View Full Version : GMSK-250
09-30-2010, 01:51 PM
I am looking for exactly what it takes to generate this mode.
Not a program, or anything like that. But the actual nuts and bolts, of what tones are generated for each character being transmitted.
I Can't find the technical details on generating GMSK-250....typical of most modes...I had to do extensive digging to finally find the technical specs on the modes I now use in my transmitter.Olivia, DominoX, packet, etc....all I can find on GMSK-250 are descriptions of PC programs to send and decode it.
Somewhere there has to be technical spec's for this mode otherwise what did these people that made programs to generate and decode it go by?
09-30-2010, 05:06 PM
Have you tried contacting the guy who wrote MMVARI? That program implements the GMSK modes, so he must have found a spec somewhere.
10-06-2010, 06:52 PM
Still no luck finding any details about this mode.
10-08-2010, 09:46 AM
You have me curious now...
Not that I want to use the mode..just that there is not much info as to how it applies....
If I post here some things I find...just ignore if you already have noted it...
"GMSK, like MSK, is a form of
frequency shifting: the MSK part specifies that the phase will be
continuous and that the frequency shift will be exactly 1/2 the bit rate;
the 'G' means that the frequency shift will be filtered with a Gaussian
profile before modulating the carrier. So sending a bunch of ones then a
bunch of zeros will give you one tone, then another."
I should give credit to the site the info came from....
10-08-2010, 12:46 PM
I'm playing with an APRS GMSK client called APRS Messenger which has GMSK250 as one of the sections. Works pretty good! here is a link (http://wa8lmf.net/APRS_PSK63) (Stephen does great work with his pages) that will get you started on the GMSK thingy... there is a section devoted to GMSK vs PSK63 and has this link (http://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/what-is-gmsk-gaussian-minimum-shift-keying-tutorial.php) for more GMSK info.
10-08-2010, 01:56 PM
Hi Doug and all,
Yup been to all these pages and have those programs also. And they are the reason I have been trying to learn HOW the signals are generated.
I know they are made of alternating two tones. just like RTTY, but the question I have YET to learn is the real bare bones of the mode. Like when it sends WB9SBD what is actually sent?
as in CW when I send WB9SBD this is what is transmitted.
.-- -... ----. ... -... -..
So, the question is when sending in GMSK250 and I send WB9SBD what actually happens in my transmitter?
10-10-2010, 08:13 AM
From what I have read , what it puts out is similar to the other modes...
Depending on the bit rate an offset frequency is used to represent the 1's and 0's..
+1 and -1 either side of the carrier...representing logic 1 and 0
The only difference using GMSK is the G...it shrinks the bandwidth compared to the regular msk.
That is what I get when doing some reading.
10-10-2010, 01:59 PM
But see yes two tones and 1's and 0's and all that.
But this must be the "CIA's" secret mode. Because NO ONE know what two tones.
On VHF-FM, 1200-baud packet data transmission is done by rapidly shifting an audio tone between two frequencies traditionally referred to as the "MARK" and "SPACE" frequencies. These tones are inserted into an FM radio's mic jack or equivalent. On 1200baud VHF packet, these two tones are 1000 Hz apart and standardized on 1200 and 2200 Hz.
On 300 baud HF, the actual RF carrier of the transmitter (essentially a continuous key-down CW transmission) is shifted between two frequencies 200 Hz apart at the 300 baud data rate in a process known as FSK (frequency shift keying). On thirty meters APRS, these two frequencies, a.k.a "mark" and "space" are 10.149 200 Mhz and 10.149 400 MHz.
while yes there are no exact "Tones" on HF (Because on how you tune it in) what is KEY is the amount of space between them and in packets case it's 200Hz.
GMSK no one has even said what the spacing is.
Never mind what the pattern of the tones to say what character.
It's the FBI's top secret!
10-16-2010, 12:57 AM
By definition of GMSK the tone spacing is 1/2 of the symbol rate. If GMSK-250 has a symbol rate of 250 symbols per second then the spacing is 125 Hz.
"MSK can also be viewed as a continuous phase frequency shift keyed (CPFSK) signal with a frequency separation of one-half the bit rate."
10-16-2010, 01:30 AM
But the top secret still is what gets transmitted.
with a cw program and I want it to send W9LM
.-- ----. .-.. --
what is sent when I send W9LM in gmsk?