Audio frequency shift keying
Audio Frequency Shift Keying ( AFSK ) is a digital modulation type , which on the Frequency Shift based (FSK), and as a special feature as a modulating signal frequencies in the low frequency range used (NF, 16 Hz to 20 kHz). In German, the process is also known as low frequency keying or low frequency frequency keying .
Procedure
With the AFSK, the high-frequency carrier oscillation is not keyed directly, as is the case with frequency shift keying, but a sub-carrier in low frequency (NF). A digital “0” ( called space ) corresponds to a low frequency, a digital “1” ( called mark ) corresponds to a high frequency. Double modulation is carried out by using a frequency-modulated sub-carrier.
Bell 202
With the widely used 1200 baud AFSK , for example, the two tone frequencies 1200 and 2200 Hz are keyed. The difference between these two sound frequencies (in this case 1000 Hz) is called the shift . These frequencies come from AT&T "Bell 202" modem that was connected directly to an FM radio.
Advantages and disadvantages
The use of an easily manageable NF modulation as a sub-carrier means that the effort for the transmission of digital data is minimal. The frequency generator of the transmitter does not have to be controlled directly. The demodulation is also largely uncomplicated and again results in an LF signal. A major disadvantage of this method is that the bandwidth efficiency is very low due to the double modulation , see also Carson's formula . Methods that modulate the RF carrier directly can be much more bandwidth efficient. The transmission quality does not increase significantly due to the double modulation.
AFSK is used in spite of the disadvantages because in simple FM radios only one LF signal can be used for (de-) modulation. In order to generate FSK with such a device, the LF signal would have to be switched in rectangular form between level 0 and the level for the desired frequency deviation . The limitation of the frequency of the LF signal (high-pass behavior of the modulator) means that the modulation signal is no longer present in a rectangular shape. The modulated signal would no longer be clearly keyed between the frequency stages, which makes reliable demodulation difficult. Maintaining the exact modulation levels that determine the frequency deviation also turns out to be a problem. With the use of finished FSK modules such as B. for Bell 202 for NF modulation, these problems can be easily avoided.
application
AFSK has applications in radio telex using the Baudot code as well as in amateur radio with 1k2 packet radio and its application APRS , with AMTOR and PACTOR-I .
Earlier 8-bit computers used AFSK to store data on a datasette . The same procedure was also used for remote data transmission by fax or modem.
AFSK in relation to SSB and FSK
The single-sideband (SSB) ensure the implementation ( modulation ) of low frequency to high frequency, without changing the waveform of the signal. A transmission of an FSK-modulated NF signal with SSB is therefore still FSK, even if the frequencies are shifted into the range of the target frequency.
AFSK is given when an LF signal is first frequency-modulated and then transmitted with frequency modulation. AFSK differs from FSK in that it has double modulation. With FSK, the high-frequency carrier frequency is modulated directly with the digital signal.
With improved methods such as GMSK, a multiple of the data rate can be achieved in the same bandwidth .
Sound samples
(0:35 min, 92 kB, OggVorbis)
