## 4. Actively Augmenting Mobile Devices
### Active Augmentation Framework
#### 5.1.2 From Virtual to Physical

3D printing can be used to materialize virtual representation of musical instrument parts under certain conditions. Thus, most elements provided to mesh2 faust[56] can be printed and turned into physical objects. #### 5.1.3 Connecting Virtual and Physical Elements

Standard hardware for digitizing mechanical acoustic waves and vice versa can be used to connect the physical and virtual elements of a hybrid instrument (see Figure 11). Piezos (contact microphones) can capture mechanical waves on solid surfaces (e.g., guitar body, string, etc.) and microphones mechanical air waves (e.g., in a tube, etc.). Captured signals can be digitized using an analog to digital converter (ADC). Inversely, digital audio signals can be converted to analog signals using a digital to analog converter (DAC) and then to mechanical waves with a transducer (for solid surfaces) or a speaker (for the air). In some cases, a unidirectional connection is sufficient as waves travel in only one direction and are not (or almost not) reflected. This is the case of the BladeAxe[42] where sound excitations (i.e., plucks) are picked up using piezos and transmitted to virtual strings. This type of system remains simple and works relatively well as the latency of the DAC or the ADC doesn't impact the characteristics the generated sound. On the other hand, a bidirectional connection (see Section 5.2.1) might be necessary in other cases. Indeed, reflection waves play a crucial role in the production of sound in some musical instruments such as woodwinds. For examples, connecting a physical clarinet mouthpiece to a virtual bore will require the use of a bidirectional connection in order for the frequency of vibration of the need to be coupled to the tube it is connected to. This type of connection extends beyond the instrument to the performer that constantly adjusts its various parameters in function of the generated sound [5]. However, implementing this type of system can be very challenging as the DAC and the ADC will add latency, which in the case of the previous example will artificially increase the length of the virtual bore. Thus, using low latency DACs and ADCs is crucial when implementing this type of systems sometimes involving the use of active control techniques [58; 59]. More generally, the use of high-end components with a flat frequency response is very important when implementing any kind of hybrid instruments. Also, hardware can become very invasive in some cases, and it is the musical instrument designer's responsibility to find the right balance between all these parameters. #### 5.1.4 Adapting This Framework to Mobile Devices

Beyond this theoretical modularity (keeping in mind that audio latency can be a limiting factor in some cases) where any part of mobile hybrid instruments can either be physical or virtual, some design \"templates\" are more efficient than others. Here, we give some guidelines/rules to restrain the scope of our framework to optimize its results when making mobile hybrid instruments. In the context of augmented mobile instruments where standalone aspects and lightness are key factors, the number of physical/acoustical elements of hybrid instruments must be scaled down compared to what is possible with a desktop-based system. Indeed, transducers are large and heavy components requiring the use of an amplifier, which itself needs a large power source other than the mobile device battery, etc. Similarly, multichannel ADCs and DACs can take a fair amount of space and will likely need to be powered with an external battery/power supply. Even though applications generated with faust2 smartkeyb (see Section 2) are fully compatible with external USB ADC/DACs, we believe that restraining hybrid mobile instruments to their built-in ADC/DACs helps preserve their compactness and playability.