![]() Normally, the transmitted audio alternates between two tones: one, the "mark", represents a binary one the other, the "space", represents a binary zero.ĪFSK differs from regular frequency-shift keying in performing the modulation at baseband frequencies. Main article: Gaussian minimum-shift keyingĪ variant of MSK called Gaussian minimum-shift keying ( GMSK) is used in the GSM mobile phone standard.Īudio frequency-shift keying (AFSK) is a modulation technique by which digital data is represented by changes in the frequency ( pitch) of an audio tone, yielding an encoded signal suitable for transmission via radio or telephone. , +0.93, +0.98, +1, and this smoother pulse is used to determine the carrier frequency, the out-of-band spectrum will be reduced. If the pulse is changed going from −1 to +1 as −1, −0.98, −0.93. In ordinary non-filtered FSK, at a jump from −1 to +1 or +1 to −1, the modulated waveform changes rapidly, which introduces large out-of-band spectrum. Gaussian filtering is a standard way to reduce spectral width it is called pulse shaping in this application. For basic data rate Bluetooth the minimum deviation is 115 kHz.Ī GFSK modulator differs from a simple frequency-shift keying modulator in that before the baseband waveform (with levels −1 and +1) goes into the FSK modulator, it passed through a Gaussian filter to make the transitions smoother to limit spectral width. It is used by Improved Layer 2 Protocol, DECT, Bluetooth, Cypress WirelessUSB, Nordic Semiconductor, Texas Instruments, IEEE 802.15.4, Z-Wave and Wavenis devices. This filter has the advantage of reducing sideband power, reducing interference with neighboring channels, at the cost of increasing intersymbol interference. Rather than directly modulating the frequency with the digital data symbols, "instantaneously" changing the frequency at the beginning of each symbol period, Gaussian frequency-shift keying ( GFSK) filters the data pulses with a Gaussian filter to make the transitions smoother. The elimination of discontinuities in the phase (and therefore elimination of sudden changes in amplitude) reduces sideband power, reducing interference with neighboring channels. In practice, many FSK transmitters use only a single oscillator, and the process of switching to a different frequency at the beginning of each symbol period preserves the phase. In general, independent oscillators will not be at the same phase and therefore the same amplitude at the switch-over instant,Ĭausing sudden discontinuities in the transmitted signal. In principle FSK can be implemented by using completely independent free-running oscillators, and switching between them at the beginning of each symbol period. Main article: continuous-phase frequency-shift keying
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