Frequency-shift keying#Audio FSK
Audio Frequency Shift Keying ( AFSK ) is a digital modulation which on the frequency shift keying ( FSK) based and uses as a special frequencies in the audio range ( 16 Hz to 20 kHz). In German, the method also Niederfrequenzumtastung or low frequency is called frequency shift keying.
Method
Wherein the high-frequency carrier wave AFSK is not keyed, as in the FSK, but a sub-carrier at the low frequency (LF). ( Called Space ) of a digital "0" here corresponds to a low frequency, a digital "1" ( Mark called ) a high frequency. The low frequency carrier can be modulated as needed below in an additional step to a radio frequency carrier, commonly frequency modulation.
Bell 202
In the widely used 1200 baud AFSK example, the two frequencies become keyed in 1200 and 2200 Hz. The difference between the two tone frequencies (in this case, therefore, 1000 Hz ) is referred to as a shift. These frequencies are from AT & T " Bell 202 " modem that was directly connected to a FM radio. So that the effort for the transmission of digital data was minimal. It has the frequency generator can not be controlled directly, and demodulation is also largely uncomplicated. A major disadvantage of this method is that the bandwidth efficiency is very low due to the double modulation. Procedures that modulate the RF carrier directly, can work much more bandwidth- efficient.
Application
Applications has AFSK when radio telex using the Baudot code and the 1k2 packet radio amateur radio. In previous 8 -bit computers, data were stored on a Datasette using AFSK. The same procedure was also used for data transmission via fax or modem to use. However, this principle allows only limited bandwidth.
AFSK in relation to (M) FSK and GMSK
AFSK differs as already mentioned by Rated by the double modulation. U is the direct change in the radio frequency carrier frequency. This change may comprise two or more stages, accordingly, called the method then FSK (Frequency Shift Keying) and MFSK ( Multiple Frequency Shift Keying). Since abrupt changes of the frequency bandwidth required as a smooth, increase the bandwidth efficiency is possible by pre-filtering the digital signals. A common method used in the form of the Gaussian bell-shaped curve to form the signal edges, accordingly, this method is called Gaussian Minimum Shift Keying ( GMSK ).
The English Piccolo system used 32 tones ( Mark D piccolo ) and later 6 tones ( Mark F piccolo ).