Amplitude-shift keying
Digital data encoded as carrier amplitude variations.
Amplitude-shift keying, or ASK, is a type of amplitude modulation where digital information is conveyed by changing the amplitude of a carrier wave. For a single-bit-per-symbol system, the carrier might be sent at full strength for a binary 1 and at a lower strength or not at all for a binary 0.
The method works by using a limited set of distinct amplitudes, each tied to a specific binary pattern. Typically, every amplitude encodes the same number of bits, and each pattern of bits forms a symbol. On the receiving end, a demodulator designed for that exact symbol set measures the incoming signal’s amplitude and translates it back to the original data, while the carrier’s frequency and phase stay fixed.
Like standard AM, ASK is linear and vulnerable to atmospheric noise, distortion, and varying propagation conditions in telephone networks. Both the modulation and demodulation processes are inexpensive. ASK is also widely used for sending digital data over optical fiber. With LED transmitters, a short pulse of light stands for binary 1, and no light stands for binary 0. Laser transmitters often run a fixed bias current that produces a low light level for binary 0, with a higher-amplitude light wave for binary 1.
The simplest and most common ASK form acts like a switch: the presence of the carrier indicates a binary 1, and its absence indicates a binary 0. This is called on-off keying (OOK) and is used at radio frequencies for Morse code (continuous wave operation). More advanced M-ary ASK methods, such as QAM, exist.
More complex encoding schemes use additional amplitude levels to represent data in groups. For example, a four-level scheme can encode two bits per amplitude shift, and an eight-level scheme can encode three bits. These higher-level ASK forms require a strong signal-to-noise ratio to be recovered reliably, because much of the signal is transmitted at reduced power.
An ASK system can be broken into three parts: the transmitter, a linear model of the channel’s effects, and the receiver’s structure. The carrier signal for transmission is denoted ht(f), the channel’s impulse response is hc(f), n(t) is channel noise, and hr(f) is the receiver filter. L is the number of levels used, and Ts is the time between symbol generations. Different symbols correspond to different voltages.
- Voltage range
- [−A, A]
- Voltage formula
- v_i = (2A/(L-1))*i - A; i = 0,1,…,L-1
- Voltage difference
- Δ = 2A/(L-1)
- Symbol time
- Ts
- Levels
- L
Lore & Background
ASK is a linear modulation scheme, sensitive to atmospheric noise, distortions, and propagation conditions. Both modulation and demodulation processes are relatively inexpensive. The simplest and most common form of ASK is on-off keying (OOK), which uses the presence of a carrier wave to indicate a binary one and its absence to indicate a binary zero; this is used at radio frequencies to transmit Morse code (referred to as continuous wave operation).
For optical fiber transmission, LED transmitters represent binary 1 by a short pulse of light and binary 0 by the absence of light. Laser transmitters normally have a fixed bias current that causes a low light level for binary 0 and a higher-amplitude lightwave for binary 1. More sophisticated M-ary encoding schemes use additional amplitude levels, such as four-level encoding representing two bits per shift or eight-level encoding representing three bits; these require a high signal-to-noise ratio for recovery.
An ASK system can be divided into three blocks: transmitter, linear channel model, and receiver. The transmitter generates impulses with areas corresponding to symbol voltages, which are filtered and sent through the channel. The receiver filters the signal and performs analog-to-digital conversion. If filters satisfy the Nyquist ISI criterion, intersymbol interference is eliminated, and transmission is affected only by noise.
Reader's Guide
ASK is significant as a foundational digital modulation technique, directly derived from amplitude modulation. Its primary legacy is its simplicity and low cost, making it suitable for applications such as optical fiber data transmission and Morse code radio transmission. ASK is linear and sensitive to atmospheric noise, distortions, and propagation conditions, which limits its use in noisy environments. The development of M-ary ASK, including QAM, allows higher data rates by encoding multiple bits per symbol, but at the cost of requiring a high signal-to-noise ratio. The mathematical framework provided in the article, including the voltage level formulas and the system block model with Nyquist ISI criterion, formalizes the design and analysis of ASK systems.
Did You Know?
- ASK uses a finite number of amplitudes, each assigned a unique pattern of binary digits.
- The simplest form of ASK is on-off keying (OOK), used to transmit Morse code at radio frequencies.
- M-ary ASK schemes, such as four-level encoding, require a high signal-to-noise ratio for recovery.
More in Radio Modulation Modes 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced