In order to transport digital information over the air, there needs to be a physical representation for the states 1 and 0 that can be imposed on a radio frequency.
Quick Overview
Amplitude Shift Keying (ASK) is a simple digital modulation technique often used in remote controls and other short range devices.
There are more digital modulation techniques such as PSK (phase-shift keying), FSK (frequency-shift keying), and QAM (quadrature amplitude modulation). ASK however is most frequently used in breakout boards and commercial remote controls because of its simplicity.
ASK is a variant of amplitude modulation: the radio frequency remains fixed. The information is expressed as a variation in amplitude of the radio waves.
ASK is used to transmit digital data over optical fibers, too. Here, a binary 1 is represented by a short pulse of light, and a binary 0 by the absence of light.
On-Off Keying (OOK)
To transport digital information via ASK, the bits need to be transformed to radio signals.
In its most simplistic and common form, the value 1 is sent at a nominal amplitude, and the value 0 is represented by the absence of a radio signal (pause). This type of modulation is called on-off keying (OOK).
Since radio transmitters can vary the amplitude, using a simple on-off scheme with a fixed amplitude is inefficient from a data transmission point of view, but very efficient from an implementation point of view.
Since remote controls transmit only very little data, OOK overall is the best solution.
Sophisticated Encoding Schemes
Once data transmission gets into the focus, i.e. when sending sensor data or text messages, more sophisticated encoding schemes can define additional symbols (groups of bits), and use different amplitude levels to identify these symbols.
This increases the data transmission speed as more information can be transmitted in the same time.
Morse code can be viewed as OOK with two symbols instead of one: there is a symbol for a short tone and a symbol for a long tone. Imagine how much longer it would take for morse code messages if there was just one symbol, and all letters would need to be defined just by short tones.
EV1527
EV1527 represents a specialized encoding scheme frequently used in simple commercial remote controls.
Actually, EV1527 is the name of an encoder chip that first introduced this encoding scheme.
You do not need a dedicated encoder chip as long as you know the encoding details (especially its timing) and use a microcontroller. There are plenty of free libraries available at github that can generate the EV1527 signals. One of the most popular library is rc-switch.
EV1527 uses OOK as carrier signal and transmits a 24-bit data packet. The first 20 bits represent a unique (random) hardware id set by the chip producer.
The remaining four bits can be used for data, i.e. to address four separate switches.
This scheme ensures that with 2^20 = 1.048.576 different combinations, remote controls are roughly as specific as typical door locks and won’t accidentally open the neighbors’ garage door.
On the receiver side, a microcontroller can decode the EV1527-encoded information (i.e. by using the rc-switch library), or encoding is done by dedicated decoder chips such as the ST3777.
Getting Into It
There is a multitude of breakout boards available for ASK, both senders and receivers.
Same Basic Principle
All ASK senders and receivers expose just one data pin.
The data to be transmitted is fed to the senders’ data pin and received at the receivers’ data pin.
The intellectual part is the data encoding: the data to be transmitted needs to be transformed into indivuidual digital bits (encoded), and on the receiver side transformed back into the original data (decoded).
Simple Route
If you do not want to wrap your head around ASK and encoding/decoding, then a number of ready-to-go libraries exist.
rc-switch is a very popular example.
Understanding Technology
If you are curious and hungry for more, this article provides a great in-depth introduction and explains the technical principles and myths attached to ASK.
I strongly recommend reading this article as it explains the tricks to use to improve data rate, stability, and reach of ASK-based solutions.
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(content created Apr 15, 2024)