GMSK Q&A2009年02月23日 22時32分

AMSAT-BB で、GMSK に関する興味ある Q&A が行われていました。
ここに転載します。翻訳は明日。
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Subject: [amsat-bb] GMSK: Time for a stupid question :-)
From: "Henk, PA3GUO"
Date: Sun, 22 Feb 2009 22:44:41 +0100 (CET)

While trying to work Cute-II, I started to think about GMSK ...
(I have a PK96 external TNC hooked-up)

My beautifull TS2000 has FM/AM/USB/LSB/CW modes.

For GMSK, do I use FM or FSK ?

Please only a gentle smile, I realize I do look silly asking this :-)
Henk
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Subject: [amsat-bb] Re: GMSK: Time for a stupid question :-)
From: "Colin Hurst"
Date: Mon, 23 Feb 2009 17:24:54 +1030

Henk,
AO-51 runs 9600 GMSK.
Most people receive AO-51's GMSK digital on PSK Decoders.
The early UoSats and their subsequent derivatives ran PSK.
Understandably there is a slight degradation in performance copying
GMSK on PSK decoders.

Connecting an oscilloscope to your TNC will identify the difference
between GMSK and PSK.
Put the TNC in test mode to verify the PSK waveform.
Use AO-51 to compare one against the other.

The TNC3S has a GMSK selector, but I cannot remember whether it was
for RX, TX or both.
Its some time since I had the covers of or perused the manual.

So your PK96 should work "OK" on GMSK.
73,
Colin VK5HI.
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Subject: [amsat-bb] Re: GMSK: Time for a stupid question :-)
From: "Howard Long"
Date: Mon, 23 Feb 2009 10:00:40 -0000

Hi Henk 

FM. FSK is for (relatively) narrow band RTTY type modes that
traditionally you'd have your radio set to USB or LSB.
Your TNC modem will expect the signal to have gone through a
limiter/discriminator, so you need the TS-2000's FM mode.

GMSK is FSK, but with two particular features.

Firstly the modulation index for GMSK is always 0.5, with state
changes occurring at zero crossings and phase continuity maintained.
Secondly, after the scrambler, the 1's and 0's go through a Gaussian
filter and that's what modulates the FM carrier: the -3dB point of
that Gaussian filter is determined by the BT.

The primary benefit of GMSK is that it reduces adjacent channel
interference at the expense of intersymbol interference.
I doubt that's the reason it was chosen for this mission though,
much more klikely it was because you can buy chips off the shelf
to do GMSK. 

There's quite a good intro to GMSK here,
http://sss-mag.com/pdf/gmsk_tut.pdf

So yes, you should be able to demodulate GMSK with a standard
9k6 G3RUH modem although theoretically it might not be optimal.

73, Howard G6LVB

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