This article contains useful information on modulation schemes used with radio systems. It can be used with your radio to enhance your listening on the bands.
Modulation is the process needed to make a radio frequency wave carry a signal that conveys some sort of information (or intelligence). It is essentially a way of distorting the pure sinusoidal radio frequency (RF) carrier so that it does something useful. A variety of modulation techniques can be heard on the air, but you will need a receiver with a BFO or SSB detector to resolve many of them.
Modulation methods include:
On-off keying. This is the simplest method of conveying information on an RF carrier: simply switch it on and off. Morse (CW) works in this way. If you do not have a BFO or SSB/CW detector, you will only hear "thumping" sounds from your radio's speaker. This mode is one of the transmission methods used for the Hellschreiber Text/FAX system.
Amplitude modulation. Various forms of amplitude modulation are used: double sideband full carrier (or conventional AM , used by broadcasters); single sideband (or SSB , used by radio amateurs, utility users and some broadcasters; vestigial sideband (wideband TV and some digital signals, seldom seen on Shortwave). You will need a BFO (or SSB detector) to resolve SSB transmissions.
Frequency modulation. This is commonly used for broadcasts on VHF (wideband FM) and for push-to-talk communications (such as radio amateurs and utilities) (narrowband FM). You can hear radio amateurs using narrowband FM on the 29 MHz band (around 29.6 MHz), otherwise this mode is seldom used on HF. Digital signals use frequency modulation in distinct steps (often two) so that a stream of digital symbols is transmitted; this is known as frequency shift keying (FSK) and is used by radio amateurs and utilities. Digital FSK signals are not limited to just two levels and up to sixteen or more may be used. HF FAX transmissions use slow speed analogue frequency shifting which can be used to define a greyscale for the transmission of an image. Slow-scan TV (SSTV ) is a more sophisticated image transmission system that can support colour or monochrome images. VHF radio can support a number of data modes including pager and selcal signals.
Phase modulation. This is used for digital signals, it involves changing the phase of the radio frequency carrier to represent digital symbols (eg +180 degrees phase shift may mean binary 1, whereas -180 degrees phase shift may mean binary 0). Phase shift keying (PSK) is not restricted to two levels and up to eight or more may be used. Phase modulated signals at high baud rates are very difficult to detect on the air since they sound just like noise.
Amplitude and phase modulation (QAM). This is combined amplitude and phase modulation to produce a high speed digital symbol stream which makes best use of the available bandwidth. Most telephone modems use QAM (eg V.32 is 16 QAM), but this modulation method is seldom, or ever, used on HF.
Spread spectrum. This technique is most often used for disguising signals to prevent them from being heard. There are a variety of techniques including high-speed on-off keying, frequency hopping and direct-sequence spread spectrum (where the narrowband signal is phase modulated with a high-speed digital spreading signal). This type of signal is difficult to hear (for obvious reasons) and will seldom be of interest to the casual listener.
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