Re: LF: Fwd: CMSK Beta released

[Andy Talbot <andy.g4jnt@googlemail.com>]

MSK does not lend itself to generation on a PIC.   It is NOT a two tone mode, but a waveform that transitions back and forth  around a circle in phase space.  Do not think of MSK as frequency Shift Keying

 

It may be possible to do it on a on a PIC.  By not touching the frequency register, but reprogramming the phase offset register in real time at a fast-enough sampling rate. 

 

But it would need a high sampling rate  to get it right while keeping alias sidebands at the sampling rate at a low enough level (sin(x) / x and all that)

 

PSK31 on a PIC, sampled at 7812.5Hz (250 samples per symbol) gives sidebands at +/- N.Fs of < 60dBc.

 

Andy
www.g4jnt.com

On 6 August 2010 09:22, Michel F5WK <f5wk@mterrier.net> wrote:

Hi Graham, LF,

> As  far as I know , only wspr  and  ros-mf have  the  ability to  drive  a
> pic/dds  vfo , with  only    mf-1/7  giving  a  tailored mf  qso  mode ?

What about asking for a mod allowing support of a DDS ?

MSK/CMSK has only two tones, whose frequencies are known. Therefore only one
bit of information is required for one symbol.
The big question is how much accurate the deltaF has to be ...

Graham, since I swapped the link between the PC and the LF/MF transmitter
from USB to BlueTooth, the timing has slightly degraded, and as a result
ROSmodem is not working anymore. I have to resync the timing at the PIC
level, definitely the correct way ...

73,

Michel - F5WK