math operations. This change adds two versions using 32
bit integer math (instead of 64 bit math): one using 10 bit audio (and
22 bit FIR filter coefficients) and one using 12 bit audio (and 20 bit
filter coefficients).
These version reduces the number of bits of both the audio and the FIR
filter to that all operations can be done by any CPU with hardware 32bit
integer math core. Compaired to the version using 32 bit * 16 bit
integer operations; it runs 2 to 3 times faster!
On the other hand; it does reduce the resolution of the audio and FIR
filter. Now, concidering some hardware gmsk modems (like the dvrptr)
only use 10 bit audio anyway (and do work very good), I do not think
that part is an issue. However, I do not have sufficient background
knowledge of DSP to evaluate if reducing the resolution the FIR-filter
from 32 bits down to 20 or 22 bits will have an effect on its quality or
its ability to deal with noise.
If somebody can give some information on that, feel free to do so.
See also my blog on some more background information on the changes:
http://villazeebries.krbonne.net/hamstuff/?p=165
(The interesting part starts in the 2nd half of the message)
If this does work good; one of the things this would allow is to port
the gmskmodem to something like the chipkit (27 euro arduino-size board,
but using a 80 Mhz 32bit PIC32/MIPS core instead of a 8 bit AVR) + some
audio output board.
Next step: implement the header of the codec2 gmsk. I've already
received feedback from somebody for the code for golay forward-error
correction!
73
Kristoff - ON1ARF
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