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What follows is a brief list of the most common methods of transmitting information by HF/VHF radio and a simplified version of their ITU emission classification. If you have looked at the full table of ITU Emission Classifications you will know that describing the modulation scheme of a radio signal can be very complicated and would require a small radio laboratory in order to discover the exact method of modulation of an unknown signal. In addition the full classification includes the bandwidth of the emitted signal, but we will ignore that information as we are concerned only with the bandwidth of the received signal, not with the bandwidth that might have been intended by the operators of the transmitter. 

Here we will concern ourselves only with the most common types of modulation and to the depth that suits the average Radio Amateur. Note that some of our usage is not quite according to the ITU rules, but these non-standard adaptions are more useful for our purposes. 

Unmodulated carrier wave
Note that if the carrier is amplitude modulated, for example, by power supply "hum" it would be proper to call the emission A3E or A3X but, since such modulation is unintentional, common practice in the MS is to call it N0N.
On-off-keyed (OOK) Morse telegraphy A1A
Facsimile by amplitude modulation (AM)
Not so common anymore. 
AM telephony
Double sideband, amplitude modulated, full carrier, carrying voice, music, etc. This is used by standard AM broadcast stations and a few Amateur stations.
Single sideband (SSB) telephony
Single sideband suppressed carrier amplitude modulation by voice, music, etc. Don't forget to add whether the upper or lower sideband is being used (J3E,U or J3E,L)
Reduced carrier SSB telephony
SSB reduced carrier amplitude modulation by voice, music, etc. The carrier of this SSB signal is not fully suppressed.
Frequency shift keyed (FSK) Morse telegraphy
Don't forget to measure or estimate the frequency shift. 
FSK teleprinter
Single channel; radio teletype (RTTY) is a common example of this. 
Facsimile by frequency modulation (FM)
Much more common than A3C. 
FM telephony
Frequency modulation carrying voice, music, etc. This is used by standard FM broadcast stations (wide bandwidth) and many Amateurs (narrow band), especially in the VHF and UHF bands.
Multichannel FSK teleprinter
More than one channel of teletype. If you know from analysis that digital data is being carried instead of teletype traffic, then F7D is the appropriate code.
Phase shift keyed (PSK) teleprinter
Single channel. The PSK31 mode of Amateur digital signals is a good example. By comparison, multichannel PSK data signals (G7D) are sometimes difficult to distinguish from multichannel FSK signals using minimum-shift-keying. When in doubt, call them all F7B.
Pulsed carrier with no modulation of the pulse train
Many over-the-horizon (OTH) radars now use sophisticated pulse modulation schemes, so P0N is actually not so common any more. These schemes are difficult to detect without special equipment.
Noise jammer
Many broadcast jamming signals are difficult to accurately describe except as "noise". In this case use the non-ITU code JAM. Note that FM "wobble" and "bubble" jamming signals are more accurately described by "FXX" than by "JAM".
Use AXX, FXX, PXX, or XXX to describe amplitude, frequency, pulse, or unknown modulations for which a more specific description can not be found. 

There are many signals with other methods of carrying information, some of them quite sophisticated, but the ones described above are the most common, and the codes given here will serve for almost all intruders. 

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