Digital data, more specifically, the binary data changes the properties of
the carrier signal
• In digital modulation, an analog carrier signal is modulated by a discrete signal
Changing amplitude, frequency or phase in proportional to the binary data, produces
digital modulated signal called
• Amplitude Shift Keying (ASK)
• Frequency Shift Keying (FSK)
• Phase Shift Keying (PSK)
ASK – Amplitude shift keying
• Amplitude-shift keying (ASK) is a form of modulation that represents
digital data as variations in the amplitude of a carrier wave.
• The amplitude of an analog carrier signal varies in accordance with
the bit stream (modulating signal), keeping frequency and phase
• The level of amplitude can be used to represent binary logic 0s and
Binary ASK(BASK) or On-Off Keying (OOK)
• Although we can have several levels (kinds) of signal
elements, each with a different amplitude, ASK is normally
implemented using only two levels. This is referred to as
binary amplitude shift keying.
• We can think of a carrier signal as an ON or OFF switch. In the
modulated signal, logic 0 is represented by the absence of a
carrier, thus giving OFF/ON keying operation and hence the
name given OOK.
MSK – Multiple shift keying
• The above discussion uses only two amplitude levels. We can have
multilevel ASK in which there are more than two levels. We can use
4,8, 16, or more different amplitudes for the signal and modulate the
data using 2, 3, 4, or more bits at a time.
Demodulator Or Detector
• The demodulator determines the amplitude of the received signal
and maps it back to the symbol it represents, thus recovering the
Pros and Cons
• ASK transmitter and receiver are simple to design.
• ASK needs less bandwidth than FSK.
• ASK transmission can be easily corrupted by noise
• early telephone modem(AFSK)
• ASK is used to transmit digital data over optical fibre.