[Lowfer] PSK31 success last night.

[John Langridge]

Hey JD - 

I really appreciate your very thoughtful response.  Lots of great info here.  Unfortunately I failed to address a number of issues in my original response.  Such is life when multitasking, I guess.  I will try to address a few of your comments here.

>I wonder what your transmission speed was in comparing 
these two, John,

I typically run 15.625 with GMSK which has been good for about -19 db SNR.  I have tried faster and slower but 15.625 seems to be a good balance for making consistent QSO's. I am limited to a BW of 62Hz.

CMSK has the 8 and 31 options, both of which I have tried in manual mode.  I've only made QSO's with it working XXM who is within ground wave range so we are able to discount skywave effects. 
 Decodes can be found running either speed with considerable difficulty in adjusting the tuning in spite of strong signals being present.  It seems with the difficulty in tuning, it would be very challenging to give CMSK a fair test via skywave.  All that said, I do not know how deep one could dig compared to GMSK in the real world because I never gotten it to work.

>Yet I would reiterate that "better" is relative in comparison of 
modulation methods

absolutely 100% agree.  As stated earlier, I use GMSK (over PSK31,for example) because its compatible with my class-D systems, I'm granted for MSK, it fits in my BW requirements, software was easily found with a google search, and maybe most importantly - the software can drive my system with ease.  Pressing MOX or hitting the foot switch to actuate all the relays in the system is ok for QSO
 mode but tough for beaconing HI!

One notable item that I have compared on RX is that the print speed of GMSK 15.625 seems to be very much on par with what you see with PSK-31.  I don't know if that's a result of the Gaussian filter processing that may be buffering data but that, along with the BPF filters included in the MMVARI interface have allowed me to dig deeper during QSB (how deep is hard to say - maybe a db or 2 better?).  My personal feeling is that GMSK seems to handle fades better on RX but I can't back that up with data, it only based on-air performance.  My comparison with PSK-31 is only based on RX of that mode.

Aside from the listening exercise above, I don't use PSK-31 here so I can't make a meaningful comparison between the two modulations methods.

Looking at W1TAG's evaluation of modes, I see that MSK31 is available in spectrum lab.  I will have to look at the help file to see what it takes to set that up and give it a try.

73 and thanks again for the detailed info!

John XIQ





________________________________
 From: JD <listread at lwca.org>
To: "Discussion of the Lowfer (US, European, & UK) and MedFer bands" <lowfer at mailman.qth.net> 
Sent: Wednesday, April 30, 2014 2:24 AM
Subject: Re: [Lowfer] PSK31 success last night.
 

XIQ wrote: "For me, I am far more interested in the QSO and the content of 
that QSO rather worrying about some astonishingly low detection limit."

I thoroughly endorse that view myself!  Software techniques that are mainly 
about automated reporting of
 spots, or which can only establish correlation 
with an ID already in a database, are OK as a technical exercise, I suppose, 
but they leave me cold because (IMO) radio should be about communication.

Similarly, though, my question pertained to robustness and performance of 
the raw modulation method itself, rather than interface convenience or 
available features.  These things are also important, and I would not claim 
otherwise.  But they are a separate matter from the modulation technique, 
the coding scheme, and any error correction method employed, that make any 
given mode work.

It's way too easy to end up comparing apples to oranges if one talks about 
GMSK being a "mode" and MSK being another "mode," without specifying actual 
parameters or even which implementation one is using.

The technique identified as "GMSK" in MMVari, for instance, appears to be a 
simple varicode data stream at
 15.625 or 31.25 baud with no processing other 
than the Gaussian filtering, before being frequency modulated onto the 
carrier with m=0.5.  (At least, I haven't yet found any mention of there 
being FEC or the like.)  However, there is no reason GMSK data has to be a 
custom varicode as it is in the MMVari engine (it could be RTTY or specially 
timed Morse or most any other coding method).

Likewise, there is no reason why plain MSK necessarily has to be represented 
by the ZL2AFP "CMSK" implementation.  CMSK has convolutional coding and who 
knows what all built in before the data stream is allowed near the carrier. 
Ergo, we can expect it to be touchier about tuning as you noted, and for it 
to fall apart more completely as one drops below detection threshold 
(although perhaps less error prone as its developers claim, when everything 
is nearly ideal), and for the data throughput to be less for a
 given baud 
rate because of the extra payload.

(Before I continue, I wonder what your transmission speed was in comparing 
these two, John, and what your bandwidth limitation is?  I know a little 
something about the modulation methods themselves, but less about the 
software that's available for amateur use.  The different software packages 
seem to have as much folklore about them on the Web as actual fact.)

Anyway...all else being _strictly_ equal, in principle plain MSK has certain 
robustness advantages over GMSK.  That's why I asked if PSK31 isn't giving 
Neil the kind of coverage he wanted, why he thought GMSK would do better? 
From the modulation standpoint alone, if the baud rate is the same and the 
coding were also the same, GMSK should perform slightly worse and/or require 
more transmitted power, and plain MSK should perform slightly better.

I suppose the practical difference
 is that different software packages do 
NOT necessarily have the same coding schemes, baud rates, and data 
throughput.  That's why it's good to hear real-world results such as yours, 
John.  Yet I would reiterate that "better" is relative in comparison of 
modulation methods when different formats and speeds of data are being sent. 
To be meaningful, it should probably also be specified which software is 
being used at what speed, and whether/what form of error correction is being 
employed.

John D.



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