What is Amplitude Modulation?

Amplitude Modulation (AM) is the modulation of one signal’s amplitude by another. You may be familiar with the term from its application in broadcast for transmitting audio on radio waves.  

Ultrahaptics’ original haptic technology modulated ultrasound waves by switching them off and then on again fast enough to stimulate the vibration-sensitive touch receptors (mechanoreceptors) in the skin of the hands, giving tactile sensation. These pulses are typically at frequencies between 40Hz and 400 Hz (40 to 400 times a second). On and off switching of ultrasound can cause unpleasant, audible effects, so the actual pulses are smoothed with a sinusoidal wave.

You can see this in the diagram below, which shows the low-frequency haptic signal in red modulated on the 40kHz ultrasound pressure wave. 


Projecting ultrasound onto an oil bath allows you to ‘see’ the shapes created by Ultrahaptics’ haptic technology and the difference between Amplitude Modulation and Spatiotemporal Modulation.

Here you can see the discrete focal points created by Amplitude Modulation:


Amplitude Modulation is suitable for providing mid-air, haptic feedback or feedforward (where you receive information prior to an event) in applications with controls such as buttons, switches and sliders or when indicating virtual spaces.

The Amplitude Modulation API

Ultrahaptics also uses the term Amplitude Modulation to refer to its first-generation modulation scheme and API. This uses a fixed waveform and requires your application to send updates to the array when you wish to change control point position and intensity. Since an update will only take effect at a maximum or minimum of a control point’s intensity, the update rate will be less than twice the modulation frequency:


For a modulation frequency of 200 Hz, the latency between update and its application is less than 2.5ms. Updates that are sent at a higher rate are ignored.

As all the Ultrahaptics software takes care of modulating the intensity, the Amplitude Modulation emitter makes generating haptic sensations very easy. 

You can read more about the Amplitude Modulation API here, and full details can be found in the documentation included in Ultrahaptics SDK.

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