One final point, and correcting a typo in my previous message.
The Subject line should read GFSK, not GMSK.
FT4 and FT8 use Gaussian Frequency Shift Keying (GFSK), *not* Gaussian
Minimum Shift Keying (GMSK). The latter is yet another possible
modulation scheme.
-- Joe, K1JT
On 7/17/2019 10:27 AM, Conrad PA5Y wrote:
Thanks both Bill and Joe, that is certainly a very worthwhile
improvement, thanks to the team for the effort! I will read everything
with interest.
Regards
Conrad PA5Y
------------------------------------------------------------------------
*From:* Joe Taylor <j...@princeton.edu>
*Sent:* 17 July 2019 15:33
*To:* WSJT software development
*Subject:* Re: [wsjt-devel] GMSK waveform generation details
Hi Conrad,
On 7/17/2019 4:09 AM, Conrad PA5Y wrote:
Hello Joe I am quite interested to learn more about the GMSK waveform
generation, where can I read more about it and the implementation in
WSJT-X and the advantages.
A good place to start would be our document describing FT4 during its
early development, especially pages 3 and 4:
http://physics.princeton.edu/pulsar/k1jt/FT4_Protocol.pdf
Bill gave you pointers to the code that generates GMSK waveforms for FT4
and FT8.
Main advantages:
- constant envelope waveform,
- peak-to-average power ratio = 1.0
- very clean spectrum
Plots of spectral measurements you may find interesting:
http://physics.princeton.edu/pulsar/k1jt/rtty_ft8_ft4_spec.png
http://physics.princeton.edu/pulsar/k1jt/FT8_FSK_GFSK.png
For comparison, the first plot shows spectra of a standard RTTY signal,
FT8 as generated with WSJT-X 2.0, FT8 with WSJT-X 2.1, and FT4.
The second plot shows examples of FSK and GFSK versions of FT8, captured
from on-the-air signals. The signals happen to have nearly identical
strengths. The much cleaner spectrum of the new FT8 is easy to see on
the waterfall as well as the graphed spectrum.
-- 73, Joe, K1JT
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