Full large-vocabulary speech recognition is unnecessary and far too expensive for many practical applications. For a table lamp, it would quite sufficient if it understands "Light off" and "Light on." Besides, most of the time, speech devices are just sitting there waiting for instructions when nobody is speaking. Speech recognition algorithms moreover use a lot of computational resources such that it would be wasteful to have them analyse sounds when there is no speech present. Voice activity detection (VAD) takes care of the first part, to detect whether speech is present or not, such that all activities are in a sleep mode when speech is not present.
Wake-word and keyword spotting refer to small-vocabulary speech recognition tasks. They are used either in very simple applications where proper speech recognition is unnecessarily complex (keyword spotting), and in pre-processing tasks, where we want to save resources by waiting for a "Hey computer!". In the latter task, wake-word spotting is thus a trigger for more complex speech processing tasks. Though the two tasks have rather different objectives, the underlying technology is very similar and they will here be discussed jointly.
Most typically wake-word and keyword spotting algorithms run on devices with limited resources. They can be limited in memory footprint and in computation resources (CPU power) or often both. Increasing amount of memory or using a larger CPU would both increase cost of device (investment cost), but would also require more power (maintenance cost). In small devices such marginal costs are a very significant part of the overall cost of the device.
Adapted from Yundong Zhang, Naveen Suda, Liangzhen Lai and Vikas Chandra, "Hello Edge: Keyword Spotting on Microcontrollers" .
In other words, keyword and wake-word spotters have a small set of accepted keywords, which are hard-coded into the software. If we have N possible keywords, then the neural network has N outputs corresponding to the probability that the input is each of those keywords. The output is then thresholded such that that keyword is chosen which has the largest probability.
The choice of structure for the neural network is highly dependent on the device constraints as well as the objective, operating environment and context of the application. Usually the structure is a deep neural network featuring some recurrent, convolutional and long-short term memory (LSTM) components.
A central challenge in training keyword spotting algorithms is finding and choosing training data. To get good quality, you would typically need several tens of thousands of utterances of the keywords, spoken by a large range of different speakers and in different environments. For example the "Speech Commands Dataset" by Google has 65.000 utterances of 30 short words. However, the choice of keywords is naturally dependent on those functions that the keyword spotter should activate, or the desired wake-word. For real-world applications we therefore often cannot use pre-collected datasets, but have to collect our own. You can just imagine the workload required to collect 65.000 utterances from over a thousand speakers!