the so-called "semi-modes"

Below an interesting article by SergUA6 (RIP) from radioscanner.ru about MSK, GMSK, SDPSK, and OQPSK: more info and examples here:
http://signals.radioscanner.ru/info/item68/ 
http://signals.radioscanner.ru/info/item281/ 

The "two-faced" signals, such as MSK and GMSK I name semi-modes, of course, it is not the standard name. It simply was required to allocate somehow this class of signals from the general PSK family, because such signals possessing both PSK signs and FSK signs.

The second harmonic of these semi-modes, has two bright spectral lines, the spacing between these lines is equal to Br, that is one of the signs of these modes.This is the necessary condition of their definition at the analysis, but not the sufficient one. Two lines in the second degree/power can be also given by both SDPSK and OQPSK modes.

SDPSK, generally, does not demand synchronism of transitions in extremums of the carrier, and because of this, it has the bigger width of the spectrum than MSK. This width of the spectrum can be reduced by filtering of the bit-stream before feeding on the modulator, this procedure is usually realizing through RRC filters. SDPSK (PSK-2 with phase rotation) in essence, has same resulting signal as MSK, only with wider spectrum. It can also be demodulated by FSK demodulator, becos of getting under definition of semi-modes.

Modern methods of creation of the various signals do often erase the distinction between various modes, for the reason, that developers are almost always (it strongly simplifies development) aspire to select a multiple relation between the clock frequency of manipulation and the frequency of the carrier.
In this case developer declares and forms, for example,that the modulation is SDPSK, but the resulting signal, in essence, is MSK. Thus, casually or deliberately some confusion is brought into diversity of the various modes and their definitions.

Very often GFSK modulation is specified,in descriptions of the signals, while in actual fact it is typical SDPSK according to all signs. The example is the signal of the Finnish radiosonde.
When we are looking at the circuit of formation of GFSK modulation, it is easy to understand that if the clock frequency and the generator will be synchronized, and the frequency spacing will be choosen as BR/2, then such “GFSK” will easily turn into one of our semi-modes, at defined parameters of Gaussian filter and high stability of generator’s parameters. Seems like developers just don’t think about it or just do not know.
By the way, in one’s time, by this reason, GFSK was mistakenly classified to these semi-modes on one of sites the modulation ACARS VHF has been declared as GFSK.

In analysis it is very desirable to define what type of modulation is used, at least approximately. It is also necessary to be oriented on the width of the spectrum, which is occupied by the signal and on its form. At pure MSK modulation, width of the spectrum is about 1.5*Br, at GMSK spectrum is lesser than this value, and in it’s limit is very close to theoretical Br, at the same time the spectrum of MSK, GMSK is obviously expressed as bell-looking-like shape/form, at SDPSK the spectrum is more then 1.5*Br. The basic sign of semi-modes is two lines in the second degree/power, the basic but not sufficient, it demands certain accuracy and attention. The spectrums form does also require certain accuracy, because often receivers distort it to unrecognizability, especially if the signal is taking off from AF’s output or from discriminator, in this sense it is much more preferable the I/Q record or IF.

Good luck!